Biomechanics

Zong-Ming Li


Everyday Activities involve extensive use of the hand and fingers. The muscles that serve the fingers are classified as either intrinsic or extrinsic. The extrinsic muscles (eg. The flexor digitorum profundus) originate outside the hand, cross multiple joints and produce mechanical effects on all of them. The intrinsic muscles (e.g. the interossei) originate and insert within the hand itself. The execution of many daily activities depends on well-coordinated muscle and finger forces.

A number of factors play a role in finger force production, including variations in muscle length, muscle and tendon compliance, joint conditions, neurological problems, pinch type, and body/joint configuration. The hand is an end-effector of the multi-link kinematic chain of the human body, and therefore a positional change in any of the proximal series of segments may influence the performance of the hand. It has been reported that hand grip strength is dependent on body posture (e.g., standing or sitting), and angular positions of the shoulder, the elbow, the forearm, the wrist joint, the metacarpophalangeal joint, and the interphalangeal joints.

Among the aforementioned factors, wrist position has been shown to be one of the most important determinants of grip and pinch strength capabilities. The articulation of the wrist joint allows motion in two planes: flexion/extension motion (FEM) in the sagital plane and radial/ulnar deviation (RUD) in the frontal plane. Axial hand rotation in the transverse plane occurs through pronation/supination of the forearm, not the wrist joint.

The normal wrist range of motion is 65 to 80° of flexion, 55 to 75° of extension, 15 to 25° of radial deviation, and 30 to 45° of ulnar deviation. Most daily activities require a significant portion of the full range of wrist motion. For example, Rye et al. evaluated 24 activities of daily living and found that the hand experienced 114° in FEM (54° of flexion, 60° of extension) and 57° in RUD (40° of ulnar deviation, 17° of radial deviation). Therefore, it is important to understand how wrist position affects hand function.

Lamoreaux and Hoffer found a significant loss of total grip strength with wrist at maximal ulnar and maximal radial deviations as compared to the anatomical neutral position. Halpern and Fernandez found that deviated wrist postures decreased pinch strength up to 33%. O'Driscoll et al found a maximal grip strength output at a self-selected optimal wrist position of 35° of extension and 7° of ulnar deviation. With the wrist in 15° of extension or 0° of radial-ulnar deviation, grip strength was reduced to two thirds to three fourths of the strength at self-selected position. Pryce measured power grip strength in nine wrist positions of ulnar deviation and flexion/extension and found that the strongest grip strength occurred at 15° extension and 0° deviation, while deviation from these values in either RUD or FEM resulted in a significant decrease in grip forces. Hazelton et al. found that the greatest total finger flexion force was achieved in wrist ulnar deviation, followed in order by the wrist positions of anatomical neutral, radial deviation, extension and flexion.

Previous studies have focused on the influence of wrist postural changes on force output of a single finger (e.g., pinch), or the total force output of multiple fingers (e.g. grip). Very few studies focused on how force production capability of individual fingers is influenced by wrist position. The execution of many tasks that involve multiple fingers may depend not only on total force output, but also on the distribution of the total force among individual fingers. In addition, previous attempts to evaluate the effect of wrist position on grip strength have concentrated on a few limited static wrist positions. Many activities of daily living (e.g. opening a tight lid), industrial work (e.g. twisting a wrench), and recreational tasks (e.g. playing golf) require forceful gripping while the wrist travels through an arc of motion. To date, it remains unclear as to how wrist position affects force production of individual fingers in functional gripping tasks involving multiple fingers.