COORDINATED MOVEMENTS OF THE HAND
The apparently simple human functions of closing the hand to grasp an object, or opening the palm to release it, are in reality tasks of considerable mechanical complexity, requiring the simultaneous contraction of many individual muscles. The isolated action of a single muscle may be inferred from the positions of its origin and insertion, and the estimated line of action (usually the centre line of the muscle) in relation to the axes of all the joints traversed by the muscle and its tendon. The limb can be regarded as a chain of joints crossed by muscles. If it is known which muscles are active, then the reason why one joint moves and others do not is a matter of simple mechanical relationships.
For example, flexor pollicis longus is considered to have a major role as a flexor of the interphalangeal joint of the thumb. However, the position of its tendon relative to more proximal joints in the limb gives it the potential for producing flexion at the metacarpophalangeal joint and also at the trapeziometacarpal and wrist joints. In the living subject the actual motion that takes place depends on which other muscle groups are acting, and so the potential for movement must be considered for each joint in the chain in turn. Motion at the wrist is generally balanced by wrist extensors. Motion at the trapeziometacarpal joint is balanced by abductor pollicis longus. Flexor pollicis longus will then have an action as a flexor of the metacarpophalangeal and interphalangeal joints only.
The factor that determines whether one or both of two joints will move is the turning moment at each. The greater the perpendicular distance from the line of muscle or tendon pull to the axis of the joint, the stronger is the turning effect of the muscle at the joint, but the smaller the range of joint motion that can be produced. In the case of flexor pollicis longus, the tendon is situated further from the axis of the metacarpophalangeal joint than from the axis of the interphalangeal joint: it will therefore tend to produce flexion preferentially at the metacarpophalangeal joint unless that joint is restrained by extensor pollicis brevis. In this way different postures of the thumb can be produced by the interplay of flexor and extensor forces. These simple guiding principles should provide an understanding of muscle action in the hand that is sufficient for most purposes.
In considering the role of a particular muscle, there is a tendency to concentrate on motion. Indeed, many muscles are named on the basis of the movements that they generate, although others – often those whose actions are the most difficult to interpret – are described according to their morphology or situation. A more important function may be the nature of the force generated. For example, although flexor pollicis longus flexes the thumb (see above), a large range of flexion is actually required in only a few activities, such as certain ripping tasks. In most pinch and manipulative tasks the role of the thumb is to apply isometric force, which it does with such precision that it is possible to pick up an egg and neither crush nor drop it. Thus for much of the time flexor pollicis longus behaves as an extremely sophisticated mechanism for the application of force, in which contraction and proprioception are equally important.
The anatomical position of the hand (palm flat and pointing anteriorly, forearm supinated) is a convenient standard for studying structural relationships. The hand in the relaxed (anaesthetized) position adopts a posture of partial flexion and mid-supination/pronation (the reader can verify this by relaxing completely and observing forearm and hand position).
