When standing upright, the centre of gravity falls in front of the axis of the ankle joint, which means that the body tends to sway forwards. The plantar flexor muscles prevent this sway, particularly soleus, which is metabolically adapted to resist fatigue. During quiet standing periodic adjustment of the plantar flexor activity is the principal balancing force for the body. Movement at the subtalar joint, inversion and eversion, is an important adaptation for controlling direction in walking.
The arches of the foot are another set of adaptations for the special function of the lower limb. The arches are important in allowing the foot to serve as a shock absorber, in allowing it to adapt to uneven or sloped ground, and in the recovery of energy during locomotion. They are maintained by a mixture of the shape of the tarsal bones, elements that hold the plantar surfaces of the bones together and suspensive elements.
Having explained adaptation to a bipedal stance, we routinely take our bodyweight on one leg. Here, the centre of gravity usually falls between the feet. Thus to maintain stability, we need to move the centre of mass to lie over the supporting limb. Thus when a limb is lifted the hip abductor muscles (gluteus medius and gluteus minimus) of the supporting limb contract, not just to prevent the pelvis from tilting down, but to tilt the hip up. Loss of the abductor muscles, such as from polio, results in a Trendelenburg gait.
Locomotion, involving both stance and swing phases requires strong muscles for propulsion. Propulsion being provided by plantarflexion and flexion of the two. In the stance phase, as the weight is moved onto the supporting limb, tension in the iliotibial tract as well as contraction of the extensor muscles of the hip and knee contract to support the body weight. In the swing phase, the iliopsoas, raise the limb, it is then brought forward and then down again. There are six small lateral rotators of the hip arranged close to the joint, similar to the rotator cuff muscles of the shoulder. The swinging leg carries the pelvis forwards and the limb tends to turn inwards. Activity in the lateral rotators will keep the foot pointing forwards to carry on walking in the same direction.
The lower limb’s function mainly allowing the intricate movements of the hand is facilitated by the range of movement at the shoulder joint. One particular example being scratching one’s own back. Often movement involves the whole of the pectoral girdle. The bony clavicle and the scapula form the pectoral girdle which provides the link between the upper limb and the trunk. Two articulations are involved in this link; the sternoclavicular joint between the medial end of the clavicle and the clavicular notch on the manubrium of the sternum; and the acromioclavicular joint between the lateral end of the clavicle and the acromion process of the scapula. The sternoclavicular joint is stabilised by the strong costoclavicular ligament, and by an intra-articular disc attached to the clavicle above and to the first costal cartilage below. The muscles that move the shoulder girdle and the glenohumeral joint allowing for mobility may be divided into three. Firstly, the muscles stabilising the glenohumeral joint. Secondly the muscles acting on the glenohumeral joint and thirdly the muscles moving the pectoral girdle. And yet, although the muscles may be grouped for clarity, they combine in various ways, grouping and regrouping as different movement proceeds.
In the glenohumeral joint, the shallow glenoid cavity of the scapula and the hemispherical head of the humerus, joined by a thin loose capsule, permit a wide range of movement, but represent a poor prospect for stability. This being in contrast to the hip joint. The only strong ligament linking the bones is the coracohumeral ligament extending from the lateral border of the coracoid process to the greater tuberosity of the humerus. The most effective provision for support for the joint is
from the four muscles of the rotator cuff surrounding it and blending closely with the capsule. The rotator cuff muscles have weak action as prime movers since their insertions are close to the joint, but they function as stabilisers in all movement at the shoulder joint.
Increased abduction occurs as a result of the scapula being rotated so that its glenoid fossa projects superiorly. It may also be rotated downwards. These movement together with depression, elevation, protraction and retraction all add to the upper limbs mobility and hence efficient function.
As well as the elbow joint, added mobility is also achieved by the ability to pronate and supinate at the proximal and distal radioulnar joints. For the hand to be able to perform delicate movements, the forearm is placed in the semiprone position. In this position the forearm bones are most stable as the interosseous membrane is taut; in other positions it is lax.
The wrist joint, a condyloid synovial joint allows for abduction, adduction, flexion, extension and circumduction. The absence of rotation at the wrist joint is compensated for by the ability to pronate and supinate the forearm. In the position of function, the wrist joint is slightly extended, allowing the long flexor and extensor muscles to work to the best of their advantage.
In contrast with the foot, which is adapted as a segmented bony plate, the hand has substantial independent mobility, especially of the thumb. Much of the importance of the hand is dependent on the pincer action of the thumb in opposition, which enables one to grasp objects between the thumb and the index finger. The extreme mobility of the first metacarpophalangeal saddle-type joint makes the thumb functionally as important as the rest of the remaining fingers combined. The intrinsic muscles of the hand, as well as the long flexor and extensor muscles allow for the fine movements in the hand. The lumbricals and interossei, being particularly of importance as they flex the metacarpophalangeal joints and extend the interphalangeal joints, allowing for intricate movements of the fingers such as in playing the piano or even holding a pen. The thenar and hypothenar eminences help cup the hand, which along with the palmar aponeurosis, allow for an increased grip.
Thus the upper and lower limbs, as opposed to the arm and the leg in the essay question which anatomically only refers to the parts between the shoulder and elbow and that between the knee joint and the ankle, are many ways adapted to their special functions. A common theme being the trade-off between stability and mobility, and yet which is more important?
