development, evolution and optimization

Walking is the major achievement of human locomotor life. It takes approximately one year for humans to reach the first stage and seven years to reach full maturity (Sutherland et al 1980). This is unlike most other terrestrial vertebrates which can run with the herd within a few hours or a few weeks after birth. Though somewhat unstable compared to the quadruped gait, bipedal motion has many advantages.

Walking on two legs is more efficient than on four and therefore allows coverage of great distances at less energy expenditure (Washburn 1960). Bipedal motion freed the hands from locomotion and provided man with structures for manipulation, carrying and throwing of objects, protection (Fisher 1982) and communication (Hewes 1977). The first language is thought to have been by hand and arm gestures but then perhaps in the competition between hand use and gesture, vocal skills developed (Hewes 1977).

The notion that low back pain is the result of abnormal stress on his spine from man's rise to the erect posture is unlikely to be correct. Twelve million years of hominoid posture and locomotion and more than one million years of erect human standing and walking is considered long enough for any deficiencies associated with uprightness to have been selected out of the gene pool (Davis 1968).

Braune and Fischer (1895) cited by Waters et al (1973) first observed that the total mechanical energy of the head and trunk is conserved between successive steps or very nearly so (Saibene 1990; Winter 1990). As the head and trunk translate and rise over the stance leg, kinetic energy of this motion is converted to potential energy.

As the center of gravity descends with the forward motion of the opposite leg the potential energy is converted to kinetic energy. By this mechanism a smooth transfer of energy occurs between steps, minimizing muscle effort. Alexander (1989) maintains that optimization is the primary determinant of all terrestrial gaits. Efficient gait minimizes time spent foraging for food, leaves energy for the seeking of mates and provides protection by maximum speed and fitness for escape from predation.

Walking is a natural part of life that we take for granted until it is lost or reduced by disease or accident or habit. The human locomotor pattern is rich in interrelationships between evolution, physics, biomechanics and neuroscience; we are yet to take full advantage of these aspects of our human function and our medicine.

kinematics of human locomotion

Taking optimization as the unifying principle of locomotor organisation, Saunders et al (1953) proposed a simplified concept of locomotion as

" the translation of the center of gravity through space along a pathway requiring the least expenditure of energy" pg 558. Vertical displacement of the center of gravity requires expenditure of energy. It occurs twice in the gait cycle, at mid stance of each leg (Saibene 1990).

Saunders et al (1953) described six major kinematic determinants in locomotion for producing the efficient translation of the center of gravity:

1. pelvic rotation
2. pelvic list ( also pelvic side bend or pelvic rotation in the frontal plane)
3. knee flexion
4. foot and ankle mechanisms
5. foot and ankle mechanisms
6. lateral displacement of the body.

The six major determinants of human walking are best and simply expressed in Inman et al's 1981 book:" Human Walking". Pelvic rotation in the horizontal and frontal planes are illustrated in figure 1.

Figure 1.  Pelvic and Thoracic rotations in three planes (A. frontal plane = side bending; B. sagittal plane= flexion and extension; C. horizontal plane rotation). Notice the pelvic tilting or listing in A with forward swing of the left leg. In C the counter rotation of the trunk and pelvis is evident. (From Stokes et al 1989).

Seven to 11 degrees of pelvic rotation occurs with the forward swinging leg  (Inman et al. 1981; Stokes et al 1989). This effectively increases stride length and reduces the angular displacement at the hip. A pelvic list or tilt or rotation in the frontal plane of 9 to 11 degrees also occurs with the forward swinging leg (Stokes et al 1989). Pelvic tilt enhances the abductor mechanism of the hip (Saunders et al 1953).

Pelvic rotation in the horizontal and frontal planes both contribute to the reduction of a vertical displacement of the center of gravity and reduce the energy requirements of stride.

The body shifts laterally over the stance leg with each stride bringing it to a vertical alignment over the stance leg. This further minimizes energy expenditure in maintaining uprightness.

