The lower limbs – introduction

The human lower limb is adapted for weight-bearing, locomotion and maintaining the unique, upright, bipedal posture. For all of these functions much more strength and stability are required than in the upper limb. The bones of the lower limb are larger and more robust than their upper limb counterparts, and vary in their characteristics in relation to muscular development and body build. Many bones, particularly the innominate and, to a lesser extent the femur, show sexual differences; variations in the female pelvis for example being an adaptation to child-bearing.

The form and structure of individual bones are adapted to the function of support and resisting mechanical stresses. The internal architecture of the bone is arranged to resist all such stresses and forces. This is particularly marked at the articular regions of the bone. During growth and life continuous modifications are being made in order to maintain the functions of support and resistance to stress as the stresses change. The attainment of an habitual, upright posture and bipedal gait has resulted in both the mechanical and functional requirements of all the bones of the lower limb. Consequently, during evolution, there have been major changes in the lower limb.

The pelvic girdle, formed by the innominate of each side articulating anteriorly at the symphysis pubis, connects the lower limb to the vertebral column via its posterior articulation with the sacrum. This posterior articulation, the sacroiliac joint, provides great strength in the region of weight transference from the trunk to the lower limb at the sacrifice of almost all mobility. The human ilium has developed so that it is no longer blade-like but is shortened and tightly curved backwards and outwards, changing the actions of the gluteal muscles. The changes in the pelvis have resulted in a shift in its positioning from an essentially horizontal to an essentially vertical position. This has enabled the trunk to be held vertically, but has necessitated a change in the orientation of the sacrum with respect to the ilium with the result that the axis of the pelvic canal lies almost at a right angle to the vertebral column. During evolution there has been a relative approximation of the sacral articular surface to the acetabulum which makes for greater stability in the transmission of the weight of the trunk to the hip joint. The increase in the magnitude of this weight has resulted in an increase in the area of contact between sacrum and ilium relative to the area of the ilium as a whole. For the same reason the acetabulum and femoral head have also increased in relative size during evolution. The shortening of the ischium that has occurred is an adaptation for speed and rapid movements, which is of great importance in bipeds. Thus power of action has been sacrified for speed.

Changes have also occurred at the knee with the femoral condyles being more parallel in humans as compared with other primates. The major change though has been in bringing the knees inwards towards the body midline, which appears to be part of the overall pattern of centring the body mass, thus reinforcing skeletal rather than muscular equilibrium.

In humans, the tibia and fibula are held tightly together, with the tibia being the weight-bearing component while the fibula is mainly for muscle attachments. There has been a loss of rotation of the fibula with respect to the tibia.

It is the foot, however, which has perhaps undergone the greatest change during evolution. These changes reflect not so much the evolution of a new function as a reduction in the original primate functions, with the foot changing from a grasping, tactile organ to a locomotor prop. Although some non-locomotor function is still possible, the foot has evolved from a generalized to a specified organ. The joints of the human foot permit much less internal mobility – an adaptation to ground walking. In locomotion, the foot acts as a lever adding propulsive force to that of the leg, with the point of pivot being the subtalar joint. The front part of the foot has been shortened relative to the posterior portion, where the main thrust in walking is developed – the power capabilities are thus accentuated.

The arches of the foot are formed by the shape of the bony elements, and are supported by ligaments and tendons. They convert the foot into a complex spring under tension and allow it to transmit the stresses involved in walking, both when body momentum is checked at heel-strike and when the foot is used in propelling the body forward. The lateral arch helps to steady the foot on the ground, while the medial arch transmits the main force of thrust in propulsion. This arched form of the foot is important in providing one of the major determinants of gait, helping to minimize energy expenditure and thus increase the efficiency of walking.

An important consequence of the upright, bipedal posture is that the centre of gravity of the body has been brought towards the vertebral column, so that in humans it lies slightly behind and at about the same level as the hip joint thereby reducing the tendency of gravity to pull the trunk forward. The centre of gravity projection then passes anterior to the knee and ankle joints. At the knee, the line of weight transmission passes towards the outside. Because of the angulation of the femur, during walking of the foot, tibia and knee joint of each leg stay close to the line followed by the centre of gravity, and thus energy expenditure is minimal in maintaining the centre of gravity above the supporting limb. Balance is thus improved and there is more time for the free leg to swing forward enabling an increase in stride length. The alteration in the line of weight transmission is carried into the foot, where it passes to the inner side. However, it must be remembered that weight is also transmitted through the outside of the foot, bringing the entire foot into use as a stabilizing element.

In order to reduce the possibility of collapse or dislocation, due to the forces to which they are subjected, the joints of the lower limb are structurally more stable than those of the upper limb. This stability is either due to the shape of the articular surfaces, the number and strength of the ligaments, or the size of the muscles related to the joint. Commonly, each of these factors contributes to a varying extent.