External power output changes during prolonged cycling with electrical stimulation

Exercise Responses During Outdoor Versus Virtual Reality Indoor Arm+FES-Leg Cycling in Individuals with Spinal Cord Injury

Background: Virtual reality (VR)-enhanced indoor hybrid cycling in people with spinal cord injury (SCI) can be comparable to outdoor hybrid cycling. Method: Eight individuals with chronic thoracic-lesion SCI performed voluntary arm and electrically assisted leg cycling on a hybrid recumbent tricycle. Exercises were conducted outdoors and indoors incorporating VR technology in which the outdoor environment was simulated on a large flat screen monitor. Electrical stimulation was applied bilaterally to the leg muscle groups. Oxygen uptake (VO2), heart rate, energy expenditures, and Ratings of Perceived Exertion were measured over a 30-minute outdoor test course that was also VR-simulated indoors. Immediately after each exercise, participants completed questionnaires to document their perceptual-psychological responses. Results: Mean 30-minute VO2 was higher for indoor VR exercise (average VO2-indoor VR-exercise: 1316 ± mL/min vs. outdoor cycling: 1255 ± 53 mL/min; highest VO2-indoor VR-exercise: 1615 ± 67 mL/min vs. outdoor cycling: 1725 ± 67 mL/min). Arm and leg activity counts were significantly higher during indoor VR-assisted hybrid functional electrical stimulation (FES) cycling than outdoors; 42% greater for the arms and 23% higher for the legs (P < 0.05). Similar responses were reported for exercise effort and perceptual-psychological outcomes during both modes. Conclusion: This study proposes that combining FES and VR technology provides new opportunities for physical activity promotion or exercise rehabilitation in the SCI population, since these modes have similar "dose-potency" and self-perceived effort. Human Research Ethics Committee of the University of Sydney Ref. No. 01-2010/12385.

Control of Cycling Limb Movements: Aspects for Rehabilitation

Walking, swimming, cycling, and running are cyclic movements that are often performed in training programs or rehabilitation protocols by athletes or people with neuromotor disorders. The muscular and kinematic activities that are acquired during cyclic movements reveal control principles, especially for the optimization and stabilization of motor performance, for a given criterion in rehabilitation processes. The influence of external loads and resistive forces on limb movements should be considered in rehabilitation protocols and when assessing physical activity levels or defining activity patterns for the artificial control of limb movements. This chapter focuses on special cyclic limb movements: lower and upper limb cycling. Two aspects of this research and applications are discussed. First, variances of movement patterns are examined at different levels of the motor system (endpoint, joint configuration, muscle) during unimanual right and left arm cycling and bimanual cycling movements. Second, it is shown that the muscle activity patterns that are acquired during lower and upper limb cycling in able-bodied people may be used to define and improve stimulation patterns for functional electrical stimulation-driven cycling movements in spinal cord-injured individuals. This report also discusses the advantages of the application and control of these types of movements for the rehabilitation of people with paralyzed limbs.

A biomechanical cause of low power production during FES cycling of subjects with SCI

BACKGROUND

The goal of Functional Electrical Stimulation (FES) cycling is to provide the health benefits of exercise to persons with paralysis. To achieve the greatest health advantages, patients should produce the highest possible mechanical power. However, the mechanical power output (PO) produced during FES cycling is very low. Unfavorable biomechanics is one of the important factors reducing PO. The purpose of this study was to investigate the primary joints and muscles responsible for power generation and the role of antagonistic co-contraction in FES cycling.

METHODS

Sixteen subjects with complete spinal cord injury (SCI) pedaled a stationary recumbent FES tricycle at 60 rpm and a workload of 15 W per leg, while pedal forces and crank angle were recorded. The joint muscle moments, power and work were calculated using inverse dynamics equations.

RESULTS

Two characteristic patterns were found; in 12 subjects most work was generated by the knee extensors in the propulsion phase (83% of total work), while in 4 subjects most work was shared between by the knee extensors (42%) and flexors (44%), respectively during propulsive and recovery phases. Hip extensors produced only low net work (12 & 7%). For both patterns, extra concentric work was necessary to overcome considerable eccentric work (-82 & -96%).

CONCLUSIONS

The primary power sources were the knee extensors of the quadriceps and the knee flexors of the hamstrings. The antagonistic activity was generally low in subjects with SCI because of the weakness of the hamstrings (compared to quadriceps) and the superficial and insufficient hamstring mass activation with FES.

