Muscle physiology and contraction

Skeletal muscle has important metabolic functions but the focus of this article is to examine its ability to generate mechanical force. Adult skeletal muscle fibres are each innervated by a single branch of the axon arising from an α-motoneuron in the spinal cord. The α-motoneuron and all the fibres it innervates constitute a motor unit, and this is the functional unit of the muscle. α-Motoneurons differ in size and excitability and it is the recruitment of these cell bodies in the spinal cord that determines which fibres within the muscle are active during a movement. Correct functioning of the neuromuscular junction is clearly critical for muscle action and it is a site at which many drugs affecting muscle have their action. Here we describe also the mechanism by which skeletal muscle generates force following activation, a process known as excitation–contraction coupling and examine the contractile properties of muscle as well as describing muscle weakness and fatigue and the assessment of muscle performance in health and disease.