Combinatorial Effects of Transcutaneous Spinal Stimulation and Task-Specific Training to Enhance Hand Motor Output after Paralysis

Abstract

Background: Despite the positive results in upper limb (UL) motor recovery after using electrical neuromodulation in individuals after cervical spinal cord injury (SCI) or stroke, there has been limited exploration of potential benefits of combining task-specific hand grip training with transcutaneous electrical spinal stimulation (TSS) for individuals with UL paralysis.

Objectives: This study investigates the combinatorial effects of task-specific hand grip training and noninvasive TSS to enhance hand motor output after paralysis.

Methods: Four participants with cervical SCI classified as AIS A and B and two participants with cerebral stroke were recruited in this study. The effects of cervical TSS without grip training and during training with sham stimulation were contrasted with hand grip training with TSS. TSS was applied at midline over cervical spinal cord. During hand grip training, 5 to 10 seconds of voluntary contraction were repeated at a submaximum strength for approximately 10 minutes, three days per week for 4 weeks. Signals from hand grip dynamometer along with the electromyography (EMG) activity from UL muscles were recorded and displayed as visual feedback.

Results: Our case study series demonstrated that combined task-specific hand grip training and cervical TSS targeting the motor pools of distal muscles in the UL resulted in significant improvements in maximum hand grip strength. However, TSS alone or hand grip training alone showed limited effectiveness in improving grip strength.

Conclusion: Task-specific hand grip training combined with TSS can result in restoration of hand motor function in paralyzed upper limbs in individuals with cervical SCI and stroke.

Keywords: hand motor function; spinal cord injury; spinal cord stimulation; stroke.

Figure 1.

Study design and experimental setup. (A) Study design included 12 training sessions incorporating grip training performed in the presence of sham or actual transcutaneous electrical spinal stimulation (TSS) interventions performed as well as four tests and 2 weeks of washout period. The circles in the figure represent the training sessions and are labeled with numbers. (B) The experimental setup involved the use of a handgrip dynamometer and visual feedback from electromyography (EMG) activities and grip force signals during training. The cathode electrode was placed between the C6 and T1 vertebrae to target forearm and hand muscles based on the motor pools distributions.

Figure 3.

Changes in forearm muscle electromyography (EMG) activities and grip strength during task-specific hand grip training. (A) Representative participant in spinal cord injury, SCC-021, during hand grip training within the session of sham transcutaneous electrical spinal stimulation (TSS) and actual TSS. (B) Representative participant in stroke, STL-007, during hand grip training within the session of actual TSS. During actual TSS intervention, stimulation waveforms consisted of biphasic 0.5 ms pulses, at a frequency of 30 Hz, delivered over cervical spinal cord between the C6 and T1 spinous processes.

Figure 2.

Changes in grip strength in paralyzed individuals. (A) Waveforms of grip strength and forearm electromyography (EMG) activities with or without task-specific hand grip training combined with sham transcutaneous electrical spinal stimulation (TSS) and actual TSS intervention in participants after cervical spinal cord injury and stroke who showed improvement across the testing sessions. (B) Changes in hand grip strength with or without task-specific hand grip training combined with sham TSS and actual TSS in four participants with cervical SCI classified as AIS A and B and two participants with cerebral stroke. Box range was set as percentage 25% to 75%, white circle in boxplots present the median values, and whiskers indicate the 95% confidence interval. Significant differences across the sessions are indicated with horizontal lines. Dotted lines: p < .05; dashed lines: p < .01; solid lines: p < .001.