The purpose of this thesis is to determine if the amount of load on the extensor muscles of the leg is involved in determining the stance-to-swing transition during human gait. This transition is a critical period during the step cycle as stability can be compromised if it occurs at an incorrect moment. Studies on reduced animal preparations show that increasing the load input prolongs the stance phase and delays the onset of swing. Healthy adult humans, healthy infants with functionally immature supraspinal descending inputs and neurologically impaired people suffering from Parkinson’s disease were studied. Parkinson’s patients were included as some patients have difficulty in initiating a step.

The results of this thesis show that changes in load will affect the generation of the walking pattern in humans. The response seen is dependent on the state of the system, the nature of the disturbance and the task at hand. For example, in sitting, electrical stimulation of group I afferents from an extensor muscle cause an inhibition of the activity of a synergist muscle in healthy adults. During walking, the same stimulus causes less inhibition, and in some subjects, it causes facilitation of the synergist muscle. Mechanically loading the extensor muscles during walking results in responses that are dependent on whether the load causes a postural instability. The amplitude and duration of the extensor activity are modified separately to compensate for the changes in load. With increased postural instability, there are changes in the duration of muscle activity In a posturally stable situation, loading results in minor changes in the duration of muscle activity. The load is mostly compensated for by an increase in the amplitude of the activity of the extensor muscles. This is not the case in the non-independently locomoting infant. Regardless of the method of introducing load, infants respond by prolonging the extensor burst and the step cycle. The ability to modulate the response to load seems to be dependent on functionally mature descending systems. In Parkinsonian subjects, electrical activation of the group I afferents does not seem to affect the output of the soleus motomeuronal pool, either during sitting or walking, though a trend towards facilitation is seen during sitting. However, it appears that this population may use more cortical control during walking as distracting attention away from their walking causes it to deteriorate. Modulation of the group I pathway is clearly altered in this population, but the mechanisms for the alteration remain unknown.