The efficiency of energy conversion by fish myotomal muscle undergoing sinusoidal length changes with brief periods of stimulation in each cycle has been found to be greater than the efficiency of the same type of muscle when tetanically stimulated. This finding is reviewed in relation to the well-known energetic properties of muscle undergoing shortening. It is suggested that an adaptation of the ideas of Lombardi & Piazzesi which were proposed to explain the behaviour of muscle during lengthening could be used to provide one possible explanation of this finding. Their theory proposes that crossbridges, when they have sufficient energy because they are strained, can detach to a crossbridge state which can rapidly reattach without having to split ATP first. This type of theory might also provide an explanation of the findings on ventricular energetics which are expressed in the time varying elastance model of Suga.