Eccentric and posteccentric contractile behaviour of skeletal muscle: a comparative study in frog single fibres and in humans.

Eccentric and posteccentric force behaviour in human skeletal muscle and in isolated frog muscle fibres was studied by imposing stretch-and-hold loading conditions during contractions with maximal voluntary effort or under tetanic stimulation in the isolated preparations. The investigations on human muscle were made on the forearm flexors of a group of kayak racers (n = 16; age: 17-22 years) and of schoolgirls (n = 15; age: 17-18 years) with both groups participating in a strength-training programme over 4 (kayak racers) or 3 (girls) months. Half of the training regime consisted of eccentric elements. In the isolated muscle fibres, it could be shown that in the posteccentric hold phase the enhanced force decayed exponentially to the original isometric value with a mean time-constant of 0.35 s (10 degrees C) and of 0.23 (20 degrees C). In the forearm flexor of human subjects similar results were obtained not only qualitatively but even quantitatively (time constant of posteccentric force decay: 0.25-0.37 s). Strength training in both groups did not lead to an enhancement in maximal isometric force alone [mean increase in force 17 (SD 10)%], a well-known and generally accepted fact, but also to a parallel shift in eccentric [21 (SD 10)%] and posteccentric force level. The close similarity between the findings in isolated muscle fibres and in human muscle in situ suggests that the eccentric and posteccentric behaviour must be primarily ascribed to the contractile properties of the muscle fibres themselves. A three-element muscle model with variable visco-elastic properties would appear to be most suitable for simulating the experimental findings.