J Appl Physiol 97: 2322–2332, 2004.
First published August 20, 2004; doi:10.1152/japplphysiol.00955.2003.
Contractile responses of the rat gastrocnemius and soleus muscles
to isotonic resistance exercise
K. M. Norenberg and R. H. Fitts
Department of Biology, Marquette University, Milwaukee, Wisconsin 53201
Submitted 5 September 2003; accepted in final form 15 August 2004
weight training; single-fiber physiology
type II fiber types (7, 12, 35, 36, 39). The increase in force and
power appears to be primarily caused by the exercise-induced
muscle hypertrophy (12). The effects of WT on maximal
shortening velocity (Vmax) are not well understood (12). Both
endurance and resistance exercise-training programs reduce the
percentage of type IIb and increase the number of IIx and IIa
fibers, a change that should reduce the velocity of fast skeletal
muscles (12). Controversy exists as to whether or not the
velocity of individual fiber types is altered by resistive exercise
programs. WT in young men had no effect on the maximal
unloaded shortening velocity (V0) of the slow type I or fast type
IIa fibers (39). In older men but not women, Trappe et al. (35,
36) observed a high-resistance exercise program to increased
V0 in both the type I and IIa fiber types.
Muscle atrophy and deleterious changes in cell function are
known to be a serious problem during normal aging and in the
microgravity environment of space. In both cases, regular
exercise training has been utilized as a countermeasure to slow
and reduce the deleterious alterations in limb skeletal muscle.
It is our hypothesis that the optimal exercise countermeasure
will require the combination of both isometric and isotonic
resistance exercises, with the former best protecting tonic
slow-twitch muscles and the latter phasic fast-twitch muscles.
This hypothesis is supported by the data of Hurst and Fitts (20),
who observed isometric exercise to better protect the rat slow
Sol than the fast calf muscles from hindlimb unloading-induced atrophy. To test this hypothesis, it is necessary to first
develop isometric and isotonic exercise protocols that can be
quantified as to the work performed and tested independently
and then in combination. We have recently developed an
isometric testing device and studied the effects of isometric
exercise-training on rat calf muscles (20). The purpose of this
study was to use a modification of a “squat”-type exercise first
reported by Klitgaard (21) to determine the effects of isotonic
WT exercise on the following: 1) the mass of the plantar and
dorsiflexors of the foot; and 2) the contractile properties of the
intact Sol as well as individual slow- and fast-twitch fibers
isolated from both the Sol and the Gast. This is the first study
examining both whole muscle and cellular contractile properties of rat muscle in response to WT resistance exercise.
DESPITE THE WIDESPREAD USE of resistive exercise and its apparent effectiveness in increasing skeletal muscle size and strength
and reducing muscle atrophy associated with aging and bed
rest, there is a paucity of data concerning the contractile
adaptations at either the whole muscle or cell level (12).
Several animal models have been developed to study the
effects of resistive exercise on skeletal muscle to include
“squat”-type exercises (13, 15, 21, 34), weighted ladder climbing (8, 38), isometric training (10, 22), plyometric training (1),
and electrically evoked contractions (3, 4, 42). A general
finding is that resistance exercise increases the muscle-to-body
weight ratio of the soleus (Sol) by 31– 45% (21, 22), the
plantaris (Plant) by 26 –34% (21, 22, 34), and the gastrocnemius (Gast) by 7–18% (21, 22, 32, 34, 42). Few studies have
investigated the contractile changes that occur as a result of
resistance exercise. In rat studies, primarily slow-twitch muscles, such as the adductor longus and Sol, have shown no
change in peak isometric tension (1, 8, 10, 32). Fast-twitch
muscles may be more responsive to weight training (WT) as
greater gains in peak forces have been reported for the rectus
femoris and medial Gast (10).
Human resistance exercise training programs routinely produce increases in muscle cross-sectional area (CSA), strength,
and power, with gains occurring in both the slow type I and fast
Animal characteristics, exercise program, and diet. All methods
were approved by the Marquette University Institutional Animal Care
and Use Committee. Due to the time involved with training, this study
was carried out in five phases with four rats comprising each phase.
Twenty male Sprague-Dawley rats were obtained from the same
vendor (Sasco, Madison, WI) and weighed ⬃250 g on arrival. Rats
Address for reprint requests and other correspondence: R. H. Fitts, Dept. of
Biological Sciences, Wehr Life Sciences Bldg., Marquette Univ., P. O. Box
1881, Milwaukee, WI 53201–1881 (E-mail: [email protected]).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
2322
METHODS
8750-7587/04 $5.00 Copyright © 2004 the American Physiological Society
http://www. jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
Norenberg, K. M., and R. H. Fitts. Contractile responses of the
rat gastrocnemius and soleus muscles to isotonic resistance exercise.
J Appl Physiol 97: 2322–2332, 2004. First published August 20, 2004;
doi:10.1152/japplphysiol.00955.2003.—Male rats were divided into
control and weight-trained (WT) groups. WT rats performed squattype exercises twice daily, 5 days/wk, for 14 wk. They averaged 36
lifts/day, with an average weight of 555 g. Muscle-to-body weight
ratio (mg/g) of the soleus (Sol) was not different from control, but it
increased 11 and 6% in the gastrocnemius (Gast) and plantaris,
respectively (P ⬍ 0.05). The normalized twitch tension of the in situ
Sol was elevated by 21%, whereas single-skinned type I fibers from
the Sol showed an increased rate constant of tension redevelopment
(Ktr) but no other contractile adaptations to WT. In contrast, the Gast
type I fibers showed an increase (P ⬍ 0.05) in maximal velocity of
shortening (25%), peak power (15%), Ktr (18%), and normalized
tension (7%). The Ktr and normalized tension of the Gast type IIa
fibers increased by 24% (P ⬍ 0.05) and 12% (P ⬍ 0.05), respectively,
whereas velocity and power showed a tendency to increase. Fiber size,
determined by myosin ATPase histochemistry, was not different for
any fiber type from the Gast or Sol. These results indicate that isotonic
resistance exercise of the calf targets the Gast (type I and type IIa
fibers) and has little effect on the Sol.
RESISTANCE EXERCISE IN RATS
Fig. 1. Isotonic training apparatus. Rats were trained to enter the Lucite
cylinder through the door, place their head through the collar, and extend their
hindlimbs to obtain a food pellet suspended above the collar from the top of the
cylinder. Vertical displacement was measured by a sliding indicator and, in
selected experiments, by the photodiode array. The product of mass lifted,
vertical displacement, and number of repetitions was used to calculate work.
See text for details regarding the training procedure.
J Appl Physiol • VOL
displacement of the collar during lifts. Weights could be added to
horizontal bars that screwed into the collar to provide resistance (Fig.
1). A small food pellet (120 –150 mg) was held in place near the top
of the cylinder by means of rubber tubing attached to a 20-cm rod. The
rod was passed through a rubber stopper that fit into a hole in the top
of the cylinder. The position of the food pellet and collar could be
adjusted to accommodate different sized rats and to maximize the
range of motion. A successful lift consisted of the rat entering the
tube, assuming a crouched posture on its hindlimbs, placing its head
through the collar, and lifting the collar and attached weights by
plantar flexion to receive a food pellet. The rat would usually withdraw from the tube to eat the pellet and then return to perform the next
lift. Rats were trained to perform the lifting motion in the following
four stages. In stage 1, small pellets of Purina Rodent Chow were
placed on the floor within the walls of the cylinder and in the food
holder ⬃15 cm from the floor of the cage. During this stage, rats were
trained to associate food with the cylinder and were encouraged to
enter the cylinder and take food from the holder. This stage lasted
⬃1–2 days. In stage 2, the food pellet was gradually moved upward
so that rats had to place their entire body within the tube. The collar
was progressively lowered so that the rats had to put their head
through the collar, plantar flex against the load of the unweighted
collar (200 g), and move it upward to get the food pellet. This stage
lasted ⬃3 days. In stage 3, the collar was lowered further so that
resistance would be met soon after the rat began to raise its body from
a crouched position. The food pellet was raised to a height that
required full range of motion at the ankle, as determined by visual
inspection, to receive a food pellet. Weight was added to the collar as
rapidly as would be tolerated. Pilot studies indicated that rats could lift
⬃200% of body weight after the first 2 wk of lifting. In stage 4, after
the initial 2 wk of training, the weight lifted was kept near 200% of
body weight for the remaining 12 wk of the study. Occasionally, rats
were unwilling to lift this load, and the weight had to be reduced. The
weight was returned to 200% of body weight as rapidly as possible.
