Influence of different stimulation frequencies on power output and fatigue during FES-cycling in recently injured SCI people

This study investigated whether power output during 30 min sessions of functional electrical stimulation (FES)-cycling can be increased by using stimulation frequencies higher than 30 Hz. The stimulation frequencies of FES-cycling training sessions of 19 recently injured para- and tetraplegics were randomly set at 30, 50, or 60 Hz and power output (PO) was measured continually. The mean PO of the 30 min, the PO of the last minute of each session, and the minimum PO were significantly greater at 60 and 50 Hz than at 30 Hz (ANOVA without cross-product). A 19% and 25% higher mean PO was reached at 50 and 60 Hz, respectively, compared to 30 Hz. The PO of the last minute of each session was almost always higher than the mean PO of the whole session and also higher at higher frequencies, which indicates that no muscle fatigue could be detected in 30 min FES-cycling at any of the tested frequencies.

I. Introduction

It is commonly known that the power output achieved by functional electrical stimulation (FES)-cycling is very small (10–25 W) [1]– [3] and many times smaller than the power output reached in voluntary cycling. The main reason for this reduced power output in FES-cycling seems to lie in the nature of the electrical stimulation and not in the physical adaptations to the spinal-cord injury (SCI), as shown in a study by Kjær et al. [4], wherein able-bodied subjects with epidural lumbar anaesthesia produced similarly low power outputs as SCI subjects. Possible reasons for the low power output may be the timing of the stimulation during crank rotation, nonoptimal stimulation parameters, and the fact that the contributing muscles can not be stimulated individually with surface stimulation but only as muscle groups resulting in coarse movements. If mobile cycling is to be used for recreation, mobility, or fitness, it is desirable to achieve higher power output. This has been a topic of little interest so far, possibly due to the fact that mainly stationary cycling systems have been used. More attention has been paid to the optimization of stimulation angles [5], and seating position [6], but very little to the choice of stimulation parameters.