JOURNAL OF MORPHOLOGY 251:323–332 (2002)
Morphometry of Macaca mulatta Forelimb. II. Fiber-Type
Composition in Shoulder and Elbow Muscles
Kan Singh,1,2 Ellen H. Melis,3 Frances J.R. Richmond,3 and Stephen H. Scott1,2*
1
CIHR Group in Sensory-Motor Systems, Queen’s University, Kingston, ON, Canada K7L 3N6
Department of Anatomy & Cell Biology, Queen’s University, Kingston, ON, Canada K7L 3N6
3
Department of Physiology, Queen’s University, Kingston, ON, Canada K7L 3N6
2
Published online xx Month 2001
ABSTRACT The present study examined the fibertype proportions of 22 muscles spanning the shoulder
and/or elbow joints of three Macaca mulatta. Fibers
were classified as one of three types: fast-glycolytic
(FG), fast-oxidative-glycolytic (FOG), or slow-oxidative
(SO). In most muscles, the FG fibers predominated, but
proportions ranged from 25– 67% in different muscles.
SO fibers were less abundant except in a few deep, small
muscles where they comprised as much as 56% of the
fibers. Cross-sectional area (CSA) of the three fiber
types was measured in six different muscles. FG fibers
The recording of neural activity during reaching
movements in nonhuman primates has become an
important paradigm for examining how the brain
plans and controls movement (for review, see Georgopoulos, 1995; Kalaska et al., 1997). These studies
identify how different regions of the brain contribute
to the central control of forelimb movement, but they
are difficult to relate to the biomechanics and kinematics of the movements, in part because relatively
little information exists on the biomechanical properties of the limb. The production of arm movements
using motor commands (i.e., muscle activation patterns) depends on these intrinsic biomechanical
properties of the limb as well as the mechanics of
multijoint motion, as dictated by Newtonian mechanics. Thus, to understand motor control it is not
enough just to record from the brain. It is necessary
to relate those signals to the activity patterns and
contractile behaviors of muscle and to the biomechanics of the body segments. Mathematical models
are one way to try to understand the function of the
peripheral motor apparatus, but these models require detailed information on the physical properties of the limb segments and muscles.
One particularly useful model for studying muscle
is that developed by Zajac (1989). This generic model
can be scaled to any individual muscle based on five
parameters: fascicle or fiber length, tendon slack
length, angle of pennation, physiological crosssectional area, and maximum shortening velocity
(Vmax). Little is known about any of the intrinsic
© 2002 WILEY-LISS, INC.
DOI 10.1002/jmor.1092
tended to be the largest, whereas SO fibers were the
smallest. While fiber-type size was not always consistent between muscles, the relative size of FG fibers was
generally larger than FOG and SO fibers within the
same muscle. When fiber CSA was taken into consideration, FG fibers were found to comprise over 50% of the
muscle’s CSA in almost all muscles. J. Morphol. 251:
323–332, 2002. © 2002 Wiley-Liss, Inc.
KEY WORDS: Macaca mulatta; monkey; histochemistry;
forelimb; biomechanical modeling; muscle
properties in the muscles used in reaching tasks,
especially in rhesus monkeys—a popular choice of
experimental animal. Our previous work examined
the first four of these parameters based on physical
measurements of each muscle (Cheng and Scott,
2000). The last of the parameters indicated by
Zajac, Vmax, has been shown to be correlated with
myosin adenosine triphosphatase (mATPase) activity within each muscle fiber (Barany, 1967). Since
Vmax differs among different types of fibers, measuring the mATPase activity through histochemical
staining techniques can provide an estimation of the
speed of the muscle of interest.
Despite the possible range of metabolic and physiological properties, skeletal muscle fibers are generally classified into one of three histochemically
defined types: type I, type IIa, and type IIb (Burke,
1981). These fiber types are considered homologous
Contract grant sponsor: the Medical Research Council (MRC) of
Canada; Contract grant number: MT-13462.
Current address for E. Melis: Canadian Physiotherapy Association,
1400 Blair Place, Suite 205, Gloucester, ON, Canada K1J 9B8.
Current address for F.J.R. Richmond: Department of Molecular
Pharmacology and Toxicology, University of Southern California, Los
Angeles, CA 90033.
