Joint mechanics as a determinant of motor unit organization in man.

To support a given weight a muscle which inserts close to the axis of a single joint must develop more force than a muscle which inserts further from the axis. Using available anatomical and histochemical data we have shown an association between the mechanical leverage of a muscle at a joint and the proportion of type II (fast twitch, large tension) muscle fibres. We suggest that the mechanical leverage at joints determines the proportion of fast and slow motor units within a muscle in a teleologically appropriate manner. This determination probably occurs before the differentiation of motorneurones into fast and slow types.