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]

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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).

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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

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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.

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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).

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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-

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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.

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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

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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

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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. LITERATURE CITED Acosta L Jr, Roy RR. 1987. Fiber-type composition of selected hindlimb muscles of a primate (cynomolgus monkey). Anat Rec 218:136 –141. Ariano MA, Armstrong RB, Edgerton VR. 1973. Hindlimb muscle fiber populations of five mammals. J Histochem Cytochem 21: 51–55. Barany M. 1967. ATPase activity of myosin correlated with speed of muscle shortening. J Gen Physiol 50:(Suppl):197–218. Braund KG, Amling KA, Mehta JR, Steiss JE, Scholz C. 1995. Histochemical and morphometric study of fiber types in ten skeletal muscles of healthy young adult cats. Am J Vet Res 56:349 –357. Brown IE, Satoda T, Richmond FJ, Loeb GE. 1998. Feline caudofemoralis muscle. Muscle fibre properties, architecture, and motor innervation. Exp Brain Res 121:76 –91. Burke RE. 1981. Motor units: anatomy, physiology, and functional organization. In: Brooks VB, editor. The nervous system (handbook of physiology, sect. 1, vol. II. Motor control, part 1). Bethesda, MD: American Physiological Society. p 345– 422. Cheng EJ, Scott SH. 2000. Morphometry of Macaca mulatta forelimb. I. Shoulder and elbow muscles and segment inertial parameters. J Morphol 245:206 –224. Cheng EJ, Brown IE, Loeb GE. 2000. Virtual muscle: a computational approach to understanding the effects of muscle properties on motor control. J Neurosci Methods 101:117–130. Cordonnier C, Stevens L, Picquet F, Mounier Y. 1995. Structurefunction relationship of soleus muscle fibres from the rhesus monkey. Pflugers Arch 430:19 –25. Corneil BD, Loeb GE, Richmond FJ, Munoz DP. 1996. EMG activity in dorsal neck muscles of the rhesus monkey during head movements and in different head positions. Soc Neurosci Abstr 22:2036. Georgopoulos AP. 1995. Current issues in directional motor control. Trends Neurosci 18:506 –510.

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Guth L, Samaha FJ. 1970. Procedure for the histochemical demonstration of actomyosin ATPase. Exp Neurol 28:365–367. Hutton RS, Enoka RM. 1986. Kinematic assessment of a functional role for recurrent inhibition and selective recruitment. Exp Neurol 93:369 –379. Kalaska JF, Scott SH, Cisek P, Sergio LE. 1997. Cortical control of reaching movements. Curr Opin Neurobiol 7:849 – 859. McIntosh JS, Ringqvist M, Schmidt EM. 1985. Fiber type composition of monkey forearm muscle. Anat Rec 211:403– 409. Richmond FJ, Bakker DA, Stacey MJ. 1988. The sensorium: receptors of neck muscles and joints. In: Peterson BW, Richmond FJR, editors. Control of head movement. New York: Oxford University Press. p 49 – 62. Richmond FJ, Singh K, Corneil BD. 1999. Marked non-uniformity of fiber-type composition in the primate suboccipital muscle obliquus capitis inferior. Exp Brain Res 125:14 –18. Richmond FJ, Singh K, Corneil BD. Neck muscles in the rhesus monkey. I. Muscle morphometry and histochemistry. J Neurophysiol. In press.

Roy RR, Bello MA, Powell PL, Simpson DR. 1984. Architectural design and fiber-type distribution of the major elbow flexors and extensors of the monkey (cynomolgus). Am J Anat 171: 285–293. Roy RR, Bodine-Fowler SC, Kim J, Haque N, de Leon D, Rudolph W, Edgerton VR. 1991. Architectural and fiber type distribution properties of selected rhesus leg muscles: feasibility of multiple independent biopsies. Acta Anat (Basel) 140:350 –356. Scott SH, Brown IE, Loeb GE. 1996. Mechanics of feline soleus. I. Effect of fascicle length and velocity on force output. J Muscle Res Cell Motil 17:207–219. Spector SA, Gardiner PF, Zernicke RF, Roy RR, Edgerton VR. 1980. Muscle architecture and force-velocity characteristics of cat soleus and medial gastrocnemius: implications for motor control. J Neurophysiol 44:951–960. Zajac FE. 1989. Muscle and tendon: properties, models, scaling, and application to biomechanics and motor control. Crit Rev Biomed Eng 17:359 – 411.