Forty-seven elite male track athletes (100-m sprinters, SPR, N = 23; and distance runners, DR, N = 24; composed of 10K (N = 14) and marathon (N = 10)) and 24 untrained male undergraduate students (controls) were recruited for this study. All athletes had been training and competing for over 7 yr (average 8.4 yr). Personal best performances ranged between 10.0 and 10.9 s for 100 m in SPR and 13.5 and 14.5 min for 5000 m or 130 and 145 min for 42.2-km marathon in DR. Control students had not participated in any recreational sports for at least 2 yr before testing. All subjects gave informed consent before testing.
Skeletal muscle distribution.
Skeletal muscle distribution was determined from measurements of muscle layer thickness across the body. As described previously (1,2), muscle thickness was measured across the muscle group by B-mode ultrasound (SSD-500, Aloka, Tokyo, Japan) at 13 anatomical sites [anterior (at 30, 50, and 70% thigh length, starting at the greater trochanter) and posterior (at 50 and 70% thigh length) thigh, anterior and posterior lower leg (at 30% proximal between the lateral malleolus of the fibula and the lateral condyle of the tibia), lateral forearm (at 30% proximal between the styloid process and the head of the radius), anterior and posterior upper arm (at 60% distal between the lateral epicondyle of the humerus and the acromial process of the radius), abdomen, subscapula, and chest). 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 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 muscle thickness. Precision and linearity of the image reconstruction have been described and confirmed elsewhere (12). The coefficient of variation (CV) of this method from test-retest (20 samples) is 0.8%.
Skeletal muscle architecture.
Muscle architecture of specific muscles within a muscle group, termed isolated, was determined by placing the transducer over the specific muscle. Isolated muscle thickness and pennation angle of the vastus lateralis (at midway between the lateral condyle of the femur and greater trochanter), gastrocnemius medialis (at a point 30% proximal between the lateral malleolus of the fibula and the lateral condyle of the tibia), and gastrocnemius lateralis (at the same level as gastrocnemius medialis) muscles were measured in vivo as described previously (2,3), using the B-mode ultrasound apparatus (SSD-500, Aloka). Briefly, mediolateral widths of the three specific muscles were determined over the skin surface, and the position of one-half of the width was used as a measurement site for each muscle. Muscle fibers are packed in bundles called fascicles. These fascicles are connected together and extend from the proximal to distal tendons (6,9,23). The echoes from interspaces of fascicles and from the superficial and deep aponeuroses were visualized (12), and the ultrasonic images were printed onto calibrated recording films (UPP-110HA, Sony, Tokyo). From the printed images, the fascicle pennation angle was determined from the angles between the echo of the deep aponeurosis of each specific muscle and interspaces among the fascicles of that muscle. Isolated muscle thickness was taken as the distance between subcutaneous adipose tissue-muscle interface and intermuscular interface. Fascicle length across the deep and superficial aponeurosis was estimated from the isolated muscle thickness and pennation angle using the following equation:MATH where α is the pennation angle of each muscle determined by ultrasound. The estimated CV of this fascicle length determination (20 samples) is 4.7%.
Results are expressed as means ± SD. For comparison by group a one-way ANOVA was used with the a priori level of statistical significance set at P < 0.05. Relationships between selected variables were examined using Pearson product correlations.
There were no significant differences among SPR, DR, and control subjects in age (SPR, 21.0 ± 2.3 yr; DR, 22.4 ± 2.3 yr; and controls, 20.9 ± 2.07 yr), standing height (172 ± 4, 172 ± 5, and 171 ± 6 cm, respectively), upper arm (31.7 ± 1.0, 31.4 ± 1.2, and 31.4 ± 1.7 cm), thigh (39.3 ± 1.7, 39.9 ± 1.7, and 38.8 ± 2.3 cm) and lower leg (39.5 ± 1.7, 40.0 ± 1.6, and 39.7 ± 2.3 cm) lengths. Body weight was significantly greater in SPR (66.1 ± 3.9 kg) compared with DR (57.6 ± 4.3 kg) and controls (58.6 ± 6.8 kg), which were similar.
Skeletal muscle distribution.
All skeletal muscle distribution data are presented in Table 1. Muscle thickness was significantly greater in SPR than DR at all sites except the forearm and 70% anterior thigh, and controls at all sites except 70% anterior thigh and the anterior lower leg. In DR, muscle thickness of posterior thigh was significantly higher, whereas all other sites were similar to control.
