Skeletal muscles are generally classified in two main morphological types, fusiform muscles in which fibers lie almost parallel to the line of action of the muscle and pennate muscles in which fibers insert into tendons at an angle to the line of action of the muscle (11). The number of sarcomeres in series of the fusiform muscle represents the true anatomical muscle length. In pennate muscle, on the other hand, the number of sarcomeres in series and muscle fascicle length are generally less than in fusiform muscle. The advantage, however, of pennate muscle is the ability to pack more crossbridges in a given area, hence more physiological cross-sectional area. The number of sarcomeres in series contributes to the specific action of the muscle: force development or shortening velocity. The velocity of shortening, although mainly determined by myosin ATPase activity (5), is modulated by architectural features, i.e., the number of sarcomeres in series and angle of pennation (22). Moreover, the differences in maximal shortening velocity between muscles is more attributed to differences in muscle fiber lengths rather than biochemical differences (19,24). Therefore, muscle architectural parameters may associate with sports performance, especially sprinting or jumping events.
Previous studies have indicated that most of the best sprinters in the world are black Americans (20) and, in both male and female children, blacks perform better than white counterparts in 30- to 50-yard dashes (9,14,17). Ama et al. (3) found that sedentary blacks have a significantly higher percentage of type II muscle fibers and higher anaerobic enzyme activities in the vastus lateralis muscle compared with whites. Unfortunately, little other scientific evidence is available to support the differences in sprint/jumping performance between blacks and whites. Thus, the purpose of this study was to determine whether there are differences between black and white athletes with respect to muscle fascicle length, pennation angles, and muscle thickness distributions.
Forty-four American, male college football players volunteered for the study (Table 1). They were divided into two groups by race; black (N = 13; 5 defensive and offensive backs, 7 defensive and offensive linemen, and 1 linebacker) and white (N = 31; 10 backs, 16 linemen, and 5 linebackers). All players competed at the NCAA Division I level for the academic year 1997–1998. None of the subjects were taking anabolic hormones. Informed consent was obtained from each subject before testing.
Measurement of fat-free mass.
Body density was measured by the hydrostatic weighing technique (1). Pulmonary residual volume was measured using the oxygen dilution method of Rahn et al. (18), and body fat percentage was calculated from the body density using Brozek et al.’s equation (7). Fat-free mass (FFM) was estimated as the difference between total body mass and fat mass. We used a two-compartment model to compare the body composition of black and white athletes. However, there are race differences in the density of fat-free tissue (21). Therefore, the calculation of body density and % fat of black athletes was modified using the following equation: % fat = 437.4/BD − 392.8 (13).
Measurement of muscle thickness.
Muscle layer thickness (MTH) was measured by B-mode ultrasound (SSD-500, Aloka, Japan) at 13 sites (chest, abdomen, anterior (at 30, 50, and 70% of thigh length) and posterior (at 50 and 70%) thigh, anterior and posterior lower leg (at 30% of lower leg length), forearm lateral (at 30% of forearm length), anterior and posterior upper arm (at 60% of upper arm length) and subscapula) as described previously (2). Briefly, the measurements were carried out while the subjects stood with their elbows and knees extended and relaxed. A 5-MHz scanning head was placed perpendicular to the tissue interface. The scanning head was coated with a water-soluble transmission gel to provide acoustic contact without depressing the dermal surface. The subcutaneous adipose tissue-muscle interface and the muscle-bone interface were identified from the ultrasonic image, and the distance from the adipose tissue-muscle interface to the muscle-bone interface was taken as MTH. Precision and linearity of the image reconstruction have been described and confirmed elsewhere (10).
Measurements of isolated muscle thickness and fascicle angles. Isolated MTH and fascicle pennation angles (PANG) of the long head of the triceps (at 60% distal between the lateral epicondyle of the humerus and the acromial process of the scapula), vastus lateralis (at midway between the lateral condyle of the femur and greater trochanter), and gastrocnemius medialis (at 30% proximal between the lateral malleolus of the fibula and the lateral condyle of the tibia) muscles were measured in vivo as described previously (2), using the B-mode ultrasound apparatus. Briefly, the ultrasound transducer was placed perpendicular to isolated MTH and parallel to PANG for each muscles. Using the ultrasonic images, the distance between subcutaneous adipose tissue-muscle interface and intermuscular interface was adopted as isolated MTH. The angles between the echo of the deep aponeurosis of the muscles and interspaces among the fascicles of the muscles were measured as PANG. From the isolated MTH and the PANG, the length of fascicles (lf) across the deep and superficial aponeuroses was estimated from the following equation:MATH 1 where φ is the PANG of the each muscle determined by ultrasound.
