Reply to R. H. Abbott

IN our article in Nature last year¹ we showed that, after a step change of length imposed during tetanic contraction, the early tension recovery varies in speed according to the size and direction of the step, being slowest in large stretches and fastest in large releases. Abbott² now proposes that this variation is due not to a change of time scale of a single process but to a change in the relative amplitudes of two exponentially decaying components each of which has a fixed time constant. He shows that the records in Fig. 2 of ref. 1 can be approximately fitted in this way and concludes that “either the results are not typical or the mathematical theory put forward by Huxley and Simmons is not correct”.