Characterisation of cardiorespiratory responses to electrically stimulated cycle training in paraplegia

Functional, electrically stimulated (FES) cycle training can improve the cardiorespiratory fitness of spinal cord injured (SCI) individuals, but the extent to which this can occur following high volume FES cycle endurance training is not known. The effect of training on aerobic endurance capacity, as determined by the appearance of respiratory gas exchange thresholds, is also unknown. The oxygen cost (O2 cost) of this type of exercise is about 3.5 times higher than that of volitional cycling, but the source of this inefficiency, and of the variation between subjects, has not yet been investigated. The electrical cost of FES cycling, measured as the stimulation charge required per Watt of power produced (stim/Pt), has neither been calculated nor investigated before. It is also not known whether a period of FES cycling can alter the O2 cost or the stim/Pt of this unique form of exercise. Additionally, the acute metabolic responses to prolonged, high intensity FES cycling after a 12-month period of high-volume training have not yet been characterised for this subject group. Accordingly, these parameters were investigated over the course of a 12-month homebased FES cycle training programme (up to 5 x 60 min per week) in 9 male and 2 female individuals with paraplegia. Outcomes were investigated using a novel, sensitive test bed that accounted for both internal and external power production (Pt). The test protocol permitted high resolution analyses of cycling power and metabolic thresholds, and a sensitive training dose-response analysis, to be performed for the first time in FES cycling. Efficiency estimates were calculated within a new theoretical framework that was developed for those with severe disability, and the stim/Pt was determined using a novel measure designed for this study. The current training programme resulted in significant improvements in cardiorespiratory fitness and peak cycling power, but only over the first 6 months when training was progressive. These improvements were positively related to the number of training hours completed during this time. It is not known whether the plateau in training response that was found after this time was due to a physiological limitation within the muscles, or to limitations in the current stimulation strategy and of the training protocol used. The efficiency of FES cycling was not significantly altered by any period of training. However, the stim/Pt of cycling had reduced over the first 6 months, probably as a result of a fibre hypertrophy within the stimulated motor units. The relationship that was found between variables after this time suggest that differences in the efficiency of FES cycling ii between subjects and over time related primarily to the stim/Pt, which determined the number of motor units recruited per unit of power produced, rather than to metabolic changes within the muscle itself. The aerobic gas exchange threshold (GET) was detected at an oxygen uptake (˙VO2) equivalent to that normally elicited by very gentle volitional exercise, even after training. This provided metabolic evidence of anaerobic fibre recruitment from the outset, as a consequence of the non-physiological motor unit recruitment pattern normally found during FES. The cardiorespiratory stress of training was found to be significantly higher than that elicited by the incremental work rate tests, calling into question the validity of using traditional, continuous incremental work rate tests for establishing the peak oxygen uptake (˙VO2peak) of FES cycling. The respiratory exchange dynamics observed over a 60 min training session were characterised and provide a unique insight into the remarkable aerobic and anaerobic capacity of trained paralytic muscles. For this particular highly motivated subject group, training for 60 min per day on more than 4 days of the week was demonstrated to be feasible, but not able to be sustained. Further work is therefore recommended to develop and to evaluate different stimulation patterns and parameters, loading strategies and training protocols. The aim would be to determine the optimal combination of training parameters that would maximise favourable training responses within a more viable and sustainable lower volume, training programme for this subject group. In conclusion, the outcomes of this multi-centre study have demonstrated the clinical significance of using otherwise redundant, paralytic leg muscles to perform functional, regular physical exercise to improve cardiorespiratory and musculoskeletal health after SCI. Additionally, the significant increases in cycling power and endurance that were achieved opened up new mobility and recreational possibilities for this group of individuals. These findings highlight the clinical and social relevance of regular FES cycle training, and the importance of integrating FES cycling into the lives of those affected by SCI. The early and judicious implementation of this form of exercise is strongly recommended for the maintenance of a healthy body, wellbeing, and of an active lifestyle after SCI.