A study was performed to compare joint torque development with EMG activities, and to investigate the relationship between peak torque and power during different phases of leg movement at different sprint velocities from the starting dash to full stride. Cinematographic recordings of sprint movement together with ground reaction forces and EMGs from seven lower limb muscles were obtained from four male sprinters at the 1st, 3rd, 5th, 9th, 13th and 19th (maximal velocity) steps from the sprint start. Joint torque and power in the hip, knee and ankle were computed from analysis of the film and ground reaction forces. The results can be summarized as follows: 1) The developmental patterns of joint torque and power, and the EMG activities were similar from the first step to the maximal velocity. 2) The phases of torque development were parallel to the related EMG activities. 3) Stretch-shortening cycles of muscle were observed in the m. Soleus during the foot contact period and in the m. Biceps femoris during the latter half of the swing period. 4) There was no peak torque or power that showed a decrease with increased sprint velocity. 5) Peak torque and power that increased with sprint velocity were (a) torque and positive power of the hip flexors during the first half of the swing period, (b) torque of the hip extensors during the latter half of the swing period, (c) torque and negative power of the knee extensors during the first half and latter half of the swing period, (d) torque of the ankle extensors during the middle of the foot contact period, (e) negative power of the ankle extensors during the first half of the foot contact period, and (f) positive power during the latter half of the foot contact period. These results suggest that muscles in the hip and knee joints cooperate with the change in swing movement, and that muscles in the ankle joint cooperate with the change in drive movement as sprint velocity increases. 6) The peak torque and power that remained high independent of sprint velocity were (a) torque and positive power of the hip extensors from the middle of the swing period to the middle of the foot contact period, (b) torque of the ankle extensors during the foot contact period, and (c) power of the ankle extensors during the latter half of the foot contact period. These results indicate the importance of the hip extensors and ankle extensors in sprinting.
