Overground Body-Weight-Supported Gait Training for ...

Physical & Occupational Therapy in Pediatrics, Early Online:1–13, 2013 C 2013 by Informa Healthcare USA, Inc. Available online at http://informahealthcare.com/potp DOI: 10.3109/01942638.2013.771719

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ARTICLE

Overground Body-Weight-Supported Gait Training for Children and Youth with Neuromuscular Impairments Max J. Kurz, PhD, Wayne Stuberg, PT, PhD, PCS, Stacey DeJong, PT, PhD, & David J. Arpin, MS Physical Therapy Department, Munroe-Meyer Institute for Genetics and Rehabilitation, University of Nebraska Medical Center, Omaha, Nebraska USA

ABSTRACT. The aim of this investigation was to determine if body-weight-supported (BWS) overground gait training has the potential to improve the walking abilities of children and youth with childhood onset motor impairments and intellectual disabilities. Eight participants (mean age of 16.3 years) completed 12 weeks of BWS overground gait training that was performed two times a week. BWS was provided during the training sessions by an overhead harness system that rolls overground. There was a significant improvement in the preferred walking speed after the training (p < .01; pre = 0.51 ± 0.2 m/s; post = 0.67 ± 0.3 m/s; Cohen’s d = 0.80) and cadence (p = .04; pre = 37 ± 7 steps/min; post = 43 ± 8 steps/min; Cohen’s d = 0.94). Our results indicate that overground BWS gait training may be an effective treatment strategy for improving the preferred walking speed of children and youth with motor impairments. KEYWORDS. Cerebral palsy, gait training, locomotion, physical therapy, rehabilitation, walking

INTRODUCTION A considerable amount evidence from animal models has shown that partially supporting the body’s weight while walking on the treadmill can promote neuroplastic changes in the nervous system that improve the ability to walk (Dietz, 2009). This reorganization is partly driven by afferent somatosensory feedback that is received from the load receptors and the hip joint (Dietz, Muller, & Colombo, 2002; Harkema et al., 1997). For example, as the trailing limb approaches terminal portion of the stance phase, the muscle spindles sense elongation of the hip flexor muscles, and Golgi tendon organs in the plantar flexor muscles sense unloading as weight is transferred forward onto the opposite limb. These sensory cues are Address Correspondence to: Max J. Kurz, PhD, Department of Physical Therapy, Munroe-Meyer Institute for Genetics and Rehabilitation, University of Nebraska Medical Center, 985450 Nebraska Medical Center, Omaha, NB 68198-5450, USA (E-mail: [email protected]). (Received 16 March 2012; accepted 18 January 2013)

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powerful signals that bring about stance-to-swing phase transitions and contribute to rhythmic, alternating lower extremity movements during gait (Dietz et al., 2002; Harkema et al., 1997). These insights from animal models have inspired the use of body-weight-supported treadmill training (BWSTT) for adults with neurological impairments. Several investigations involving adults with motor impairments have demonstrated that BWSTT results in clinically relevant improvements in walking speed and walking endurance (Mulroy et al., 2010; Sullivan, Knowlton, & Dobkin, 2002). Similar outcomes can be achieved with children that have motor impairments. For example, BWSTT can be used to assist children with Down syndrome to achieve their walking motor milestone sooner (Ulrich, D. A, Ulrich, B. D., Angulo-Kinzler, & Yun, 2001). Moreover, several studies have shown that BWSTT can result in improvements in the walking speed, walking endurance, and gross motor function of children with cerebral palsy (CP) (Cherng, Liu, Lau, & Hong, 2007; Dodd & Foley, 2007; Kurz, Corr, Stuberg, Volkman, & Smith, 2011a; Kurz, Stuberg, DeJong, 2011b; Kurz, Wilson, Corr, Volkman, 2012; Mattern-Baxter, Bellamy, & Mansoor, 2009). These encouraging results infer that supporting the body weight during treadmill training may be a task-specific therapeutic modality that provides essential afferent feedback for neural connections that govern a successful stepping pattern. This conclusion is partially supported by research that has shown that BWSTT promotes reorganization of the somatosensory cortices in children with CP (Kurz et al., 2012; Phillips et al., 2007). Although BWSTT appears to be a beneficial therapy, there is a growing amount of scientific evidence that overground gait training may provide equivalent improvements in the walking abilities of adults and children with neurological impairments (Dobkin et al., 2006; Kosak & Reding, 2000; Nilsson et al., 2001; Willoughby, Dodd, Shields, & Foley, 2010). Moreover, the use of overground walking with or without BWS may have greater face value since it can be readily translated into clinical practice across environments. Overground gait training is a more conventional physical therapeutic approach where the stepping pattern is intensely practiced while the body weight is partially supported by the arms using parallel bars or an assistive mobility device (i.e., wheeled walkers, lower extremity orthoses). This suggests that improvements in walking may not be directly dependent upon the ability of the treadmill’s motion to enforce the progression of the stepping pattern or the facilitation of afferent feedback for motor learning. Rather improvements in walking appear to be a result of practicing a high number of steps. Conventional overhead BWS systems reduce the metabolic cost of walking, which allows for longer duration training sessions (Nilsson et al., 2001; Unnithan, Kenne, Logan, Collier, & Turk, 2006). Furthermore, the additional BWS can enhance the patient’s confidence in completing the therapeutic regimen since the harness stabilizes the torso and removes the possibility of a fall (Kosak & Reding, 2000). Hence, there are direct clinical benefits of using an overhead BWS system during gait training. Potentially, performing gait training overground while the body weight is supported may represent an ideal task-specific approach for improving the gait of children and adolescents with gait impairments. Overground BWS gait training could be accomplished by placing the BWS system on wheels and moving the system to match the forward progression of the participant’s overground walking pattern (Figure 1).