Measurements of sprint/jumping performance.
Running and jumping tests were conducted on an indoor tartan track. The best score for each test was used for data analysis. For the 40-yard dash, subjects began from a standing position with a self-start. Time was measured with an electronic timing system (nearest 0.01 s, Model VT 131, Vanguard, Inc.). Three consecutive trials with 2–5 min of recovery between trials were performed by each subject. Vertical jump was tested using a jump and reach system (nearest 0.1 cm). Subjects began with one foot forward and jumped off of that foot. Subjects were not allowed to change his foot position before jumping. Arm swing was allowed. Three trials were conducted.
Results are expressed as means ± SD. For comparison by race, a one-way ANOVA was used with the a priori level of statistical significance set at P < 0.05. Relationships between isolated MTH and muscle PANG of the selected muscles were examined using Pearson product correlation.
There were no significant differences between black and white athletes in age, standing height, body weight, body mass index, limb circumferences, and arm and leg lengths. The ratio of leg length to standing height was significantly (P < 0.05) higher in blacks compared with whites. Although these differences did not reach statistical significance (P = 0.08), the ratio of arm length to standing height tended to be higher in blacks than in whites. Percentage body fat and FFM were similar between the two groups of athletes (Table 1).
Vertical jump (77.7 ± 6.1 cm) and 40-yard dash (4.63 ± 0.13 s) of the black defensive and offensive backs (N = 5) was significantly greater compared with the white counterparts (N = 10; 69.1 ± 6.9 cm and 4.90 ± 0.15 s, respectively). However, sprint/jumping performances in the defensive and offensive linemen and linebackers were similar between two racial groups. Significant correlations were observed between 40-yard dash versus % fat (r = 0.80, P < 0.01) and total fat mass (r = 0.84, P < 0.01) and vertical jump versus % fat (r = 0.45, P < 0.05) and fat mass (r = 0.52, P < 0.01) in the linemen group, but not in backs.
MTH was significantly larger in blacks at 30%-quadriceps and 50%-hamstrings (upper position of thigh) whereas other the measured sites (50%- and 70%-quadriceps, and 70%-hamstrings) of the thigh were not significantly different between black and white athletes. The MTH of biceps and abdomen were significantly higher in blacks than in whites. However, no significant difference was found in isolated MTH, PANG, and fascicle length of the long head of the triceps, vastus lateralis, and gastrocnemius medialis between groups (Table 2).
A significant positive correlation was observed between MTH and PANG for the long head of the triceps (r = 0.467, P < 0.01) muscle, but not for vastus lateralis (r = 0.152) and medialis gastrocnemius (r = 0.01) muscles. Normalizing MTH by limb length did alter these observations as a significant correlation existed between relative MTH and PANG for the long head of the triceps only (r = 0.558, P < 0.01) (Fig. 1).
In general, blacks (children and adult) have been shown to perform better in sprinting/jumping events than whites (3,9,14,17,20). In the present study, we also found that black backs had significantly greater sprint/jumping performances than that of the white backs, although there was no significant difference between linemen. However, few studies exist making it difficult to interpret the origin of these differences. Superior sprint/jump performance may be related to greater anaerobic capacities in blacks (4,6). Ama et al. (4) reported that although there were no significant differences between sedentary blacks and whites in maximal voluntary knee extension strength and in total peak power output performed during 90-s of repetitive knee extensions exercise, blacks were less resistant to fatigue than whites. Further, differences in muscle histochemical and biochemical characteristics may also contribute to performance differences (3). For example, Ama et al. (3) also reported that whites had a significantly higher percentage of type I fibers, whereas blacks had a significantly higher percentage of type IIa fibers. Moreover, blacks had a significantly higher activities for creatine kinase, hexokinase, phosphofructokinase, and lactate dehydrogenase in the vastus lateralis muscle compared with those of whites, suggestive of greater speed of movement and anaerobic capacity.