WT rats were trained twice daily, for 30 min/session, 5 days/wk,
for 14 wk. During each training session, rats performed as many
repetitions as possible. The first training session occurred in the
morning, approximately 1 h into the light cycle. Six hours later the
rats were trained again.
Feeding pattern. To initiate training, all rats were fasted for 18 h.
WT rats received small food pellets when they completed a lift
successfully. After the morning training session, the amount of food
eaten by the WT rat was measured, and its corresponding C rat was
fed an equal amount. This was repeated for the afternoon training
session approximately 6 h later. Typically, the amount of food eaten
during lifting was between 3 and 8 g/day. In addition to the food
received while lifting, all rats were fed an additional bolus of 11 g of
food after the second training session. After the 5th day of training,
rats received a bolus of 50 –55 g, which lasted until the first training
session of the next week. In pilot studies, this amount of food left the
rats motivated to perform the exercise but was sufficient to allow an
increase in body weight over the course of the study. The average
number of calories ingested exceeded the minimal daily requirements
for the maintenance of body size as determined by the National
Research Council (29). This value was determined according to the
following equation: kcal/day ⫽ 110 kcal/kg body mass0.75. Purina
Rodent Chow contains 3.04 kcal/g of metabolizable energy and 4.00
kcal/g of gross energy. Using these values, a 300-g rat would require
14.7 g of food per day.
Calculation of work. Work performed during lifting was calculated
as the product of mass lifted, vertical displacement, and number of
repetitions. A sliding indicator attached to the side of the lifting
apparatus measured vertical displacement. To calculate more precisely the amount of work performed, a force plate on which the rat
would stand was mounted below the lifting apparatus, and a photodiode array (Positron Development) was attached to the collar to
measure vertical displacement (Fig. 1). Force and position traces and
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
were handled for 3 days to minimize contact stress. During this
period, all rats were individually exposed to the cage in which WT
took place. Those rats that readily entered the exercise tube and took
food from the holder (see below) were assigned to the WT group that
performed resistance exercises, twice daily, 5 days/wk, for 14 wk (n ⫽
11). The remaining rats were assigned to a sedentary control (C) group
that was restricted to cage activity (n ⫽ 9). Rats were housed
individually in standard rat cages and maintained on a 12:12-h
light-dark photoperiod. They had free access to water and were pair
fed, as described below.
WT. The model of isotonic WT used in this study was a modification of that originally reported by Klitgaard (21). The training
apparatus (Fig. 1) consisted of a Lucite cylinder, 30 cm high by 10 cm
diameter, mounted vertically at one end of a lucite platform. A door
10 cm high was cut into the front of the cylinder to allow rats to enter.
A doughnut-shaped disk (or “collar”) was suspended from the top of
the cylinder and could be easily moved vertically within the cylinder.
The diameter of the hole in the collar through which the rat passed its
head could be shimmed to a diameter of 3, 3.5, or 4 cm. Collars were
chosen to ensure that the weight rested on the shoulder girdle of the
rat and that only the head of the rat could pass through. A sliding
indicator and ruler on the side of the tube indicated the vertical
2323
2324
RESISTANCE EXERCISE IN RATS
Fig. 2. Records collected during a representative lift using a photodiode array
to measure vertical displacement of the weights (A) and a force plate to
measure weight lifted (B). The body mass of the rat (⫺280 g) was not
subtracted from the total force. Electromyography (EMG) of the soleus (Sol;
C) and lateral gastrocnemius (Gast; D) are also shown. The EMG electrode
implants and recordings were performed exactly as described previously (20).
J Appl Physiol • VOL
ms (Positron Development). The analog signals for force and length
were displayed on an oscilloscope, amplified, converted to a digital
signal, and sent to a personal computer for data analysis. The optimal
length (LO) for force development was determined, and then twitch
contractions were elicited. From each contraction, peak isometric
twitch tension (Pt), time to peak tension (TPT), one-half relaxation
time, and the peak rate of force development (⫹dP/dt) and decline
(⫺dP/dt) were measured. TPT and one-half relaxation time are defined as the time elapsed from the onset of force production to Pt and
the time required for force to decline from Pt to one-half of Pt,
respectively. To measure peak tetanic force (PO), the Sol was stimulated with supramaximal, square-wave, biphasic pulses at 100 Hz for
1,000 ms. From each contraction, PO, ⫹dP/dt, and ⫺dP/dt were
measured. Each contraction was separated by a minimum of 1 min.
Slack test. Maximal V0 of the muscle was measured by the slack
test method (26, 38). The muscle was stimulated to contract tetanically for 750 ms at which point the computer triggered shortening of
the muscle by a predetermined distance, followed by an additional 200
ms of stimulation. The tension in the shortened muscle dropped to
zero until cross-bridge cycling could take up the induced slack and
begin to restore force. The period of unloaded shortening, which was
defined as the time from when the length change (⌬L) was completed
until the force was discernibly above baseline, was measured for each
slack length. LO was reset, and the process was repeated to give a total
of five different ⌬L values. The duration of unloaded shortening was
plotted vs. the slack step distance. The slope of this relationship
represents V0. To compensate for muscles of different lengths, absolute V0 (mm/s) was divided by muscle length (ML), and V0 was
expressed as ML per second.
Force velocity. To construct a force-velocity curve, the muscle was
maximally activated, and, after the muscle had generated peak tension, the computer triggered a series of isotonic shortenings (23, 40).
The position motor was integrated into a feedback servomechanism,
whereby force was kept constant at a submaximal load by shortening
the muscle at a constant rate necessary to maintain the selected force.
During each contraction, the muscle was stepped through three submaximal loads. The velocity of shortening and relative force were
measured during the final one-half of each load step. This process was
repeated four to five times for each muscle at different loads to
generate 12–15 data points per curve. The Vmax was determined for
each muscle by fitting the data to the Hill equation: (P ⫹ a)(V ⫹ b) ⫽
(PO ⫹ a)b, where P is the isometric force, V is the velocity, a is a
constant in units of force, and b is a constant with units of velocity
(19). The data were fit by using an iterative nonlinear curve-fitting
procedure (Marquardt-Levenberg algorithm), as previously described
(40). The maximal velocity of shortening was normalized to ML. The
parameters describing the force-velocity relationship, Vmax (the velocity
axis intercept), a/PO (a unitless parameter describing the curvature of the
relationship), and PO were used to calculate peak power (40).
Skinned fiber contractile properties. To determine the fiber-typespecific effects of isotonic resistance exercise on contractile function,
single-fiber studies were conducted. These studies were particularly
important in determining the exercise training-induced changes in the
heterogeneous Gast, where intact muscle studies of function are
limited to isometric measurements. The direct Ca2⫹ activation of the
individual skinned fiber allowed the effects of exercise-training on PO
to be assessed independently of the excitation-contraction coupling
process. Additionally, the measurements of the rate constant of
tension redevelopment (Ktr) and fiber elastic modulus (E0) provide
important information about the cross-bridge binding rate and force
per cross bridge that cannot be obtained from whole muscle studies.