*Correspondence to: Dr. Stephen Scott, Department of Anatomy &
Cell Biology, Queen’s University, Kingston, ON Canada K7L 3N6.
E-mail: [email protected]
324
K. SINGH ET AL.
TABLE 1. Sampling of fibers for analysis in shoulder
and elbow muscles
Muscle
Biceps, long
Biceps, short
Brachialis
Brachioradialis
Coracobrachialis
Deltoid, anterior
Deltoid, middle
Deltoid, posterior
Dorsoepitrochlearis
Extensor carpi
radialis brevis
Extensor carpi
radialis longus
Infraspinatus
Latissimus dorsi
Pectoralis major
Rhomboid, major
Subscapularis
Supraspinatus
Teres major
Teres minor
Triceps, lateral
Triceps, long
Triceps, medial
Abbrev.
n
(monkeys)
n
(areas)
n
(fibers)
BL
BS
B
Br
Cb
DA
DM
DP
De
ECRB
3
3
3
3
1
3
3
2
2
3
34
14
24
12
4
19
42
11
11
20
3679
1256
2529
1324
347
1954
4027
1003
1078
1847
ECRL
3
17
1745
Is
LD
PM
Rh
Sb
Sp
TMa
TMi
TLa
TLo
TMe
3
3
3
1
3
3
3
1
3
3
2
31
15
57
6
31
44
34
8
93
90
14
3369
1386
5830
493
2862
4403
3416
798
9837
7466
1762
to the metabolic classification of slow-oxidative (SO),
fast-oxidative-glycolytic (FOG), and fast-glycolytic
(FG) fibers. As the names imply, fast fibers tend to
have higher maximum shortening velocities than
slower fibers (Cordonnier et al., 1995). In the cat
soleus, made up exclusively of slow fibers, Vmax has
been reported to be between 4.5 and 4.8 L0/s (Scott et
al., 1996; Spector et al., 1980). Cheng et al. (2000)
estimated Vmax in feline caudofemoralis, made up
exclusively of fast fibers, to be 14 L0/s.
Although previous studies have examined the histological properties of forelimb muscles of nonhuman primates (Roy et al., 1984; McIntosh et al.,
1985), they provide an incomplete picture, since
these studies focused on a limited number of muscles spanning only the elbow or wrist joints. The
purpose of the present study was to provide a complete histochemical analysis of the muscles involved
in reaching that span both the elbow and shoulder
joints in Macaca mulatta.
MATERIALS AND METHODS
Twenty-two shoulder and elbow muscles were dissected from up to three adult female rhesus monkeys (Macaca mulatta) within 3 h of death during
postmortem examinations (Table 1). The monkeys
were previously involved in physiological studies of
reproductive hormone cycling and were euthanized
for necessary histological analyses in those studies.
Thus, no monkeys were euthanized specifically for
this study. All animal care and experimental procedures were carried out in accordance with the guide-
lines of the Canadian Council on Animal Care. The
animals, ranging in weight from 4.7–5.7 kg, were
housed in a light- and temperature-controlled environment. Animals were anesthetized with a mixture
of Saffan (alphaloxalone, alphadolone acetate, Cooper’s Agrofarm, Ajax) and ketamine hydrochloride
(Rogar/STB Inc., 6 –9 mg/kg) injected intravenously.
The carotid arteries were catheterized and the brain
was perfused, first with PBS and then with 4% paraformaldehyde solution, to euthanize the animals.
The perfusion procedure was restricted to the head
and neck to ensure that the limb musculature remained unperfused.
Each muscle was divided into blocks (⬃2 cm2) and
mounted in a recorded orientation onto numbered
cryostat chucks using embedding medium (Cryomatrix, Shandon, Pittsburgh, PA). The blocks were covered with talcum powder and immersed in liquid
nitrogen. Frozen blocks were later warmed to ⫺20°C
and transverse sections (16 ␮m) were cut on a cryostat. Sections (two to six per block) were mounted
on gelatin-coated slides and air-dried before staining. Serial sections were stained with hematoxylin
and eosin (H&E) and for mATPase activity after
alkaline preincubation at pH 10.4, according to the
procedure described by Guth and Samaha (1970)
with some minor modifications. Briefly, sections
were kept in a sealed container with a desiccating
compound for up to 2 h after being cut to minimize
hydration of tissue and loss of enzyme reactivity.