These determinants are critical features of human locomotion. Alteration, loss or inhibition of these motions have effects of energy efficiency and muscle and joitn strain locally and at a distance.

trunk and shoulder movements during locomotion

Lower limb mechanics has been the primary target of most major locomotion studies in humans (Saunders et al 1953; Murray 1967). More recently the trunk and shoulder girdle have been given greater attention in humans (Gregerson and Lucas 1967; Stokes et al 1989; Krebs et al 1992; Ohsato 1993). The involvement of the trunk in animal locomotion is clear (see figure 3).

Interest in trunk motion during human locomotion arises because the trunk comprises more than 50-65% of body mass and whilst the legs contribute to the primary propulsive movements of locomotion, the trunk assists by maintenance of body equilibrium and interaction with the limbs to produce efficient motion (Thorstensson et al 1984; Stokes et al 1989; Krebs et al 1992; Ohsato 1993). According to Gurfinkel et al (1981) the trunk is the main object of posture regulation with the legs as levers for regulating the position of the trunk. Some maintain in fact that the trunk is the major source of human propulsion and not the legs (Gracovetsky 1980).

Throughout the gait cycle pelvic rotation is matched by a counter rotation  of the trunk and shoulder (Gregerson and Lucas 1967; Stokes et al 1989; Krebs et al 1992; Ohsato 1993) (See figure 1&2). While the pelvis rotates in one direction 7-11 degrees the shoulder girdle rotates in the opposite direction 7-11 degrees.

With the shoulders and pelvis rotating in opposite directions it is the mid-thoracic region that rotates maximally to accommodate this affect. The thorax has articular facets that allow this rotation. Without the rotation of the trunk opposite to the pelvis during locomotion the head and shoulders would rotate from side to side (Evans 1992) and at higher walking speeds the individual would be unable to proceed in a straight line (Inman et al 1981). The counter rotation also provides a mechanism for maintaining a stable head position, so that the sensors of the head (visual and vestibular) remain relatively unaffected by locomotion, leaving them free to perceive the environment unimpeded.

Notice from figure 2 that 7-11 degree rotation of the pelvis and lumbar spine in the horizontal plane is not acheived by the mobility of the lumbar spine. Rotation between lumbar vertebrae reaches approximately only 0.03 degrees and lumabr discs are very suceptable to the effects of excessive rotational stresses (Farfan 1970).

Loss of trunk rotation in locomotion results in a 10% increase in energy expenditure (Ralston 1965). Evans (1992) postulated that  reduced trunk rotational movements may cause increased rotational strain on the lumbar spine. Rotational stress on the lumbar discs is a recognized factor in disc rupture (Farfan 1970).

Obviously a mobile thorax that complements and accommodates both pelvic and shoulder motion is essential for efficient forward translation during locomotion.

For me, trunk counter rotation and the mobility of the thorax would be the third major determinant of human locomotion on Saunders's list. Pelvic action is always accompanied by and significantly affected by its matching and complimentary thorax mobility. So I would elevate the thorax to third place if not higher.

Figure 2.  Pelvic and Thoracic rotations in the horizontal plane. Counter rotation of the trunk. Notcie that the pelvis and lumbar spine rotation 8 degrees in space but little rotation occurs within the lumbar spine 0.03 degrees. (From Gregersen & Lucas 1967).

Figure 3.  Trunk motion during horse, cheetah and newt locomotion. It is easy to see how in these animals trunk motion contributes significantly to the function and power of the legs during locomotion (from Grillner 1975)

reaching and locomotion

Reaching and locomotion are closely linked by evolutionary and neurophysiological connections. Georgopoulos and Grillner (1989) suggest that the fine control of the limb for reaching has evolved from the neural system for the precise positioning of the limb during each step cycle and therefore the two are closely related. For humans, now freed from four legged motion, precision placement of the forelimb has evolved to precision reaching and manipulatory ability. Both functions involve the corticospinal system. These very frequent human functions are often employed simultaneously or at least in close relation in the work place. Reaching has been the subject of extensive research to discover the functions of the motor cortex in motor behaviour (Georgopoulos 1986). For these reasons the organisation of the spine for reaching and locomotion are relevant to arm function as well as leg function.