Fiber PO, Ktr, maximal V0, Vmax, peak power, and E0 were determined for single-fiber segments isolated from the Sol and Gast. All
contractile tests were performed on each fiber segment. If fiber
deterioration occurred (PO decline of ⬎10%), the data for that fiber
were discarded. The measurement of fiber V0 (slack test), Vmax
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
the corresponding electromyogram (EMG) traces for the Sol and
lateral Gast during a representative lift are presented in Fig. 2. The
signals from the force plate and photodiode were digitized and later
analyzed by using custom software. The software calculated peak
force, peak displacement, and the duration of the lift. Data were
collected by using this system during one session, once a week for two
of the WT rats.
Determination of whole muscle contractile properties. To avoid
experimenter bias, all contractile tests were performed blind. The
identities of the rats were kept by a third party and not revealed until
the completion of the study. Due to the force limitations of the
servomotor force transducer employed (300 g maximal), we were only
able to test the contractile properties of the Sol and not the Plant or
Gast. In situ contractile studies were performed as previously described (24). On completing the 14-wk training period, rats were
anesthetized to a surgical plane (pentobarbital sodium, 50 mg/kg ip).
An incision was made along the centerline of the dorsal side of the left
leg from the ankle to the hip. The hamstrings and medial and lateral
heads of the Gast and the Plant were dissected to expose the underlying Sol, with care taken not to disrupt the blood or nerve supply. The
surrounding muscles were denervated, and a suction electrode was
placed on the sciatic nerve to apply stimulation. The distal tendon of
the Sol was cut and attached, by using a loop of 2– 0 suture, to a
servo-controlled transducer (model 305, Cambridge Technologies,
Cambridge, MA) capable of measuring force and displacement. The
preparation was covered with gauze and kept moist with rat Ringer
solution. Temperature probes were inserted into the rectum and the
lateral head of the Gast to monitor core and muscle temperatures,
respectively. Core and muscle temperatures were maintained at 37 and
35°C, respectively, by carrying out the experiments in a temperaturecontrolled chamber.
Isometric contractile properties. The muscle was stimulated by
using supramaximal, constant-current, square-wave pulses lasting 0.5
2325
RESISTANCE EXERCISE IN RATS
droxymethyl)aminomethane, 1% SDS, 2 mg/ml bromophenol blue,
15% glycerol, and 5% ␤-mercaptoethanol. To determine the myosin
heavy chain composition, each fiber was analyzed by SDS PAGE, as
described previously (38). Fibers were identified as fast (type IIb, IIx,
IIa) or slow (type I), based on their mobility compared with known
standards.
Myosin ATPase histochemistry. One-half of the unstimulated Sol
was pinned to an index card at LO. The deep, red portion of the lateral
head of the Gast was separated and also pinned to an index card at a
slightly stretched length. The tissues were rapidly frozen in a slurry of
Freon-22 cooled with liquid nitrogen and stored under liquid nitrogen
until staining. Before staining, the muscle was mounted in Tissue Tek
and sectioned into 10-␮m sections, at ⫺20°C, by using a ReichertJung cryostat. Muscle sections from each group were placed on each
slide to avoid differences due to variations in the staining procedure.
All slides were run in duplicate. Muscle sections were stained for acid
and alkaline myofibrillar ATPase activity by using a modification of
the methods of Guth and Samaha (16). Fibers types were distinguished by either a 15-min preincubation at pH 10.4 or an 8-min
preincubation at pH 4.35. Fast fibers are alkaline stable, and slow
fibers are acid stable.
After staining, sections were analyzed by using a light microscope
interfaced to a personal computer. Fibers were classified as slow type
I, fast type IIa, or fast type IIb. CSAs were measured by computerized
digitizing morphometry (Bioquant II, R&M Biometrics, Nashville,
TN). Before each digitization session, one fiber was digitized five
times to determine the coefficient of variation. A minimum of 50
fibers of each fiber type were measured when possible (the Sol
contains no type IIx or IIb fibers). To determine fiber-type distribution, fiber-type percentages were determined from four visual fields
per muscle (⬃200 fibers total).
Statistics. Comparisons between groups were made by using a
one-tailed, Student’s t-test for independent samples. All values are
reported as means ⫾ SE. The level of significance was established as
either P ⱕ 0.10 or P ⱕ 0.05.
RESULTS
Work performed. Eleven rats were successfully trained to
perform the isotonic resistance exercise. After the first 2 wk of
training, the average maximal weight lifted was 419 ⫾ 5 g.
This corresponded to 155 ⫾ 10% of body mass. The maximum
weight lifted was increased and maintained as close to 200% of
body mass as possible for the remainder of the study. Due to an
increase in body mass throughout the study, the average weight
lifted increased weekly and was 631 ⫾ 10 g or 192 ⫾ 5% of
body mass at the end of the study. Typically, rats began each
training session with five repetitions at a weight between 150
Table 1. Muscle weight and muscle-to-body weight ratios
C
n
Body mass, g
9
363⫾6
WT
11
344⫾8†
P Value
C
167⫾5
1,783⫾81
376⫾13
683⫾22
175⫾8
1,003⫾30
165⫾6
1,865⫾30
378⫾9
674⫾21
175⫾6
977⫾31
P Value
0.08
Wet weight, mg
Sol
Gast
Plant
TA
EDL
Heart
WT
Muscle-to-body weight ratio, mg/g
0.40
0.16
0.46
0.38
0.46
0.28
0.46⫾0.02
4.90⫾0.19
1.04⫾0.03
1.88⫾0.04
0.48⫾0.02
2.77⫾0.08
0.48⫾0.02
5.45⫾0.15*
1.10⫾0.02*
1.94⫾0.05
0.50⫾0.01
2.85⫾0.10
0.21
0.02
0.04
0.16
0.16
0.26
Values are means ⫾ SE; n, no. of animals in each group. C, control; WT, weight trained; Sol, soleus; Gast, gastrocnemius; Plant, plantaris; TA, tibialis anterior;
EDL, extensor digitorum longus. Significantly different from C: *P ⬍ 0.05 and †P ⬍ 0.10.
J Appl Physiol • VOL
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
(determined from force-velocity relationship), and peak power were
exactly as described for the intact Sol. The relaxing (pCa 9.0) and
activating (pCa 4.5) solutions, fiber preparation, measurement of PO,
and stiffness were performed as previously described (23, 25, 26).
Briefly, on completing the in situ contractile tests, the contralateral
Sol and Gast muscles were removed, trimmed of excess fat and
connective tissue, and placed in cold relaxing solution (pCa ⬍ 9.0).
The muscles were divided longitudinally, and one-half were used for
single-fiber contractile studies and the other for ATPase histochemistry. The deep red portion of the lateral Gast was used as it contains
a mix of fast- and slow-fiber types. The section for single-fiber
analysis was dissected into small bundles (⬃150 fibers) that were tied
to capillary tubes and placed in skinning solution and stored at ⫺20°C
for up to 4 wk (26, 38). Individual fibers were isolated and transferred
to the experimental chamber filled with relaxing solution (pCa 9.0 and
15°C). The fiber was suspended between a force transducer (Cambridge Technology, model 400A) and the arm of a position controller
(Cambridge Technology, model 300B), as described previously (26,
38). The sarcomere length was set to 2.5 ␮m, and fiber diameter was
determined by a Polaroid photograph taken while the fiber was briefly
suspended in air. The fiber CSA was calculated based on the assumption that the fiber forms a circular cross section when suspended in air
(38). Fibers were activated by transferring the fiber to a chamber
containing a high calcium content (pCa 4.5). Peak force (PO) was
measured, and V0 and Vmax were determined. Peak power for each
fiber was calculated from PO, Vmax, and a/PO (40). Composite forcevelocity and force-power curves were constructed by calculating
individual fiber velocity and power values at 1% intervals between 0
and 100% of PO and then plotting the mean velocity and mean power
obtained at each interval (40).
Fiber stiffness was measured by triggering a sinusoidal ⌬L equal to
0.05% of fiber length at 1.5 kHz and measuring the resulting force
change (⌬F). Peak fiber stiffness (⌬F/⌬L) was determined by computer subtraction of the stiffness recorded in relaxing solution from
that measured in activating solution. The E0 of each fiber was
calculated as follows: E0 ⫽ ⌬F/⌬L ⫻ fiber length/fiber CSA.
