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IMPROVING FES-LEG CYCLE ERGOMETER PERFORMANCE IN INDIVIDUALS WHO HAVE PLATEAUED DURING LONG-TERM TRAINING

Thomas W.J. Janssen, Roger M. Glaser, José W. Almeyda, D. Drew Pringle, Thomas Mathews Institute for Rehabilitation Research and Medicine, Wright State University School of Medicine; Rehabilitation Institute of Ohio, Miami Valley Hospital; and Dayton VA Medical Center, Dayton, OH

ABSTRACT

The purpose of this study was to determine if an interval training program (ITP) with an enhanced functional electrical stimulation leg cycle ergometer (FES-LCE) could increase exercise performance in subjects who had plateaued during long-term training on the original FES-LCE with the standard continuous protocol. Modifications to the FES-LCE included: increased FES current (140 to 300 mA), adding shank muscles, increasing FES firing angle ranges (+55o), and using an external resistance controller for continuous load setting. Four men with spinal cord injury (SCI) who had trained on the original FES-LCE for 6±1 years were trained for 6 weeks (3x/wk) using the enhanced FES-LCE and an ITP. Power output and metabolic rate increased significantly following training. Results of this study showed that a short-term ITP with the enhanced FES-LCE can elicit marked improvements in subjects whose performance has plateaued during training on the original FES-LCE.

BACKGROUND

FES-LCE technology was developed to permit individuals with SCI to pedal via their paralyzed quadriceps, hamstring, and gluteal muscle groups. It has been shown that FES-LCE exercise training can provide physiological and psychological benefits in individuals with SCI (1,2). These benefits may be unattainable with conventional arm exercise modes (3). However, a problem frequently encountered with long-term FES-LCE therapy relates to the person's inability to exercise at sufficiently high intensity levels to elicit continuous gains in exercise performance and corresponding cardiopulmonary training adaptations. Typically, individuals initiate training at a power output (PO) of 0 W, progress to 6-12 W after several weeks, and plateau at this level for long periods. This mediocre exercise performance results in limited exercise responses and training effects, which can discourage patients and health care providers from using this therapy. However, recent research suggests that the efficacy of the original FES-LCE technology may be markedly improved by appropriate modifications of FES parameters and muscle groups activated. For instance, increasing maximal FES current from the original 140 mA to 300 mA resulted in notably improved exercise responses, including significant increases in PO and cardiopulmonary variables (4). It was further shown that augmented metabolic and cardiopulmonary responses could be obtained when the gastroc-soleus and anterior tibialis muscle groups were also incorporated (5). In addition, Schutte et al. (6) predicted, based on a biomechanical modeling technique, that FES firing angles ranges could be substantially widened from those originally used to provide a smoother and more continuous propulsive action by increasing the contraction duty cycle. Thus, a training regime using combinations of these modified parameters could provide greater overload capability to enhance muscular and cardiopulmonary adaptations. The plateau in training effects may also be related to the recommended protocol for FES-LCE training, which requires up to 30 min of continuous exercise at a constant PO level during each session. This protocol does not appear to provide the overload necessary to markedly improve exercise performance. Therefore, it may be desirable to develop an ITP, where each session consists of several bouts of shorter duration but higher intensity. Therefore, the purpose of this study was to determine if an enhanced FES-LCE, used in conjunction with an ITP, could increase exercise performance in subjects who had plateaued during long-term training on the original FES-LCE.

METHOD

Subjects. Four men with SCI (age 44±14 yr, time since injury 13±8 yr, lesion level C5/6, C6/7, C6/7, T6) who had trained on the original FES-LCE for 6±1 years, volunteered to participate in this study. Subjects were medically screened and signed an IRB-approved consent form prior to participation.

FES Instrumentation. FES-LCE was performed on a modified Therapeutic Alliances, Inc. model ERGYS I. A custom-built 10-channel stimulator system, which had the capability to boost maximal current output (bi-phasic rectangular wave, 300 µs, 35 Hz) from the original 140 mA to 300 mA, was used to stimulate the quadriceps, hamstring, gluteal, gastroc-soleus, and anterior tibialis muscle groups. The gastroc-soleus and anterior tibialis muscles were co-contracted with the hamstrings and the quadriceps, respectively. FES firing angles were widened by 55o from the original ERGYS I using a custom EPROM chip. Max current was set to 300 mA for the gluteal and thigh muscles and 110 mA for the shank muscles. One subject, who had partial sensate skin, could only tolerate up to 180 mA. An external load controller was designed and constructed that could be set to the desired load resistance (continuously variable) by turning a dial before and during exercise. This eliminated the need to discontinue exercise to reset the load.

FES-LCE Exercise Performance. A continuous, progressive intensity exercise protocol was designed to determine peak metabolic and cardiopulmonary responses before (for both the original and enhanced FES-LCE) and after (only the enhanced FES-LCE) the training period. The protocol initiated with exercise at 0 kp, and resistance was increased every 2 min by 1/16 kp (3.1 W at 50 rpm) until pedaling velocity dropped from 50 to below 35 rpm, at which time exercise was terminated. During exercise, expired gases were collected by a metabolic cart, and maximal values for metabolic rate (METS) and pulmonary ventilation (VE) were determined. Heart rate (HR) was continuously monitored via ECG signals. Immediately after exercise cessation, cardiac output (CO) was non-invasively assessed by impedance cardiography. Five minutes after exercise a finger tip blood sample was analyzed for blood lactate concentration to estimate the anaerobic energy supplementation.

