Power Grip and Precision Handling

The recent study by Napier (1956) of the prehensile movements of the human hand is of outstanding importance because it points out that the "nature of the intended activity finally influences the pattern of the grip" (p. 906). He distinguishes between the "precision grip" and the "power grip" and states that these patterns, "either separately or in combination, provide the anatomical basis for all prehensile activities, whether skilled or unskilled" (p. 902). In the precision grip the thumb is abducted in both the metacarpophalangeal and the carpometacarpal joints, whereas in the power grip both joints are addicted. With the abduction (combined with flexion) the thumb also rotates medially, so that the pulp surface becomes directly "opposed to the pulp surface of one or all of the remaining digits for the purposes of (precision) prehension" (p. 908). Napier (1952) positively stated that this abduction with related rotation took place not only in the carpometacarpal joint, but also in the metacarpophalangeal joint; this had been previously observed by Duchenne (1867) and Bunnell (1938), but neither they nor Napier offered any substantial evidence in support of this idea. In the power grip the combined fingers form one jaw of the clamp with the palm as the other jaw. The fingers are more or less flexed according to the size of the object, and they are laterally rotated and inclined towards the ulnar side of the hand. The power grip should therefore be manifested in grasping a cylindrical object. The thumb-in the adducted position-is pressed with the pulp surface against the object to be grasped, and this, according to Napier, introduces an element of precision into the power-grip complex. If the object is heavier and the possibility of precision more limited, then the thumb will gradually become more abducted and will be thrown over the object. In the precision grip the abduction of the thumb ensures that the pulp surfaces of the thumb and fingers can place themselves opposite to each other; the fingers are now described as being flexed and abducted at the metacarpopbalangeal joints. The characteristics of both grips come into play in grasping globular or cylindrical objects. Napier states that the position of the hand is determined not so much by the shape of the object as by the quality of the grip, which in its turn depends on the purpose for which the instrument is grasped. For example, a pinhammer has about the same diameter as a pen or pencil and, although in both cases an element of precision is involved, a pinhammer is clearly held and used differently from a pen. The problem which Napier raises is extremely important, not only in the actual anatomy, but also in the evaluation of incongruity, the correction of deformities, and the reconstruction and rehabilitation of the hand. He gives very apt examples of objects with the same shape that can be used in different ways, according to the power or precision required. It is necessary for those concerned in the rehabilitation of crippled or deformed hands to study the whole subject of the various positions which the normal hand is capable of assuming according to the object grasped and the use to which it is to be put. Napier uses the terms "precision grip" and "power grip" "in a dynamic as well as a static sense in the same way that flexion and extension are employed to describe both the posture and the movement" (p. 903). The dynamics of gripping produce a particular grip, and the static concept indicates the final state of gripping. This concept is given little consideration by Napier; he does not say whether both forms of grip may be regarded as a variant of the final state, or whether the movements leading to this final state may be taken in both cases as variants of an equivalent movement pattern. Consequently it seems justifiable to attempt to discover how the dynamic and static stages of these