Kinetics of Muscular Contraction-I **

The kinetics of muscular contraction is basic for the quantitative description of muscle and understanding its molecular mechanism. However, the subject has been embarrassed by several historical accidents. First, due to the lack until recently of adequate high (mechanical) impedance transducers, attention was focussed on isotonic measurement; this led to the idea of muscle as a shortening generator in series with an elastic impedance-an idea which is quite misleading. Second, the facts surrounding the concept of activation have been confused and oversimplified, partly because of the above, but mainly because the experiments have been done almost exclusively on the frog sartorius which turns out to be a special case, unsuited to elucidating the general picture. Activation as a precursor to the development of tension originates with Gasser and Hill.' By applying a quick stretch at different times relative to the stimulus they showed that (a) Practically coincident with tension development there is a rise in elastic modulus (b) After a transient overshoot, the sustained tension, subsequent to such a stretch, occurs earlier than in the unstretched muscle. The implication from (b) is that early in the onset of contraction a muscle is potentially able to develop some sizeable fraction of its full tension. Thus it appears there are two possible measures of activation: (1) Potential ability to develop tension (2) Elastic modulus. Hillu has given evidence based on (1) and (2) that activation is established rapidly in comparison with the rate of development of tension. Apparently impressive in relation to (1) is the following': the rate of recovery of tension on quick release to zero from tetanus tension is only 10 per cent higher than that on initial stimulation. This is consistent with his view, which this essentially negative experiment seems to confirm, namely, that activation is a function of time only and is no way influenced