The present study demonstrates that the knee extensor MA is greater in sprinters than in non-sprinters, and this morphological structure in sprinters is associated with sprint performance. Therefore, for the first time, we provided evidence that a greater knee extensor MA in sprinters may be an advantageous for achieving superior sprint performance.

Although recent studies have reported that the forefoot bones are longer in sprinters than in non-sprinters, these reports included a relatively small number of subjects. Moreover, while computer simulation suggested that longer forefoot bones may contribute to higher sprint performance by enhancing plantar flexor moment during sprinting, the correlation between forefoot bone length and sprint performance in humans has not been confirmed in observational studies. Thus, using a relatively large sample, we compared the length of the forefoot bones between sprinters and non-sprinters. We also examined the relationship between forefoot bone length and performance in sprinters. The length of forefoot bones of the big and second toes in 36 well-trained male sprinters and 36 male non-sprinters was measured using magnetic resonance imaging. The length of forefoot bones in the big and second toes was significantly longer in sprinters than in non-sprinters. After dividing the sprinters into faster and slower groups according to their personal best time in the 100-m sprint, it was found that the forefoot bone length of the second toe, but not that of the big toe, was significantly longer in faster group than in slower group. Furthermore, the forefoot bone length of the second toe correlated significantly with the personal best time in the 100-m sprint. This study supported evidence that the forefoot bones are longer in sprinters than in non-sprinters. In addition, this is the first study to show that longer forefoot bones may be advantageous for achieving superior sprint performance in humans.

[Purpose] Ultrasonography (US) is widely applied to measure the muscle size in the limbs, as it has relatively high portability and is associated with low costs compared with large clinical devices such as magnetic resonance imaging (MRI). However, the applicability of US for evaluating trunk muscle size is poorly understood. This study aimed to examine whether US-measured muscle thickness (MT) in the trunk abdominal and back muscles correlated with MT and muscle cross-sectional area (MCSA) measured by MRI. [Subjects and Methods] Twenty-four healthy young males participated in this study. The MT and MCSA in the subjects were measured by US and MRI in a total of 10 sites, including the bilateral sides of the rectus abdominis (upper, central, and lower parts), abdominal wall, and multifidus lumborum. [Results] The interclass correlation coefficients of US-measured MT on the total 10 sites showed excellent values (n=12, 0.919 to 0.970). The US-measured MT significantly correlated with the MRI-measured MT (r=0.753 to 0.963) and MCSA (r=0.634 to 0.821). [Conclusion] US-measured MT could represent a surrogate for muscle size measured by MRI. The application of US for evaluating trunk muscle size may be a useful tool in the clinical setting.

This study aimed to determine the relationship between Achilles tendon (AT) length and running performance, including running economy, in well-trained endurance runners. We also examined the reasonable portion of the AT related to running performance among AT lengths measured in three different portions. The AT lengths at three portions and cross-sectional area (CSA) of 30 endurance runners were measured using magnetic resonance imaging. Each AT length was calculated as the distance from the calcaneal tuberosity to the muscle-tendon junction of the soleus, gastrocnemius medialis (GM ), and gastrocnemius lateralis, respectively. These AT lengths were normalized with shank length. The AT CSA was calculated as the average of 10, 20, and 30 mm above the distal insertion of the AT and normalized with body mass. Running economy was evaluated by measuring energy cost during three 4-minutes submaximal treadmill running trials at 14, 16, and 18 km/h, respectively. Among three AT lengths, only a GM correlated significantly with personal best 5000-m race time (r=-.376, P=.046). Furthermore, GM correlated significantly with energy cost during submaximal treadmill running trials at 14 km/h and 18 km/h (r=-.446 and -.429, respectively, P<.05 for both), and a trend toward such significance was observed at 16 km/h (r=-.360, P=.050). In contrast, there was no correlation between AT CSA and running performance. These findings suggest that longer AT, especially GM , may be advantageous to achieve superior running performance, with better running economy, in endurance runners.

Compared to pure isometric contractions, isometric muscle force at a given length is larger when the eccentric contraction is conducted before the isometric contraction. This phenomenon is widely known as residual force enhancement, and has been confirmed consistently in isolated muscle experiments. The purpose of this study was to confirm whether residual force enhancement also occurs in human plantar flexors and to examine its joint angle dependence. Eleven men participated in this study. Isometric joint torque was measured in a Control trial (pure isometric contraction) and Residual force enhancement (RFE) trial (isometric contraction after eccentric contraction) at plantar flexion 0° (Short condition) and dorsiflexion 15° (Long condition). Fascicle length and pennation angle of the medial gastrocnemius were measured simultaneously to evaluate the influence of architectural parameters on isometric joint torque. Isometric joint torque observed in the Short condition was not significantly different between the Control and RFE trials (Control: 42.9 ± 8.0 Nm, RFE: 45.1 ± 8.4 Nm) (p = 0.200). In contrast, significant differences in isometric joint torque were observed in the Long condition between Control and RFE trials (Control: 40.5 ± 9.3 Nm, RFE: 47.1 ± 10.5 Nm) (p = 0.001). Fascicle length and pennation angle were not different between Control and RFE trials in the Short and Long conditions. Isometric joint torque was larger when eccentric contraction was conducted before isometric contraction while architectural differences were not observed, indicating that residual force enhancement occurs in human plantar flexors. However, the influence of residual force enhancement may be limited in dorsiflexed positions because the magnitude of residual force enhancement is considered to be prominent in the descending limb (long muscle length condition) and small in the ascending limb (short muscle length condition) where human plantar flexors operate in plantar flexed positions.