Effect of temperature and velocity of stretching on stress relaxation of contracting frog muscle fibres.

1. Active muscle resists stretch with a tension greater than it can develop at constant length, but at the end of lengthening the extra tension disappears, at first rapidly and then more slowly. 2. This unexplained trend of muscle stress relaxation was studied at two different temperatures (4 and 14 degrees C) and after ramp stretches of different velocity (0.2-2.2 fibre lengths s-1) on frog muscle fibres near slack length. 3. The velocity of the fast fall in tension increases with temperature much more than that of the slow fall. In addition, the amplitude of the fast fall in tension increases with the velocity of stretching whereas that of the slow fall decreases. 4. It is hypothesized that some of the energy absorbed by the muscle during stretching is used to raise the chemical energy level of the cross-bridges, and this energy transfer occurs during and after stretching.