Superior sprint performance is achieved using gross torques of the lower limb joints potentially by increasing peak vertical ground reaction force. The Achilles tendon (AT) plays an important role in storing and returning elastic energy during the stance phase of human locomotion, including sprinting.4 Thus, the favorable AT morphology may increase in the ankle plantar flexor torque and peak vertical ground reaction force and may help achieve superior sprint performance. Using magnetic resonance imagining (MRI), we and others previously reported a positive correlation between longer AT length and better running performance in endurance runners. Furthermore, one study by Monte and Zamparo determined that longer AT correlated with better sprint performances, assessed using personal best 100-m sprint time and sprint variables during a 20-m sprint in sprinters. The longer AT may be an important morphological factor in achieving superior performance in sprinters, similar to endurance runners, which may be due to an increase in the plantar flexor torque and peak vertical ground reaction force. Although the study of Monte and Zamparo measured the AT length using ultrasonography (US), MRI is more appropriate for measuring morphological variables, including the AT length. Additionally, although they measured only the AT length of the gastrocnemius medialis (GMAT), the AT length can be measured from 3 different portions of the triceps surae, including the soleus (SOLAT) and gastrocnemius lateralis (GLAT). Therefore, the impact of the AT length on sprint performance has not been fully understood. The plantar flexor torque is a major source of torques required for body support and propulsion during the stance phase while sprinting. However, previous studies determined that although the plantar flexor torque was increased by changing velocity from walking to running, it remained unchanged when altering velocity from running to sprinting. Thus, the plantar flexor torque may not play an important role in increasing peak vertical ground reaction force and achieving superior sprint performance during 100-m sprinting; in particular, a longer AT is not required for better sprint performance. Based on this background, we hypothesized that the AT length might not be related to sprint performance in sprinters. To test this hypothesis, using MRI, we examined the relationships between 3 AT lengths of the triceps surae and 100-m sprint time in sprinters.
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