The effects of FES cycling combined with virtual reality racing biofeedback on voluntary function after incomplete SCI: a pilot study

Abstract

Background: Functional Electrical Stimulation (FES) cycling can benefit health and may lead to neuroplastic changes following incomplete spinal cord injury (SCI). Our theory is that greater neurological recovery occurs when electrical stimulation of peripheral nerves is combined with voluntary effort. In this pilot study, we investigated the effects of a one-month training programme using a novel device, the iCycle, in which voluntary effort is encouraged by virtual reality biofeedback during FES cycling.

Methods: Eleven participants (C1-T12) with incomplete SCI (5 sub-acute; 6 chronic) were recruited and completed 12-sessions of iCycle training. Function was assessed before and after training using the bilateral International Standards for Neurological Classification of SCI (ISNC-SCI) motor score, Oxford power grading, Modified Ashworth Score, Spinal Cord Independence Measure, the Walking Index for Spinal Cord Injury and 10 m-walk test. Power output (PO) was measured during all training sessions.

Results: Two of the 6 participants with chronic injuries, and 4 of the 5 participants with sub-acute injuries, showed improvements in ISNC-SCI motor score > 8 points. Median (IQR) improvements were 3.5 (6.8) points for participants with a chronic SCI, and 8.0 (6.0) points for those with sub-acute SCI. Improvements were unrelated to other measured variables (age, time since injury, baseline ISNC-SCI motor score, baseline voluntary PO, time spent training and stimulation amplitude; p > 0.05 for all variables). Five out of 11 participants showed moderate improvements in voluntary cycling PO, which did not correlate with changes in ISNC-SCI motor score. Improvement in PO during cycling was positively correlated with baseline voluntary PO (R2 = 0.50; p < 0.05), but was unrelated to all other variables (p > 0.05). The iCycle was not suitable for participants who were too weak to generate a detectable voluntary torque or whose effort resulted in a negative torque.

Conclusions: Improved ISNC-SCI motor scores in chronic participants may be attributable to the iCycle training. In sub-acute participants, early spontaneous recovery and changes due to iCycle training could not be distinguished. The iCycle is an innovative progression from existing FES cycling systems, and positive results should be verified in an adequately powered controlled trial.

Trial registration: ClinicalTrials.gov, NCT03834324. Registered 06 February 2019 - Retrospectively registered, https://ichgcp.net/clinical-trials-registry/NCT03834324. Protocol V03, dated 06.08.2015.

Keywords: Biofeedback; Cycling; Functional electrical stimulation; ISNC-SCI motor score; Spinal cord injury; Virtual reality.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

a Block Diagram of iCycle. The cadence control sets the voltage applied to the DC motor. The measured torque-cadence curves are shown in (d); the stiffness was made intentionally low to avoid musculo-skeletal damage. When the torque is positive, the power supply is absorbing power which is dissipated in a heat sink. Stimulation was applied during alternate revolutions of the pedals and the output of the torque transducer is averaged over revolutions without stimulation to give the signal called effort. For each of the six stimulation channels, the output was gated so that the 30 Hz pulses were applied from a switch-on to a switch-off angle, which could be set for each participant. A commercial virtual reality cycling game was adapted for the iCycle. The hardware interface for this game has an output which is approximately the slope of the road in the rolling scenery. This slope is subtracted from the effort and the difference frequency-modulated by a voltage-controlled oscillator (VCO) before being fed back as the wheelspeed. There is also a pulse signal for every revolution of the crankshaft which is switched on to start the game and synchronises the avatar’s pedals with the real pedals. The VCO has an S-shaped characteristic, shown in (c), which limits the avatar’s speed to 0–12 m/s. The controls labelled effort, offset and slope are used to set the working range as indicated below the graph. The feint lines in (d) are the function where V is the voltage applied to the motor, T is the torque (N.m) and Ω is cadence (r.p.m.). The iCycle can deliver or absorb 30 W. b The iCycle is used with participants sitting in their own wheelchairs in front of the VR screen

Fig. 2

a Time schedule of outcome measures and intervention (b) CONSORT flow diagram

Fig. 3

Change in ISNC-SCI motor scores at baseline (B), end of training (EOT) and follow up (FU). Participant number is provided beside each data set. Participants with chronic injuries are shown in the left panel and those with sub-acute injuries are shown on the right. The graph shows the changes on a scale from 0 (complete paralysis) to 100 (able-bodied)

Fig. 4

Change in ISNC-SCI motor scores at follow up (FU) relative to baseline (B), plotted against (a) age (years) at time of enrolment to the study, (b) time since injury (years), (c) initial ISNC-SCI score, (d) initial power output (watts), (e) cycling duration per session (min), (f) average stimulation amplitude (stimulator setting). Participants with chronic injuries are shown in black, those with sub-acute injuries in grey

Fig. 5

Raw plots from two participants during an iCycle session. The plots show torque data from the stimulated (yellow) and non-stimulated (orange) revolutions, and the game on signal (blue traces). The three velodrome laps (V1, V2, V3) and free cycling (route) are indicated. The upper panel represents a typical plot when no negative torque was recorded, and the lower panel represents a typical plot when negative torque occurred at the onset of voluntary effort

Fig. 6

Power output from the stimulated (black) and non-stimulated (grey) revolutions during one velodrome lap for each participant across sessions 1–12

Fig. 7

Change in power output (regression slopes taken from Fig. 6) plotted against (a) initial ISNC-SCI score, (b) initial power output (watts), (c) cycling duration per session (min). Participants with chronic injuries are shown in black, those with sub-acute injuries in grey