J Appl Physiol 99: 890 – 897, 2005.
First published May 5, 2005; doi:10.1152/japplphysiol.00243.2005.
Muscle endurance is greater for old men compared with strength-matched
young men
Sandra K. Hunter,1,2 Ashley Critchlow,1 and Roger M. Enoka1
1
Department of Integrative Physiology, University of Colorado at Boulder, Boulder, Colorado; and
Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin
2
Submitted 2 March 2005; accepted in final form 5 May 2005
muscle fatigue; elbow flexor muscles; electromyography; torque fluctuations
OLD ADULTS ARE OFTEN MORE fatigue resistant when performing
sustained and intermittent contractions than young adults (6, 7,
11, 21, 22, 33, 38), although not always (2, 4, 21, 49, 50) (for
review see Ref. 3). The mechanism responsible for the reduced
fatigability may depend on the details of the task performed
(3), but it appears to be located distal to the sarcolemma (29,
32, 39), although there may be a contribution by central
mechanisms for some muscle groups (6, 10, 50).
In young adults, the time to failure for a sustained contraction is related to the absolute target force exerted during the
task (23, 26), probably due to the stronger young adults
experiencing greater intramuscular pressures (48), blood flow
occlusion (5, 28), accumulation of metabolites, impairment of
oxygen delivery to the muscle, and an earlier onset of task
failure during a sustained contraction. When comparisons are
made with weaker individuals, therefore, the lower absolute
target force exerted may explain the longer time to failure, such
Address for reprint requests and other correspondence: S. K. Hunter,
Exercise Science Program, Dept. of Physical Therapy, PO Box 1881, Marquette Univ., Milwaukee, WI 53201 (e-mail: [email protected]).
890
as has been observed for old adults. Accordingly, Hunter et al.
(22) found that the time to task failure for a group of young and
old adults was inversely related to target force. However, when
the times to failure for old adults (men and women) were
analyzed separately, there was no relation between target force
and time to failure, which suggests that age-related reductions
in strength may have a limited role in muscle fatigability of old
adults.
These observations led to the following question: What is
the contribution of the reduction in strength to the increased
fatigue resistance exhibited by old adults during a sustained
submaximal contraction? The purpose of this study was to
compare the time to failure for a sustained isometric contraction performed at a submaximal intensity with the elbow flexor
muscles by young and old men who were matched for strength.
Strategies used by subjects to perform the task were characterized by the measurement of electromyographic (EMG) activity of various muscles, mean arterial pressure (MAP), heart
rate, and fluctuations in motor output during the fatiguing
contraction.
METHODS
Eight young men (21.5 ⫾ 4.4 yr; range, 18 –31 yr; 176 ⫾ 5 cm, and
65 ⫾ 5 kg) and 8 old men (71.3 ⫾ 2.9 yr; range, 67–76 yr; 181 ⫾ 6
cm, and 81 ⫾ 13 kg) volunteered to participate in the study. All
subjects were healthy with no known neurological or cardiovascular
diseases and were naive to the protocol. Before participation in the
study, each subject provided informed consent, and the Human
Subjects Committee at the University of Colorado approved the
protocol.
The physical activity level for each subject was assessed with a
questionnaire (36) that estimated the relative kilocalorie expenditure
of energy per week. Arm dominance was estimated using the Edinburgh Handedness Inventory (45); all subjects were right handed.
Mechanical Recording
Subjects were seated upright in an adjustable chair with the nondominant arm abducted slightly and the elbow resting on a padded
support. The elbow joint was flexed to 1.57 rad so that the forearm
was horizontal to the ground and the force at the wrist was directed
upward when the elbow flexor muscles were activated during a
voluntary contraction. Two nylon straps were placed vertically over
each shoulder to restrain the subject and minimize shoulder movement. The hand and forearm were placed in a modified wrist-handthumb orthosis (Orthomerica, Newport Beach, CA), and the forearm
was placed midway between pronation and supination. The forces
exerted by the wrist in the vertical and horizontal (side-to-side)
directions were measured with a force transducer (Force-Moment
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Hunter, Sandra K., Ashley Critchlow, and Roger M. Enoka.
