In an exercise consisting of repetitive small jumps on both feet at a frequency of 116/min, the mechanical work performed and the O2 consumption at steady state were measured. Of the positive work performed in the jump only 40% appears to be due to the chemical transformations taking place in the contractile componenet of the muscle fibres; the remaining 60% appears to be due to the elastic energy accumulated in the elastic elements of the contracted stretched muscles of the lower limbs during the falling phase of the previous jump, when the body hits the ground.