Tension responses to rapid length changes in skinned muscle fibres of the frog.

Tension responses due to rapid length changes completed in 50 and 150 microseconds were obtained from activated skinned single fibres of the ileofibularis muscle of the frog. The natural frequency of the force transducer was about 50 kHz. The length changes ranged between -1% and +0.5% of the fibre segment length. The sarcomere length was adjusted to 2.15 micron. The temperature was maintained at 2.5 degrees C. The transmission velocity estimated from these recordings obtained on fibre segments with different length was 230 m/s in fully activated segments and 112 m/s in relaxed segments. The initial part of the responses during the length changes consisted of an abrupt change in tension reaching an extreme value T1, which depended on the amplitude as well as the duration of the length change. A partial rapid recovery towards a plateau occurred after the length change. The reciprocal half-time of this recovery increased with the amplitude of the displacement both for stretches as well as releases up to about 5 nm/half sarcomere.