Muscle endurance is greater for old men compared with strengthmatched young men. J Appl Physiol 99: 890 – 897, 2005. First published May 5, 2005; doi:10.1152/japplphysiol.00243.2005.—The purpose was to compare the time to task failure for a sustained isometric
contraction performed at a submaximal intensity with the elbow flexor
muscles by young and old men who were matched for strength. Eight
young men (18 –31 yr) and eight old men (67–76 yr) sustained an
isometric contraction at 20% of maximal voluntary contraction
(MVC) torque until the target torque could no longer be achieved for
at least 5 s. The maximal torque exerted at the wrist was similar for
the young and old men before the fatiguing task (65.9 ⫾ 8.0 vs.
65.4 ⫾ 8.7 N 䡠 m; P ⬎ 0.05), and they experienced similar reductions
in MVC torque after the fatiguing contraction (31.4 ⫾ 10.6%; P ⬍
0.05). The time to task failure was longer for the old men (22.6 ⫾ 7.4
min) compared with the strength-matched young men (13.0 ⫾ 5.2
min; P ⬍ 0.05), despite each group sustaining a similar torque during
the fatiguing contraction (P ⬎ 0.05). The increases in torque fluctuations, electromyographic (EMG) bursting activity, and heart rate
were greater for young men compared with the old men, and they
were less at task failure for the old men (P ⬍ 0.05). Mean arterial
pressure increased at a similar rate for both groups of men (P ⬎ 0.05),
whereas the averaged EMG activity and rating of perceived exertion
reached similar values at task failure for the young and old men (P ⬎
0.05). These findings indicate that the longer time to task failure for
the old men when performing the submaximal contraction was not due
the absolute target torque exerted during the contraction.
AGING AND TASK FAILURE IN STRENGTH-MATCHED MEN
Sensor, JR-3, Woodland, CA) that was mounted on a custom-designed, adjustable support. The orthosis was rigidly attached to the
force transducer. The forces detected by the transducer were recorded
on line using a Power 1401 A-D converter and Spike2 software
[Cambridge Electronics Design (CED), Cambridge, UK]. The force
exerted in the vertical direction was displayed on a 17-in. monitor
located 1.5 m in front of the subject. The force signal was digitized at
500 samples/s.
Electrical Recordings
Cardiovascular Measurements
Heart rate and blood pressure were monitored during the fatiguing
contractions because these adjustments involve both central and
peripheral processes (14, 18, 42, 47). Heart rate and blood pressure
were monitored with an automated beat-by-beat, blood pressure monitor (Finapres 2300, Ohmeda, Madison, WI). The blood pressure cuff
was placed around the middle finger of the relaxed, dominant hand
with the arm placed on a table adjacent to the subject at heart level.
The blood pressure signal was recorded online by a computer at 500
samples/s.
Experimental Protocol
Each subject visited the laboratory for an introductory session to
become familiar with the equipment and perform maximal voluntary
contractions (MVC) and to participate in one experimental session.
The protocol for the experiment comprised an assessment of the MVC
force for the elbow flexor and elbow extensor muscles, determination
of the EMG-force relations for the elbow flexor muscles, and performance of a fatiguing contraction. Within 10 s of completing the
fatiguing contraction, an MVC was performed with the elbow flexor
muscles.
MVC torque. Each subject performed three MVC trials with the
elbow flexor muscles, followed by three trials with the elbow extensor
muscles. The MVC task consisted of a gradual increase in force from
zero to maximum over 3 s, with the maximal force held for 2–3 s. The
force exerted by the wrist was displayed on a monitor, and each
subject was verbally encouraged to achieve maximal force. There was
a 60-s rest between trials. If the peak forces from two of the three trials
were not within 5% of each other, additional trials were performed
until this was accomplished. The greatest force achieved by the
subject was taken as the MVC force and used as the reference to
calculate the 20% target level for the fatiguing contraction. The MVC
J Appl Physiol • VOL
torque was calculated as the product of MVC force and the distance
between the elbow joint and the point at which the wrist was
connected to the force transducer.
