Interaction of transcutaneous spinal stimulation and transcranial magnetic stimulation in human leg muscles.

Transcutaneous spinal stimulation is a noninvasive method that can activate dorsal and/or ventral roots depending on the location and intensity of stimulation. Reflex root-evoked potentials (REPs) were studied in muscles that traditionally evoke large (soleus) and small H-reflexes (tibialis anterior), as well as muscles where H-reflexes are difficult to study (hamstrings). This study characterizes the interaction of the REP and the motor-evoked potential (MEP). Transcranial magnetic stimulation (TMS) delivered 11-25 ms before spinal stimulation resulted in more than linear summation of the two responses. Because of overlap, the modulation was quantified after subtracting the contribution of the conditioning MEP or REP. At rest, the mean-rectified soleus response was facilitated by up to ~250 µV (21-times the MEP or 161% of the REP). The increases were more reliable during a voluntary contraction (up to ~300 µV, 517% of the MEP or 181% of the REP). At the 13-ms interval, the mean-rectified response in the pre-contracted hamstrings was increased by 227% of the MEP or 300% of the REP. In some subjects, TMS could also eliminate the post-activation depression produced using two spinal stimuli, confirming that the interaction can extend to presynaptic spinal neurons. The spatiotemporal facilitation in tibialis anterior was not significant. However, the large MEP was facilitated when the spinal stimulus preceded TMS by 100-150 ms, presumably because of rebound excitation. These strong interactions may be important for inducing motor plasticity and improved training procedures for recovery after neurological damage.

Effect of a foot-drop stimulator and ankle-foot orthosis on walking performance after stroke: a multicenter randomized controlled trial.

BACKGROUND: Studies have demonstrated the efficacy of functional electrical stimulation in the management of foot drop after stroke. OBJECTIVE: To compare changes in walking performance with the WalkAide (WA) foot-drop stimulator and a conventional ankle-foot orthosis (AFO). METHODS: Individuals with stroke within the previous 12 months and residual foot drop were enrolled in a multicenter, randomized controlled, crossover trial. Subjects were assigned to 1 of 3 parallel arms for 12 weeks (6 weeks/device): arm 1 (WA-AFO), n = 38; arm 2 (AFO-WA), n = 31; arm 3 (AFO-AFO), n = 24. Primary outcomes were walking speed and Physiological Cost Index for the Figure-of-8 walking test. Secondary measures included 10-m walking speed and perceived safety during this test, general mobility, and device preference for arms 1 and 2 for continued use. Walking tests were performed with (On) and without a device (Off) at 0, 3, 6, 9, and 12 weeks. RESULTS: Both WA and AFO had significant orthotic (On-Off difference), therapeutic (change over time when Off), and combined (change over time On vs baseline Off) effects on walking speed. An AFO also had a significant orthotic effect on Physiological Cost Index. The WA had a higher, but not significantly different therapeutic effect on speed than an AFO, whereas an AFO had a greater orthotic effect than the WA (significant at 12 weeks). Combined effects on speed after 6 weeks did not differ between devices. Users felt as safe with the WA as with an AFO, but significantly more users preferred the WA. CONCLUSIONS: Both devices produce equivalent functional gains.

An implantable neural stimulator for intraspinal microstimulation.

This paper reports on a wireless stimulator device for use in animal experiments as part of an ongoing investigation into intraspinal stimulation (ISMS) for restoration of walking in humans with spinal cord injury. The principle behind using ISMS is the activation of residual motor-control neural networks within the spinal cord ventral horn below the level of lesion following a spinal cord injury. The attractiveness to this technique is that a small number of electrodes can be used to induce bilateral walking patterns in the lower limbs. In combination with advanced feedback algorithms, ISMS has the potential to restore walking for distances that exceed that produced by other types of functional electrical stimulation. Recent acute animal experiments have demonstrated the feasibility of using ISMS to produce the coordinated walking patterns. Here we described a wireless implantable stimulation system to be used in chronic animal experiments and for providing the basis for a system suitable for use in humans. Electrical operation of the wireless system is described, including a demonstration of reverse telemetry for monitoring the stimulating electrode voltages.

Restoring stepping after spinal cord injury using intraspinal microstimulation and novel control strategies.

The overall objective of this project is to develop a feedback-driven intraspinal microstimulation (ISMS) system. We hypothesize that ISMS will enhance the functionality of stepping by reducing muscle fatigue and producing synergistic movements by activating neural networks in the spinal cord. In the present pilot study, the controller was tested with ISMS and external sensors (force plates, gyroscopes, and accelerometers). Cats were partially supported in a sling and bi-laterally stepped overground on a 4-m instrumented walkway. The walkway had variable friction. Limb angle was controlled to within 10° even in the presence of variable friction. Peak ground reaction forces in each limb were approximately 12% of body weight (12.5% was full load bearing in this experimental setup); rarely, the total supportive force briefly decreased to as low as 4.1%. Magnetic resonance images were acquired of the excised spinal cord and the implanted array. The majority of electrodes (75%) were implanted successfully into their target regions. This represents the first successful application of ISMS for overground walking.

