Physical activity is an important lifestyle behavior for individuals with physical disabilities (23). Specific to this population, regular physical activity reduces risk of secondary disease associated with disability (14), improves lipid profiles (13), and increases functional capacity (5). However, many individuals with physical disabilities do not regularly engage in physical activity due to various exercise barriers (18,27,29). Individuals with ambulation limitations, such as those that rely on electric or power wheelchairs, are most at risk of a sedentary lifestyle and reduced opportunities to participate in physical activity.
Wheelchair sport opportunities are increasing for individuals who rely on power wheelchairs for ambulation. Power wheelchair athletes include individuals with a variety of physical disabilities, including arthrogryposis, cerebral palsy (CP), muscular dystrophy (MD), spina bifida, and spinal cord injury (SCI). Although the use of an electric wheelchair theoretically equates the energy demand and limits exercise intensity for participants, exercise responses to power wheelchair competition may vary due to mechanisms specific to disability type. Several authors have documented higher energy expenditure during low-intensity activity for children and adults with CP compared with able-bodied controls (16,19,20,22,24,25). Unnithan and researchers (25) have documented that total body work, or mechanical power output, explains 87% of variability in oxygen cost of children with CP during submaximal walking. Inefficient movement patterns elicit greater work output among individuals with CP, and wasteful movements or simultaneous contraction of opposing muscle groups may explain achievement of moderate exercise intensities (i.e., 3–7 METs) during light activity in this population (19–20). Higher energy expenditure during rest and low-intensity activity has also been documented for children with MD compared with the general population (3,26), but the causes are less understood. Because balancing and body position adjustments among power chair users may also elicit elevated work output compared with the able-bodied population, it seems prudent to document physiological response variations among power wheelchair athletes to improve the understanding of acute and chronic training outcomes that are disability specific.
To examine the influence of disability on power wheelchair activity responses, a pilot study was conducted on three power wheelchair athletic teams. Heart rate (HR) responses of power soccer athletes were monitored before and during power soccer scrimmages and official games. Power soccer is similar to able-bodied soccer, except that teams are limited to four players using power wheelchairs (i.e., electric wheelchairs) to score an oversized ball on a hard-court surface. Participants consisted of six athletes with spastic CP and involvement in four limbs (i.e., quadriplegia). The comparison group consisted of four individuals without CP (e.g., spina bifida and SCI). Mean HR for pregame and game conditions as well as the change score between game and pregame HR (RESPONSE) were measured. Visual inspection of the data substantiated that disability type influences HR response during power wheelchair sport (Fig. 1). The magnitude of the HR increase from pregame conditions among athletes with CP, 46 ± 15 bpm, resembled an exercise intensity associated with physical training in the general population; however, the HR increase was not as high among individuals without cerebral palsy (i.e., 15 ± 4 bpm). Although factors such as environment and anxiety can increase HR from resting conditions, a sustained rise in HR during power wheelchair sport could indicate achievement of a training threshold and associated cardiorespiratory fitness benefits for power wheelchair users.
Despite an increase in health promotion literature specific to individuals with disabilities, research is limited regarding the physiological responses to exercise among individuals with physical disabilities (18). Based on pilot study results and the need for empirical investigations addressing severe disability, the purpose of this study was to determine the influence of disability type on HR response during competitive power soccer competition. The secondary purpose was to determine the extent to which HR responses among individuals with various physical disabilities meet intensity recommendations for the general population that elicit cardiorespiratory fitness benefits. Due to the elevated work output associated with involuntary movement, our hypothesis was that athletes with CP would demonstrate a significantly greater HR response during competition than athletes with alternative disabilities due to disability-specific mechanisms.