EMG activity. The EMG activity of the involved muscles was
recorded in standardized tasks so that the EMG-torque relation could
be compared between the young and old men in the nonfatigued state.
Each subject performed a sustained constant-torque contraction with
the elbow flexor muscles for 6 s at target values of 20, 40, and 60%
MVC. The subject was given a 30-s rest between each contraction.
These relations for the two groups of subjects were examined to
ensure that changes in EMG during the fatiguing contraction represented physiological adjustments and were not due to differences in
recording conditions between the two groups.
Fatiguing contraction. The fatiguing contraction of the elbow
flexor muscles was performed at a target value of 20% MVC torque.
The subject was required to match the target torque as displayed on
the monitor and was verbally encouraged to sustain the torque for as
long as possible. The fatiguing contraction was terminated when
either the torque declined by 10% of the target value for greater than
⬃5 s or when the subject lifted the elbow off the support for greater
than ⬃5 s, despite strong verbal encouragement to maintain the task.
Neither the subject nor the investigator who terminated the task knew
the time during the task.
An index of perceived effort, the rating of perceived exertion
(RPE), was assessed with the modified Borg 10-point scale (8). The
subjects were instructed to focus the assessment of effort on the arm
muscles performing the task. The scale was anchored so that 0
represented the resting state and 10 corresponded to the strongest
contraction that the arm muscles could perform. The RPE was
measured at 30-s intervals during the fatiguing contraction.
Data Analysis
All data collected during the experiments were recorded on line
using a Power 1401 A-D converter (CED) and analyzed offline using
the Spike2 data-analysis system (CED).
The MVC torque was quantified as the peak value achieved during
the MVC. Similarly, the maximal EMG for each muscle was determined as the average value over a 0.5-s interval that was centered
about the peak rectified EMG. The rectified EMG of the constanttorque contractions for the elbow flexor muscles performed at 20, 40,
and 60% of MVC torque was averaged over the middle 4 s of the 6-s
contraction.
The fluctuations in torque during the fatiguing contraction were
quantified in the vertical direction as the coefficient of variation (CV)
for torque (SD/mean ⫻ 100) for the first 30 s; 15 s on both sides of
25, 50, and 75% of time to task failure; and the last 30 s of the task
duration.
The EMG activity of the elbow flexor muscles and elbow extensor
muscles during the fatiguing contraction was quantified in two ways:
1) for statistical purposes, as averages of the rectified EMG (AEMG)
over the first 30 s; 15 s on both sides of 25%, 50%, and 75% of time
to task failure; and the last 30 s of the time to task failure for the
fatiguing contraction; and 2) for graphic presentation, as the AEMG
for every 1% of the time to task failure. The EMG was normalized to
the peak EMG obtained during the MVC.
Low-intensity contractions of long duration are characterized by
bursts of EMG activity, which were quantified by the process described previously (23, 27). The rectified EMG signal was 1)
smoothed with a low-pass filter at 2 Hz for surface EMG signals and
at 3.8 Hz for the intramuscular EMG (brachialis), 2) differentiated
over five-point averages, and 3) divided by the average of the rectified
EMG so that muscles with different EMG amplitudes could be
compared. The differentiated signal represents the rate of change for
the low-pass-filtered EMG signal and was used to identify rapid
changes in the EMG signal. A burst was identified when the
smoothed, differentiated EMG signal increased by more ⬎0.20 s⫺1
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EMG signals were recorded with bipolar surface electrodes (AgAgCl, 8-mm diameter; 16 mm between electrodes) that were placed
over the long head of biceps brachii, the short head of biceps brachii,
brachioradialis, and triceps brachii muscles. Reference electrodes
were placed on a bony prominence at the elbow or shoulder. The
EMG of the brachialis muscle was measured with an intramuscular
bipolar electrode comprised of two stainless steel wires (100-␮m
diameter) that were insulated with Formvar (California Fine Wire,
Grover Beach, CA). Intramuscular recordings were necessary for
brachialis because of the contamination of surface recordings by
volume-conducted signals from more superficial muscles. One wire in
each pair had the insulation removed for ⬃2 mm to increase the
recording volume of the electrode. The electrode was inserted into the
muscle 4 –5 cm proximal to the antecubital fold with a hypodermic
needle that was removed immediately after insertion. A surface
electrode (8-mm diameter) placed on a bony prominence served as the
reference electrode. The EMG signal was amplified (500 –2,000⫻)
and band-pass filtered (13–1,000 Hz for the surface and intramuscular
EMG signals) with Coulbourn modules (Coulbourn Instruments,
Allentown, PA) before being recorded directly to computer using the
Power 1401 A-D converter (CED) and displayed on an oscilloscope.
