Effects of intramuscular trunk stimulation on manual wheelchair propulsion mechanics in 6 subjects with spinal cord injury

Abstract

Objective: To quantify the effects of stabilizing the paralyzed trunk and pelvis with electrical stimulation on manual wheelchair propulsion.

Design: Single-subject design case series with subjects acting as their own concurrent controls.

Setting: Hospital-based clinical biomechanics laboratory.

Participants: Individuals (N=6; 4 men, 2 women; mean age ± SD, 46 ± 10.8y) who were long-time users (6.1 ± 3.9y) of implanted neuroprostheses for lower extremity function and had chronic (8.6 ± 2.8y) midcervical- or thoracic-level injuries (C6-T10).

Interventions: Continuous low-level stimulation to the hip (gluteus maximus, posterior adductor, or hamstrings) and trunk extensor (lumbar erector spinae and/or quadratus lumborum) muscles with implanted intramuscular electrodes.

Main outcome measures: Pushrim kinetics (peak resultant force, fraction effective force), kinematics (cadence, stroke length, maximum forward lean), and peak shoulder moment at preferred speed over 10-m level surface; speed, pushrim kinetics, and subjective ratings of effort for level 100-m sprints and up a 30.5-m ramp of approximately 5% grade.

Results: Three of 5 subjects demonstrated reduced peak resultant pushrim forces (P≤.014) and improved efficiency (P≤.048) with stimulation during self-paced level propulsion. Peak sagittal shoulder moment remained unchanged in 3 subjects and increased in 2 others (P<.001). Maximal forward trunk lean also increased by 19% to 26% (P<.001) with stimulation in these 3 subjects. Stroke lengths were unchanged by stimulation in all subjects, and 2 showed extremely small (5%) but statistically significant increases in cadence (P≤.021). Performance measures for sprints and inclines were generally unchanged with stimulation; however, subjects consistently rated propulsion with stimulation to be easier for both surfaces.

Conclusions: Stabilizing the pelvis and trunk with low levels of continuous electrical stimulation to the lumbar trunk and hip extensors can positively impact the mechanics of manual wheelchair propulsion and reduce both perceived and physical measures of effort.

Keywords: Electrical stimulation; FEF; GME; MANOVA; Neural prostheses; Posture; ROM; Rehabilitation; SCI; Spinal cord injuries; Torso; URS; Usability Rating Scale; fraction of effective force; gross mechanical efficiency; multivariate analysis of variance; range of motion; spinal cord injury.

Figure 1

Experimental set-up for assessing effects of stimulating the trunk and hip extensors on level propulsion at self-selected speeds with instrumented pushrim and reflective markers for digital motion capture.

Figure 2

Average (±SD) peak resultant wheelrim force (a) and peak sagittal shoulder moment (b) with and without stimulation to the hip and trunk extensors for each subject during level self-paced propulsion. (* = statistically significant at p<0.05)

Figure 3

Average (±SD) stroke length (a) and cadence (b) with and without stimulation to the hip and trunk extensors for each subject during level self-paced propulsion (*=statistically significant at p<0.05).

Figure 4

Average (±SD) Fraction of Effective Force (a), and average maximal forward trunk lean (b) during level self-paced propulsion with and without stimulation for each subject (* = statistically significant at p<0.05)

Figure 5

Change in subjective perceptions of effort with activation of the hip and trunk muscles relative to propulsion without stimulation during (a) level sprints of 100m and (b) ascent of a 30.5m ramp. S5 could not complete the ramp maneuver either with or without stimulation. (* = statistically significant at p<0.05)