ABSTRACT
Human movement is produced by the rotation of segments around the instantaneous axis of rotation of the joints. Mechanically, rotation is only possible with the application of a moment. Muscles generate muscle forces that are transmitted to the bones via tendons. As an example, we will consider a knee extension motion in the sagittal plane only, around an instantaneous axis of rotation that is perpendicular to the plane of the segment motion. Assuming that there is only one active muscle-tendon unit, the moment producing the rotation can be determined simply as the product of the muscle force and its shortest distance from the axis of rotation or moment arm (Figure 6.1).