Power soccer players participating in the 2003 National Power Soccer Tournament were recruited to participate in a nonrandomized pretest posttest design. Individuals with CP (N = 31), SCI (N = 10), and MD (N = 7) volunteered to participate. Athletes with CP reported spastic, ataxic, or mixed symptoms. Nine athletes with SCI had injuries ranging from level C4 to C7. One athlete had a complete injury at the T7 level. Initial recruitment efforts were made through team coaches, and a project description was provided to all teams before competition. Methods were approved by institutional review for both pilot and main study participants. Pilot data were collected approximately 4 months before the national tournament, but were not included in the study sample. Consent was obtained from all athletes and from parents of those athletes who were under 18 yr of age.
Groups consisted of athletes with CP, MD, and SCI. Participants with CP were delimited to involvement in all four limbs (i.e., quadriplegia). Athletes with MD and SCI were delimited to individuals using power wheelchairs during competition. Because variability across disability type may influence physical responses to activity, secondary analyses were conducted on experimental groups that were further delimited based on common symptoms. The subsample consisted of athletes with spastic CP (N = 13) and athletes with cervical spinal injuries (N = 9). No further delimitations were applied to individuals with MD.
Polar S610TM HR monitors (Polar Electro, Inc., Woodbury, NY) were used to record all HR data. The S610TM model enables HR to be measured at intervals of 5, 15, or 60 s, and data to be stored and downloaded. The S610TM has a unique receiver for each transmitter and prevents multiple unit interference. The investigators fitted monitors to participants before game activities and attached the receiver to either the subject’s wrist or armrest.
Heart rate was recorded every 5 s throughout pregame and game conditions, and later downloaded. Data collection started approximately 10 min before scheduled warm-ups and occurred throughout the duration of each tournament match. Data were collected from two to six participants during each match, and visual observation of matches was used to classify scores into the appropriate category: PRE (lowest 5-s HR before team warm-ups), GAME (average HR during the match), PEAK (highest 5-s HR during competition), RANGE (difference score between PRE and PEAK HR), and RESPONSE (difference score between mean GAME and PRE HR). Data collection during GAME conditions occurred over two 25-min halves, totaling 50 min. Although the HR instrument is intended to prevent multiple unit interference, all downloaded data were visually screened to ensure that scores were consistent and not affected by extremes due to electrical malfunction (e.g., PEAK scores did not exceed maximum predicted values, PEAK scores were within 5 bpm of subsequent high score). Nongame activities (e.g., time outs, half time) were excluded from data analysis.
Based on the Levene test, data did not meet homogeneity of variance assumptions for ANOVA. Therefore, to determine the effect of disability type on exercise response during power soccer competition, the Kruskal–Wallis one-way analysis of variance by ranks test ((21), pp. 397–410, represented by the symbol H) was conducted on the dependent measure, RESPONSE, to determine whether a significant difference existed among medians for athletes with CP, SCI, and MD (P < 0.05). Power analyses, based on pilot study results, were conducted to ensure adequate sample size. Using the sample size estimate procedure suggested by Cohen ((4), pp. 380–390), we concluded that six participants per group, in combination with the documented effect size (d = 2.57), should result in a power greater than 0.80. The power estimate is consistent with an observed power of 0.89 in the pilot study derived from the general linear model. The effect size, d, indicates that the RESPONSE for athletes with CP was approximately 2.5 standard deviations greater than the RESPONSE for athletes without CP. This index is computed by dividing the difference score between group means by the pooled standard deviation ((4), pp. 20–26). Mann–Whitney U post hoc comparisons between groups were conducted after a significant difference ((21), pp. 181–194). Because Type I error can be inflated by performing multiple statistical tests, the Dunn–Bonferroni adjustment was made to experiment-wide alpha reducing each post hoc comparison to an alpha level of 0.01 (6). An additional Kruskal–Wallis analysis was conducted to determine whether a significant median difference existed among athletes with spastic CP, cervical SCI, or MD (P < 0.05).
