Fig 4. Schematic illustration depicting the proposed length changes of the contractile element (CE, representing muscle fascicles) and elastic element (EE, representing tendon) of plantaris longus (PL) muscle during the propulsive phase of a frog jump made based on the observations in the current study.

The length of the PL muscle-tendon unit, L_o, remains constant in the early part of the propulsive phase, and only starts to decrease after 40ms when it continues to shorten until the end of the propulsive phase. However, the contractile element length, C_o, shortens throughout the propulsive phase. Likewise, the angle of pennation, α, increases throughout the entire propulsion phase [31]. Due to the series arrangement and the passive property, the series elastic element is stretched continuously in the propulsive phase until the peak muscle force is reached at 76ms. This is followed by a shortening of the EE. At the end of the propulsive phase (109ms), the EE length is shorter than E₂ and E₁, but remains longer than the initial EE length, E_o. Note that the timings shown in the figure are mean values presented in Table 1.