Biomechanics of the Upper Limb

Description of Motion

Biomechanically, anatomical joints are described according to joint axes and degrees of freedom. For example, the interphalangeal (IP) joint is considered to be a uniaxial joint with one degree of freedom, allowing motion in one plane. The type and range of movement of a joint are dependent on passive constraint provided by the shape and contour of the joint surfaces, ligaments, and soft tissue, as well as active facilitation by the neuromuscular system. The description of motion becomes detailed when multiaxial joints, such as the glenohumeral joint, are studied in three-dimensional motion.

Normal Range of Motion Arcs Versus Functional Motion Requirements

When studying upper limb motion, it is important to delineate between the normal arc of movement for specific joints and the functional arc of motion required for most daily activities. For example, although the elbow has a normal arc of flexion–extension of 0 to 150 degrees and pronation and supination of 75 to 85 degrees, respectively, the full arc of motion is generally not used for most activities of daily living. A study of the functional elbow arc of motion conducted by Morrey et al. revealed that 15 activities of daily living can be carried out with an arc of motion of 30 to 130 degrees of flexion–extension ( Fig. 11.1 ). Furthermore, these same activities required an equal amount of pronation and supination (50 degrees of each). Those activities that use an arc of motion require it be about equally centered between pronation and supination. Note that the activities studied were related to activities of daily living. Special requirements for other activities, including occupational tasks, have not been clearly elucidated. Fig. 11.2 illustrates the importance of forearm supination and pronation in positioning the upper extremity for feeding and dressing tasks.

Similarly, the wrist has a normal arc of flexion of 80 to 85 degrees, approximately 70 degrees of extension, 20 degrees of radial deviation, and 30 degrees of ulnar deviation. Ryu et al. examined 40 normal subjects (20 men and 20 women) to determine the range of motion required to perform activities of daily living. The amount of wrist flexion and extension, as well as radial and ulnar deviation, were measured simultaneously using a biaxial wrist electrogoniometer. The entire battery of evaluated tasks could be accomplished with 60 degrees of extension, 54 degrees of flexion, 40 degrees of ulnar deviation, and 17 degrees of radial deviation. Most hand placement and range of motion tasks studied in this project could be accomplished with 70% of the maximal range of wrist motion ( Fig. 11.3 ). This equates to 40 degrees each of wrist flexion and extension and 40 degrees of combined radial–ulnar deviation (30 degrees of ulnar deviation, 10 degrees of radial deviation). Knowledge of functional range of motion requirements for activities of daily living can be beneficial when developing orthotic assistive devices. It should be an objective to provide devices that will allow motion to perform functional tasks and, ideally, occupational tasks.

Compensatory Motion in the Upper Limb

Contrary to conventional thought, the primary functions of the shoulder complex and elbow are mutually exclusive. It has been observed clinically that patients can tolerate elbow flexion contractures of about 30 degrees without much functional impairment. On the other hand, it also is well recognized that with motion loss greater than 30 degrees, patients readily complain of functional impairment. To better understand the implications of this loss of motion, it should be recognized that the shoulder functions as a ball and socket joint, allowing the hand to move in a spherical boundary in space. As the elbow moves into a more extended position, a new sphere surface may be described. Thus the effective sphere of influence of the hand extends from the patient as far into space as the link at the elbow allows. It has been calculated that a loss of elbow motion of 30 degrees, 45 degrees, and 60 degrees leads to loss of functional volume of the hand by 28%, 39%, and 60%, respectively.

A study by O'Neill et al. analyzed compensatory motion in the upper extremity after simulated elbow arthrodesis. The 3Space Tracker System (Polhemus, Inc., Colchester, VT) was used to measure shoulder motion, a biaxial wrist goniometer was used to measure wrist compensation, and all subjects were videotaped to qualitatively observe other compensatory motion. Ten healthy male subjects were asked to complete a series of tasks that represented use of the elbow in functional tasks. They were fitted with a custom adjustable brace that simulated elbow arthrodesis at 50, 70, 90, and 100 degrees of flexion and asked to repeat the tasks. Unlike other joints, elbow arthrodesis at any angle resulted in a significant impairment, because the adjacent shoulder and wrist joints could not compensate to allow completion of activities. In upper limb orthotic design, it is important to apply this information to understand the role and limits of compensatory motion of adjacent joints in providing function after injury or disease.

Synergistic Motion of the Wrist and Hand

Awareness of the relationship between wrist joint position and tendon excursion is essential for understanding motor control of the fingers and hand. Positioning the wrist in the direction opposite that of the fingers alters the functional length of the digital tendons so that synergistic finger movement can be attained. Wrist extension is synergistic to finger flexion and increases the length of the finger flexor muscles, allowing increased flexion with stretch. In contrast, wrist flexion is synergistic to finger extension, with wrist flexion placing tension on the long extensors, facilitating finger extension. This relationship has application in tendon-splinting techniques with respect to affecting tendon excursion through positioning and the potential benefits of synergistic wrist motion and metacarpophalangeal (MP) joint motion in promoting flexor tendon gliding after repair. Also, synergistic wrist motion principles are the basis for many orthotic designs that provide assistive function of the hand with tetraplegia.