Current American College of Sports Medicine (ACSM) recommendations indicate that aerobic exercise ≥ 55% HRmax (i.e., 4.8 METs for young adults) elicits improvement in cardiorespiratory fitness in low-fit individuals (1). To determine the extent to which HR responses meet ACSM intensity recommendations, frequencies were conducted on GAME HR to determine the number of participants that averaged 55% of predicted HRmax for 30 min during competition. Also, GAME HR was used to estimate MET values sustained during activity from four regression equations appropriate for the current sample (i.e., children with CP, adults with CP, children without CP, and adults without CP). The regression equations for children were developed on individuals aged 7–17 yr (19), whereas the adult equations were developed on individuals 18 yr and older (22). Because participants ranged in age from 8 to 55 yr, participant age and disability type dictated the regression equation selected. All analyses were conducted with SPSS 11.0 (Chicago, IL).
Athletes with SCI (mean = 32.2 ± 12.8 yr) and CP (mean = 28.4 ± 11.5) were significantly older (t = 3.35, df = 45, P < 0.05) than athletes with MD (mean = 14.1 ± 2.9). Heart rate increased from PRE conditions across all groups, and descriptive statistics are presented in Tables 1 and 2 for the entire sample and subsample, respectively. A significant difference on RESPONSE existed among athletes with CP, SCI, and MD (H = 10.99, df = 2, P < 0.05) with an observed power of 0.90. This power statistic is based on the general linear model, and actual power is lower for nonparametric tests (i.e., Kruskal–Wallis). However, power of the one-way analysis by ranks approximates the parametric F test (21). The median RESPONSE for athletes with CP was 12 bpm higher than athletes with SCI. This post hoc comparison was significant (U = 49.0, df = 1, P < 0.01) and represented a large effect (d = 1.35). No additional post hoc comparisons were significant; however, the effect sizes between disability groups were noteworthy. A large effect existed between MD and SCI (d = 1.12), and a medium effect existed between CP and MD (d = 0.52). It is not surprising that these group differences were not statistically significant. Sample size estimates were based on distinct disability group responses during the pilot study, but greater functional overlap exists in the current sample (i.e., participants from different groups were able to demonstrate similar limb and trunk function); therefore, power for post hoc comparisons is probably lacking.
A significant median difference on RESPONSE existed among athletes with spastic CP, cervical SCI, and MD (H = 7.91, df = 2, P < 0.05) with an observed power of 0.85. Athletes with spastic CP demonstrated a median RESPONSE score 15 bpm higher than athletes with cervical SCI. This post hoc comparison was significant (U = 20.0, df = 1, P < 0.01) and represented a large effect (d = 1.35). A large effect existed between MD and cervical SCI (d = 1.06) and a medium effect existed between spastic CP and MD (d = 0.60), although these post hoc comparisons were not significant.
Twenty-two athletes with CP (71%) exceeded a cardiorespiratory fitness training threshold for a minimum of 30 min despite use of a power wheelchair for competition (i.e., ≥ 55% HRmax predicted). Nineteen of these 22 athletes exceeded this threshold for the entire 50-min competition and specific types of CP (e.g., spastic, mixed) did not distinguish responders from nonresponders. Based on GAME HR, athletes with CP averaged an exercise intensity of 4.2 METs during competition. Five athletes with MD (71%) exceeded 55% HRmax for 30 min. Based on the prediction equation for children without CP, athletes with MD averaged 3.8 METs during competition. One athlete with cervical SCI (10%) surpassed 55% HRmax for a minimum of 30 min.
Disability type influences the HR of athletes during power soccer competition (Fig. 2). Findings from the current study reveal higher HR responses for athletes with CP than athletes with SCI, potentially suggesting increased activity demands for athletes with CP compared with SCI during power wheelchair sport. Athletes with CP reached and maintained higher HR intensities during competition than athletes with SCI, as indicated by PEAK, GAME, and RESPONSE scores. Results were similar for athletes delimited into subsample groups based on symptoms. Responses for athletes with spastic CP were reflective of athletes with CP as a whole (i.e., spastic, athetoid, and mixed), indicating that type of CP may not prohibit an increased HR response to activity. These findings have important implications relative to the understanding of acute and chronic physiological adaptations to disability sport.
