Contractile properties of human motor units: Is man a cat?

A major goal in neuroscience is to understand how the CNS controls posture and movement in humans. This requires an understanding of individual human motor unit properties and how they interact within the muscle to perform different tasks. This article describes differences and similarities between the contractile properties of human motor units and those of the cat prototype medial gastrocnemius (MG) muscle, on which so many studies have been conducted. The article describes the methods available for measuring human motor unit properties and their limitations, and it discusses how far the behavior of whole muscles can be predicted from their histochemistry. It questions the extent to which human motor units conform to the conventional criteria by which S (slow, fatigue resistant), FR (fast but fatigue resistant) and FF (fast, fatigable) unit types are usually classified. An important difference between human and cat MG data is that weak human motor units are not necessarily slow, nor strong ones fast; that is, generally, human unit force is not correlated with contractile speed. Also, unlike cat MG, the few human muscles studied so far contain few if any FF units but a high proportion of units with intermediate fatigue resistance (Flnt). These apparently aberrant human properties, however, are also found in other cat and rat muscles. Thus, cat MG may not be the best model for motor unit behavior generally. Finally, the influence of human motor unit properties on force output by recruitment and/or rate coding is discussed.