Ktr was measured by using the methods of Brenner and Eisenberg
(2). The fiber was activated at pCa 4.5, and, after the development of
PO, the fiber was slacked 400 ␮m. After a brief pause, the fiber was
reextended to its original length. The pause duration was 45 and 25 ms
for fibers with V0 less than or equal to and greater than 2.0 FL/s,
respectively. Based on fiber force, these values produced the greatest
degree of cross-bridge detachment on reextension of the fiber to LO.
The recovery of force after reextension was fit by using the monoexponential equation to calculate Ktr (28).
Electrophoretic determination of myosin heayy chain composition.
After the contractile test was performed, the fiber was removed from
the chamber and placed in 10 ␮l of SDS sample buffer and stored at
⫺80°C. Sample buffer contained 6 mg/ml EDTA, 0.06 M tris(hy-
2326
RESISTANCE EXERCISE IN RATS
Table 2. Isometric contractile properties of the soleus studied in situ (35 °C)
⫹dP/dt
Peak Force
Group
Twitch
C
WT
P value
Tetanic
C
WT
P value
n
TPT, ms
RT1/2, ms
N
kN/m2
8
9
61⫾3
54⫾3†
0.06
79⫾5
85⫾9
0.22
0.31⫾0.02
0.35⫾0.03†
0.10
53⫾2
64⫾4*
0.02
12.4⫾1.1
14.2⫾1.5
0.18
1.78⫾0.07
1.88⫾0.10
0.19
305⫾13
358⫾28†
0.06
14.4⫾1.1
15.1⫾1.7
0.37
8
9
N/s
⫺dP/dt
N/s
kN䡠m⫺2䡠s⫺1
2.10⫾0.15
2.75⫾0.39†
0.08
⫺3.19⫾0.30
⫺3.29⫾0.31
0.41
⫺0.54⫾0.03
⫺0.62⫾0.07
0.16
2.47⫾0.21
2.94⫾0.43
0.19
⫺16.7⫾1.7
⫺17.7⫾2.1
0.36
⫺2.87⫾0.32
⫺3.39⫾0.48
0.20
kN䡠m⫺2
䡠s
⫺1
Values are means ⫾ SE, n, no. of animals. TPT, time to peak tension; RT1/2, one-half relaxation time; ⫹dP/dt, peak rate of tension development; ⫺dP/dt, peak
rate of tension decline. Significantly different from C: *P ⬍ 0.05 and †P ⬍ 0.10.
J Appl Physiol • VOL
Whole muscle contractile properties. Twitch and tetanic
contractile properties of the Sol are presented in Table 2.
Training produced a significantly greater normalized twitch
force (C: 53 ⫾ 2 kN/m2; WT: 64 ⫾ 4 kN/m2, P ⬍ 0.05).
Representative twitches from C and WT Sol muscles are
shown in Fig. 3. WT decreased TPT and increased absolute
twitch force (N) and ⫹dP/dt (P ⬍ 0.10). Following WT, the
Sol PO normalized for muscle CSA (kN/m2) was higher than C
(P ⬍ 0.10), whereas no differences were observed in tetanic
⫹dP/dt or ⫺dP/dt between groups. For the C group, the Sol
Vmax determined by the slack test (V0) was 2.35 ⫾ 0.19 ML/s,
and this parameter was unaltered by WT. The force-velocity
characteristics of the Sol (studied in situ at 35°C) are shown in
Table 3. WT had no effect on Vmax, a/Po, %Po at peak power,
or peak power (absolute or relative).
Single-fiber diameter and force. Results from slow type I
fibers from the Sol and Gast and type IIa fibers from the Gast
are presented in Table 4. Due to insufficient numbers, those
fibers expressing more than one MHC or other fast isoforms
were excluded from analysis. There were no differences between groups in mean fiber diameter for any fiber type. Nor
were there differences for absolute and normalized peak force
of type I Sol fibers (Table 4). Absolute peak force from WT
type I and type IIa Gast fibers was not significantly different
than C. However, when normalized for fiber CSA, the tension
of type I and type IIa fibers was 7 and 14% greater, respec-
Fig. 3. Representative soleus twitch traces from control (C; solid line) and
weight-trained (WT; dashed line) rats.
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
and 175% of body weight, after which the weight was increased to the final amount. Although WT rats were fairly
consistent in their performance, occasionally they were unwilling to lift, and the maximum weight lifted had to be reduced to
facilitate exercise. Rats averaged 36 ⫾ 1 repetitions/day over
the course of the study. Each lift lasted an average of 1.00 ⫾
0.02 s, as measured from the photodiode trace (Fig. 2), and did
not change over the course of the study. Vertical displacement
increased from 2.93 ⫾ 0.04 cm in the first week of lifting to
3.93 ⫾ 0.01 cm in the last week. There was a ⬍5% difference
in vertical displacement and weight lifted between measurements made using the photodiode and force plate and measurements made by the sliding indicator and summing the added
weights. The average work performed was 7.4 ⫾ 1.3 J/day.
The daily average increased from 5.9 ⫾ 0.5 J/day during the
second week of lifting to 9.0 ⫾ 0.6 J/day in the final week. The
increased amount of work is the result of an increase in the
absolute amount of weight lifted and increased vertical displacement, because the number of lifts performed did not
change over the course of the study.
Muscle and body weights. Body and muscle weights are
summarized in Table 1. At the onset of the study, the average
body weight was not different between groups (C ⫽ 269 ⫾
11 g; WT ⫽ 262 ⫾ 12 g), whereas, following the 14 wk of
exercise-training, the body weights of C rats were significantly
greater than those of WT (P ⬍ 0.10; Table 1). Muscle weights
(mg) showed no significant differences between groups for any
of the muscles studied. However, when normalized to body
mass, the Gast and Plant muscles of the WT rats were 11 and
6% greater, respectively, than those of C (P ⬍ 0.05). The Sol,
tibialis anterior, extensor digitorum longus, and heart showed
no adaptations either in absolute weight or when normalized to
body weight. In an attempt to determine whether there was a
correlation between muscle weights and work performed, the
muscle weights and muscle-to-body weight ratios for the Sol
and Gast of the individual WT rats were correlated with the
average daily work performed by each rat. Surprisingly, we
observed a negative (albeit nonsignificant) correlation between
the amount of work performed and the muscle wet weights for
both the Sol and Gast. There was a stronger negative correlation between the amount of work performed and the muscleto-body weight ratios, and for the Gast this correlation (r2 ⫽
0.474) was significant (P ⬍ 0.05). For the Sol, ML was also
measured, and it was unaffected by the resistance exercise
(28.5 ⫾ 1.0 vs. 27.1 ⫾ 1.3 mm for WT and C, respectively).
2327
RESISTANCE EXERCISE IN RATS
Table 3. Maximal shortening velocity, force-velocity characteristics, and peak power of the soleus studied in situ (35°C)
V0
Peak Power
Group
n
mm/s
ML/s
a/Po
%Po at Peak Power
mN䡠ML䡠s⫺1
C
WT
7
7
64⫾7
71⫾5
2.35⫾0.19
2.43⫾0.13
0.27⫾0.02
0.23⫾0.03
31.1⫾0.7
29.9⫾1.1
151⫾22
154⫾16
W/l
26⫾3
30⫾3
Values are means ⫾ SE; n, no. of animals. V0, maximal unloaded shortening velocity determined from the slack test; Po, peak isometric force; a/Po, unitless
parameter describing curvature of the force-velocity relationship; ML/s, muscle lengths per second. Values expressed in ML/s can be converted to fiber lengths/s
(FL/s) by dividing by the rat soleus FL-to-ML ratio of 0.58 (41). The relative power unit of watts/liter (W/l) is equivalent to kN䡠m⫺2䡠ML䡠s1.
was 20% higher in WT compared with C, and this difference
was significant at P ⬍ 0.05 (Table 5).