Overground Body Weight Support Gait Training

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FIGURE 1. Body weight support system used during the overground gait training. The system consisted of an overhead cable-pulley system that was connected to a torso harness and leg straps. The body weight support system was on wheels, which allowed for the therapist to push the support system to match the walking speed of the participant during the training sessions. The amount of body weight supported was measured with a load cell that was integrated into the system and provided a digital readout of the amount of body weight supported.

In this exploratory investigation, we examined whether overhead BWS during overground gait training improved the walking abilities of children and youth with motor impairments. Consistent with previous research, our primary outcome measure was preferred walking speed. Our secondary outcome measures were changes in cadence, Gross Motor Function Measure Section E (Russell, Rosenbaum, Avery, & Lane, 2002), the Supported Walker Ambulation Performance Scale (SWAPS) (Malouin, Richards, Menier, Dumas, & Marcoux, 1997), and the Observational Gait Scale (OGS) (Mackey, Lobb, Walt, & Stott, 2003). Lastly, we described differences between younger and older participants. METHODS Subjects All of the participants in this study were enrolled in a school for children with severe or profound cognitive disabilities. Individuals were eligible to participate if they


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could walk a minimum of 10 m continuously with or without an assistive device and were excluded from participating if they were unable to follow verbal instructions, a congenital heart defect or other cardiac disorder, orthopedic surgery of the spine or lower extremities within the past year, or botulinum toxin injections to their lower extremity muscles within the past 6 months. Children who had orthopedic surgery or a significant change in spasticity management medication during the course of the study were also excluded from participating. Eight participants with childhood onset motor disabilities were recruited to participate in this investigation (age = 16.3 ± 5 years; height = 1.42 ± 0.1 m; mass = 36.2 ± 12 kg). Four of the participants had spastic diplegic or quadriplegic CP, 3 had chromosomal disorders, 2 had Rett syndrome, and 1 had a history of traumatic brain injury in infancy. All of the participants could be described as having a Gross Motor Function Classification Score of II or III (Palisano et al., 1997). Six of the participating children routinely used an assistive device for community ambulation. Further details on the participants and their assistive devices are found in Table 1. Although almost all of the participants required an assistive mobility device for community ambulation, the devices were not incorporated into the overground training protocol. The parents of the participants gave their written consent for their child’s participation in the investigation, and the Medical Center’s Committee for the Protection of Human Subjects approved the experimental protocol. Outcome Measures Preferred Walking Speed Pre–post gait assessments were performed by having each participant walk down a 16-m walkway while two 60-Hz cameras (Panasonic AG-HVX200P) recorded video in two dimensions (sagittal and frontal plane). The outputs of the respective cameras were fed into a digital video mixer (Panasonic Digital AV Mixer WJ-MX30), which generated a split-screen video of the walking performance. One experienced pediatric physical therapist (WS) performed all pre- and post-training assessments. Walking speed during gait analysis involving children and youth with an intellectual disability is dependent upon motivation, fitness level, and motor control of the stepping pattern. To overcome some of these sources of variability we took an average of the walking speed across four sessions. Average walking speed for the first four and last four treatment sessions was measured to determine if the participants increased their walking speed during the treatment session by the end of the 12 weeks. Walking speed was calculated by measuring the total distance a participant walked during the training session and dividing by the training time. Pretest walking speed was the average speed for the first four training sessions and posttest walking speed was the average speed for the last four training sessions. This method was chosen to ensure that the measured speeds were representative of the participants’ preferred speeds. Observational Gait Scale The OGS was used to assess changes in the walking pattern before and after the overground BWS gait training (Mackey et al., 2003). This assessment consisted of a 4-point Likert scale that rated the knee’s position at midstance, position of the


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TABLE 1. Demographics of the Participating Children Age (yrs)

Gender

Height (m)

Mass (kg)

