Oxygen uptake for negative work, stretching contractions by in situ dog skeletal muscle

Oxygen uptake for negative work, stretching twitch contractions by in situ gastrocnemius-plantaris muscle was calculated from measurements of venous outflow and arterial and venous blood oxygen contents. Contractions were produced by valving air at high pressure into the pneumatic lever 10-50 ms before stimulation of the muscle. The loads produced were up to about 2.5 times isometric. Muscle length was always below optimal isometric length. Oxygen uptake for shortening contractions increased with increasing load up to isometric load. Oxygen uptake for stretching contractions decreased with increasing loads above isometric load. Velocity of shortening decreased with increasing loads up to isometric load whereas velocity of stretching increased with increasing loads above isometric. In shortening contractions external work done by the muscle was greatest at intermediate loads, but in stretching contractions the work done on the muscle increased with increasing loads. In stretching contractions the ratio of the energy equivalent of the work absorbed by the muscle reached 8.0 times the energy equivalent of the oxygen uptake. Since this ratio cannot exceed 1.0 for an engine, muscles must act as brakes during stretching contractions.