Histochemistry. Results of the histochemical analysis are
presented in Table 6. Resistance exercise produced no significant differences in fiber CSA for any fiber type or in the
distribution of fiber types in either the Sol or Gast.
DISCUSSION
Effectiveness of the model. A primary hypothesis of this
study was that high-resistance, squat-type exercise would
cause relatively greater adaptations in the fast Gast compared
with the slow Sol. To test this hypothesis, it was necessary to
successfully train rats to perform a squat-type resistance exercise with moderate to heavy loads. A potential criticism of this
work is that the imposed load (⬃200% of body weight) was too
light to elicit optimal adaptations in muscle mass or function.
Studies using electrically evoked contractions in anesthetized
rats show that one leg can move in excess of 1,500 g (4), and
it has been demonstrated that rats can volitionally produce
forces upwards of 350 – 400% of body weight during activity
(6). Roy et al. (32) were able to get rats to lift 300% of body
weight. We attempted to increase the %load throughout the
study, but, when the load was increased to ⬃225% of body
mass, the number of repetitions fell to less than five per 30-min
session, and sometimes the rats were completely unwilling to
lift the greater load. However, it is important to realize that,
although the weight lifted remained at 200% of body weight
throughout the study, the absolute load lifted increased weekly
due to gains in body weight. This plus a 1-cm improvement in
vertical displacement allowed for a 1.5-fold increase in the
work performed from the beginning to the end of the training
period. The significant inverse relationship between work performed and the Gast-to-body mass ratio suggests that the rats
may have overtrained. Multiple training sessions were selected
Table 4. Diameter, force, elastic modulus, and Ktr of single muscle fibers
Po
Group
Sol-Type I
C
WT
Gast-Type I
C
WT
Gast-Type IIa
C
WT
n
Diameter, ␮m
40
44
76⫾2
75⫾1
42
43
23
26
N⫻
kN/m2
E0, kN/m2 ⫻ 10⫺4
Po/E0
Ktr, s⫺1
5.24⫾0.15
5.05⫾0.12
119⫾3
116⫾3
2.51⫾0.12
2.54⫾0.11
45.0⫾2.1
43.6⫾1.3
3.25⫾0.08 (35)
3.61⫾0.15*(40)
67⫾1
67⫾1
4.00⫾0.14
4.22⫾0.16
113⫾3
121⫾3*
2.53⫾0.11
2.72⫾0.11
43.7⫾1.7
43.1⫾1.3
3.03⫾0.08 (39)
3.57⫾0.12*(40)
60⫾2
59⫾2
3.29⫾0.15
3.54⫾0.17
119⫾4
133⫾6*
1.99⫾0.11
2.17⫾0.13
58.5⫾2.5
58.0⫾3.8
9.16⫾0.68 (22)
11.36⫾0.69*(24)
10⫺4
Values are means ⫾ SE; n, no. of fibers studies. Po is peak tension at pCa 4.5; E0, elastic modulus; Ktr, peak rate of tension redevelopment. Number of fibers
studied for Ktr is given in parentheses next to the values. *Significantly different from C, P ⬍ 0.05.
J Appl Physiol • VOL
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
tively, than C (Table 4). Fiber stiffness and force-to-stiffness
ratio were not different for any of the fiber types (Table 4).
Maximal V0 and Ktr. WT had no significant effect on V0 for
any fiber type (Table 5). In contrast, the Ktr of Sol type I and
Gast type I and IIa fibers were 11, 18, and 24% greater,
respectively, than C (Table 4). Figure 4 shows superimposed
records of the recovery of force after the slack-reextension
protocol for three different fibers. The Ktr of the WT type I
fibers was clearly elevated above C but still less than that of
type IIa fibers.
Force-velocity-power. Table 5 summarizes the force-velocity-power characteristics. The force-power relationships for
Sol type I and Gast type I and IIa fibers are shown in Fig. 5.
Vmax determined by the Hill equation was not different between groups in the Sol. In the Gast type I and IIa fibers, Vmax
of WT fibers was 16% (P ⬍ 0.05) and 13% (P ⬍ 0.1) greater,
respectively, than C.
The a/Po, which describes the curvature of the force-velocity
relationship, was not different between WT and C for the Sol
type I or Gast type IIa fibers. In WT rats, the a/Po of the Gast
type I fibers was significantly less, indicating that there was a
greater curvature to the force-velocity profile for this fiber type
after resistance exercise.
Peak power was generated at the same percentage of peak
force in the Sol type I and Gast type IIa fibers. In the type I
fibers of the Gast, peak power was generated at a lesser relative
force after WT. Peak power was not different in the type I
fibers of the Sol, either in absolute terms or when normalized
for fiber CSA (Table 5). In the type I fibers of the Gast, the
higher velocity of shortening and higher forces compared with
C resulted in greater peak power (Table 5 and Fig. 5). Absolute
peak power and normalized peak power were both 15% greater
than C (Table 5). In the type IIa fibers of the Gast, there was
a trend for greater absolute peak power in the WT group (17%
higher than C with P ⫽ 0.12). In contrast, normalized power
2328
RESISTANCE EXERCISE IN RATS
Table 5. Force-velocity-power characteristics of single muscle fibers
Peak Power
Group
Sol-Type I
C
WT
Gast-Type I
C
WT
Gast-Type IIa
C
WT
Vmax, FL/s
a/Po
% Po at Peak Power
␮N䡠FL䡠s⫺1
W/l
n
V0, FL/s
40
44
1.14⫾0.03
1.11⫾0.02
0.62⫾0.03
0.62⫾0.03
0.09⫾0.01
0.09⫾0.01
22.0⫾0.3
22.3⫾0.3
14.7⫾0.6
14.4⫾0.5
3.41⫾0.18
3.31⫾0.13
42
42
1.01⫾0.04
1.07⫾0.03
0.52⫾0.02
0.65⫾0.04*
0.10⫾0.01
0.09⫾0.01*
23.2⫾0.3
22.0⫾0.4*
10.9⫾0.5
12.5⫾0.6*
3.14⫾0.15
3.61⫾0.16*
23
26
2.95⫾0.13
3.12⫾0.15
1.64⫾0.11
1.85⫾0.08†
0.12⫾0.01
0.12⫾0.01
24.3⫾0.5
24.1⫾0.6
33.0⫾3.2
38.7⫾3.4
11.8⫾1.1
14.2⫾1.0*
Values are means ⫾ SE; n, no. of fibers studied. Vmax, maximal shortening velocity determined from force-velocity relationship; %Po at peak power, the
percentage of peak force at which peak power was obtained. The relative power unit of watts/liter (W/l) is equivalent to kN䡠m⫺2䡠FL䡠s⫺1. Significantly different
from C: *P ⬍ 0.05 and †P ⬍ 0.10.
Fig. 4. Force records used to determine the rate constant of tension redevelopment (Ktr). Single fibers were maximally activated and subjected to a
400-␮m shortening reextension protocol to break active cross bridges. The
subsequent redevelopment of force is shown for C Gast type IIa (top trace,
Ktr ⫽ 11.27), WT Gast type I (middle trace, Ktr ⫽ 3.56), and C Gast type I
(bottom trace, Ktr ⫽ 3.04).
J Appl Physiol • VOL
to-body weight ratio was likely primarily due to the reduced
body weight in the WT group.
Muscle-specific adaptations were noted in single-fiber function, where slow and fast single fibers from the Gast showed
contractile alterations, but slow fibers from the Sol were
unaffected except for Ktr. Several factors may have contributed
to the preferential effect of the WT on Gast compared with Sol
fibers. First, the movement during lifting was very rapid and
short in duration (⬃1 s). The Sol, which is composed of
80 –90% type I fibers, may become unloaded during the rapid
contractions of the fast Gast and Plant muscles (37). Second,
the Sol spans only the ankle joint, whereas the Gast spans both
the ankle and the knee. This affects the extent and velocity of
shortening during lifting. During a lift, there is both extension
at the knee and plantar flexion at the ankle. The Sol is
undergoing shortening as a result of plantar flexion and thus
operating at continually shorter sarcomere lengths. The Gast
shortens with plantar flexion, but also lengthens as a result of
knee extension. Thus the overall change in ML is not as great
for the Gast as it is for the Sol. Thus the Gast operates closer
to optimal filament overlap for a greater portion of the lift.
