Although physical activity options for people with spinal cord injury (SCI) are limited, one option for exercise is functional electrical stimulation leg cycle ergometry (FES-LCE). It provides computerized electrical stimulation to hamstring, gluteal, and quadriceps muscles to enable active leg cycling at a preset cadence (measured in pedal revolutions per minute [rpm]). Previous literature highlighting the benefits of FES-LCE includes increases in muscle mass,1,2 improved cardiovascular fitness,3 and slower rate of bone density loss.4 Long-term users report decreased pressure sores and pain, allowing them to remain in their wheelchair for extended periods of time.5 Furthermore, FES-LCE, like any other exercise, enhances self-image, maintains a healthy appearance, and boosts energy levels.5
However, not all people with SCI can use the FES-LCE. Potential barriers include insufficient contractile response to electrical stimulation,6 intolerable pain in response to the FES, autonomic dysreflexia (AD),7 and excessive lower extremity spasticity.2,8 To our knowledge, there has been no clear documentation of the screening procedures used to determine appropriate entry into an FES-LCE exercise program. Previous literature reports medical screening (general physical examinations, x-rays) before using the FES-LCE.2,3 Although these tests are a necessary precautionary measure, they do not indicate successful response to the FES-LCE. Because FES-LCE use depends on both physiologic health and successful response to the stimulation, a complete screening protocol requires two steps: eliminating individuals who (1) have known contraindications to FES-LCE use and (2) do not successfully respond to the FES-LCE.
For those who can use FES-LCE regularly, the literature suggests an exercise duration of 30 minutes at a cycling cadence >35 rpm.1,5 However, many participants are initially unable to maintain 30 minutes of cycling at this speed. Consequently, the need for a pretraining period, or habituation (building tolerance to FES-LCE), has been documented.3 Despite this acknowledgment, the lack of detailed information regarding the procedures or time course of this habituation phase has been noted.1
Previously documented habituation procedures vary from short preconditioning phases to electrically induced knee extension training (Table 1). However, documentation is rarely thorough enough to allow replication of the procedure or prediction of the time course required to attain the target exercise duration and speed. Consequently, there is no established protocol for FES-LCE habituation.
It would benefit clinicians and researchers to clearly document the stepwise process involved before initiating an FES-LCE exercise program. Such information could assist the knowledge translation of research findings into practice, thereby facilitating the implementation of this intervention. Therefore, our main objective was to document the screening and habituation process used before a three-month study of FES-LCE exercise for people with SCI. Specifically, we outlined the screening process used before trying the FES-LCE and the protocol used to attain target duration and cycle speed.
All participants met the following criteria: (1) sustained a traumatic SCI, (2) chronic SCI (injury duration more than one year), and (3) able to communicate in English. We excluded participants who had (1) other neurologic conditions in addition to SCI, (2) known unstable cardiovascular disease, ie, cardiac arrhythmia or high blood pressure (systolic blood pressure >140 mm Hg; diastolic blood pressure >90 mm Hg), (3) unhealed wounds or pressure ulcers, (4) history of fragility fracture(s), and/or (5) abnormal bone formations that limited hip or knee range of motion (eg, heterotopic ossification). No participants had previous experience using FES-LCE. Eligible participants gave written informed consent. The study was approved by local hospital and university research ethics boards.
The cycle training was performed using a computer-controlled leg cycle ergometer (Ergys2, Version H.6, Therapeutic Alliances Inc., Fairborn, OH). This device provides electrical stimulation using six self-adhesive surface electrodes (Uni-Patch, Wabasha, MN) placed in three areas: quadriceps, hamstrings, and gluteals (Fig. 1). A small screen, visible to both the FES-LCE user and the attending researcher, displayed stimulation level, pedaling cadence, and elapsed time. FES-LCE boot pedals, designed to restrict sagittal and transverse plane movements at the ankle, were used by all participants.