The secondary purpose of the study was to determine the extent to which competition heart rate met exercise intensity recommendations for cardiorespiratory fitness. The ACSM promotes that a training intensity of 55% HRmax is sufficient to elicit improvement in fitness for the general population (1). Seventy-one percent of athletes with CP and MD exceeded this threshold for 30 min during competition. GAME intensity ranged from 3.5 to 6.9 METs for athletes who surpassed the HR criterion (meanCP = 4.9 ± 1.0; meanMD = 4.4 ± 0.5). GAME intensity ranged from 1.0 to 3.2 METs for athletes that did not meet the target HR criterion (meanCP = 2.4 ± 0.7; meanMD = 2.3 ± 0.1). Maximal HR among individuals with complete cervical SCI typically does not exceed 120 bpm due to sympathetic nervous system impairment, and therefore predicted heart rate values based on age are inappropriate. However, training intensities from 30 to 80% heart rate reserve among individuals with quadriplegia may approximate 50–85% in the general population (12). If the lower threshold is viable, eight of the nine participants with cervical SCI exceeded this threshold based on PRE, PEAK, and GAME HR.
The efficacy of ACSM training recommendations for individuals with CP, SCI, and MD has been documented (7). Regular training at moderate intensities improves submaximal exercise efficiency, improves maximal power output and peak aerobic power, and reduces functional decline due to inactivity in study populations ((7), pp. 237–294). Although fewer longitudinal training studies for individuals with physical disabilities are present in the literature compared with the able-bodied population, empirical evidence supports the use of these guidelines. Training at 135 bpm 4× wk−1 over a 9-month period resulted in a 35% aerobic capacity increase among children with CP (28). The current participants were able to reach and sustain this intensity, indicating the ability to obtain significant functional gains through competition. The current findings also support the use of ACSM intensity guidelines for individuals with CP and MD based on the ability of GAME HR to distinguish between individuals that did or did not maintain moderate-intensity MET values. Despite the inability of all participants to sustain a training intensity, it is important to document the ability of power wheelchair athletes to reach training thresholds associated with important health benefits through disability sport.
PEAK HR values obtained in the current study on athletes with quadriplegic CP were similar to peak HR values reported among non–power wheelchair users with CP. Tobimatsu and colleagues (22) investigated the cardiorespiratory responses to arm ergometry in 12 men with CP. Five individuals were community ambulators and seven were manual wheelchair users. The mean peak HR for participants (spastic, athetotic, and ataxic symptoms) was 154 bpm, identical to the current study. Unnithan and colleagues (24) reported a mean HR of 142 bpm at 90% of the fastest walking speed among nine children with spastic CP (seven with diplegia), and Rose and colleagues (19) reported a mean HR of 160 bpm at maximum walking speed in a similar sample. A HR of 154 bpm was reported as the anaerobic threshold in one group of boys with spastic CP that used elbow crutches (15), and the ability of athletes to reach this intensity reinforces the possibility of a training effect during power wheelchair sport.
Despite similar PEAK HR responses to non–power wheelchair users, RESPONSE scores in the current sample were lower than HR increases reported in samples with diplegia and hemiplegia that performed continuous exercise. Unnithan and colleagues (24) reported an increase of 56 bpm from rest to 90% of the fastest walking speed. This increase is approximately 20 bpm higher than in the current study (Table 1). Rose and associates (19) reported a 69-bpm increase from rest to the fastest walking speed in four children with hemiplegia and nine with diplegia. This response was almost identical to a follow-up study that included the difference between resting HR and the most economical walking speed (20). Within the literature, Tobimatsu and colleagues (22) reported the highest increase in HR (74 bpm) from rest to peak physical working capacity on an arm ergometer. Important to note, the current study design necessitated the use of pregame HR values rather than true resting HR to determine RESPONSE. Because net increases in HR are typically evaluated by the change from true resting conditions, it is possible that RESPONSE is actually underestimated in the current study.
