A motor neurone and all the muscle fibres it controls. In skeletal muscle, each muscle fibre is innervated by just a single motor neurone, but the axon of a single neurone typically branches to form synapses with multiple muscle fibres. In muscles used for very fine movement, as in the fingers, a single neurone may connect with only one or two muscle fibres, whereas in powerful muscles, such as the quadriceps of the upper leg, a single neurone may form synapses with hundreds of muscle fibres, distributed throughout the body of the muscle. Recruitment of more and more motor units is a key mechanism to increase the force of muscle contraction. Motor units can be categorized according to the number of muscle fibres, the speed at which those fibres contract, and their resistance to fatigue into slow units, fast-fatigable units, and fast fatigue-resistant units; these are deployed according to whether a muscle is used for sustained contraction, e.g. in maintaining posture, or for rapid, forceful exertion, e.g. running.