How the Arm and the Leg are Adapted for Their Special Function

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How the Arm and the Leg are 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. This stability is in marked

contrast with the upper limb. Forces are transmitted from the pelvis

to the femur at the hip joint. As a ball and socket, the hip is

intrinsically stable. The acetabulum being deepened by the acetabular

labrum, as well as strong capsular ligaments provide for a strong hip

joint. As the centre of gravity falls behind the hip joint, the

tendency when standing is for the hip to be hyperextended. The strong

iliofemora...

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...d, 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?

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