The greater speed of movement, sprint/jump performance, in blacks could certainly be explained by the greater number of fast-twitch fibers. However, differences in maximal shortening velocity between muscles is more highly related to fiber length, architecture, than to biochemical differences (19,24). Typically, muscle can be divided into two morphological types; fusiform or pennate (11). Muscles that are fusiform have a fiber orientation that is parallel to the line of force generation. These muscle tend to be longer and have a low physiological cross-sectional area. This particular design promotes velocity of shortening at a given myosin ATPase activity. Fibers of pennate muscle are oriented at an angle to the line of force generation, with shorter fiber length and relatively high physiological cross-sectional area. This design promotes force generation rather than speed of movement. Thus, it is possible that differences in sprint performance may be related to race-related differences in muscle architecture, and to our knowledge, no such data have been reported. The present data show that fascicle length and pennation angle are similar in blacks and whites, and thus, it would appear that there is not a race-related difference in muscle architecture. A previous report (3) has shown that mean fiber area of the type I and type II muscle fibers was similar between sedentary black and white subjects. Greater or increased (i.e., hypertrophy) fiber area at a constant muscle length, fiber length (same fascicle length reported here), and fiber number must result in differences in PANG (12). However, PANG was the same in blacks and whites. Taken together, the same fiber area, fascicle length, and PANG are similar in blacks and whites, which provides indirect evidence that the number of sarcomeres in series between the two groups is similar. This would suggest that better sprinting performance in blacks does not appear to be a result of race differences in architectural characteristics of skeletal muscle.
There were no significant differences in FFM or FFM-to-height ratio between black and white football players. However, there were differences in the distribution of skeletal muscle between the two groups of athletes. Our data show that blacks had significantly greater MTH in upper portion of the thigh (5.8% at the 30%-quadriceps and 6.9% greater at the 50%-hamstrings sites, Table 2) compared with whites. Given that absolute leg lengths were similar between groups, the differences in MTH along the quadriceps and hamstrings suggests a limb-specific distribution of skeletal muscle by race. Or simply, the shape of the quadriceps and hamstring muscles are different in blacks compared with whites. Narici et al. (15,16) reported a change in the “shape” of skeletal muscle after high-intensity weight training. They indicated that the nonuniform changes in cross-sectional area along the length of the quadriceps muscle after high-intensity resistance training was the result of varying degrees of hypertrophic response in each of the constituent muscles of the quadriceps. However, with respect to the present data, it is not clear if differences in muscle shape between black and white football players is an inevitable consequence of race variation or ultimately related to mode of muscular training. These football players have all followed a similar weight training program for 2–3 yr as part of the varsity football program. So it appears that one can rule out training differences and suggest a possible race difference in muscle shape, limb-specific muscle distribution. If or how race-dependent, limb-specific distribution of skeletal muscle affects sprint/jump performance is not known. Clearly, this conclusion is preliminary and much more work is needed to clarify the issue.
Tanner (23) showed that among Olympic athletes, black sprinters (100 m and 400 m) had relatively longer legs than whites. In similar studies comparing physique/stature between black and white athletes, leg and arm lengths relative to sitting height and standing stature are usually greater in black athletes (8). The present data confirm these observations as ratio of leg length to standing height was significantly greater in black college football players than in white counterparts, although ratio of arm length to standing height did not reach statistical significance (P = 0.08).
Generally, FFM is used to estimate total skeletal muscle mass. In general, FFM is about 50% muscle mass in reference man (12). Therefore, a large FFM reflects a large quantity of skeletal muscle and typically greater levels of muscular strength. This, presumably is advantageous for sports performance. Wilmore et al. (25) reported that the average FFM and % body fat were 81.9 ± 6.4 kg and 9.4 ± 4.0% for offensive backs and 95.8 ± 10.1 kg and 18.2 ± 5.4% for defensive linemen, respectively, in the National Football League players. We found no differences in body composition in black and white football players; FFM was 89.9 kg and 89.1 kg, respectively, and % fat was 18.8% and 17.2%, respectively. Thus, it appears that the body composition of the present population of football players is consistent with that of high-level football players.
In conclusion, although it appears that there are some race differences in anatomical stature (limb length/height and limb-specific muscle distribution) muscle architecture is likely independent of race. Thus, differences in sprint/jump performance do not appear to be a result of different architectural characteristics of skeletal muscle. The possible role of race-related differences in limb-specific muscle distribution needs to be investigated.
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