Muscle fiber architecture may also play a role in force development. The muscle fibers of the Sol are arranged parallel to
the axis of force generation, and so a change in overall ML has
a greater effect on fiber and hence sarcomere length than that
observed for the pinnate-arranged Gast. The high-velocity
movement and extensive shortening experienced by the Sol
both act to limit the force generated by the Sol during lifting.
During a high-velocity isotonic contraction, the Sol would be
shortening at a relatively high percentage of its Vmax, and,
consequently, the force developed would be low. Prilutsky et
al. (30) used EMG and force records in cats to estimate that,
during rapid (1.8 m/s) treadmill locomotion, the Sol would
only be capable of generating 6 –20% of its PO. Additionally,
the Sol has been shown to be near maximally recruited during
even low-intensity activities such as standing (37). So, imposing a high external load may not increase the activation of the
Sol. The additional load may be imparted to the Gast and Plant
muscles, which can increase EMG activity severalfold. To
determine whether this was the case, stainless steel bipolar
electrodes were surgically implanted to record EMG activity
from the Sol and lateral head of the Gast of a rat (20). The rat
was then trained to perform the same type of resistance
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
because Klitgaard (21) reported that three training sessions per
day maximized the amount of work performed. Also, there are
reports from human studies that indicate that greater strength
gains are achieved when the same amount of work is performed in multiple sessions (17). Future studies are needed to
test the hypothesis that overtraining reduced the functional
adaptations.
Muscle-specific response. Our hypothesis that WT would
increase the mass of fast- but not slow-twitch muscle was not
supported by the data, as the exercise-training had no effect on
the wet weight of any of the hindlimb muscles studied (Table
1). However, the muscle-to-body weight ratios of the Gast and
Plant, but not the Sol, of WT rats were greater than C (Table
1). Because fiber hypertrophy was not observed in either the
Sol or the Gast (Tables 4 and 6), the increased Gast muscle-
RESISTANCE EXERCISE IN RATS
2329
exercise presented in this study. Figure 2 shows the displacement, force, and EMG activity measured during the lift. The
results indicate that the Gast was inactive except during the lift.
In contrast, the Sol was always active except for brief quiescent
Table 6. Fiber-type cross-sectional area and distribution
determined by myosin ATPase histochemistry
CSA, ␮m2
Muscle
Fiber Type
Sol
Type
Type
Type
Type
Type
Gast
I
IIa
I
IIa
IIb
Fiber-type Distribution, %
C (n ⫽ 6)
WT (n ⫽ 7)
C (n ⫽ 6)
WT (n ⫽ 8)
3,206⫾265
2,405⫾355
2,942⫾254
2,541⫾126
2,171⫾176
3,508⫾263
2,948⫾378
3,112⫾185
2,844⫾164
2,353⫾135
89.4⫾3.1
10.6⫾3.1
40.9⫾3.0
38.0⫾2.0
21.1⫾1.2
92.0⫾2.9
8.0⫾2.9
45.9⫾3.0
33.5⫾3.9
20.6⫾1.9
Values are means ⫾ SE; n, no. of animals. CSA, cross-sectional area.
J Appl Physiol • VOL
periods immediately before and after the lift when the rat was
stepping to and from the force plate. Thus the increase in EMG
activity caused by the lift was highest in the Gast. All of these
factors may potentially limit the amount of force generated by
the Sol during high-velocity movements.
Whole muscle contractile properties. Despite the lack of
increase in Sol wet weight, there were changes in the whole
muscle contractile properties. Twitch properties were more
rapid in WT rats (Table 2). TPT was less and ⫹dP/dt was
greater than C (P ⬍ 0.10). The former may reflect a change in
the kinetics of calcium release and reuptake, whereas the latter
could reflect an increased rate of SR calcium release or myosin
binding to actin. The latter possibility is supported by the
finding that single-fiber Ktr was greater in the WT group (Table
4). The duration of muscle activation, which varies with the
mode of exercise, may be an important determinant of con-
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
Fig. 5. Composite force-velocity and forcepower relationships for Sol type I (A and D),
Gast type I (B and E), and Gast type IIa (C
and F) fibers. Solid lines represent C, and
dashed lines represent WT fibers for all
plots. FL, fiber length.
2330
RESISTANCE EXERCISE IN RATS
J Appl Physiol • VOL
cross-bridge kinetics, or 4) a combination of the aforementioned. Sarcomere stability is a critical issue in the determination of Ktr. Brenner and Eisenberg (2) reported that Ktr is
reduced by 50% if there is internal shortening of sarcomeres by
only 2.5% at 15°C. Chase et al. (5) found that, when fiber
length, as opposed to sarcomere length, was controlled, Ktr was
19% less. However, they also reported that there was a very
high correlation (r2 ⫽ 0.97) between Ktr values obtained using
these two different techniques. Therefore, even though the
present study controlled overall fiber length, and not sarcomere
length, the data should qualitatively reflect differences in Ktr.
Also, our values of Ktr for rat type I Sol fibers are similar to
those published by other laboratories in which sarcomere
length was held constant (27, 28).
The functional consequences of the increased Ktr are uncertain. Hakkinen et al. (18) observed an increased ⫹dP/dt in
humans after resistance training, and, in this study, we found a
31% higher (P ⫽ 0.08) normalized twitch ⫹dP/dt in the WT
Sol (Table 2). Because Ktr is higher in skinned fast fibers than
slow fibers, it is thought that this may partially explain the
higher ⫹dP/dt seen in intact fast-twitch muscle. While this
probably holds true between muscles of different muscle fiber
compositions, we found no correlation between ⫹dP/dt normalized for peak force (during twitches or tetanic contractions)
and the Ktr of fibers isolated from the contralateral muscle of
the same subject. In fact, the correlation was fairly strong and
negative (r2 ⫽ 0.57), suggesting that the calcium transient may
be more important than Ktr in dictating whole muscle ⫹dP/dt
in muscles of similar fiber-type composition.
In the skinned fiber preparation, the increased peak force of
the Gast fibers post-WT (Table 4) may have resulted from an
increased number of cross bridges formed or an increase in the
amount of force generated per cross bridge. It has been shown
that fiber stiffness varies directly with the amount of filament
overlap (14) and normalized force (2), which has lead to the
conclusion that stiffness is an indication of the number of cross
bridges in the strong bound state. The increase in normalized
force of the type I and type IIa fibers of the Gast was mirrored
by nonsignificant increases in stiffness of approximately the
same magnitude. Because fiber diameter was not different
between groups, this suggests that the increase in force is the
result of an increased number of cross bridges per cross section
of muscle fiber. This would be consistent with an increase in
myofibrillar protein concentration as a result of resistance
exercise, a response that has not been documented in rats,
despite increases in Sol muscle wet weight (42). Also, there
was no change in the PO/E0, which has traditionally been used
as an indication of the force generated per cross bridge, again
suggesting that the increased force was the result of an increase
in the number of cross bridges per CSA. It should be noted that
it is theoretically possible that other factors, such as thin
filament compliance, may be responsible for the change in
stiffness. The end compliance of the fiber, as determined from
the y-intercept of the slack test, was similar between groups,
indicating that this is unlikely.
Widrick et al. (39) and Trappe et al. (35, 36) recently studied
the effects of a 12-wk WT exercise program on single-fiber
function in humans. Unlike our rat data, in humans the resistance exercise increased fiber CSA and absolute force (N),
whereas force per CSA (kN/m2) was unaltered. This suggests
that, in these studies, rats and humans increased force output in
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
tractile adaptations. Periods of activation lasting several minutes, such as that performed during isometric training, have
been found to prolong contraction time (9). However, contraction time was reduced in the Sol after plyometric training
where the duration of muscle activation was very short (1).
