Metabolic efficiency of volitional and electrically stimulated cycling in able-bodied subjects

This study compared the metabolic efficiency of volitional cycling and functional-electrical-stimulation (FES) cycling within a subject group of able-bodied individuals, with a view to further elucidating the mechanisms underlying the low efficiency of FES cycling. Previous studies estimated the metabolic efficiency of volitional cycling and anaesthetised FES cycling in able-bodied subjects, and of FES cycling in subjects paralysed by spinal cord injury. The rationale for the experimental model chosen here, i.e. non-anaesthetised able-bodied subjects, was that this lies between normal cycling and paralysed cycling: while using FES, this group has artificial muscle activation and timing like the paralysed group; but it does not have disrupted sensory feedback and vasomotor control; this measurement therefore allows delineation of the magnitude of reduction in metabolic efficiency resulting from: (i) the FES itself and (ii) paralysis (where there is disrupted sensory feedback and vasomotor control). Furthermore, we used the same methods employed previously for estimation of metabolic efficiency in subjects with motor- and sensory-complete paraplegia. The mean metabolic efficiency of volitional cycling was found to be 29.8% and that of FES cycling was 16.4% (n=11). The low efficiency of FES cycling can be explained in large part by the crude timing of muscle activation and by non-physiological muscle fibre recruitment. In FES cycling with paralysed subjects, disrupted sensory feedback and vasomotor control may play a further, albeit smaller, role in the reduced efficiency.