Efficiency of skeletal and cardiac muscle

In the human physiology literature values for skeletal and cardiac mechanical efficiency are being reported that seem to be much higher than are found in isolated tissue studies. We consider some of the different efficiency definitions and explore some possible reasons for low isolated tissue values, these include the experimental protocols used, the time at which measurements are made, afterloaded versus sinusoidal contractions, the effects of varying activation levels and recovery heat uncertainty. We examine some of the mechanical and energetic differences between cardiac and skeletal muscle--the absence of shortening heat, the linear relationship between energy per beat and pressure-volume area, constant contractile efficiency, and a larger crossbridge (CB) working stroke. Some observations are made on muscle energetics and loose and tight coupling CB models.