Environmentally induced mechanical feedback in locomotion: Frog performance as a model

At first glance, the strategy for generating propulsive impulses for both jumping and swimming in frogs is quite similar. Both modes rely on powerful extension of the hind limbs. However, in Rana esculenta (the semi-aquatic green frog), propulsive impulses for jumping were found to be much larger than those generated during swimming [Nauwelaerts and Aerts, 2003. Propulsive impulses as a covarying performance measure in the comparison of the kinematics of swimming and jumping in frogs. J. Exp. Biol. 206, 4341–4351]. The hypothesis that differences in propulsive impulse between swimming and jumping are largely caused by specific environmental constraints rather than being due to changes in motor control is tested in the present study. To assess this question, the actuator of a simple mathematical model, mimicking a frog with symmetrically kicking hind limbs, is first tuned to perform frog-like jumps. Next, the same actuator activation is applied to drive the model in an ‘aquatic environment’. Despite the entirely identical activation, the resulting in silico propulsive swimming impulse was less than half that produced during jumping, just as observed in vivo. Although duration of limb extension is similar for both locomotor modes (both in vivo and in silico), this conspicuous difference in model behaviour is entirely explained by the actuator working at different positions along its force–velocity curve. These findings suggest that the same environmentally induced effects are also involved in real swimming and jumping as well, thus explaining the apparent difference in performance level.