How molecular motors work in muscle

Howard replies — Huxley points out that there is a discrepancy between recent mechanical recordings from single myosin molecules in vitro and earlier mechanical measurements from muscle fibres in vivo: the single-molecule recordings indicate a working distance and maximum duty ratio of 4-6 nm and 10-20%, respectively, only about half the values deduced from the fibrestudies. Although it is possible that technical limitations, for example misorientated heads, make the single-molecule recordings suspect, I argued in my ReviewArticle1 that the discrepancy might be only an apparent one. In particular, the smaller working distance does conform to many of the results from muscle, and the lower duty ratio can even provide a more satisfactory explanation for some of the experiments with muscle fibres. Huxley challenges this view by discussing three additional observations from fibres. But I believe that these too are mainly consistent with the smaller working stroke and duty ratio.