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* To whom correspondence should be addressed. E-mail: y.ivanenko{at}hsantalucia.it.
FMRI imaging of motoneuron activity in the human spinal cord is still in its infancy and it will remain difficult to apply to walking. Here we present a viable alternative for documenting the spatiotemporal maps of a-motorneuron (MN) activity in the human spinal cord during walking, similar to the method recently reported for the cat (Yakovenko et al. 2002). We recorded electromyographic (EMG) activity from 16-32 ipsilateral limb and trunk muscles in 13 healthy subjects walking on a treadmill at different speeds (1-7 km/h), and mapped the recorded patterns onto the spinal cord in approximate rostrocaudal locations of the motoneuron pools. This approach can provide information about pattern generator output during locomotion in terms of segmental control rather than in terms of individual muscle control. A striking feature we found is that nearly every spinal segment undergoes at least 2 cycles of activation in the step cycle, thus supporting the idea of half-center oscillators controlling MN activation at any segmental level. The resulting spatiotemporal map patterns seem highly stereotyped over the range of walking speeds studied, though there were also some systematic redistributions of MN-activity with speed. Bursts of MN activity were either temporally aligned across several spinal segments or switched between different segments. For example, the center of mass of MN activity in the lumbosacral levels generally shifted from rostral to caudal positions in 2 cycles for each step, revealing 4 major activation foci, 2 in the upper lumbar segments and 2 in the sacral segments. The results are consistent with the presence of at least 2 and possibly more pattern generators controlling the activation of lumbosacral MNs.
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