We have demonstrated that well-controlled, fatigue-resistant skeletal muscle activation can be achieved using interleaved multielectrode stimulation of motoneurons in the peripheral nerves of the cat hind limb. In this poster, we describe experiments directed to the production of normal-appearing sit-to-stance and stance behaviors in an anesthetized cat. We are using intrafascicular multielectrode stimulation of the nerves innervating hip, knee, and ankle muscles. Pulse-width modulated stimuli are delivered using Utah Slanted Electrode Arrays implanted in either or all of the femoral, sciatic, or muscular branch of the sciatic nerves. Work is ongoing in a number of subprojects.

Automated mapping of implanted electrodes to excited muscles. We use a custom 1100-channel, constant-voltage, pulse-width modulated neural stimulator to activate the muscles of the hind limb. The stimulator is computer controlled, which allows the automated mapping of stimulated electrodes to specific muscles in the hind limb. This system also allows the determination of force-recruitment curves evoked by stimulation via each implanted electrode, and the quantification of overlap between motor unit pools activated by groups of electrodes.

Use of force vectors at the foot to map implanted electrodes to excited muscles. In a paralyzed subject, the use of EMGs in the mapping process described above would be tedious and could result in infection. We are investigating the measurement of force vectors, sensed with a force plate under the foot of an implanted and anesthetized cat, to achieve non-invasive mapping of electrodes to extensors and flexors of the hip, knee, and ankle.

Production of ripple-free ballistic forces using interleaved, multielectrode stimulation. Muscle twitch forces evoked by different electrodes in the implanted array have different kinetics and amplitudes. Thus, the amplitudes of the stimuli delivered in an interleaved fashion via an implanted array must be adjusted to produce a low-ripple, tonic contraction at a variety of force levels. We have developed algorithms to achieve such ballistic forces.

Production of pursuit tracking of continuously varying force targets using interleaved, multielectrode stimulation. We are building adaptive feedback controllers that provide optimized ripple reduction and tracking of time-varying target forces. To date, this is done using computer models. These models will be tested in the anesthetized cat.

These experiments are directed at producing natural-appearing, fatigue-resistant stance in wheelchair-bound subjects.

Topic: Sensory-Motor and Functional Electrical Stimulation.

Supported by NIH RO1-NS-039677.