Skeletal muscle architecture.
All skeletal muscle architecture data are presented in Table 2. Isolated muscle thickness of vastus lateralis and gastrocnemius medialis and lateralis muscles was significantly greater in SPR compared with DR and controls; which were similar. Pennation angle was similar in SPR and controls in all three selected muscles. Compared with SPR, DR had a greater pennation angle in vastus lateralis and gastrocnemius medialis, but not gastrocnemius lateralis muscle. Pennation angle in DR was significantly greater than controls in all three selected muscles.
In vastus lateralis muscle, fascicle length (absolute and relative to limb length) was greatest in SPR, least in DR, with control values between the athlete groups. In gastrocnemius medialis muscle, absolute and relative fascicle length was greater in SPR than in DR and controls, which were similar. In gastrocnemius lateralis muscle, absolute and relative fascicle length was significantly greater in SPR than DR. Absolute fascicle length was similar between DR and controls; however, relative fascicle length was lesser in DR.
Relative fascicle length and isolated muscle thickness (both normalized to limb length) were positively correlated in SPR and DR in the vastus lateralis (r = 0.40, P < 0.05 and r = 0.74, P < 0.01, respectively), gastrocnemius medialis (r = 0.69 and r = 0.82, both P < 0.01), and lateralis (r = 0.80, P < 0.01 and r = 0.44, P < 0.05), whereas in controls relative fascicle length correlated to isolated muscle thickness in the gastrocnemius medialis (r = 0.63, P < 0.01) and lateralis (r = 0.52, P < 0.01), but not in vastus lateralis (r = 0.24) (data not shown).
In the SPR group, pennation angle demonstrated a significant negative correlation to relative fascicle length in vastus lateralis (r = −0.79, P < 0.01), gastrocnemius medialis (r = −0.65, P < 0.01), and lateralis (r = −0.77, P < 0.01). In DR, gastrocnemius medialis (r = −0.54, P < 0.01) and lateralis (r = −0.74, P < 0.01) pennation angle were negatively correlated to relative fascicle length, but not in the vastus lateralis (r = −0.22). In controls, pennation angle correlated to relative fascicle length in the vastus lateralis (r = −0.83, P < 0.01) and gastrocnemius lateralis (r = −0.82, P < 0.01), but not in gastrocnemius medialis (r = −0.36) (data not shown).
It is known that muscle fascicle length plays a significant role in determining maximum shortening velocity of muscle, based on studies of animal skeletal muscle (7,21,24). However, it is unknown whether there is a relationship between human skeletal muscle fascicle length and running performance. The major finding in the present study demonstrate that the fascicle length of vastus lateralis and gastrocnemius medialis and lateralis, relative to limb length, is significantly greater in elite SPR than that observed in elite DR or controls. These findings demonstrate for the first time in human skeletal muscle that sprint performance appears to be related to differences in muscle fascicle length.
The reasons for the differences in fascicle length among SPR, DR, and controls are unclear, but several possibilities exist. First, the role of genetic predisposition must be considered. Certain individuals may be born with longer fascicle length per given limb length, providing them a greater potential for muscle shortening velocity, the magnitude of which would be advantageous in sprint performance. On the contrary, certain individuals may be born with shorter fascicle length, giving them a limited potential for sprinting. Thus, it is possible that these individuals selected the “proper” sports activity based on fascicle length and muscle shortening velocity capacity.
However, the possibility exists that the differences in fascicle length are secondary to training and the muscle enlargement observed in SPR as greater muscle thickness (Table 1), whereas DR and controls were similar. In the present study, we find that fascicle length is positively correlated to isolated muscle thickness for all selected leg muscles in SPR and DR. The relationship between fascicle length and increased muscle thickness lends intriguing support to the possibility that under certain training conditions fascicle lengthening may occur in humans and may play a role in muscle enlargement (14). Although this observation is novel in humans, several studies have shown muscle fascicle lengthening associated with muscle enlargement in animals (11,16,25). In two of these studies, muscle fascicle lengthening and muscle enlargement were observed after chronic muscle stretch (11,25). Although the chronic stretch model is not directly applicable to the present scenario, running performance, it does support the possibility of fascicle lengthening as an adaptive response in muscle enlargement. One animal study with direct application to the present study showed fascicle lengthening in leg muscles of rats trained with downhill running (16). This study shown that fascicle lengthening is possible in intact muscle training with physiologically and anatomically relevant repetitive contractions, muscle loading, and shortening cycles. This would support the present observation that sprinters may lengthen fascicle in response to sprint training. Likewise, whether the shorter fascicle length observed in DR is associated with training-specific adaptations in muscle due to distance running remains to be investigated. Certainly, additional studies are necessary to fully understand the possibility and role of muscle fascicle lengthening or shortening as a training adaptation to running training.