The electrical stimulation was applied in a coordinated sequence of muscle contractions that resulted in cycling. The sinusoidal wave pulse was set at a 500-μs duration, and the stimulation frequency was set at 60 Hz and synchronized for each pair of electrodes. The ergometer was programmed to increase the stimulation current from 0 to 140 mA to achieve the desired pedaling cadence. The maximum stimulation current was set on an individual basis, depending on comfort. Threshold settings, used during the warm-up and cooldown, were set to palpable muscle contraction in each participant.
Pedaling cadence (rpm) is a frequency range defined by preset upper maximum and lower minimum levels. If the pedaling cadence approached the upper maximum, the stimulation current would decrease to slow down the pedaling cadence. If stimulation was at its maximum current and the pedal speed dropped below the preset lower minimum, the ergometer would automatically stop and go into cooldown mode.
Participants who met the inclusion and exclusion criteria started on the FES-LCE at the minimum upper and lower cadence levels (32 and 18 rpm, respectively). Participants who did not respond to the FES or who responded to the FES-LCE with (1) intolerable pain, (2) severe spasticity upon stimulation (stimulation at the lowest stimulation levels caused an increase in muscle tone and extensor muscle spasms that prohibited the cycling motion), and/or (3) severe AD were excluded from the study. Pain was considered to be intolerable if participants required cessation of stimulation immediately upon use of the FES-LCE at the lowest stimulation levels. Severe AD was defined as that which caused adverse symptoms (eg, headache/sweating above the neurologic level of injury) sufficient for participants to request the cessation of electrical stimulation and/or an increase in systolic blood pressure to 150 mm Hg (or 40 mm Hg above baseline). Blood pressure and heart rate were measured before exercise, on initial stimulation, and periodically during exercise.
All participants were scheduled for three sessions per week at the local rehabilitation center. Each session started with a one-minute passive warm-up. Stimulation was pulsed between 50% and 100% of the preset threshold setting, and the participant manually kept the speed above the preset lower cadence by pushing alternately on his or her knees, which turned the crankshaft. In cases of higher tetraplegia, the research assistant turned the crankshaft manually. Low stimulation during the passive (ie, stimulation not sufficient to initiate/maintain leg movement) warm-up and cooldown sessions was used to facilitate blood flow in the lower extremities to prevent blood pooling and potential hypotension. After the warm-up, stimulation of the leg muscles started, and active FES-LCE training continued until the participant could no longer maintain a pedaling cadence >18 rpm. Once the speed decreased to <18 rpm, the working muscles were considered to be fatigued, and the FES-LCE automatically began a two-minute cooldown mode, during which the stimulation was pulsed between 50% and 100% of the threshold setting and the participant would again push alternatively on his or her knees or the research assistant turned the crankshaft manually to keep the pedaling frequency higher than the preset minimal cadence level. This was followed by a 5-minute rest period. This warm-up, training, cooldown, and rest cycle constituted one “run.” The procedure was repeated to a maximum of five runs or 30 minutes of active FES-LCE. Once participants were able to complete 30 minutes of active cycling in one run for two consecutive sessions, the upper and lower bound speeds were increased to the next speed increment and the procedure was repeated until the target cadence (maximum, 49 rpm; minimum, 35 rpm [49/35]), was reached. We used four preset increments on the Ergys2 to move from 35/18 rpm to 49/35 rpm (39/21, 42/25, 46/32, 49/35 rpm). If the subject was able to achieve a good muscle contraction with a moderate amount of current with no adverse effects on the first day, a range of speeds was attempted to ascertain the highest speed at which the participant could sustain active pedaling. Stimulation was stopped immediately upon reports of intolerable discomfort/pain or when systolic blood pressure increased to >40 mm Hg above baseline.