Systematic variation of staining variables for mATPase staining showed that consistent differences between fiber types were obtained by fixing sections in
5% formalin for 2.5 min rather than 5 min. Sections
were preincubated in alkali solution for 4 min rather
than 15 min and then incubated in an ATP, KCl, and
CaCl2 solution. After washing, sections were placed
in 1% ammonium sulfide for 1 min rather than 3
min. To control for the staining of any nonspecific
alkaline phosphatase, the ATP substrate was not
added to the incubating solution in some cases. No
staining was evident in these sections.
Fiber-type proportions were determined by examining stained sections under a light microscope (Fig.
1). As observed previously (McIntosh et al., 1985;
Richmond et al., 1999), fibers showed one of three
levels or intensities of staining. Lightly stained fibers were classified as SO (or type I), darkly stained
fibers were classified as FG (or type IIb), and fibers
with an intermediate staining intensity were classified as FOG (or type IIa). Based on the size of the
muscle, two to nine areas, each containing approximately 100 fibers, were selected from different areas
of the muscle (specific number of samples for each
muscle are listed in Table 1). The fiber-type proportions were calculated for each area and these values
were then averaged to obtain fiber-type percentages
for each muscle.
Different fiber types are known to vary in crosssectional area (CSA) (for review, see Burke, 1981).
HISTOCHEMISTRY OF M. MULATTA FORELIMB MUSCLES
325
lated from the average fiber-type CSAs and later
used to determine the relative CSA contributed to
each muscle by each fiber type.
RESULTS
Fig. 1. Macaca mulatta. Typical mosaic of three fiber types in
biceps, long, stained for myofibrillar adenosine triphosphatase
(mATPase, preincubation pH 10.4) activity. Lightly stained fibers
were classified as SO, darkly stained fibers as FG, and fibers with
intermediate staining intensity as FOG. Scale bar ⫽ 100 ␮m.
The CSAs of the different fiber types were measured
in up to six muscles obtained from each of the three
animals. For an individual section, 20 fibers of each
type were selected from two to three representative
areas. Fiber CSAs were measured using Image-Pro
software (Media Cybernetics, Silver Springs, MD).
An average CSA for each fiber type was calculated in
the six muscles and those values were used to calculate an overall average CSA for each fiber type.
FOG-to-FG and SO-to-FG CSA ratios were calcu-
All shoulder and elbow muscles studied here contained a mixture of FG, FOG, and SO fibers, but
proportions varied (Table 2). FG fiber content
ranged from 25– 67%, whereas SO fiber proportions
rarely exceeded 40% (range: 13–56%). The leastrepresented fiber type were the FOG fibers, which
had similar proportional representation in all muscles (18 –28%).
We quantified the relationships between the different fiber types by plotting the percentages of FG
or FOG fibers against the slow-fiber percentages
(SO) for each of the muscles studied (Fig. 2). The
proportions of FG and SO fibers were correlated
inversely with each other (r2 ⫽ 0.93, P ⬍ 0.005),
whereas the proportion of FOG fibers was not correlated with either of the other two fiber types (r2 ⫽
0.02 and 0.01, P ⬎ 0.05 for SO and FG fibers, respectively).