Twenty-six weeks of ladder climbing produced no differences
in contraction time or relaxation time in the Sol or extensor
digitorum longus (8). Resistance exercise did not result in
substantially greater force production of the Sol. However,
when normalized for muscle CSA, there were increases in peak
twitch (P ⫽ 0.02) and tetanic (P ⫽ 0.06) tension. The latter
effect was small and not observed in single-fiber analyses
where Sol type I fiber PO was not different between groups.
Gardiner et al. (15) found peak force in the Sol to be 25%
lower than control after weight lifting. This, however, could be
explained by the lower muscle to body weights of the trained
rats because normalized tension was not different. In contrast,
Klitgaard et al. (21, 22) found peak forces of the Sol and Plant
to be higher in very old WT rats relative to age-matched C. The
effects of WT on shortening velocity are controversial. This
study found no change in V0 or Vmax in the intact Sol. The only
other accounts of shortening velocity in rats after WT report a
reduced Vmax in the adductor longus (32) and an increase in the
Sol (1). Heavy resistance training has been shown to decrease
the percentage of fast-type IIb and increase IIx and IIa fibers in
fast-twitch muscles of both humans and rodents (3, 4, 12).
However, no changes have been observed in either the percentage of type I fibers or myosin heavy chain composition (3,
4, 12). Thus one would not expect resistance training to alter
the V0 of a primary slow-type I muscle such as the Sol.
Single-fiber contractile properties. This is the first report on
the effects of resistance exercise training on the contractile
properties of single, skinned muscle fibers in rats. Consistent
with the change observed in the muscle-to-body weight ratio,
the fibers from the Gast but not the Sol showed adaptation in
contractile function. The type I Gast fibers displayed the
greatest adaptation with increased normalized force, Vmax,
absolute and relative power, and Ktr.
Perhaps the most novel finding of this study is that rapid (⬍1
s), moderate-intensity resistance exercise increased Ktr in all
fiber types. Ktr, first reported by Brenner and Eisenberg (2), is
thought to reflect the rate of transition from the weakly bound
low-force state (AM䡠ADP䡠Pi) to the strongly bound high force
state (AM䡠ADP) of the cross bridge. Ktr has been shown to
vary with myosin heavy chain isoform, free Ca2⫹, ionic
strength, phosphate, and internal shortening of sarcomeres (2,
5, 25, 26). In the present study, the differences in Ktr were not
likely due to differences in MHC content, because gel electrophoresis was used to ensure that all fibers expressed the same
MHC isoform. It is possible that slow fibers from the exercisetrained muscles may have expressed an alternate form of the
type I MHC with different kinetics that was undetectable using
our gel protocol (11). Also undetectable amounts of fast MHC
isoforms may have been present, but these would account for
⬍2.5% of total fiber protein (26). The differences were not due
to Ca2⫹ concentration, ionic strength, pH, or phosphate, as all
of these were controlled in the solutions used for skinned fiber
contractile tests. Remaining possible explanations include 1)
differences in the stability of the sarcomeres during the slackrestretch protocol, 2) posttranslational alterations in myosin (or
actin) that affect binding rate, 3) other unknown modulators of
RESISTANCE EXERCISE IN RATS
J Appl Physiol • VOL
ACKNOWLEDGMENTS
The authors extend thanks to Janell Romatowski for expert assistance with
gel electrophoresis and Danny Riley and Kalpana Vijayan for assistance with
the histochemical procedures.
Present address of K. M. Norenberg: Dept. of Biology, Xavier University of
Louisiana, I Drexel Dr., Box 85B, New Orleans, LA 70125.
GRANTS
This research was supported by National Aeronautics and Space Administration Grant NAG 9 –1156 to R. H. Fitts.
REFERENCES
1. Almeida-Silveira MI, Perot C, Pousson M, and Goubel F. Effects of
stretch-shortening cycle training on mechanical properties and fiber type
transition in the rat soleus muscle. Pflügers Arch 427: 289 –294, 1994.
2. Brenner B and Eisenberg E. Rate of force generation in muscle:
correlation with actomyosin ATPase in solution. Proc Natl Acad Sci USA
83: 3542–3546, 1986.
3. Caiozzo VJ, Hadia F, Baker MJ, and Baldwin KM. Influence of
mechanical loading on myosin heavy-chain protein and mRNA isoform
expression. J Appl Physiol 80: 1503–1512, 1996.
4. Caiozzo VJ, Ma E, McCue SA, Smith E, Herrick RE, and Baldwin
KM. A new animal model for modulating myosin isoform expression by
altered mechanical activity. J Appl Physiol 73: 1432–1440, 1992.
5. Chase PB, Martyn DA, and Hannon JD. Isometric force redevelopment
of skinned muscle fibers from rabbit activated with and without Ca2⫹.
Biophys J 67: 1994 –2001, 1994.
6. Dooley PC, Bach TM, and Ruff AR. Effect of vertical jumping on the
medial gastrocnemius and soleus muscles of rats. J Appl Physiol 69:
2004 –2011, 1990.
7. Duchateau J and Hainaut K. Isometric or dynamic training: differential
effects on mechanical properties of a human muscle. J Appl Physiol 56:
296 –301, 1984.
8. Duncan ND, Williams DA, and Lynch GS. Adaptations in rat skeletal
muscle following long-term resistance exercise. Eur J Appl Physiol 77:
372–378, 1998.
9. Exner GU, Staudte H, and Pette D. Isometric training of rats: effects
upon fast and slow muscle and modification by an anabolic hormone
(Nandrolone Decanoate). Pflügers Arch 345: 1–14, 1973.
10. Exner G, Staudte H, and Pette D. Isometric training of rats– effect upon
fast and slow muscle and modification by an anabolic hormone (Nandrolone Decanoate). II. Male rats. Pflügers Arch 345: 15–22, 1973.
11. Fauteck S and Kandarian S. Sensitive detection of myosin heavy chain
composition in skeletal muscle under different loading conditions. Am J
Physiol Cell Physiol 268: C419 –C424, 1995.
12. Fitts RH. Effects of regular exercise training on skeletal muscle contractile function. Am J Phys Med Rehabil 82: 320 –331, 2003.
13. Fluckey JD, Vary TC, Jefferson LS, and Farrell PA. Augmented
insulin action on rates of protein synthesis after resistance exercise in rats.
Am J Physiol Endocrinol Metab 270: E313–E319, 1996.
14. Ford LE, Huxley AF, and Simmons RM. The relation between stiffness
and filament overlap in stimulated frog muscle fibers. J Physiol 311:
219 –249, 1981.
15. Gardiner PF, Hibl B, Simpson DR, Roy R, and Edgerton VR. Effects
of a mild weight-lifting program on the progress of glucocorticoid-induced
atrophy in rat hindlimb muscles. Pflügers Arch 385: 147–153, 1980.
16. Guth L and Samaha FJ. Procedure for the histochemical determination
of actomyosin ATPase. Exp Neurol 28: 365–367, 1970.
17. Hakkinen K and Kallinen M. Distribution of strength training volume
into one or two daily sessions and neuromuscular adaptations in female
athletes. Electroencephalogr Clin Neurophysiol 34: 117–124, 1994.
18. Hakkinen K, Komi PV, and Allen M. Effect of explosive type strength
training on isometric force- and relaxation-time, electromyographic and
muscle fiber characteristics of leg extensor muscles. Acta Physiol Scand
125: 177–185, 1985.
19. Hill AV. The heat of shortening and dynamic constants of muscle. Proc R
Soc Lond B Biol Sci 126: 136 –195, 1938.
20. Hurst J and Fitts R. Hindlimb unloading-induced muscle atrophy and
loss of function protective effect of isometric exercise. J Appl Physiol 95:
1405–1417, 2003.
