walking

walking is an activity that we normally take for granted; we consciously start or stop, and give attention to avoidance of obstacles, but otherwise this complex, co-ordinated procedure is seemingly simple and automatic.

Walking, in technical terms, is a form of bipedal (or quadripedal) progression in which there are periods of double support, when both feet are on the ground, alternating with periods of single support. This distinguishes walking from faster gaits in which ground contact is absent for brief periods. Whilst it is commonplace, its mechanics and neurological control are complex. Many neurological disorders affect walking efficiency.

Events in both limbs are essentially identical but are phase shifted. The sequence of events in the right limb begins with heel contact with the ground. At this point the left foot is still on the ground (the first phase of double support). The body moves forward and the centre of gravity passes in front of the left toes. At this point the weight is supported by the right limb alone, the left limb flexes and swings forward. The left limb begins to extend later in the swing phase, causing left heel contact with the ground. This initiates a second period of double support, which is followed by the swing phase in the right limb terminating in right heel contact. The smooth forward movement of the centre of gravity includes lateral movements so that the centre of gravity lies over the right foot during single support on that limb. There are symmetrical leftward sways during left limb stance. In addition, the centre of gravity rises and falls by some 50 mm.

Young adults typically select a range of normal walking speeds of between 80 and 100 metres per minute. This corresponds to a stride length of about 1.4 m, i.e. a step length of 0.7 m, and a stride duration of about 1 second. The duration of the stance phase for each foot is about 0.65 second. The peak vertical force rises to about 120% of body weight during the stance phase.

Limb muscles generally show single bursts of activity during each step. Extensor (leg straightening) muscle activity typically begins just before heel contact to prepare the limb for load bearing, whilst flexor muscle activity is confined to the swing phase after toe-off, to allow the limb to swing through to its new landing position. The bulk of the forward propulsive force comes from a second short burst of activity in the knee and ankle extensors just before toe-off.

The patterns of muscle activity during walking are generated by networks of neurons located in the spinal cord and accorded the description central pattern generator. These networks, which generate a simple locomotor rhythm, draw upon, and are influenced by, reflexes evoked by sensory inputs from the muscles, skin, and joints, particularly the hip joint. In many animals — the cat, dog, rat, and mouse, for example — and for swimming movements in fish, this locomotor network can express its rhythmic activity entirely independently of control from higher centres in the brain, hence the other name, spinal locomotor centre. In the case of the rabbit the movement is a bilaterally synchronous ‘hopping’ movement. Attempts to demonstrate that such mechanisms can be activated in (spinal man) (when the spinal cord is cut off by injury from the higher parts of the nervous system at a level above the segments that control walking) have however failed. If they could have been elicited it would have facilitated the development of prosthetic devices that could enable spinal man to walk. As it is, even extremely complicated computer control aimed at stimulating muscles with the same pattern as in natural walking has been only partially effective; this emphasizes the importance of the control exerted by the brain despite the automated nature of walking. Nevertheless, it remains likely that when one wishes to walk, or to stop walking, the commands issued from the brain are actually turning on, or off, a spinal pattern generator comparable to that demonstrated in animals.

R. H. Baxendale

See also gait; movement, control of; posture.