How molecular motors work in muscle

Much of Howard's Review Article1 concerns the results of experiments with single myosin molecules and actin filaments. It was a huge surprise when reports of such experiments first appeared (see, for example, ref. 2), and much is being learnt from them that cannot be deduced from experiments on whole muscle fibres, whether intact or after removal of the membrane. But single-molecule experiments do not yet approach the time resolution or the freedom from brownian noise that are easily attainable on larger assemblies of myosin and actin filaments, and their interpretation is subject to many uncertainties — due, for instance, to compliance in the actin filaments and in their attachments to beads or other force-measuring components, and the attachment of myosin molecules or fragments to the base. No doubt the time course of the working stroke of a single myosin head will one day be recorded, but until that is achieved the results of experiments on whole fibres and myofibrils deserve more careful attention than has been given to them by Howard.