In the last years it has become possible to regain some locomotor activity in patients suffering from an incomplete spinal cord injury (SCI) through intense training on a treadmill. The ideas behind this approach owe much to insights derived from animal studies. Many studies showed that cats with complete spinal cord transection can recover locomotor function. These observations were at the basis of the concept of the central pattern generator (CPG) located at spinal level. The evidence for such a spinal CPG in cats and primates (including man) is reviewed in part 1, with special emphasis on some very recent developments which support the view that there is a human spinal CPG for locomotion.

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Walking can be a very automated process, and it is likely that central pattern generators (CPGs) play a role in the coordination of the limbs. Recent evidence suggests that both the arms and legs are regulated by CPGs and that sensory feedback also regulates the CPG activity and assists in mediating interlimb coordination. Although the strength of coupling between the legs is stronger than that between the arms, arm and leg movements are similarly regulated by CPG activity and sensory feedback (e.g., reflex control) during locomotion.

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Dual-task designs have been used widely to study the degree of automatic and controlled processing involved in postural stability of young and older adults. However, several unexplained discrepancies in the results weaken this literature. To resolve this problem, a careful selection of dual-task studies that met certain methodological criteria are considered with respect to reported interactions of age (young vs. older adults)×task (single vs. dual task) in stable and unstable postural conditions. Our review shows that older adults are able to perform a postural dual task as well as younger adults in stable conditions. However, when the complexity of the postural task is increased by dynamic conditions (surface and surround), performance in postural, concurrent, or both tasks is more affected in older relative to young adults. In light of neuroimaging studies and new conceptual frameworks, these results demonstrate an age-related increase of controlled processing of standing associated with greater intermittent adjustments.

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Phase-dependent reflex modulation was studied by recording the electromyographic (EMG) responses in ankle flexors (Tibialis Anterior, TA) and extensors (Gastrocnemius Medialis, GM and Soleus, SOL) to a 20 ms train of electrical pulses, applied to the tibial or sural nerve at the ankle, in human volunteers walking on a treadmill at 4 km/h. For low intensity stimuli (i.e. 1.6 times perception threshold), given during the swing phase, the most common response was a suppression of the TA activity with a latency of 67 to 118 ms. With high intensity of stimulation (i.e. 2.8 x T), a facilitatory response appeared in TA with a latency of 74 ms. This latter response was largest during the middle of the swing phase, when it was correlated with exaggerated ankle dorsiflexion. The TA reflex amplitude was not a simple function of the level of spontaneous ongoing activity. During stance, TA responses were small or absent and accompanied by a suppression of the GM activity with a latency ranging from 62 to 101 ms. A few subjects showed an early facilitatory, instead of a suppressive, GM response (88 to 136 ms latency). They showed a phase-dependent reflex reversal from a dominant TA response during swing to a facilitatory GM response with an equivalent latency during stance. The GM facilitation occurred exclusively during the early stance phase and habituated more than the TA responses. It is concluded that phase-dependent gating of reflexes occurs in ankle muscles of man, but only when vigorous extensor reflexes are present. More commonly, a phase-dependent modulation is seen, both of facilitatory and suppressive responses.

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