Effects of endurance and strength-directed electrical stimulation training on the performance and histological properties of paralyzed human muscle: a pilot study

Electrical stimulation (ES) improves muscle properties after spinal cord injury (SCI), but cycling power output (PO) remains low. We investigated the effect of endurance and strength ES training on these parameters. Assessments of quadriceps strength and fatigue resistance, cycling PO, and muscle biopsies were made in four well-trained SCI subjects (three cyclists and one rower) before and after additional weight training in the cyclists and once in the rower. Weight training improved muscle strength, but cycling PO was low in all subjects. There was no effect of training type on biopsy data. Biopsies showed non-specific signs of pathology, predominance of type IIa fibers, and uniform metabolic activity. Oxidative activity was low, as were capillary:fiber ratios in the cyclists. Cycling PO is limited by factors other than muscle strength. Future ES training studies should attempt to improve muscle oxidative capacity to optimize the potential benefits of ES exercise.

Why is the metabolic efficiency of FES cycling low?

The potential benefits of functional electrically stimulated (FES) cycling for people with spinal cord injury (SCI) are limited by the power output (PO) attainable. To understand why PO and metabolic efficiency are low, it is helpful to distinguish the effect of the SCI from the effects of electrical stimulation. The purpose of this study was to determine the performance of electrically stimulated (ES) muscle under simpler conditions and in able-bodied people in order to answer two questions about the causes of the poor efficiency in FES cycling. Fifteen able-bodied subjects (26.6 years, six male) performed 5 min of intermittent isometric quadriceps contractions at 40% maximum voluntary contraction during both voluntary and ES activation. Subsequently, nine of them performed 5 min of ES intermittent concentric contractions at the same intensity. This intermittent quadriceps activation imitated the muscles' activity during FES cycling at 35 rpm. Metabolic measurements were recorded. Input power relative to the integral of torque produced (W/Nm x s) was significantly higher during ES than voluntary isometric contractions. Efficiency of ES concentric contractions was 29.6 +/-2.9%. Respiratory exchange ratio was high during ES (1.00-1.01) compared with voluntary (0.91) contractions. ES is less economic than voluntary exercise during isometric contractions, probably due to the greater activation of fast muscle fibres. However, during ES concentric contractions, efficiency is near to the expected values for the velocity chosen. Thus there are additional factors that affect the inefficiency observed during FES cycling.

Oxygen consumption during functional electrical stimulation-assisted exercise in persons with spinal cord injury: implications for fitness and health

A lesion in the spinal cord leads in most cases to a significant reduction in active muscle mass, whereby the paralysed muscles cannot contribute to oxygen consumption (VO2) during exercise. Consequently, persons with spinal cord injury (SCI) can only achieve high VO2 values by excessively stressing the upper body musculature, which might increase the risk of musculoskeletal overuse injury. Alternatively, the muscle mass involved may be increased by using functional electrical stimulation (FES). FES-assisted cycling, FES-cycling combined with arm cranking (FES-hybrid exercise) and FES-rowing have all been suggested as candidates for cardiovascular training in SCI. In this article, we review the levels of VO2 (peak [VO2peak] and sub-peak [VO2sub-peak]) that have been reported for SCI subjects using these FES exercise modalities. A systematic literature search in MEDLINE, EMBASE, AMED, CINAHL, SportDiscus and the authors' own files revealed 35 studies that reported on 499 observations of VO2 levels achieved during FES-exercise in SCI. The results show that VO2peak during FES-rowing (1.98 L/min, n = 17; 24.1 mL/kg/min, n = 11) and FES-hybrid exercise (1.78 L/min, n = 67; 26.5 mL/kg/min, n = 35) is considerably higher than during FES-cycling (1.05 L/min, n = 264; 14.3 mL/kg/min, n = 171). VO2sub-peak values during FES-hybrid exercise were higher than during FES-cycling. FES-exercise training can produce large increases in VO2peak; the included studies report average increases of +11% after FES-rowing training, +12% after FES-hybrid exercise training and +28% after FES-cycling training. This review shows that VO2 during FES-rowing or FES-hybrid exercise is considerably higher than during FES-cycling. These observations are confirmed by a limited number of direct comparisons; larger studies to test the differences in effectiveness of the various types of FES-exercise as cardiovascular exercise are needed. The results to date suggest that FES-rowing and FES-hybrid are more suited for high-intensity, high-volume exercise training than FES-cycling. In able-bodied people, such exercise programmes have shown to result in superior health and fitness benefits. Future research should examine whether similar high-intensity and high-volume exercise programmes also give persons with SCI superior fitness and health benefits. This kind of research is very timely given the high incidence of physical inactivity-related health conditions in the aging SCI population.