Thirteen people expressed interest in the study (Table 2). Participant 11 had a previous fragility fracture and was excluded; the remaining 12 participants met the initial inclusion criteria and were deemed safe to try the FES-LCE at minimal intensity. Five people were excluded because of insufficient and/or inappropriate response to the FES-LCE. Participants 8 and 9 did not respond sufficiently to the FES-LCE to independently cycle the ergometer. Participant 8 obtained a hamstring and gluteal, but not a quadriceps, response to the stimulation, whereas participant 9 had no palpable response to the FES-LCE and was unable to move the pedals. Participants 10 and 12 could not pedal the ergometer because the stimulation triggered extreme knee extensor muscle spasms that prevented knee flexion. Participant 12 also exhibited AD, as displayed by a spike in systolic blood pressure to 210 mm Hg and a severe headache. Participant 13 was unable to cycle because the stimulation caused AD that presented as intolerable head and whole body pain. Thus, seven participants began the habituation training (Fig. 2).
After one session of the habituation training (no adverse effects noted), participant 7 withdrew because of the time commitment necessary to complete the exercise program (three sessions per week). The remaining six participants completed the habituation phase of FES-LCE training. Table 3 reports the time course of the exercise sessions (in exercise sessions and minutes), including the time to achieve 30 minutes of FES-LCE and 30 continuous minutes of FES-LCE at 35 rpm. A scatter plot (Fig. 3) highlights the positive relationship between length of time to achieve 30 continuous minutes and length of time to achieve 30 continuous minutes at the target cadence (49/35 rpm). The number of sessions to achieve 30 continuous minutes at 49/35 rpm ranged from one to 26, over a time frame of one day to 23 weeks.
Adverse reactions to the FES-LCE were minimal for those participants who passed the screening phase and were included in the training study (Table 3). Two of the six participants did not experience any adverse reactions. Three participants experienced mild AD (ie, subjective symptoms not accompanied by large increases in blood pressure). For one, this presented as a mild headache that subsided naturally as she became accustomed to using the ergometer. For the two others, AD was minimized with a gradual habituation process. In all cases, blood pressure was monitored and stimulation continued at participant discretion. Participants were scheduled for three sessions per week. One subject was able to cycle for the 30-minute duration at 49/35 rpm on the first day, whereas the other five attended 1.3–2.7 times per week.
General Eligibility Criteria
We did not allow one participant to use the FES-LCE because of the presence of a previous fragility fracture. Previous literature has shown that the knee-joint torques during FES-LCE cycling may be great enough to produce a fracture in the compromised lower limbs of an individual with SCI,8 and others have been equally conservative in their exclusion of participants due to a history of bone fracture and/or osteoporosis.8,10,15
Response to the FES-LCE
Insufficient Response to the FES-LCE
Two of our participants did not respond to the FES-LCE sufficiently to independently cycle the ergometer. Successful stimulation of the muscle for functional purposes requires an intact lower motor neuron (LMN). Although LMN damage is often associated with traumatic injuries occurring at or below the lower thoracic (T10-12) segments, many of these individuals show preserved LMN function.19 In our study, one of three individuals with low thoracic injuries had a good response to the FES-LCE. Thus, response to the FES-LCE cannot be predicted solely by injury level and should be determined on an individual basis.
The AD resulting from the FES-LCE was severe enough to stop or modify training in four participants. For two, the presence of severe AD (increasing blood pressure and/or intolerable pain) was apparent within 30–60 seconds of electrical stimulation and required immediate cessation of the machine. Thus, AD prevented further FES-LCE use for these two individuals. For the two others, AD was not as extreme (blood pressure did not reach 40 mm Hg above baseline and was not accompanied by pain) and subsided with gradual progression of stimulation intensity. These individuals were able to continue FES-LCE habituation with careful monitoring and progression. Thus, although AD does not necessarily prevent FES-LCE use, its presence cannot be ignored (regardless of subjective symptoms) and must be monitored closely.
The time to reach 30 minutes at 35 rpm varied between participants but was as long as 31 sessions (778 minutes) over 23 weeks. This time frame, while similar to other habituation phases,8,9,16 was notably lengthy.