The EMG signals were digitized at 2,000 samples/s.
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AGING AND TASK FAILURE IN STRENGTH-MATCHED MEN
Statistical Analysis
Data are reported as means ⫾ SD within the text and displayed as
means ⫾ SE in the figures. Separate analysis of variances (ANOVA,
SPSS version 12.0) were used to compare the time to task failure and
percent decline in MVC torque between the old and young men.
Separate ANOVAs (age ⫻ time) with repeated measures on time were
used to compare the dependent variables of heart rate, MAP, RPE, CV
of torque, EMG-torque relation for the 6-s constant-torque contractions, EMG burst rate, and AEMG during the fatiguing contraction.
Consequently, each age group was included in the one ANOVA for
each variable. Post hoc analyses (Tukey) were used to test for
differences among groups and time intervals when appropriate. Because EMG bursts were sometimes absent during a one-third interval
of the task for some subjects, averages of the burst duration are
reported, and the results of independent t-tests are indicated where
these analyses were possible. Independent t-tests were used to test for
differences among pairs of means when appropriate; a Bonferroni
correction for statistical significance was used in these instances.
Associations were determined between some variables using Pearson’s correlation analysis. A significance level of P ⬍ 0.05 was used
to identify statistical significance, except when modified by the
Bonferroni correction.
RESULTS
There was a trend for the old men to be less physically active
compared with the young men [51 ⫾ 41 metabolic equivalents
(MET)䡠h/wk vs. 132 ⫾ 115 MET䡠h/wk; P ⫽ 0.08], but this did
not reach statistical significance because of the large variance
within groups.
MVC Torque and Time to Task Failure
The eight young men and eight old men were matched for
strength within 5% of maximal torque of the elbow flexor
muscles (Fig. 1A). The maximal torque exerted at the wrist was
similar for the young (65.9 ⫾ 8.0 N䡠m) and old men (65.4 ⫾
8.7 N䡠m) before the fatiguing task (P ⬎ 0.05), which meant
that the average torque exerted during the fatiguing contraction
was similar for the two groups. Although the moment arm for
the old men (27.0 ⫾ 0.9 cm) was slightly longer than for the
young men (26.1 ⫾ 0.4 cm; P ⬍ 0.05), the maximal force
J Appl Physiol • VOL
Fig. 1. Maximal voluntary contraction (MVC) torque (A) and time to failure
(B) for the young men (n ⫽ 8) and old men (n ⫽ 8) who were matched for
strength within 5%. A: line of identify indicates a perfect match for strength
between pairs of subjects. B: data points lie above the line of identity,
indicating that the time to failure for the old man of each pair was longer than
for his strength-matched young man.
exerted at the wrist was similar for both groups (P ⬍ 0.05).
After the fatiguing contraction, the groups of men showed
similar and significant reductions in MVC strength (31.4 ⫾
10.6%; P ⬍ 0.05). However, the time to task failure was longer
for the old men (22.6 ⫾ 7.4 min) compared with the strengthmatched young men (13.0 ⫾ 5.2 min; P ⬍ 0.05; Fig. 1B).
