How do we accelerate while running?

Abstract

Running biomechanics are well established in terms of lower extremity joint kinetics as is the direct relationship between these variables and running speed. Many studies have investigated the differences in these variables when running velocity was increased in discrete increments but investigations of accelerated running in which velocity is continually increasing are almost non-existent. One investigation of the acceleration phase of running showed that joint torques did not increase while accelerating. These results cannot be aligned with the fully established results of running biomechanics at different speeds. We expected the joint torques to increase in magnitude for each step during the acceleration phase based on the previous research investigating increases in running velocity. The purpose of this study was to quantify lower extremity joint torques and powers during constant speed running and during running while accelerating at two rates of acceleration between a baseline velocity of 2.50 ms⁻¹ to a maximal velocity of 6.00 ms⁻¹. It was hypothesized that lower extremity sagittal plane joint torques and joint powers would positively and linearly increase throughout the acceleration phase of running. 15 young, healthy runners (n = 8 females) between the ages of 18 and 22 were analyzed on an instrumented treadmill while accelerating at 0.40 ms⁻² (A1) and 0.80 ms⁻² (A2) from the initial to final velocities. Inverse dynamics were used to determine lower limb joint torques and powers using ground reaction forces and kinematic data collected by 3D motion capture. Correlation and regression analyses were used to identify the relationships between mean, maximum hip, knee, and ankle torques and power to step number during the constant velocity and acceleration phase. The results of this study showed a significant increase in the joint torques and joint powers per step in both conditions A1 and A2 at the hip, knee, and ankle joints during the acceleration phase when the regression beta weights and correlation coefficients were tested for significance (p < 0.05). It was also observed that the knee and ankle joint torques and the hip, knee, and ankle joint powers had significantly greater increases per step in condition A2. There was no significant difference in the beta weights in hip joint torque between conditions A1 and A2. The constant state, pre- and post-acceleration phases had no relationship between joint torque and step number and joint power and step number in almost every variable, with three exceptions. There was a significant, direct increase in magnitude in hip joint power during the pre-acceleration period of condition A1, as well as hip joint torque during the post-acceleration period of condition A2. Additionally, a significant inverse relationship was seen in ankle joint power in condition A2 in the post-acceleration period. Finally, it was observed that the hip and ankle are the primary contributors to accelerating while running based on the magnitude of the beta weights of these variables, with the knee also contributing but not as much as the hip and ankle. In conclusion, in contrast to a previous study, our data suggest that hip, knee, and ankle torques doincrease during accelerated running on a step by step basis as do hip, knee, and ankle joint powers. Therefore, the tested hypothesis was supported based on the results of this study.