Does functional electrical stimulation for foot drop strengthen corticospinal connections?

BACKGROUND: Long-term use of a foot-drop stimulator applying functional electrical stimulation (FES) to the common peroneal nerve improves walking performance even when the stimulator is off. This "therapeutic" effect might result from neuroplastic changes. OBJECTIVE: To determine the effect of long-term use of a foot-drop stimulator on residual corticospinal connections in people with central nervous system disorders. METHODS: Ten people with nonprogressive disorders (eg, stroke) and 26 with progressive disorders (eg, multiple sclerosis) used a foot-drop stimulator for 3 to 12 months while walking in the community. Walking performance and electrophysiological variables were measured before and after FES use. From the surface electromyogram of the tibialis anterior muscle, we measured the following: (1) motor-evoked potential (MEP) from transcranial magnetic stimulation over the motor cortex, (2) maximum voluntary contraction (MVC), and (3) maximum motor wave (M(max)) from stimulating the common peroneal nerve. RESULTS: After using FES, MEP and MVC increased significantly by comparable amounts, 50% and 48%, respectively, in the nonprogressive group and 27% and 17% in the progressive group; the changes were positively correlated (R(2) = .35; P < .001). Walking speed increased with the stimulator off (therapeutic effect) by 24% (P = .008) and 7% (P = .014) in the nonprogressive and progressive groups, respectively. The changes in M(max) were small and not correlated with changes in MEP. CONCLUSIONS: The large increases in MVC and MEP suggest that regular use of a foot-drop stimulator strengthens activation of motor cortical areas and their residual descending connections, which may explain the therapeutic effect on walking speed.

Long-term therapeutic and orthotic effects of a foot drop stimulator on walking performance in progressive and nonprogressive neurological disorders.

BACKGROUND: Stimulators applying functional electrical stimulation (FES) to the common peroneal nerve improve walking with a foot drop, which occurs in several disorders. OBJECTIVE: To compare the orthotic and therapeutic effects of a foot drop stimulator on walking performance of subjects with chronic nonprogressive (eg, stroke) and progressive (eg, multiple sclerosis) disorders. METHODS: Subjects with nonprogressive (41) and progressive (32) conditions used a foot drop stimulator for 3 to 12 months while walking in the community. Walking speed was measured with a 10-m test and a 4-minute figure-8 test; physiological cost index (PCI) and device usage were also measured. The subjects were tested with FES on and off (orthotic effect) before and after (therapeutic effect) stimulator use. RESULTS: After 3 months of FES use, the nonprogressive and progressive groups had a similar, significant orthotic effect (5.0% and 5.7%, respectively, P < .003; percentage change in mean values) and therapeutic effect with FES off (17.8% and 9.1%, respectively, P < .005) on figure-8 walking speed. Overall, PCI showed a decreasing trend (P = .031). The therapeutic effect on figure-8 speed diverged later between both groups to 28.0% (P < .001) and 7.9% at 11 months. The combined therapeutic plus orthotic effect on figure-8 speed at 11 months was, respectively, 37.8% (P < .001) and 13.1% (P = .012); PCI decreased 18.2% (P = .038) and 6.5%, respectively. CONCLUSIONS: Subjects with progressive and nonprogressive disorders had an orthotic benefit from FES up to 11 months. The therapeutic effect increased for 11 months in nonprogressive disorders but only for 3 months in progressive disorders. The combined effect remained significant and clinically relevant.

Spinal reflexes in ankle flexor and extensor muscles after chronic central nervous system lesions and functional electrical stimulation.

OBJECTIVE: Spinal reciprocal inhibitory and excitatory reflexes of ankle extensor and flexor muscles were investigated in ambulatory participants with chronic central nervous system (CNS) lesions causing foot drop as a function of time after lesion and stimulator use. METHODS: Thirty-nine participants with progressive (eg, secondary progressive MS) and 36 with generally nonprogressive (eg, stroke) conditions were studied. The tibialis anterior (TA) and soleus maximum H-reflex/M-wave (Hmax/Mmax) ratios and maximum voluntary contractions (MVC) were measured and compared with those in age-matched control participants. Reciprocal inhibition was measured as a depression of the ongoing electromyographic (EMG) activity produced by antagonist muscle-nerve stimulation. RESULTS: Participants with CNS lesions had significantly higher soleus Hmax/Mmax ratios than control participants, and reduced voluntary modulation of the reflexes occurred in both muscles. Reciprocal inhibition of soleus from common peroneal (CP) nerve stimulation was not significantly different from controls in either group. Inhibition of the TA by tibial nerve stimulation decreased and was eventually replaced by excitation in participants with nonprogressive disorders. No significant change occurred in progressive disorders. Use of a foot drop stimulator increased the TA, but not the soleus MVC overall. H-reflexes only showed small changes. Reciprocal inhibition of the TA increased considerably, while that of the soleus muscle decreased toward control values. CONCLUSIONS: Disorders that produce foot drop also produce reflex changes, some of which only develop over a period of years or even decades. Regular use of a foot drop stimulator strengthens voluntary pathways and changes some reflexes toward control values. Thus, stimulators may provide multiple benefits to people with foot drop.