Energetics of paraplegic cycling: a new theoretical framework and efficiency characterisation for untrained subjects

Complete lower-limb paralysis resulting from spinal cord injury precludes volitional leg exercise, leading to muscle atrophy and physiological de-conditioning. Cycling can be achieved using phased stimulation of the leg muscles. With training there are positive physiological adaptations and health improvement. Prior to training, however, power output may not be sufficient to overcome losses involved in rotating the legs and little is known about the energetics of untrained paralysed muscles. Here we propose efficiency measures appropriate to subjects with severe physical impairment performing cycle ergometry. These account for useful internal work (i.e. muscular work done in moving leg mass) and are applicable even for very low work rates. Experimentally, we estimated total work efficiency of ten untrained subjects with paraplegia to be 7.6 +/- 2.1% (mean +/- SD). This is close to values previously reported for anaesthetised able-bodied individuals performing stimulated cycling exercise, but is less than 1/3 of that of able-bodied subjects cycling volitionally. Correspondingly, oxygen cost of the work (38.8 +/- 13.9 ml min(-1) W(-1)) was found to be approximately 3.5 times higher. This indicates the need, for increased power output from paralysed subjects, to maximise muscle strength through training, and to improve efficiency by determining better methods of stimulating the individual muscles involved in the exercise.

Functional Output Improvement in FES Cycling by Means of Forced Smooth Pedaling

PURPOSE

Investigation of the influence of forced smooth and normal (nonsmooth) pedaling on the functional output of outdoor functional neuromuscular electrical stimulation (FES)-propelled cycling of spinal cord-injured subjects.

SUBJECTS

Twelve subjects with complete spinal cord injury (T4-T12) and limited previous FES training.

METHOD

Each subject participated in two separate outdoor sessions: once while pedaling a tricycle in a fixed gear, and a second time while free pedaling the same tricycle; both times with FES. Data on distance covered until exhaustion, cadence, and pedal forces were collected. Energy balance calculations led to evaluations of jerk loss and joint-related concentric/eccentric work.

RESULTS

First-trial and total session distances were 68 and 103% longer, respectively, in the forced smooth cycling session than in the free cycling session (P < 0.001). Significantly more additional crank work (accompanied by increased concentric work production) was generated in nonsteady cycling phases to overcome increased jerk losses during free than during fixed-gear pedaling. During fixed-gear pedaling, timing and joint location of muscle work generation were more similar to the cycling of able-bodied subjects than during freewheel pedaling, because most work was generated by knee extensors in the power phase during the former pedaling mode.

CONCLUSIONS

The superiority of forced smooth cycling to free cycling, as regards functional output distance, is based on less energy expenditure (less jerk loss and muscle tension) and on more efficient production of energy (more efficient timing and joint location of work production). Some energetic mechanisms that are advantageous for fixed-gear cycling act predominantly in unsteady phases; others work continuously during all phases of cycling.

Muscle oxygenation during prolonged electrical stimulation-evoked cycling in paraplegics

This study investigated cardiorespiratory responses and muscle oxygenation during prolonged electrical stimulation (ES)-evoked leg cycling in individuals with paraplegia (PARA). Four PARA and 6 able-bodied (AB) persons participated in this study. Subjects performed 10 min of passive cycling and 40 min of active cycling (PARA, ES cycling; AB, voluntary cycling) at workloads selected to elicit an equivalent oxygen uptake between groups. Cycling power output, cardiorespiratory responses, mechanical efficiency, and quadriceps muscle oxygenation (measured with near-infrared spectroscopy) were measured over the duration of the exercise. Oxygen uptake was similar in both groups during active cycling (PARA, 737 ± 177 mL·min–1; AB, 840 ± 90 mL·min–1). The cycling power output for PARA individuals commenced at 8.8 W, but varied considerably over 40 min. PARA individuals demonstrated markedly lower gross mechanical efficiency (~1.3%) during ES cycling compared with AB individuals performing voluntary exercise (~12.6%). During ES cycling, muscle oxygen saturation (SO2) decreased to approximately 72 ± 19%, whereas SO2 during volitional cycling was unaltered from resting levels. Muscle oxygenated haemoglobin initially decreased (–23%) during ES cycling, but returned to resting levels after 10 min. Deoxygenated haemoglobin initially rose during the first 5 min of ES cycling, and remained elevated by 28% thereafter. Upon cessation of ES cycling, lower-limb muscle oxygenation increased (+93%), suggesting reactive hyperaemia in PARA individuals after such exercise. During ES cycling, muscle oxygenation followed a different pattern to that observed in AB individuals performing voluntary cycling at an equivalent VO2. Equilibrium between oxygen demand and oxygen delivery was reached during prolonged ES cycling, despite the lack of neural adjustments of leg vasculature in the paralyzed lower limbs.

