UPPER EXTREMITY BIOMECHANICS AND ROLE IN POSTURE MAINTENANCE

Each upper extremity consists of 32 bones, spanning from the shoulder girdle, the arm, forearm, wrist and bones of hand that together form different types of joints, along with ligaments, muscles and bursae at different sites. These geometrically complex joints allow the upper limb to undertake different movements at the point of articulation of the bones, with the ligamentous, soft tissue and capsular structures stabilizing the joints. The upper limbs have their own capacity to function, established by the shoulder, elbow, wrist joints and the hand, with various functional envelopes. For normal limb segments, the availability of surrounding space is the limiting factor for their functional capacity. The upper extremity formed by the mobile shoulder girdle, the foldable arm, forearm segments and the prehensile hand is a multifaceted dexterous member that allows for holding, grasping and manipulating objects.

The shoulder complex formed by the humerus, clavicle and scapula consists of the sternoclavicular, acromioclavicular and glenohumeral joints along with an articulation between the thorax and the scapula. A major component of the shoulder joint stability is dependent on non-bony structures including ligaments and muscles since the only bony connection between the body and shoulder joint is through the sternoclavicular joint. The movements allowed by the shoulder joint include flexion-extension, abduction-adduction, rotation, scapular protraction-retraction, depression as well as elevation.

The elbow joint formed by the humerus, radius and ulna is made up of humeroulnar and humeroradial joints as well as the radio-ulnar joints. Elbow complex allows flexion-extension along with forearm pronation-supination occurring at the radio-ulnar joints. The joint in conjunction with its muscles functions to provide mobility to the hand for ADLs, by increasing or decreasing the length of the upper limb, according to activity requirements. Most of the muscles crossing the elbow have their points of action at the shoulder or the wrist, linking them together to strengthen the hand functions.

The wrist-hand complex consists of 27 bones and multiple joints that include the radiocarpal joint (wrist joint), intercarpal joints, carpometacarpal joints, metacarpal joints, metacarpophalangeal joints and phalangeal joints. The wrist joint is formed by the articulation of the radius with the proximal row of carpal bones that are the scaphoid, lunate, triquetrum and pisiform. It has 3 degrees of freedom, that is, flexion-extension, radial-ulnar deviation and rotation. The wrist-hand complex is responsible for regulating the length-tension relationship in the multiple muscles of the hand, allowing for grasping and prehensile activities.

In addition to the joints, the hand also has 3 arches – longitudinal arch (carpometacarpophalangeal) that runs from the wrist to each digit, oblique arch that runs from the base of hypothenar region to the second metacarpal head and the transverse arch that lies across the palm. The arches play a vital in maintaining the hand-wrist stability and also enhance gripping and lifting of various objects.

Significance of the Upper Limb in Body Posture and Balance

For an upright posture, that is balanced and stabilized, the body’s center of mass needs to be over its base of support, to be in equilibrium. Body movements, upper extremity motions and interactions of the body with the surroundings along with the gravitations forces render the body unstable and maintaining the center of mass over the base of support during ADLs is done through compensatory and anticipatory strategies of balance and posture. These are afforded by the movements of the arm and forearm that offer support and stability through grasp, help in absorbing shock, protecting the head during falls and also work to maintain the center of mass over the base of support.

Therefore, the upper limb is vital in maintaining body posture through coordination between arm and forearm movements that offer stability to the body. This is done by the arms providing counterweight to the body through the generation of force reactions and also by changing the body’s moment of inertia. Another contributing factor to postural balance and body sway is through grasp. When an individual grasps a solid object, it improves their balance through a combination of motor support as well as sensory feedback, altered only by the amount of manual contact. The firmer the grip, more will be the proprioceptive response, leading to an increased support for maintaining body posture and keeping the body upright.

Musculocutaneous disorders or amputations leading to the loss of upper and/or lower limb have a disastrous impact on the functional capacity of the individual and on postural stability. For patients with lower limb amputations, the upper limb prosthetics may also serve to fulfill the requirement of weight bearing by carrying a significant amount of body weight, thus helping to maintain body stability and posture.