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A motor neurone and all the muscle f ibres that it fifa 15 coins supplies (innervates) is termed a motor unit. The constituent fibres of individual motor units are found to be distributed throughout the muscle and are homogenous, possessing identical structural, metabolic and contractile properties. The number of muscle fibres innervated by a single motor neurone varies from muscle to muscle. Those muscles that control precise movements such as the eye have few muscle fibres per motor unit. Conversely muscles such as the quadriceps, which are respon- sible for powerful gross movements of body segments, have approximately 2000 muscle f ibres per motor unit. When a motor neurone is stimulated all the muscle f ibres which it innervates contract simultaneously. Motor units are progressively recruited and ‘decruited’ in order of size so that force may be modulated in a step-wise manner. Small fatigue-resistant motor units (type I) are recruited, most frequently providing control at the low forces required to perform everyday activities (walking, standing from sitting) and maintain pos- ture. Larger motor units (type II) are recruited when strong and fast contractions are required (running, jumping and kicking a ball).
Muscles span joints and it is the contraction, or shortening, of the muscle belly that produces movement at a joint. Common terms for types of movement produced about a joint are flexion (usually a reduction of the joint angle) and extension (usually increasing the joint angle), abduction (movement away from the body), adduction (movement towards the body), medial rotation (towards the body) and lateral rotation (away from the body). Common movements at the ankle joint are dorsiflexion (movement of the foot towards the shin), plantarflexion (pointing of the foot downwards), inversion (turning the foot inwards) and eversion (turning the foot outwards). The type of movement produced when muscles contract depends on the attachments of the muscle and the architecture of the joint itself.
Muscles initiating or maintaining a movement are termed prime movers or agonists, for example, the quadriceps extending the knee joint during kicking. Muscles that oppose this movement, or initiate and maintain its converse, are termed antagonists, for example, the hamstrings opposing knee extension during kicking. When prime movers and antagonists contract together as fixators they stabilize a joint, for example, the quadriceps and hamstrings to stabilize the knee joint during a block tackle.
The contraction of a prime mover that acts across a single joint may produce the desired movement. Many muscles cross more than one joint, for example, rectus femoris is an extensor of the knee and a flexor of the hip, and many joints are multiaxial. The unrestrained contraction of such muscles may produce additional and unnecessary movements. The restriction of unwanted movements occurs by the contraction of a partial antagonist muscle acting as a synergist. An example of this is powerful flexion of the f ingers by the long flexors, for example, during a throw-in. Unrestricted contraction of these muscles would also produce flexion of the wrist joint and would thus reduce function. Function is improved by contraction of the long extensors acting as synergists to prevent wrist flexion.