Comparison of stimulation patterns for FES-cycling using measures of oxygen cost and stimulation cost

AIM

The energy efficiency of FES-cycling in spinal cord injured subjects is very much lower than that of normal cycling, and efficiency is dependent upon the parameters of muscle stimulation. We investigated measures which can be used to evaluate the effect on cycling performance of changes in stimulation parameters, and which might therefore be used to optimise them. We aimed to determine whether oxygen cost and stimulation cost measurements are sensitive enough to allow discrimination between the efficacy of different activation ranges for stimulation of each muscle group during constant-power cycling.

METHODS

We employed a custom FES-cycling ergometer system, with accurate control of cadence and stimulated exercise workrate. Two sets of muscle activation angles ("stimulation patterns"), denoted "P1" and "P2", were applied repeatedly (eight times each) during constant-power cycling, in a repeated measures design with a single paraplegic subject. Pulmonary oxygen uptake was measured in real time and used to determine the oxygen cost of the exercise. A new measure of stimulation cost of the exercise is proposed, which represents the total rate of stimulation charge applied to the stimulated muscle groups during cycling. A number of energy-efficiency measures were also estimated.

RESULTS

Average oxygen cost and stimulation cost of P1 were found to be significantly lower than those for P2 (paired t-test, p<0.05): oxygen costs were 0.56+/-0.03l min-1 and 0.61+/-0.04l min-1 (mean+/-S.D.), respectively; stimulation costs were 74.91+/-12.15 mC min-1 and 100.30+/-14.78 mC min-1 (mean+/-S.D.), respectively. Correspondingly, all efficiency estimates for P1 were greater than those for P2.

CONCLUSION

Oxygen cost and stimulation cost measures both allow discrimination between the efficacy of different muscle activation patterns during constant-power FES-cycling. However, stimulation cost is more easily determined in real time, and responds more rapidly and with greatly improved signal-to-noise properties than the ventilatory oxygen uptake measurements required for estimation of oxygen cost. These measures may find utility in the adjustment of stimulation patterns for achievement of optimal cycling performance.

Control strategies for integration of electric motor assist and functional electrical stimulation in paraplegic cycling: utility for exercise testing and mobile cycling

AIM

The aim of this study was to investigate feedback control strategies for integration of electric motor assist and functional electrical stimulation (FES) for paraplegic cycling, with particular focus on development of a testbed for exercise testing in FES cycling, in which both cycling cadence and workrate are simultaneously well controlled and contemporary physiological measures of exercise performance derived. A second aim was to investigate the possible benefits of the approach for mobile, recreational cycling.

METHODS

A recumbent tricycle with an auxiliary electric motor is used, which is adapted for paraplegic users, and instrumented for stimulation control. We propose a novel integrated control strategy which simultaneously provides feedback control of leg power output (via automatic adjustment of stimulation intensity) and cycling cadence (via electric motor control). Both loops are designed using system identification and analytical (model-based) feedback design methods. Ventilatory and pulmonary gas exchange response profiles are derived using a portable system for real-time breath-by-breath acquisition.

RESULTS

We provide indicative results from one paraplegic subject in which a series of feedback-control tests illustrate accurate control of cycling cadence, leg power control, and external disturbance rejection. We also provide physiological response profiles from a submaximal exercise step test and a maximal incremental exercise test, as facilitated by the control strategy.

CONCLUSION

The integrated control strategy is effective in facilitating exercise testing under conditions of well-controlled cadence and power output. Our control approach significantly extends the achievable workrate range and enhances exercise-test sensitivity for FES cycling, thus allowing a more stringent characterization of physiological response profiles and estimation of key parameters of aerobic function. We further conclude that the control approach can significantly improve the overall performance of mobile recreational cycling.