The muscles studied here could be divided for
descriptive purposes into three groups (Fig. 2). The
largest group (15 of 22 muscles) had SO fiber proportions ranging from approximately 25– 40%. The
two smaller groups had larger or smaller percentages of SO fibers (boxed regions in Fig. 2). Muscles
with the highest proportion of slow fibers (Cb, Sp,
TMi; boxed on the right) were usually small and
situated deep, often lying adjacent to bone. Muscles
TABLE 2. Relative proportions of fiber types, contributions of each fiber type to whole muscle CSA, and the estimated
Vmax values in the forelimb muscles
Mean % (range)
CSA %
Vmax
Muscle
FG
FOG
SO
FG
FOG
SO
L0/s
cm/s
BL
BS
B
Br
Cb
DA
DM
DP
De
ECRB
ECRL
Is
LD
PM
Rh
Sb
Sp
TMa
TMi
TLa
TLo
TMe
55 (50–65)
52 (48–58)
49 (46–53)
67 (63–72)
25
44 (37–52)
36 (34–38)
60 (56–64)
51 (42–60)
43 (37–51)
48 (44–51)
39 (36–43)
46 (40–51)
51 (45–54)
54
42 (35–48)
28 (21–32)
44 (42–48)
25
47 (45–49)
42 (35–48)
41 (39–42)
25 (23–27)
28 (26–30)
24 (19–26)
19 (18–22)
19
23 (19–28)
25 (19–31)
23 (15–31)
21 (19–23)
28 (23–30)
19 (10–27)
24 (23–24)
22 (20–24)
20 (18–22)
18
26 (23–31)
23 (21–25)
26 (20–30)
23
23 (20–24)
22 (21–24)
21 (15–26)
20 (11–26)
20 (16–23)
28 (27–28)
13 (9–19)
56
33 (26–44)
39 (36–45)
17 (12–21)
28 (17–39)
30 (19–36)
34 (29–39)
37 (33–41)
32 (28–40)
29 (27–33)
28
32 (25–42)
49 (44–58)
30 (24–38)
51
31 (27–34)
37 (31–44)
39 (31–46)
66
63
62
76
39
58
50
70
64
55
62
53
60
64
67
56
41
57
39
60
56
55
23
27
23
17
24
23
27
21
20
28
19
25
22
20
18
26
27
26
28
23
23
22
10
10
15
6
38
19
23
8
15
16
19
22
18
16
15
18
31
17
34
17
21
23
12.6
12.7
12.3
13.1
10.2
11.9
11.5
12.8
12.2
12.0
12.0
11.6
12.0
12.2
12.4
12.0
10.6
12.1
10.6
12.1
11.7
11.6
68.2
84.0
52.9
145.3
18.4
62.0
31.0
60.4
70.7
37.2
68.2
29.0
138.3
96.7
63.6
22.8
27.5
68.8
19.0
52.1
44.6
48.5
326
K. SINGH ET AL.
Fig. 2. Macaca mulatta. Relative proportions of FG (filled circles) and FOG (open circles) vs. SO fibers. Note the inverse
relationship between the proportion of FG fibers and that of the SO fibers (r2 ⫽ 0.93). The boxed data points on the left and right were
muscles in which the percentage of SO fibers was unusually low and high, respectively. Those muscles with low SO proportions were
primarily whole-limb flexors (Br, BL, BS, and DP), whereas those with high SO proportions (Cb, Sp, and TMi) were small and deep.
with the lowest proportions of SO fibers (boxed on
the left) were primarily whole-limb flexors whose
primary action was to flex the elbow or extend the
shoulder (Br, BL, BS, and DP). However, BL and BS
(elbow flexors) are anatomically biarticular muscles
that also flex the shoulder. When muscles were
grouped according to their primary function, we
found that elbow flexors had a lower proportion of
SO fibers than the elbow extensors (P ⬍ 0.05). There
were no significant differences in the proportions of
SO fibers among anatomical groups at the shoulder
(flexors vs. extensors, abductors vs. adductors, internal vs. external rotators).
The distribution of fiber types within individual
muscles was not uniform and, in some cases, a
strong gradient was observed. In the majority of
muscles the proportion of SO fibers tended to increase in the deeper portions of the muscle (see, for
example, Fig. 3). To quantify this finding, we determined the proportion of each fiber type in regions
throughout the depth of BL in all three animals and
calculated the ratio of SO-to-FG fiber percentages
(Fig. 4). In the most superficial regions (Fig. 4, re-
gions 0, 1) the SO-to-FG ratio was almost always
less than 0.25. The deeper regions (Fig. 4, regions 4,
5) had higher proportions of SO fibers with the SOto-FG ratio increasing to above 0.5.
Fiber types differed in size across muscles (Fig.
5A). The size of a given fiber type varied from muscle
to muscle, such that the FG fibers in some muscles
were smaller than the FOG fibers in another muscle.