EMG-Torque Relation
The AEMG (% peak EMG) for the elbow flexor muscles
was determined during isometric contractions held at 20, 40,
and 60% of MVC. AEMG increased similarly with contraction
intensity for the young and old strength-matched men (P ⬍
0.05). For the elbow flexor muscles, the AEMG for the 20, 40,
and 60% of MVC was 10.7 ⫾ 3.4, 28.6 ⫾ 7.2, and 52.6 ⫾
11.6%, respectively, for the young men and 14.8 ⫾ 5.0, 29.7 ⫾
7.8, and 51.6 ⫾ 7.8% for the old men. The AEMG of the
brachialis muscle was significantly greater than the other elbow
flexor muscles at the 20 and 40% contraction (muscle ⫻ force
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for the surface EMG and ⬎0.23 s⫺1 for the intramuscular EMG.
These values represented 3 SDs above the mean of the smoothed,
differentiated EMG signal. The 3-SD criterion was based on EMG
records from a subset of fatiguing contractions of the present data set
when the EMG signal displayed minimal bursting during the contraction. The end of a burst was identified as the time when the smoothed
EMG signal decreased to the same amplitude as at the start of the
burst. When the EMG signal did not decline to the same EMG
amplitude at the start of the burst, however, the end of the burst was
then identified as the time that the differentiated EMG signal became
most negative before the start of the next burst. This criterion
represented the time at which the signal decreased most rapidly before
the beginning of the next burst. The start of a second burst was
constrained to be ⬎2 s apart from the previous burst, and the minimal
burst duration was 0.5 s.
Heart rate and MAP recorded during the fatiguing contraction were
analyzed by comparing ⬃15-s averages at 25% intervals throughout
the fatiguing contraction. For each interval, the blood pressure signal
was analyzed for the mean peaks [systolic blood pressure (SBP)],
mean troughs [diastolic blood pressure (DBP)], and the number of
pulses per second (multiplied by 60 to determine heart rate). MAP was
calculated for each epoch with the following equation: MAP ⫽ DBP
⫹1/3(SBP ⫺ DBP).
AGING AND TASK FAILURE IN STRENGTH-MATCHED MEN
interaction; P ⬍ 0.05), but this difference was similar for both
groups of men. Because the EMG-force relations for the young
and old men were similar across the muscles and contraction
intensity, any differences in EMG during the fatiguing contraction represented physiological adjustments and were not
due to differences in recording conditions between the two
groups.
AEMG During the Fatiguing Contraction
and it was similar among the elbow flexor muscles: short head
of biceps brachii (6.0 ⫾ 3.3 s), long head of biceps brachii
(6.4 ⫾ 5.0 s), brachioradialis (5.0 ⫾ 2.5 s) and brachialis
(3.9 ⫾ 3.0 s).
RPE During the Fatiguing Contraction
RPE increased during the fatiguing contraction (P ⬍ 0.05),
but it was similar for old and young men at the beginning and
end of the fatiguing contraction (P ⬎ 0.05). The RPE values
progressed for the young and old men from 1.4 ⫾ 1.1 and
1.3 ⫾ 0.8, respectively, at the start of the contraction to 5.1 ⫾
1.3 and 6.4 ⫾ 2.2 at 25% of time to failure, 7.9 ⫾ 1.1 and
7.9 ⫾ 1.9 at 50% of time to failure, 9.4 ⫾ 0.7 and 9.4 ⫾ 0.7
at 75% of time to failure, and 10 ⫾ 0.0 for both groups at task
failure. However, the rate of increase in the RPE was more
gradual for the old adults because of the longer duration of the
contraction.
Fluctuations in Torque During the Fatiguing Contraction
The normalized amplitude of the vertical fluctuations in
torque (CV) increased for the young and old men (P ⬍ 0.05).