Our findings are consistent with previous studies that documented increased heart rate and energy expenditure among individuals with CP during various forms of ambulation (3,9,17,19,20,24,25). Both the current findings and literature support that power wheelchair exercise responses, and therefore benefits, may be disability specific. Unnithan et al. (24) revealed that simultaneous contraction of agonist and antagonist muscles, or “cocontraction,” contributes to a higher energy cost among individuals with CP. Use of an electric wheelchair would not prevent cocontraction from occurring, and therefore would not prevent elevated energy expenditure during competition. It seems reasonable that in an effort to balance and position oneself in a power wheelchair, energy expenditure and HR are higher than would be expected due to mechanisms specific to CP.
Different mechanisms may explain the acute response among individuals with spinal cord injuries. Figoni (8) noted that oxygen extraction is greater among individuals with spinal cord injuries than the able-bodied population. Because exercise performance does not appear to be limited by central factors such as reduced maximal stroke volume (11), compensatory mechanisms such as greater oxygen extraction by working muscles may explain the ability of athletes with SCI to reach a training threshold. Specifically, Hooker and Wells (10) documented that a training intensity of 50% HRmax resulted in a 10% increase in peak aerobic capacity for an individual with a cervical SCI injury. If minimal overload thresholds exist for individuals with severe physical disabilities, power wheelchair sport may provide a meaningful physical activity for individuals without CP.
Few researchers have addressed the acute responses of exercise among individuals with MD. Increased energy expenditure has been documented for a variety of muscular dystrophies, and it seems plausible that energy costs are greater due to muscle weakness (3). High energy costs may also contribute to muscle deterioration (26). Individuals with MD demonstrate autonomic dysfunction that worsens over time (30), resulting from a reduction in parasympathetic activity, accompanied by an increase in sympathetic activity. Coupled, this mechanism results in sympathetic overdrive and likely blunts heart rate response to mild and moderate activity due to desensitization of β-adrenergic receptors. In heart failure, another condition in which sympathetic overdrive exists, β-receptors downregulate to protect cardiac myocytes from harmful adrenergic stimulation (2). Thus, it seems plausible that a similar physiological adaptation may occur in individuals with muscular dystrophy. However, further study relating to physical activity responses is needed in this population.
The major limitation of the current study is nonassessment of endocrine influences on heart rate responses. It is important to determine the extent that cardiac acceleration is driven by psychological arousal. Stress in the current sample of athletes with CP and MD was similar to competition responses of able-bodied athletes. Heart rate increased before the start of competition, continued to increase at the onset of competition, and incrementally increased during competition before reaching a plateau. Figure 3 provides a visual representation of HR consistency across competition. The stress response was distinct in the sample of athletes with spinal cord injury. Unlike changes associated with exercise, heart rate decreased at the onset of competition, and continued to decline during competition until reaching a plateau. A variety of stressors can cause a temporary rise in HR (e.g., anxiety, temperature). However, the neutral climate environment, consistent elevation of responses, and consistency of results with pilot findings leads one to hypothesize that exercise intensity was the predominant stimulus of HR changes. Nonetheless, future investigations should control for nonactivity influences on HR by utilizing a repeated measures design to compare competition responses to observational responses.
In conclusion, disability type influences acute HR response to power wheelchair sport. Athletes with CP demonstrated higher heart rate responses during competition than athletes with SCI or MD, and 71% of athletes with CP and MD were able to sustain a training intensity associated with improved cardiorespiratory fitness in the general population. Further, 88% of athletes with cervical SCI exceeded adjusted HR intensities reflective of moderate training. Findings from the current study substantiate the importance of training studies on power wheelchair sport and associated acute and chronic adaptations that may occur through disability-specific mechanisms.
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Keywords:©2005The American College of Sports Medicine
PHYSICAL DISABILITY; AEROBIC TRAINING; DISABLED SPORT; TARGET HEART RATE