Fig 1.
(A) Digital photograph and (B) schematic illustration of a fully instrumented frog plantaris longus (PL) muscle prior to closing the incision, which shows an E-shaped tendon force transducer on the PL tendon, a pair of sonomicrometry crystals at the proximal and distal ends of a central fascicle, and two EMG electrodes embedded in the mid-belly of PL. (C) Schematic illustration of the sonomicrometry placement in PL relative to fascicle orientation.
Table 1.
Muscle properties of plantaris longus (PL) and jumping performance of frogs.
Only near maximal jumps (jumps of > 50cm) were included in the analysis. The muscle properties were averaged across all animals tested (Nmuscle = 2), while the jumping performances were averaged across all analyzed jumps (Njump = 15). Data were expressed in mean ± standard error of the mean.
Fig 2.
Sequential event of a 65cm jump by a 26g frog.
The images represented lateral views and were displayed chronologically from left to right: (i) beginning of the propulsive phase (the time when resultant GRF exceeds 1 body weight), (ii) the moment when the PL MTU started to shorten, (iii) the moment when maximal force was recorded by the tendon force transducer, (iv) end of the propulsive phase (the time when the frog completely left the force plate and started the aerial phase). The lower limb joints (hip, knee, ankle, tarsometatarsal, and metatarsophalangeal) were digitized and connected with superimposed red lines to show the changes in hind limb geometry throughout the take-off phase.
Fig 3.
Representative curves of (A) muscle force, (B) length changes, (C) shortening velocity, and (D) shortening power of a plantaris longus (PL) muscle (black solid curves) and/or fascicle (red dashed curves) during a 65cm jump as shown in Fig 2. The duration of muscle activation is indicated by the yellow-highlighted region on the time axes. Time zero is defined as the beginning of the propulsive phase (see ‘Methods’ section for details). The inset in (B) shows the sarcomere force-length curve of a frog [41], with the sarcomere range observed in the current study highlighted by a bold red line (from 1.91±0.08 μm to 2.07±0.09 μm; Nfiber = 6). Peak shortening power occurred when peak muscle force and peak shortening velocity were achieved.
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, Lo, 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, Co, 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, Eo. Note that the timings shown in the figure are mean values presented in Table 1.
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.
Ri and ri (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]. Mj and mj (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 ri to Ri. 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 ri [10] but a decrease in Ri. CW–clockwise; ACW–anticlockwise.