The major problem limiting exercise training, particularly when beginning FES-LCE, is rapid muscle fatigue. Although some state that FES knee extension training is a necessary antecedent to FES-LCE because of limited lower extremity strength,6 others have found no relationship between this training and the ability to obtain a continuous cycling motion on the FES-LCE.3 Muscle fatigue is experienced faster when pedaling at a high cycling cadence (50 rpm) than when pedaling at a slower cadence (15 rpm).14 Consequently, we started habituation at a low pedal cadence to facilitate longer exercise durations and gradually increased the cadence once participants could sustain the cycling motion. In contrast, previous habituation procedures held the cadence constant while increasing resistance. Because results from able-bodied individuals suggest that pedaling at higher cadences facilitates muscular endurance and cardiorespiratory fitness,20 we worked toward the faster target cadence of 35 rpm to maximize fitness gains.
Individual reasons contributed to differences in the time to reach the target duration and cadence. Although a previous study of FES-LCE exercise noted no correlation between habituation time and age, sex, or medical history,1 it appeared that individuals with higher lesion levels required longer habituation phases in our study. Participants 5 and 6 (C4 American Spinal Injury Association Impairment Scale B; injury duration, 2.5–3 years) required long habituation periods (>20 sessions). Other individuals required very gradual increments in stimulation intensity to overcome adverse effects (headaches, pain at electrode sites). Thus, the habituation protocol should be an individualized process that addresses the needs and reactions of each participant.
The importance of adherence to the scheduled sessions cannot be overlooked. There was a marked division based on level/severity of SCI. Those requiring greater amounts of assistance for daily activities had lower attendance levels. The two individuals with tetraplegia in our study relied on care workers, modified vans, and government-operated accessible transportation service. These participants missed more sessions because of practical problems (transportation, care attendant cancellations), sickness, and fatigue than did the participants with paraplegia.
Feasibility of Using the FES-LCE as a Rehabilitation Tool
Not all participants are able to use this exercise modality. Both clients and rehabilitation professionals should be aware of the time requirements associated with this program.
Our subject pool was small, which reduces the generalizability of the results. Participants were highly heterogeneous with respect to the duration and neurologic classification of their injuries. Despite the small sample size and large diversity in participant demographics, our documented habituation procedures were successful (all subjects were able to complete 30 minutes of cycling at 49/35 rpm). However, a larger sample size would provide more data to determine relationships between other variables (eg, injury duration) and time to habituation.
The low attendance of participants in the present study was not ideal but represents “real-world” barriers to people with SCI who want to participate in treatment interventions at an institution. Similar attendance levels have been noted during long-term studies of FES-LCE exercise.3,4,6,8 Although the FES-LCE is a costly purchase, using it as a home device, where transportation to a site is not required, may be a better option for some individuals. Our results show that habituation to the FES-LCE, using the above-mentioned procedures, is possible despite low adherence rates.
About half of the participants could not pass the screening phase of FES-LCE training, mostly because of inappropriate responses to the electrical stimulation. All individuals who meet the inclusion criteria for FES-LCE use would benefit from trying the device, as response to the stimulation cannot be determined by demographic information (ie, American Spinal Injury Association classification, duration of injury, etc.) The habituation phase can be a time-consuming process and requires individualized protocols to address differences in the response to electrical stimulation. Although mild AD and/or small increases in blood pressure may be experienced by some participants, these issues can be ameliorated by gradual progression of the stimulation intensity and are not necessarily prohibitive factors to FES-LCE use. The participants in this study reported few adverse effects from using the FES-LCE once they completed the habituation phase of training.
We thank the Rick Hansen Man in Motion Foundation for providing the FES-LCE, the Michael Smith Foundation for Health Research, and the Canadian Institutes of Health Research for career scientist awards (to J.E. and D.W.).
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Keywords:© 2008 Neurology Section, APTA
spinal cord injury; rehabilitation; exercise; functional electrical stimulation; cycling; screening; habituation