However, the amplitude of the fluctuations was greater at task
failure for the young men than for the old men, and the
fluctuations for the young men increased at a greater rate (P ⬍
0.05; Fig. 4). The young men increased by 226 ⫾ 135% from
the first 30 s to the last 30 s of the fatiguing contraction,
whereas the old men increased to 162 ⫾ 84%. There was a
positive association between the absolute increases in CV for
torque and the burst rate of EMG throughout the contraction
(r2 ⫽ 0.68; n ⫽ 16; P ⬍ 0.05), indicating that 68% of the
increase in the torque fluctuations was explained by the increase in EMG burst rate.
Bursts of EMG Activity During the Fatiguing Contraction
There was a progressive increase in the number of bursts in
EMG activity during the fatiguing contraction for the young
and old men (P ⬍ 0.05). However, a time ⫻ age interaction
(P ⬍ 0.05) indicated that the young men had greater EMG
bursting activity compared with the old men for the middle and
last third of the contraction (Fig. 3A). The increase in burst rate
was similar among the elbow flexor muscles (P ⬎ 0.05; Fig. 3,
B and C).
The burst duration was similar (P ⫽ 0.2) for the young
(4.4 ⫾ 2.2 s) and old men (6.2 ⫾ 5.4 s) when all the elbow
flexor muscles were pooled. The burst duration did not differ
across the fatiguing contractions for either group (P ⬎ 0.05),
MAP and Heart Rate
MAP increased during the fatiguing contractions for the
young and old men (P ⬍ 0.05; Fig. 5A). There was no main
effect for age, indicating that the MAP was similar for the
young and old men (P ⬎ 0.05). At task failure, MAP was
137 ⫾ 18 mmHg for the young men and 142 ⫾ 11 mmHg for
the old men (P ⬎ 0.05). Furthermore, the rate of rise in MAP
was not different (P ⫽ 0.09) between the young (3.2 ⫾ 1.8
mmHg/min) and old men (1.8 ⫾ 1.0 mmHg/min), despite
differences in the time to failure.
Heart rate also increased during the fatiguing contraction for
the young and old men (P ⬍ 0.05, Fig. 5B). There was a
Fig. 2. Average rectified electromyogram
(AEMG, expressed as a percentage of the
MVC value) of the elbow flexor muscles
during the fatiguing contraction for the young
men (A) and old men (B). Each data point
represents the mean AEMG amplitude for 1%
of the time to task failure for each of the
elbow flexor muscles with every second data
point plotted. The rate of increase in AEMG
was greater for the young men compared with
the old men (P ⬍ 0.05).
J Appl Physiol • VOL
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The amplitude of the AEMG (% peak EMG) for each elbow
flexor muscle increased during the fatiguing contraction for the
young and old men (P ⬍ 0.05; Fig. 2). The AEMG for the
elbow flexor muscles pooled was similar for the young and old
men at the start (11.3 ⫾ 8.8 and 14.9 ⫾ 8.9%) and end (32.2 ⫾
14.6 and 31.6 ⫾ 14.5%) of the fatiguing contraction. Because
the old men had a longer time to failure, however, the rate of
increase in AEMG for the old men was less for all muscles
compared with the young men (P ⬍ 0.05).
The AEMG differed across muscles (P ⬍ 0.05). The AEMG
for the entire contraction for brachialis (27.7 ⫾ 11.1%) was
greater (P ⬍ 0.05) than the other elbow flexor muscle for both
the young and old men, including the short head of biceps
brachii (16.0 ⫾ 4.2%) and long heads of biceps brachii (20.4 ⫾
4.3%) and brachioradialis (21.1 ⫾ 8.5%). Furthermore, the
AEMG of the biceps brachii was less than all the other elbow
flexor muscles for both the young and old men (P ⬍ 0.05). The
triceps brachii was activated during the fatiguing contractions,
but the AEMG was considerably less than for the elbow flexor
muscles (Fig. 2; P ⬍ 0.05). The triceps brachii increased to
similar amplitudes for the young and old men during the
fatiguing contraction from 2.1 ⫾ 1.4% at the start of the
contraction to 3.6 ⫾ 1.8% at the conclusion (P ⬍ 0.05).
