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How Are the Arm and the Leg Adapted for Their Special Function?

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How Are the Arm and the Leg Adapted for Their Special Function? Both the arm and the leg are adapted in a variety of ways to accommodate their special functions. The primary function of the lower limb is to support the weight of the body and to provide a stable foundation in standing, walking and running. Thus, in general it can be seen that in the lower limb, adaptation to stability takes precedence over adaptation to mobility. The upper limb is the organ of manual activity, and as such is a multijointed lever freely movable on the upper trunk at the shoulder joint. At the distal end of the upper limb is the important prehensile organ- the hand. Much of its importance is dependent on the adaptations that allow the pincer-like motion of the thumb. Much of the stability of the lower limb is attributable to the pelvic girdle. The body mass acts through the vertebral column on the pelvic girdle, which in turn transmits forces to the lowerlimbs. Whereas the pectoral girdle of the upper limb is united to the trunk by only a small joint, the sternoclavicular joint, the two hip bones articulate posteriorly with the trunk at the sacroiliac joints and anteriorly with each other at the symphysis pubis. ...read more.


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. ...read more.


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? ...read more.

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