However, the relative size of each fiber type remained fairly constant within each muscle (Fig. 5B).
The only muscle that did not conform to this pattern
was PM, but it was examined in only one animal. In
order to use the fiber CSA data to estimate the
contribution of the different fiber types to wholemuscle CSA, the mean ratios of FOG-to-FG CSA
(0.78) and SO-to-FG CSA (0.43) were applied to the
fiber-type proportions within each muscle. Before
taking fiber CSAs into consideration, the FG fiber
contribution exceeded 50% in only six muscles. That
number increased to 18 after accounting for differences in fiber CSA (Table 2).
We can now estimate Vmax of each forelimb muscle
based on its fiber-type proportions listed in Table 2.
HISTOCHEMISTRY OF M. MULATTA FORELIMB MUSCLES
327
Fig. 3. Macaca mulatta. Photomicrographs were obtained from the right BL in a single animal.
Differing proportions of fiber types with increasing depth were observed. Proportions of SO fibers
typically increased with depth. Scale bar ⫽ 100 ␮m.
Vmax for a given muscle was estimated as the sum of
Vmax for the three fiber types each linearly weighted
by their respective proportion of the muscle’s CSA.
While estimates of Vmax have been obtained for
mammalian SO (4.5 L0/s, Scott et al., 1996) and FG
fibers (14 L0/s, Cheng et al., 2000), there are no
direct measures of Vmax for FOG fibers. We have
assumed Vmax for FOG fibers to be 12.8 L0/s based
on measures of a mixed muscle, feline medial gastrocnemius (Spector et al., 1980).
328
K. SINGH ET AL.
Fig. 4. Macaca mulatta. Fiber-type proportions in three BL muscles (each symbol represents a different animal) were measured at
varying depths (superficial depths are denoted by low numbers, 0 being the most superficial, and deeper samples are indicated by
higher numbers, 5 being the deepest). The ratio of SO-to-FG fiber proportions increased with increasing depth, indicating that SO
fibers were more abundant in the deep portions of the BL.
Estimates of Vmax for monkey forelimb muscles
range from a low of 10.2 L0/s for Cb to 13.1 L0/s for
Br, with an overall mean of 11.9 L0/s (Table 2).
Table 2 also displays Vmax in units of cm/s by
multiplying Vmax (initially in L0/s) and optimal
fascicle length for each muscle taken from Cheng
and Scott (2000) and Richmond et al. (2001). There
is almost an order of magnitude variation in Vmax
scaled in units of cm/s, from 18.4 cm/s for Cb to
145.3 cm/s for Br.
We compared fiber-type proportions with previously measured values of fascicle length and physiological cross-sectional area for each muscle (Cheng
and Scott, 2000, Richmond et al., 2001). There was a
statistically significant correlation between fascicle
length and FG and SO fiber-type proportions for the
forelimb muscles (Fig. 6, r2 ⫽ 0.47 and 0.37, respectively, P ⬍ 0.05). There was virtually no correlation
between fascicle length and FOG fibers (r2 ⫽ 0.08,
P ⬎ 0.05), although this is not surprising, given the
rather consistent proportion of FOG fibers found
across the forelimb muscles. No significant relationships were found when physiological cross-sectional
area was plotted against the different fiber types
(P ⬎ 0.05).
DISCUSSION
Reaching movements in nonhuman primates have
become a popular animal model to study how regions of the brain are involved in controlling movement. One way to understand the physics of these
motor tasks is to develop biomechanical models of
the upper limb. The force–velocity relationship of
muscle is usually included in those models but its
shape varies for each type of muscle fiber (Zajac,
1989; Cheng et al., 2000). The present study was
conducted to determine the histochemical properties
of reaching muscles in the rhesus monkey in order to
estimate the Vmax for each muscle spanning the
shoulder and elbow joints.
