An attempt is made to relate the structure and properties of the principal extensor muscles and bones of the frog leg, to their performance in jumping and swimming. The following data are presented in turn. First, the muscles are described and their dimensions, and moment arms about the joints, are given. Secondly, the maximum isometric tension of each muscle, measured in tests with excised muscles, is reported. Stresses are calculated from these forces and the dimensions previously given. Thirdly, measurements are presented of the changing angle of each joint as the leg extends in jumping, and of the magnitude and direction of the forces exerted on the ground (measured by means of a force platform). The angles are used, in conjunction with the moment arms, to calculate the range and rate of shortening of each muscle, and hence the strain and rates of strain of their component fibres. The forces are used, with the moment arms, to calculate the forces and stresses exerted by the muscles. Fourthly, measurements taken from film of frogs swimming are used to estimate the force exerted on the water during the propulsive stroke. Muscle performance in swimming and jumping is compared. Finally, measurements of the stiffness and strength in bending of the femur and tibiofibula are presented, and related to the forces which act in jumping.

The Discussion section of the paper includes an attempt to define the optimal working conditions for a muscle subjected to inertial loading.