Fig 5. Sequential free body diagrams of frog jumping drawn according to Fig 2 to illustrate the interplay between the plantaris longus (PL) extensor moment, and the ground reaction force (GRF) flexor moment at the ankle joint during jumping.

R_i and r_i (i = 0, 1, 2) are the ground reaction force moment arms and PL muscle moment arms relative to the ankle joint, respectively, and are estimated based on previous studies [7, 9, 10]. M_j and m_j (j = 0, 1, 2) are the ground reaction force flexor moments and PL muscle extensor moments about the ankle joint, respectively. The effective mechanical advantage (EMA) is defined as the ratio of r_i to R_i. In the first 40ms of the propulsive phase, the GRF flexor moment is bigger than the PL extensor moment. Therefore, virtually no movement is produced due to the joint constraints although the frog’s joints are slightly flexed almost like a small counter movement. The ankle joint angle and PL muscle-tendon unit length remain almost constant in the first 40ms, although there is a slight stretch of the PL MTU from about 10-35ms in Fig 3B, which attests to the slight compression of the frog. After 40ms, the PL extensor moment exceeds the GRF flexor moment, thus causing ankle extension and shortening of the PL muscle-tendon unit. This process is accompanied by an increase in EMA [7,9], with constant r_i [10] but a decrease in R_i. CW–clockwise; ACW–anticlockwise.