Histochemical Properties of Proximal
Forelimb Muscles
In the present study, all of the muscles examined
had all three fiber types represented in varying pro-
HISTOCHEMISTRY OF M. MULATTA FORELIMB MUSCLES
329
Fig. 5. Macaca mulatta. A: Mean CSAs (with standard deviation bars) for three fiber types (FG-black, FOG-gray, SO-white) in six
muscles (number of animals examined in parentheses). Within all muscles (except PM), mean FG CSA was greater than that of FOG
fibers. FOG fibers were in turn larger than SO fibers. Note that FOG fibers in some muscles (e.g., TLo) were larger than FG fibers in
other muscles (TMa, BL, etc.). B: The FOG-to-FG and SO-to-FG CSA ratios were calculated in each of the six muscles. The values were
fairly consistent, with means of 0.78 and 0.43, respectively.
330
K. SINGH ET AL.
Fig. 6. Macaca mulatta. Fiber-type proportions were
plotted against fascicle lengths obtained from previous studies
(Cheng and Scott, 2000; Richmond et al., In press). A significant
correlation was observed between the FG (filled circles) and SO
(open circles) fiber proportions and fascicle length (r2 ⫽ 0.47 and
0.37, respectively). FOG fiber proportions (gray triangles) were not
significantly correlated with fascicle length (r2 ⫽ 0.08, P ⬎ 0.05).
portions. In no instance did a muscle consist of one
or two fiber types exclusively, as has been reported
in certain cat muscles, soleus (Ariano et al., 1973), or
caudofemoralis (Brown et al., 1998). Nevertheless,
FG fibers were found to be the predominant fiber
type in most forelimb muscles. McIntosh et al.
(1985) also found FG fibers to be the predominant
fiber-type in their smaller sample of monkey forearm muscles, suggesting that the rhesus upper arm
and forearm are both well-adapted for strong, rapid
contractions.
Interestingly, FOG fiber percentages were relatively constant across all muscles studied (from 18 –
28%). McIntosh et al. (1985) also showed that in
eight rhesus forearm muscles the FOG fiber proportion varied only between 19 –30%. Roy et al. (1984)
demonstrated that, while FG and SO percentages
vary from superficial to deep regions within a given
muscle in the cynomolgus monkey, the FOG propor-
tion remained constant. A similar consistency in the
proportion of FOG fibers has also been reported for
muscles of the rhesus neck (Richmond et al., In
press) and hindlimb (Roy et al., 1991). As a result,
differences in fiber-type proportions appear to arise
primarily from changes in the proportions of the
other two fiber types.
It is not clear why rhesus monkeys show such a
consistent composition of FOG fibers. A constant
percentage of FOG fibers has not always been seen
in other mammals. A study conducted on the cynomolgus hindlimb revealed much greater variation in
the percentage of FOG fibers (0 –38%), not only between muscles, but also within individual muscles
(Acosta and Roy, 1987). Although a study of 10 feline
muscles revealed that only one limb muscle had
FOG fiber percentages that were not between 12–
25% (soleus: 0.78%), a number of cranial muscles
showed very high variability (Braund et al., 1995).
SO fiber proportions tended to be lower than that
of the FG fibers, but percentages usually increased
with depth within a muscle. The trend for increasing
proportions of SO fibers was also evident when fiber
contents of layered groups of muscles were examined (although the sample size of the three muscles
in which such a pattern was observed was small). A
full analysis of fiber-type regionalization within individual muscles was only carried out on one muscle
in the present study, but the trend was consistent in
all of our experimental animals. Similar variations
in fiber types within a muscle has been shown previously (Roy et al., 1984; McIntosh et al., 1985). A
very striking example of nonuniform fiber-type distribution was seen in flexor carpi ulnaris, in which a
tendon separated the two histochemically distinct
regions (McIntosh et al., 1985). Another example of
fiber-type regionalization was observed by Richmond et al. (1999) in the rhesus neck muscle,
obliquus capitis inferior. Such regionalization can
have important implications for interpreting and
designing electromyographic studies of movement.
Fine wire or epimysial patch EMG electrodes that
are implanted on or near the superficial surface of
the muscle may disproportionately represent the
signals from FG fibers, which presumably are active
only during phasic activities. Thus, little to no activity might be recorded during weaker muscle contractions, as might occur during postural tasks, when
slow-twitch fibers deep in the muscle are most likely
to be recruited in preference to fibers with poorer
oxidative capacity (Richmond et al., 1988; Corneil et
al., 1996).
