Propulsive impulse as a covarying performance measure in the comparison of the kinematics of swimming and jumping in frogs.

Animals have to modulate their locomotor behavior according to changes in external circumstances. The locomotor requirements are expected to be most extreme for species that move through different physical environments, such as water versus land. In this study, we examine the use of the propulsive impulse as a covariate in the comparison of the kinematics of locomotion of a semi-aquatic frog Rana esculenta, across land and through water. We focused on the propulsive phase because it is functionally the most significant phase of the locomotor cycle in both jumping and swimming, and it is also the most comparable. The frog alters the joint angles of its legs in order to adjust its performance (i.e. impulse) within both locomotor modes. The kinematics and this modulation of the propulsive phase differ between the two modes; however, we found that the impulse ranges of swimming and jumping do not fully overlap. Possible explanations for this include larger lateral forces during swimming, a reduced force transmission due to a lower external load during swimming and reduced muscle recruitment due to differences in coordination patterns.