A multicenter trial of a footdrop stimulator controlled by a tilt sensor.

OBJECTIVES: To test the efficacy and acceptance of a footdrop stimulator controlled by a tilt sensor. METHODS: A nonrandomized, test-retest study of 26 subjects with footdrop of more than 1 year's duration, resulting from various central nervous system disorders, was performed in 4 centers for at least 3 months. Speed of walking in a straight line, speed around a figure of 8, and physiological cost index (PCI) were measured with and without the device. Hours/day and steps/day using the device were recorded. RESULTS: All but 2 subjects used the tilt sensor at home, rather than a foot switch. Walking speed increased by 15% after 3 months (n = 26; P < 0.01), 32% after 6 months (n = 16; P < 0.01), and 47% after 12 months (n = 8; P < 0.05), while PCI decreased. The number of steps taken per day of use increased significantly over time, and increased speed was directly correlated with usage. Walking speed also increased with the stimulator off, but to a lesser extent, indicating a training effect. Subject feedback from a questionnaire indicated satisfaction with the stimulator. CONCLUSIONS: Both efficacy and acceptance of the stimulator were good in a population of subjects with chronic footdrop.

BIONic WalkAide for correcting foot drop.

The goal of this study was to test the feasibility and efficacy of using microstimulators (BIONs) to correct foot drop, the first human application of BIONs in functional electrical stimulation (FES). A prototype BIONic foot drop stimulator was developed by modifying a WalkAide2 stimulator to control BION stimulation of the ankle dorsiflexor muscles. BION stimulation was compared with surface stimulation of the common peroneal nerve provided by a normal WalkAide2 foot drop stimulator. Compared to surface stimulation, we found that BION stimulation of the deep peroneal nerve produces a more balanced ankle flexion movement without everting the foot. A three-dimensional motion analysis was performed to measure the ankle and foot kinematics with and without stimulation. Without stimulation, the toe on the affected leg drags across the ground. The BIONic WalkAide elevates the foot such that the toe clears the ground by 3 cm, which is equivalent to the toe clearance in the unaffected leg. The physiological cost index (PCI) was used to measure effort during walking. The PCI is high without stimulation (2.29 +/- 0.37; mean +/- S.D.) and greatly reduced with surface (1.29 +/- 0.10) and BION stimulation (1.46 +/- 0.24). Also, walking speed is increased from 9.4 +/- 0.4 m/min without stimulation to 19.6 +/- 2.0 m/min with surface and 17.8 +/- 0.7 m/min with BION stimulation. We conclude that functional electrical stimulation with BIONs is a practical alternative to surface stimulation and provides more selective control of muscle activation.

Movements elicited by electrical stimulation of muscles, nerves, intermediate spinal cord, and spinal roots in anesthetized and decerebrate cats.

Electrical stimulation offers the possibility of restoring motor function of paralyzed limbs after spinal-cord injury or stroke, but few data are available to compare possible sites of stimulation, such as muscle, nerve, spinal roots, or spinal cord. The aim of this study was to establish some characteristics of stimulation at these sites in the anesthetized and midcollicular decerebrate cat. The hind limb was constrained to move in the sagittal plane against a spring load. Ventral-root stimulation only produced movements down and back; the direction moved systematically backward the more caudal the stimulated roots. In contrast, dorsal-root stimulation only produced movements up and forward. Thus, neither method alone could produce the full range of normal movements. Muscle, nerve, and intraspinal stimulation within the intermediate regions of the gray matter generated discrete, selective movements in a wide range of directions. Muscle stimulation required an order of magnitude more current. Single microwire electrodes located in the spinal gray matter could activate a synergistic group of muscles, and generally had graded recruitment curves, but the direction of movement occasionally changed abruptly as stimulus strength increased. Nerve stimulation produced the largest movements against the spring load (>80% of the passive range of motion) and was the most reproducible from animal to animal. However, recruitment curves with nerve stimulation were quite steep, so fine control of movement might be difficult. The muscle, nerve, and spinal cord all seem to be feasible sites to restore motor function. The pros and cons from this study may be helpful in deciding the best site for a particular application, but further tests are needed in the chronically transected spinal cord to assess the applicability of these results to human patients.

Electrical stimulation for therapy and mobility after spinal cord injury.

This article reviews the use of therapeutic and functional electrical stimulation in subjects after a spinal cord injury (SCI). Muscles become much weaker and more fatigable, while bone density decreases dramatically after SCI. Therapeutic stimulation of paralyzed muscles for about 1 h/day can reverse the atrophic changes and markedly increase muscle strength and endurance as well as bone density. Functional electrical stimulation can also improve the speed and efficiency of walking in people with an incomplete SCI. Finally, a modified wheelchair is described in which electrical stimulation or residual voluntary activation of leg muscles can produce movements of a footrest that is coupled to the wheels. The wheelchair can provide greater mobility and fitness to persons who are not functional walkers and currently use their arms to propel a wheelchair.