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AGING AND TASK FAILURE IN STRENGTH-MATCHED MEN
Fig. 3. Burst rate of the rectified EMG of the elbow flexor muscles during the
fatiguing contractions. Values are means ⫾ SE for the first third, middle third,
and last third of the time to task failure for the elbow flexor muscles. A: old
men had a lower burst rate compared with the young men (P ⬍ 0.05). B and
C: burst rate of the individual elbow flexor muscles for the young (B) and old
men (C).
time ⫻ age interaction (P ⬍ 0.05) because heart rate was
similar at the start of the task for the young (82 ⫾ 13
beats/min) and old men (75 ⫾ 13 mmHg/min), but it was
greater for the young men (107 ⫾ 16 beats/min) at task failure
compared with the old men (93 ⫾ 15 mmHg/min). Accordingly, heart rate increased at a lower rate (P ⬍ 0.05) for the old
J Appl Physiol • VOL
Fig. 5. Changes in mean arterial pressure (MAP) and heart rate during the
fatiguing contraction for the young and old men. Values are means ⫾ SE of
15-s intervals at 25% increments of the time to task failure. The increase in
MAP (A) was similar for the young and old strength-matched men (P ⬎ 0.05),
but the increase in heart rate (B) was greater for the young men (P ⬍ 0.05).
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Fig. 4. Fluctuations in torque in the vertical direction were quantified as the
coefficient of variation (CV) for the torque exerted at the wrist. Mean (⫾ SE)
CV of the torque is shown for the young and old men for 30-s intervals at the
beginning and 25, 50, 75, and 100% of the time to task failure. CV was less for
the old men compared with the strength-matched young men during the
contraction and at failure of the task (P ⬍ 0.05).
AGING AND TASK FAILURE IN STRENGTH-MATCHED MEN
men (0.8 ⫾ 0.3 beats/min) compared with the young men
(2.2 ⫾ 1.5 beats/min).
DISCUSSION
J Appl Physiol • VOL
tween young and old adults did not influence the time to failure
(49).
Despite a similar torque exerted during the contraction, the
old men exhibited a slower rate of rise in the average EMG
compared with the young men, although the values at task
failure were similar for each group. The relative EMG activity
was similar in the young and old men when compared for
nonfatiguing contractions at various intensities of contraction.
Thus the different EMG activity at the same absolute time
during the task for the young and old men indicated different
rates of either recruitment or alterations in discharge rate for
the two groups of men. In young adults, the increase in EMG
activity during a low-force fatiguing contraction is largely due
to recruitment of larger motor units as the muscle becomes
progressively fatigued, with minimal contribution from discharge rate (9, 13, 19, 41, 44). When young and old adults
exert a similar relative submaximal force, the old adults can
have reduced discharge rates (46) but not always (30). If the
change in discharge rates were similar for the young and old
men during the task, then the EMG recordings suggest that
recruitment of the motor unit pool was likely more gradual for
the old men. Because the AEMG was similar at task failure, it
is likely that a similar proportion of the motor unit pool was
recruited by the two groups of men during the contraction (13,
19, 44). However, the difference in bursts of EMG activity
indicates that the young men had to rely more on the transient
recruitment of motor units (24, 25, 35) to sustain the task.
Presumably, the additional motor units that were activated for
brief bursts of activity were necessary to compensate for the
decline in the force capacity of the active muscle fibers. Given
the preferential loss of type II fiber area that has been observed
in old adults (29, 34, 39, 40), perhaps the old men experienced
a more gradual reduction in the force capacity of the active
muscle fibers, which were likely to have greater oxidative
capacity.