Maxwell et al. (17) proposed a theoretical model describing the interaction of the variables that determine muscle architecture, and the relationship between subsequent changes in these variables. From this model, Maxwell et al. proposed that at a given muscle length, fiber number, and fiber length, an increase in fiber cross-sectional area (CSA) must be accompanied by an increase in pennation angle. This relationship between muscle hypertrophy (increased CSA or muscle thickness) and increased pennation angle has been shown experimentally (2,12,13). However, varying pennation angle and muscle thickness within Maxwell et al.’s model suggests that fascicle length should change in some settings, i.e., a longer fascicle length would result from lesser pennation angle with whole muscle length and fiber CSA being constant. The present finding that SPR had greater muscle thickness in leg muscles compared with DR or controls supports previous findings of greater muscle fiber CSA in elite sprinters (8). However, counter to previous findings (2,12,13), pennation angle was similar between SPR and controls and lesser than DR, despite the greater muscle thickness. This can only be accounted for by changes in fascicle length and is further supported by the significant, negative correlation between pennation angle and fascicle length. We have observed this phenomenon in Japanese sumo wrestlers compared with untrained controls (15; preliminary data presented in abstract form). Thus, it appears that muscle enlargement (increased muscle thickness or CSA) does not have to result in an increased pennation angle, if fascicle length increases. Keeping with the Maxwell et al. model, increasing the fascicle length would limit change in pennation angle associated with muscle enlargement (14). This may be physiologically significant because the potential increase in force-generating capacity is more fully realized as force per unit muscle CSA of the hypertrophied muscle is reduced by increases in pennation angle (12,13). Whether inherited or a specific adaptation to training, longer fascicle length and lower pennation angle would be advantageous for sprint performance. The greater pennation angle, with similar muscle thickness in DR compared with controls, could be explained by the shorter fascicle length and/or the increases in muscle CSA due to increased slow muscle fiber CSA (8) at the same whole muscle area (muscle CSA) induced by distance running. This architectural arrangement appears to favor distance running.
Another interesting aspect of the present data relating muscle architecture to sprint performance is apparent differences in “muscle shape”. Our data show that SPR had significantly greater muscle thickness in upper portion of the anterior thigh (13.4 and 10.7% greater at the 30%- and 50%-thigh length, respectively), but not in the lower portion (70%-thigh length) as compared with controls and DR. Given that thigh length was similar among groups, the shape of the quadriceps muscle is therefore different in SPR. Previously, we have observed race differences (black vs white) in muscle shape (3). In that study, the altered muscle shape of the quadriceps and hamstrings muscles in blacks correlated with faster 40-yard sprint times. Thus, these observations (ref. 3 and present data) are consistent with the hypothesis that muscle shape (thicker upper portion of quadriceps and hamstrings) is associated with better sprint performance. With respect to the present result, it is not clear whether differences in muscle shape among the groups (present data) or blacks (3) is an inevitable consequence of genetic variation or ultimately related to mode of muscular training. Narici et al. (19,20) reported a change in the “shape” of skeletal muscle after high-intensity weight training resulting from nonuniform changes in CSA along the length of the quadriceps muscle after high-intensity resistance training. They suggest that there are varying degrees of hypertrophic response in each of the constituent muscles of the quadriceps. This may apply to sprinting as well; however, more work is needed to clarify this phenomenon.
In conclusion, it appears that sprinters have greater muscle thickness (fiber CSA) and longer fascicle length in selected leg muscles, and an altered “shape” of the quadriceps muscle group. These architectural characteristics appear to coincide with the determinants of maximum velocity of shortening, and are, therefore, consistent with faster sprint performance.
The authors wish to thank athletes and coaches of the Japan Track & Field Association and the several universities track & field teams who participated in this study, in particular Mrs. Yasuhiro Harada, Kazuhisa Kawamoto, Susumu Takano, Masuhiko Mizuno, and Toshinobu Sato.
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