214


PART III THE UPPER EXTREMITY


FUNCTIONAL USE OF THE WRIST AND HAND


The function of the hand is interdependent on the move- ment and function of the wrist in that the position of the wrist facilitates strength or fi ne motor manipulation of the hand. Both complexes are the fi nal link in the chain of upper extremity joints that requires proximal stability and control at the scapulothoracic complex and control and directional reach at the shoulder, elbow, and forearm. Impairment in one or more links of this chain can alter the function of the entire extremity.


FUNCTIONAL POSITION


For the hand to manipulate objects during daily activi- ties, it must be able to hold an object on the palmar surface with the fi ngers and thumb fl exed to maintain contact, between the fi ngers, or between the fi ngers and the thumb. For the fi ngers to provide a fl exion force, the wrist must be stabilized in a position of approximately 20° of extension and slight ulnar deviation. This position is called the functional position of the wrist and hand and is characterized by wrist complex extension (20°), ulnar deviation (10°), MCP fl exion (45°), PIP fl exion (30°), and slight DIP joint fl exion. In this position, the fi nger fl exors are at an optimal length for facilitating fl exion, and the wrist complex muscles all are under an equal amount of tension (Fig. 10.28).


PREHENSION


The hand performs the fi nal component of an upper extremity task as it grasps an object when the shoulder complex and elbow reach for the object. The hand also functions in a closed chain position as it pushes on the arms of chairs when a person is standing from a sitting


position. It can support the arm on a stable surface while the person uses the opposite arm for an activity. The hand manipulates objects when typing on a keyboard and assists communication through the use of hand gestures and expressions. Prehension is the ability of the fi ngers and thumb to grasp and hold an object. Prehension is either a grip or a pinch. A grip is a form of prehension in which all the digits are functioning, whereas a pinch most often involves only the thumb and the index fi nger. The categories of grip and pinch are further divided into movements needed to produce power and movements recruited for precision activities.


Grips


The power grip is used when power and strength are needed to hold and use an object. An example of a power grip is holding and using a tool such as a hammer. During a power grip, the wrist musculature perform a strong iso- metric contraction to stabilize the wrist in some extension and facilitate strong fi nger fl exion with assistance from the intrinsic muscles. The thumb opposes the fi ngers or adducts and is fl exed with the position depending on the size and weight of the object being held in the entire hand. A cylindrical grip is a type of power grip in which the fi nger fl exors and hand intrinsic muscles work to maintain the grasp. Typically, the wrist is maintained in neutral and slight ulnar deviation. Table 10.3 summarizes the muscle activity that contributes to power grips. The precision grip (often called a spherical grip) holds the object between one or more fi ngers and the thumb. In contrast to the power grip, in which the thumb is adducted, in the precision grip, the thumb is abducted and often positioned to oppose the fi ngers. The fi ngers are often spread wider than in a cylindrical grip, requiring more activity from the interossei muscles. The precision grip is used when control and accuracy are required. The


TABLE 10.3 Muscle Control With Power Grips Muscles


Function


Extrinsic fi nger fl exors


Extensor digitorum Interossei


Isometric contraction provides major gripping force


Provides compressive forces at MCP joints


Rotate the fi rst phalanx to assist with compression and fl ex the MCP joint


Fourth lumbricalis Only lumbricalis to function in a power grip


Figure 10.28 Functional position of the hand with wrist extension, ulnar deviation, and flexion of the MCP and IP joints.


Thenar muscles, adductor pollicis


Compress thumb against an object


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