Fig. 1 Schematic description of the ITP protocol.

Interval Training Program. Subjects were trained for 6 weeks (3x/wk) using the enhanced FES-LCE and an ITP. The goal of each training session, consisting of at least 3 exercise bouts, was to achieve 25-30 min of accumulative exercise. Target time for each progressive exercise bout was between 5-10 min. Each bout was followed by a 5-min rest interval. The systematic increase in resistance during the exercise bout at each of 26 training levels is illustrated in Fig. 1. The initial level was established by the pre-training stress test. If subjects exercised between 5-10 min, they started at Level 3, which had the same load progression as the stress test. If they were unable to exercise for 5 min, they started at a lower level; and if they exercised longer than 10 min, they started at a higher level. The level was adjusted each subsequent bout to maintain the 5-10 min target, ensuring continuous overload as exercise capability increased. This pattern was followed throughout the 6-week training period.

RESULTS

The load resistance at which these subjects plateaued while training on the original FES-LCE with the continuous protocol was 0.19±0.07 kp (9.4 W at 50 rpm). During the first training session with the enhanced FES-LCE, the subjects reached load resistance levels of 0.21±0.l1 kp (10.5 W), whereas at session 18 load resistance significantly increased to 0.52±0.21 kp (25.8 W). The subject with paraplegia who was used to training on the original FES-LCE at 12.5 W, achieved training levels of 41 W on the enhanced FES-LCE, which is close to the upper limit (44 W) for the original FES-LCE. Fig. 2 shows the physiologic results of the progressive exercise test before and after the training period. Although PO on the original and enhanced FES-LCE were not significantly different, metabolic and cardiopulmonary responses were markedly higher using the enhanced system. After the training period, PO and metabolic rate were significantly (p<0.05) improved. The other variables showed tendencies towards increases, but changes were not significant.

DISCUSSION

The results of this study show that a short-term ITP with the enhanced FES-LCE can elicit significant improvements in exercise performance in subjects whose performance has plateaued during long-term training on the original FES-LCE. The enhanced system clearly increased the the subjects' metabolic and cardiopulmonary response magnitudes, and the ITP appeared to appropriately adjust the load resistance to the progressively increasing exercise capabilities. The perceived increase in exercise intensity was indicated by the subjects by their comments about how much harder they had to work with the enhanced FES-LCE compared to the original FES-LCE. Recovery time needed after the training session was extended accordingly. Due to the intense exercise and the added activation of the shank muscles vasodilation in the legs appeared to be notably augmented. While essentially no problems in blood pressure regulation were encountered during exercise, upon completion of

Fig. 2 Exercise responses on the original and on the enhanced FES-LCE before (E-pre) and after (E-post) training. exercise bouts, lower-limb blood pooling and hypotension occurred in some subjects due to cessation of the skeletal muscle pump. In such cases, the subject was placed in the recumbent position to facilitate venous return and reduce light-headedness. To alleviate this situation, it may be advantageous to continue FES-LCE at zero load during rest intervals to maintain skeletal muscle pump activity. The lack of a change in CO after the training may be due to a lowering of total peripheral resistance since lower-limb blood vessels were most likely dilated to a greater extent. Longer training periods at the higher PO levels may be required to elicit adaptations for improving this response.

Conclusion. The results of this study suggest that the enhanced FES-LCE, used in conjunction with an appropriate ITP, can be useful to increase the exercise performance of persons who have plateaued on the original ERGYS FES-LCE.

REFERENCES

Ragnarsson KT. Physiologic effects of functional electrical stimulation-induced exercises in spinal cord-injured individuals. Clin Orthop Rel Res, 233:253-263, 1988.

Glaser RM. Functional neuromuscular stimulation: exercise conditioning of spinal cord injured patients. Int J Sports Med 15:142-8, 1994.

Figoni SF, RM Glaser, DM Hendershot, et al.. Hemodynamic responses of quadriplegics to maximal arm-cranking and FNS leg cycling exercise. Proc 10th Ann Conf IEEE Exper Med Biol Soc 1636-1637, 1988.

Glaser RM, SF Figoni, WP Couch, SR Collins, RA Shively. Effects of increased maximum current during electrical stimulation leg cycle ergometry. Med Sci Sports Exerc 26:S111, 1994.

Figoni SF, MM Rodgers, RM Glaser. Effects of electrical stimulation of shank musculature during ES-leg cycle ergometry. Med Sci Sports Exerc 26:S77, 1994.

Schutte LM, MM Rodgers, FE Zajac, RM Glaser. Improving the efficacy of electrical stimulation-induced leg cycle ergometry: an analysis based on a dynamic musculoskeletal model. IEEE Trans Rehabil Eng 1:109-25, 1993.

ACKNOWLEDGMENTS

This study was supported by the Rehabilitation Research and Development Service of the US Department of Veterans Affairs.

Thomas W.J. Janssen, Ph.D. Wright State University 3171 Research Blvd. Kettering, OH 45420, USA Tel: (513) 259-1326 Fax: (513) 259-1310 E-mail: tjanssen@desire.wright.edu