21. Klitgaard H. A model for quantitative strength training of hindlimb
muscle of the rat. J Appl Physiol 64: 1740 –1745, 1988.
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
response to resistance exercise in distinctively different ways.
The humans relied on fiber hypertrophy, whereas the rats in our
study depended on an increase in force per CSA (kN/m2).
Although muscle hypertrophy has not been observed in rats
trained with volitional exercise paradigms, it is apparent that it
can occur, as Caiozzo et al. (3) observed an 11% increase in rat
Gast muscle mass in response to resistance training that employed direct electrical stimulation of the muscle at high
frequency. Additionally, Widrick et al. (39), studying young
men, and Trappe et al. (35), studying older women, found WT
in humans to have no effect on the Vmax of either slow- or
fast-twitch fibers, whereas we observed an increased Vmax in
the Gast type I (P ⫽ 0.002) and IIa (P ⫽ 0.06) fiber. This
finding was similar to that observed in older men in whom
Trappe et al. (36) found resistance exercise to increase Vmax in
both the slow type I and fast type IIa fiber.
The mechanisms for the increased Vmax in the Gast fibers
following WT are unknown. In an earlier study, Schluter and
Fitts (33) observed a similar 23% increase in V0 in the type I
fiber of the Sol after a regular program of endurance training.
The authors attributed the change in V0 to an increased myosinATPase activity caused by a shift in the myosin light chain
(MLC) composition of the fibers. Contrary to the findings
presented here, endurance training caused an 11% decrease in
the V0 of type IIa fibers from the Gast (33). It was thought that
a lower V0 might provide for greater efficiency of fast muscle
during slow speeds of locomotion because peak power is
generated at velocities ⬍50% of V0. In support of this hypothesis, Reggiani et al. (31) found that efficiency was reduced in
fast muscle fibers that expressed greater amounts of MLC3f.
Consequently, the muscles from endurance-trained animals
may sacrifice power in favor of a more efficient performance.
The increased peak power of the Gast type I fibers following
WT suggests that resistive exercise training favors greater peak
power over efficiency. This is consistent with the unique
functional demands of a short-duration, power-type activity,
where efficiency is not a limiting factor in performance. The
increased velocity of the slow type I and fast type IIa fibers
does not necessarily translate into a faster Gast. Both endurance and strength exercise training programs have been shown
to reduce the percentage of fast type IIb fibers by transformation into the somewhat slower fast type IIx fiber (3, 4, 12).
Thus whole muscle Vmax might stay the same or even decrease.
What the WT adaptation does do is allow the slow type I and
fast type IIa fibers to maintain a higher velocity of shortening
and power when contracting against light to moderate loads
(Fig. 5). Additionally, these fibers are less likely to produce
significant internal drag to the faster contracting type IIx and
IIb fibers.
Summary. Resistance exercise training did not result in fiber
hypertrophy or any changes in fiber-type distribution in either
the Sol or the Gast. Except for a small increase in whole
muscle Pt and PO and an increased single-fiber Ktr, the Sol
muscle was unaltered by isotonic WT. The primary adaptations
to WT occurred in the Gast, especially the type I fibers, which
showed significantly greater forces per CSA, higher absolute
and normalized power, Vmax, and Ktr compared with the control
fibers. These results indicate that isotonic resistance exercise
training in rats selectively affected the Gast, improving singlefiber peak force and power without inducing fiber hypertrophy.
2331
2332
RESISTANCE EXERCISE IN RATS
J Appl Physiol • VOL
32. Roy RR, Wilson R, and Edgerton VR. Architectural and mechanical
properties of the rat adductor longus: response to weight-lifting training.
Anat Rec 247: 170 –178, 1997.
33. Schluter JM and Fitts RH. Shortening velocity and ATPase activity of
rat skeletal muscle fibers: effect of endurance exercise training. Am J
Physiol Cell Physiol 266: C1699 –C1713, 1994.
34. Tamaki T, Uchiyama S, and Nakano S. A weightlifting exercise model
for inducing hypertrophy in the hindlimb muscles of rats. Med Sci Sports
Exerc 24: 881– 886, 1992.
35. Trappe S, Godard M, Gallagher P, Carroll C, Rowden G, and Porter
D. Resistance training improves single muscle fiber contractile function in
older women. Am J Physiol Cell Physiol 281: C398 –C406, 2001.
36. Trappe S, Williamson D, Godard M, Porter D, Rowden G, and Costill
D. Effect of resistance training on single muscle fiber contractile function
in older men. J Appl Physiol 89: 143–152, 2000.
37. Walmsley B, Hodgson JA, and Burke RE. Forces produced by medial
gastrocnemius and soleus muscles during locomotion in freely moving
cats. J Neurophysiol 41: 1203–1216, 1978.
38. Widrick JJ and Fitts RH. Peak force and maximal shortening velocity of
soleus fibers after non-weight-bearing and resistance exercise. J Appl
Physiol 82: 189 –195, 1997.
39. Widrick J, Stelzer J, Shoepe T, and Garner D. Functional properties of
human muscle fibers after short-term resistance exercise. Am J Physiol
Regul Integr Comp Physiol 283: R408 –R416, 2002.
40. Widrick JJ, Trappe SW, Costill DL, and Fitts RH. Force-velocity and
force-power properties of single muscle fibers from elite master runners
and sedentary men. Am J Physiol Cell Physiol 271: C676 –C683, 1996.
41. Witzman FA, Kim DH, and Fitts RH. Hindlimb immobilization: lengthtension and contractile properties of skeletal muscle. J Appl Physiol 53:
335–345, 1982.
42. Wong TS and Booth FW. Skeletal muscle enlargement with weightlifting exercise by rats. J Appl Physiol 65: 950 –954, 1988.
97 • DECEMBER 2004 •
www.jap.org
Downloaded from http://jap.physiology.org/ by 10.220.33.2 on June 15, 2017
22. Klitgaard H, Marc R, Brunet A, Vandewalle H, and Monod H.
Contractile properties of old rat muscles: effects of increased use. J Appl
Physiol 67: 1401–1408, 1989.
23. McDonald KS, Blaser CA, and Fitts RH. Force-velocity and power
characteristics of rat soleus muscle fibers after hindlimb suspension.
J Appl Physiol 77: 1609 –1616, 1994.
24. McDonald KS, Delp MD, and Fitts RH. Fatigability and blood flow in
the rat gastrocnemius-plantaris-soleus after hindlimb suspension. J Appl
Physiol 73: 1135–1140, 1992.
25. McDonald KS and Fitts RH. Effect of hindlimb unloading on rat soleus
fiber force, stiffness, and calcium sensitivity. J Appl Physiol 79: 1796 –
1802, 1995.
26. McDonald KS and Fitts RH. Effect of hindlimb unweighting on single
soleus fiber maximal shortening velocity and ATPase activity. J Appl
Physiol 74: 2949 –2957, 1993.
27. Metzger JM and Moss RL. Calcium-sensitive cross-bridge transitions in
mammalian fast and slow skeletal muscle fibers. Science 247: 1088 –1090,
1990.
28. Metzger JM and Moss RL. pH modulation of the kinetics of a Ca2⫹sensitive cross-bridge state transition in mammalian single skeletal muscle
fibers. J Physiol 428: 751–764, 1990.
29. NRC. Nutrient requirements of laboratory animals. In: National Research
Council (4th Ed.). Washington, DC: National Academy Press, 1995.
30. Prilutsky BI, Herzog W, and Allinger TL. Mechanical power and work
of cat soleus, gastrocnemius and plantaris muscles during locomotion:
possible functional significance of muscle design and force patterns. J Exp
Biol 199: 801– 814, 1996.
31. Reggiani C, Potma EJ, Bottinelli R, Canepari M, Pellegrino MA, and
Stienen GJ. Chemo-mechanical energy transduction in relation to myosin
isoform composition in skeletal muscle fibers of the rat. J Physiol 502:
449 – 460, 1997.