Effect of NMES on Balance and Fall Risk in Chronic Stroke

Functional impairment after a stroke often includes slowed gait velocity and increased fall risk attributed to foot drop (the inability to dorsiflex the ankle during the swing phase of gait) and lower limb muscle weakness. Damage in the motor cortex or corticospinal tract often results in significant, persistent distal muscle weakness, including the sensorimotor control of the ankle joint, typically because of a combination of weakness of the agonist ankle dorsiflexor muscles and spasticity of the antagonist plantar flexor muscle. This results in slower and abnormal gait which leads to gait compensation strategies such as hip hitching, excess circumduction during gait, reduced foot clearance, and high energy expenditure, all of which are factors which could increase the risk of falls in individuals with stroke.

Functional electrical stimulation (FES) corresponds to the application of an electric field across the motor neurons of a muscle to induce an artificial, involuntary contraction to perform a functional movement. Numerous benefits of FES have been reported throughout literature such as increased muscle mass, increased bone mineral density, and improved cardiovascular parameters, among others. Previous studies in which authors assessed the effect of FES on individuals with stroke have demonstrated that the common peroneal nerve stimulates the tibialis anterior muscle to produce foot dorsiflexion during the swing phase of the gait cycle and reduces foot drop by facilitating increased voluntary muscle activity, which together improves the quality and symmetry of gait. Additionally, others studies showed that FES improves walking speed and energy expenditure in individuals with stroke.

In the last years, FES systems have been used as neuroprosthetic devices in rehabilitative interventions such as gait training. Stimulator triggers, implemented to control stimulation delivery, range from open- to closed-loop controllers.12 Finite-state controllers trigger stimulators when specific conditions are met and utilize preset sequences of stimulation. Thus, wearable sensors provide the necessary input to differentiate gait phases during walking and trigger stimulation to specific muscles.13 This technology has been largely used to improve gait parameters in stroke participants, however, it has not been well described how this technology could help stroke participants during the loss of balance or during reactive balance.

On the other hand, the literature suggests that a direct transcortical loop does not trigger the initial phase of postural responses to external perturbations, but it seems likely that the cerebral cortex becomes involved in the later phases of the reactive response.14 Thus, given that postural response lasts for many hundreds of milliseconds, it may be that the brainstem or spinal cord circuits initiate a response, and then the response subsequently becomes modified by cortical circuits during its later phases.15 In this context, the effect of peripheral stimulation to the muscles involved in the reactive response to an unexpected external perturbation on recovery performance has not yet been described.

This project aims to describe whether a specific pattern of lower limb muscle stimulation could modify the recovery response after an unexpected perturbation in the form of a slip and/or trip in individuals with stroke. Also, this project aims to examine if a specific pattern of lower limb muscle stimulation provided by FES can modify kinematic and spatio-temporal gait parameters during a standardized walking test under different sensory conditions.