This study investigated the relationships between the kinetic factor of the swing leg and step frequency (SF) and step length (SL) during the top speed phase of a sprint. Sixteen male sprinters (age 19.3±0.6 years, height 1.74±0.06 m, weight 66.1±5.2 kg) performed maximum effort 60-m sprints. Video data from the 43.5- to 50-m section of the sprint were collected using a high-speed camera (300 Hz). SF index and SL index were calculated to exclude the influence of body height on the outcomes of interest. Torque and torque power of the hip and knee joints of the right leg were calculated during the swing phase of the right leg. The time of the swing phase of the right leg was normalized so that the take-off of the right foot, touchdown of the left foot, take-off of the left foot, and touchdown of the right foot were 0%, 100%, 200%, and 300%, respectively. For every 5% of normalized time, partial correlation analysis was conducted between the right leg kinetics and SF index (controlling SL index) and SL index (controlling SF index).
The SF index was associated with a large hip flexion torque and a large hip extension torque during 10–60% and 250–280% of the swing phase, respectively. Moreover, large peaks of the hip flexion torque and positive power were associated with a high SF index (r = -.718, p <0.01; r = .531, p <0.05, respectively).
The SL index was associated with a hip flexion torque during 20–30% of the swing phase, although there was no significant partial correlation between the SL index and peak hip flexion torque (r = -.381, p = .161). In addition, a high SL index was associated with early appearance of the peak hip flexion torque power (r = -.759, p <0.01). In conclusion, throughout the top speed phase of a sprint, a high SF index requires a large torque and hip joint power for the leg swing over a short duration, and a high SL index requires an early increase of hip flexion torque power.