The series elastic component of frog gastrocnemius.

1. The force-extension curve of the series elastic elements of the frog gastrocnemius has been determined up to the force attained during stretching by means of the isotonic quick-release method, modified in order to allow the release of the muscle immediately after stretching.2. The series compliance decreases progressively up to a stress value of about 2 kg/cm(2) and then it remains constant up to the maximal values of stress attained (about 10 kg/cm(2)).3. The lengthening of the series elastic elements when the stress rises to its full isometric value (P(0)l(0)/M = 5.2 kg/cm(2)) is about 3% of the muscle resting length, l(0). The Young's modulus in the linear part of the diagram is 294 kg/cm(2).4. The elastic energy stored up to P(0) is on average 55 g cm/g; an additional amount of 63 g cm/g is stored during stretching the contracted muscle.