The mathematical model of muscle described by
Zajac (1989) scales the force–velocity relationship of
muscle based on fiber-type composition. A common
approach for estimating Vmax in a muscle containing
a mix of fibers is to linearly weight Vmax for each
fiber type based on its relative proportion within the
muscle. Such estimates assume that SO muscle fibers shortening at supramaximal speeds (speeds
HISTOCHEMISTRY OF M. MULATTA FORELIMB MUSCLES
greater than 4.5 Lo/s) will impede, to some degree,
the motion of the muscle. That is, it is assumed that
SO fibers act as viscous elements to impede force
production in the surrounding fast fibers. Experiments by Hutton and Enoka (1986) demonstrated
that the soleus (predominantly SO fibers) does not
impede lateral gastrocnemius (considerably fewer
SO fibers) contractile speed in the rat hindlimb. If
SO fibers did not impede motion of surrounding
fibers, then physiologically measured Vmax values of
mixed muscles containing FG fibers under maximal
stimulation rates should always equal the Vmax of
FG fibers. However, Vmax in the feline medial gastrocnemius, made up of a mix of fiber types (25% SO,
14% FOG, 61% FG; Ariano et al., 1973) was found to
be 12.8 L0/s (Spector et al., 1980). This reduction in
Vmax below that observed for FG fibers suggests
force production by FG fibers is hampered, to some
degree, by the surrounding slow fibers.
The correlation between fascicle length and the
proportion of fast fibers in each forelimb muscle has
important implications for the force-generating capabilities of different muscles under dynamic conditions. Muscle force output during movement is dependent not only on muscle cross-sectional area, but
also on fascicle length and fiber type. Longer muscle
fascicles have more sarcomeres in series so that,
during movement, each sarcomere undergoes less
motion (i.e., shortens at a lower velocity) as compared to a muscle with shorter fascicles. Thus, the
muscle with longer fascicles can generate more force
because force production depends on sarcomere velocity (i.e., the force–velocity relationship). An improvement in force production during motion also
occurs as the proportion of fast fibers increases
within a muscle. Therefore, the observed correlation
between fascicle length and the proportion of fast
fibers in rhesus forelimb muscles tends to expand
differences in force-generating capabilities between
various muscles. This correlation also suggests that
both the morphometric and histochemical properties
of muscle may be influenced by the force-generating
requirements of each muscle. That is, an increase in
force production in a specific muscle during dynamic
conditions may lead to both an increase in fascicle
length and in the proportion of fast fibers.
Variations in Fiber Cross-Sectional Area
A common assumption in methods used to scale
Vmax is that the cross-sectional area of each fiber is
similar across fiber types. This assumption is clearly
not correct for Macaca mulatta forelimb muscles. In
five of six muscles examined, we found that the FG
fibers had larger CSAs than FOG fibers and FOG
fibers had larger CSAs than SO fibers (but see PM,
Fig. 5A). Previous studies have shown similar variations in CSA between fiber types (see Burke, 1981,
for review). Our findings in the forelimb muscles,
however, were substantially different from those of
331
Roy et al. (1984), who studied cynomolgus monkeys.
Their study indicated that the SO fibers were always
larger than the FOG fibers, which were in turn
larger than the FG fibers. However, their calculation
of the proportion of muscle CSA contributed to by
each fiber type appeared to show that FG fibers were
in fact larger than SO fibers.
When fiber-type compositions were corrected for
variations in fiber CSA observed in the present
study, the predominance of FG fibers in the Macaca
mulatta forelimb increased. For example, if we were
to use 50% FG fibers as the criterion for classifying
a muscle as fast, 7 of 22 (32%) muscles would be
considered fast before correcting for variation in fiber size. However, if the criterion of 50% of the
muscle CSA was used, the number of fast muscles
increases to 18 (82%). These observations suggest
that future studies should document not only fibertype distributions but also fiber-type CSAs, so that
results could be used more effectively for modeling
purposes.
ACKNOWLEDGMENTS
The authors thank M.-J. Bourque, J. Creasy, and
K. Moore for expert technical assistance. KS and
EHM were supported by MRC Doctoral Awards and
SHS by an MRC Scholarship.
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