Energy storage during stretch of active single fibres from frog skeletal muscle

Muscles act as the brakes of the body as well as the motors.During locomotion the optimum performance of muscles as brakes is just as important as their performance as motors, yet braking has been far less studied.Whereas a motor converts chemical energy into mechanical work and heat, brakes convert mechanical work into some other form of energy.In the simplest case, the work done on the brakes immediately becomes heat.However, studies of muscles acting as brakes suggest that they are not operating in this simplest way (Hill & Howarth, 1959;Constable et al. 1997).In these experiments, both the heat produced in the muscle and the work done on the muscle by the apparatus stretching it were measured.If the muscles had simply converted the work directly into heat (and did nothing else), the sum would have been zero during the time the stretch was occurring.However, this is not what happened.Instead the total amount of energy in the form of heat and work declined during the stretch, and it can be concluded that energy is absorbed and stored by the muscle during stretch.Our first aim was to gain experimental evidence about the degree of uniformity of heat production during stretch.Extreme non-uniformity could lead to serious artefacts and make the conclusion about energy storage unsafe.Several authors have been concerned that such an artefact could have arisen in the following way (Hill & Howarth, 1959;Woledge et al. 1985; Constable et al.1997).In these experiments heat was measured from a part of the muscle only, and it was assumed that this part was representative of the entire muscle.The concern was that a part of the muscle gave way under the extra force during stretch.In the most extreme case the entire stretch might have occurred in one place while the rest of the muscle was not stretched at all.If the muscle were acting as a simple brake, converting all of the work done on it during stretch into heat, then all of this heat would be produced at the site of the stretch and none elsewhere.If heat were measured from the part that was not stretched (and thus not converting work into heat) the results would falsely indicate that the muscle absorbed energy during stretch.