One consequence of the greater EMG bursting activity was
a more rapid increase in the torque fluctuations for the young
men (r2 ⫽ 0.68). The amplitude of the fluctuations in torque
increased more rapidly for the young men and was greater at
task failure for the young men compared with the old men,
even though the target torque and decline in maximal force
capacity (fatigue) was similar for both groups. At the start of
the isometric contractions, the old men had greater fluctuations
in torque than the young men (Fig. 4), as has been observed in
other isometric tasks for young and old adults (12). The greater
amplitude of torque fluctuations for the old adults is likely due,
at least in part, to a more variable discharge rate of the active
motor units (37, 51). In contrast, the young men had a much
greater increase in fluctuations of torque as fatigue developed
during the sustained contraction, which probably reflects a
more rapid rate of recruitment of higher threshold motor units
that have greater discharge rate variability (43) during the task
and at task failure.
In conclusion, the time to task failure for a submaximal
contraction with the elbow flexor muscles was longer for old
men compared with strength-matched young men. Measurements of fluctuations in torque and EMG suggest that motor
unit activity increased most rapidly during the fatiguing contraction for the young men. Consequently, the mechanisms
responsible for the longer time to failure for the old men were
99 • SEPTEMBER 2005 •
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The purpose of this study was to compare the time to task
failure for a sustained isometric contraction performed at a
submaximal intensity with the elbow flexor muscles by young
and old men who were matched for strength. The main finding
was that the time to task failure was longer for the old men,
despite similar target torques and reductions in the MVC
torque performed after the fatiguing task. The two groups of
men started and ended the contraction at similar values for
MAP, AEMG, and RPE. However, the fluctuations in torque,
bursting activity of the EMG signal, and heart rate were lower
at task failure for the old men compared with the young men.
Thus it appears that the contribution of strength to the agerelated decrease in muscle endurance for this task is limited.
The difference in time to task failure between the young and
old men was not due to a difference in motivation or relative
performance. The decline in maximal strength (MVC force)
recorded after task failure, which is the consensus index for the
amount of fatigue experienced during physical activity (16),
was similar for the young and old strength-matched men.
Furthermore, the effort exerted at task failure (indicated by the
RPE) was similar for the two groups.
In young adults, the longer time to task failure for a sustained contraction performed by a group of women, who were
weaker than a group of men, was primarily explained by the
absolute force exerted when performing a submaximal fatiguing contraction (20, 23, 26). In the present study, however, two
groups of subjects with comparable strength exhibited marked
differences in the time to task failure. Young men were only
able to sustain the task for 42% of the duration achieved by the
strength-matched old men. This age-related difference in the
time to failure is consistent with results obtained previously
when young men, who were 40% stronger than the old men,
could sustain the submaximal contraction for 37% of the
duration achieved by the old men (22). The present results
indicate that the longer to time to failure for the old adults was
not due to differences in absolute strength.
Some clues to the potential physiological mechanisms that
can account for the difference in the time to failure are
suggested by the variables that were measured during the
fatiguing task. There was no difference between the two groups
in either the initial and final values for MAP or its rate of
increase. Consequently, those peripheral and central regulatory
mechanisms (14, 31, 42, 47) that attempt to sustain muscle
perfusion during a sustained contraction operated at similar
levels for the strength-matched young and old men. In contrast,
average EMG and RPE began and ended at similar values for
the two groups of subjects, but the rates of increase were
greater for the young men. Furthermore, the rate of increase in
heart rate, and the final values of the old men at task failure,
were less than those for the young men. Although heart rate is
controlled primarily by central command (1, 17, 18) during a
fatiguing contraction, the lesser heart rate for the old men at
task failure may be explained by the decline in maximal heart
rate that occurs with advancing age (15). Consistent with these
findings, others reported similar results for a fatiguing contraction indicating that the differential increase in heart rate be-
895
896
AGING AND TASK FAILURE IN STRENGTH-MATCHED MEN
not related to strength but seemed to involve a reduced need for
the transient recruitment of motor units.
GRANTS
This research was supported by a Research Endowment Award from the
American College of Sports Medicine (to S. K. Hunter) and National Institute
of Neurological Disorders and Stroke Grant NS-43275 (to R. M. Enoka).
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