A recent report by Gerasimenko et al in the Journal of Neuroscience (Gerasimenko Y, Gorodnichev R, Machueva E, et al. Novel and Direct Access to the Human Locomotor Spinal Circuitry. 2010;30:3700–3708) utilized spinal electromagnetic stimulation (SEMS) to activate spinal locomotor circuitry to induce involuntary locomotor-like movements in noninjured humans placed in a gravity-neutral position.
It is well known that in the absence of brain input, cyclic locomotor-like movements can still be generated in noninjured individuals placed in a gravity neutral position via the intrinsic circuitry of the lumbosacral spinal cord. The 2 most successful strategies for facilitating these movements were continuous vibration of the quadriceps and hamstring muscles, and stimulation of the peroneal or sural nerve. Intrinsic lumbosacral circuitries respond in a similar fashion to electrical stimulation of the lumbar spinal cord in the epidural space in animal models of spinal cord injury as well as in human individuals with clinically defined complete spinal cord injury.
This group hypothesized that electromagnetic stimulation would be able to induce similar stimulation of intrinsic lumbosacral circuitries to produce locomotor-like movements in a completely noninvasive fashion.
Investigators studied 65 normal individuals using a 2-legged suspension system where subjects were laid on their side and allowed to perform air stepping movements with large amplitudes. Subjects were instructed to not voluntarily intervene with movements induced by SEMS, and placebo control conditions were applied to insure that movements were a result of SEMS. Investigators experimented with several different stimulation frequencies, strengths, and vertebral locations to determine optimal stimulation conditions, and then paired SEMS with vibration to see if there was an additive effect.
Electrodes were placed on the rectus femoris and biceps femoris to record electrical muscle stimulation and activity, and a camera tracked movements of reflective markers on the hip, knee, and ankle joints. Electromyography (EMG) and video recordings were synchronized, and mean cycle period and amplitudes for hip, knee, and ankle displacements were determined. None of the study participants reported discomfort in response to either SEMS or vibration. Electromagnetic spinal cord stimulation impulses were generated by a coil Magstim Rapid stimulator placed on the skin over the spinal column.
Though response to SEMS was not universal, 10% of the study subjects showed consistent locomotor response. The most effective motor responses to SEMS were induced at the T11–12 vertebral level, corresponding to previous reports that the most effective epidural stimulation occurred at the L2 spinal segment. Leg movements began with hip extension followed by knee flexion, and cyclic movements were observed at 5, 10, and 20 Hz frequencies, with the latency of movement induction decreasing as SEMS frequency increased. The lowest SEMS frequency threshold for some EMG and movement initiation was found to be 3 Hz, and at this threshold, SEMS 40% strength was able to induce some rhythmic low amplitude EMG activity and movement. Predictably, mean cyclic periods lengths shortened and amplitude of movement at the hip joint increased with increasing SEMS strength. SEMS produced more consistent EMG activity and locomotor-like movement than vibration alone, but the combination of the two resulted in the most robust EMG bursting and movement amplitudes. Finally, in contrast to normal, voluntary locomotion, SEMS did not produce locomotion with tight linkage of the three joints that is typical in normal locomotion (similar cycle period lengths), and hip and knee movement amplitudes were also significantly smaller than normal.
This is a very early, preliminary study of the possible application of SEMS for spinal cord stimulation. The main drawback of the study is that it was conducted with normal individuals, not patients with spinal cord injury, where the greatest potential exists for clinical application of this type of modality. Because of the degree of neural remodeling that occurs in the spinal cord after injury, it is questionable whether or not SEMS could be efficacious for therapy, especially in patients with long standing injury where remodeling could have already disrupted the intrinsic spinal locomotor circuitry SEMS stimulates to induce movement. SEMS would probably be most effective in the setting of early intervention after injury, where stimulation could possibly help preserve spinal cord circuitry for basic function like ambulation. Despite this, SEMS still represents an exciting and extremely novel approach to spinal cord stimulation that could eventually have profound implications as a noninvasive clinical tool for injury assessment and rehabilitation.
JOHN H. CHI