1

9

Female

1.25

20.1

Cerebral palsy with spastic diplegia

2

9

Female

1.24

19.4

Cerebral palsy with Posterior ataxic quadriplegia walker

3

10

Male

1.36

29.2

4

16

Female

1.35

28.4

5

19

Male

1.67

49.2

6

20

Female

1.47

51.2

Cerebral palsy with spastic quadriplegia Rett syndrome with dystonic quadriplegia Traumatic brain injury, with spastic diplegia Mosaic tetrasomy 12P, with spastic diplegia

7

21

Female

1.46

40.1

8

21

Female

1.56

38.0


Subject

Diagnosis

Rett syndrome with quadriplegia 11–22 chromosome damage, with spastic quadriplegia

Assistive mobility Posterior walker

Posterior walker No assistive device Anterior walker Anterior walker

Anterior walker No assistive device

Orthoses Bilateral solid ankle–foot orthosis Bilateral supermalleolar orthoses, thoracolumbosacral orthosis Bilateral shoe insert Bilateral solid ankle–foot orthosis Bilateral ankle–foot orthosis Bilateral cascade #3 ankle–foot orthoses with pretibial strap None Bilateral solid ankle–foot orthosis

foot at initial contact, position of the foot at midstance, timing of heel rise, position of the hind foot at midstance, width of the base of support, and use of an assistive device. These ratings were summed and a value of 21 indicated a normal walking ability. Supported Walker Ambulation Performance Scale The SWAPS was also used to assess changes in the walking performance of the participants (Malouin et al., 1997). The SWAPS assesses the amount of support the participant needs for walking, standing postural alignment, the quality of the steps taken, and the number of steps taken. Each section of the SWAPS uses a 4-point Likert scale. The scores were summed and expressed as a percentage; 100% indicates normal walking ability. Gross Motor Function Measure-–Section E Items from Section E of the GMFM-88 were used to assess the participant’s ability to perform walking, running, and jumping tasks (Russell et al., 2002). Items include the ability to walk forward, walk backward, kick a ball, walk up and down stairs, or walk on a line. The score for Section E was expressed as a percentage of the

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72 possible points. All GMFM assessments were administered by the same experienced pediatric physical therapist (WS). All outcome measures except the GMFM were scored from the video recordings of gait, which could be replayed in slow motion and/or paused to ensure accurate observations.


INTERVENTION The participants completed an overground gait-training program that was performed 2 days a week for 12 weeks, with a minimum of 1 day of rest between the training sessions. The overground gait training was performed at the school and was integrated into the participant’s day-to-day schedule. BWS was provided during the training sessions by a commercially available overhead harness system that had the capability to roll overground (Unweighting System; Biodex Medical Systems, NY; Figure 1). The BWS walking sessions were conducted in a long 27-m hallway at the school. The participant was provided with verbal encouragement to walk as long and as fast as possible during the gait-training sessions, as the overhead BWS system was pushed to match the walking speed. The therapeutic prescription consisted of initially supporting 40% of the participant’s body weight in Week 1, and gradually reducing the amount of BWS by 5% every other week. The percentage of BWS was determined from a load cell that was integrated into the support system and provided a digital reading of the amount of BWS. Each training session included 20 min of walking time, with one or two breaks provided if fatigue was evident. No hands-on assistance was provided during the training sessions. The participants attended 94 ± 0.03% of the scheduled gait-training sessions. All participants completed the study, and no adverse events occurred. Data Analysis Preferred walking speed was calculated from the video by measuring the time it took to walk across the middle 6 m of the walkway. The speeds were scaled on the basis of the participant’s height using Equation 1 (Hof, 1996). v vdimensionless = √ Lg

(1)

where vdimensionless is the nondimensionalized velocity, v is the velocity in meter per second, g is gravity (e.g., 9.81 m/s2 ), and L is the leg length of the participant in meters. This scaling procedure adjusts the walking speed based on the leg length of the participant. This is done because individuals with longer leg lengths may have faster walking speeds. Lastly, the cadence was calculated by dividing the number of steps by the time it took to walk the 6 m. Paired t-tests were used to determine if there were significant differences between pre and post outcome measures. Statistical significance was assumed when p < .05. Cohen’s d was calculated to determine the effect size of the change in the respective outcome measures (Cohen, 1998). The calculated effect sizes are interpreted as: 0.2 small, 0.5 medium, and greater than 0.8 as large. Medium to large effects are clinically relevant because they are large enough to be seen by observation (Oeffinger et al., 2008).


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We also separated the data from the respective young (subjects 1–3; ages 9–10 years) and older (subjects 4–8; ages 16–21 years) participants to qualitatively evaluate if aged may have influenced the percent change in the outcome measures.


RESULTS The individual and group pre- and posttest values are presented in Table 2. Average walking speeds with BWS were significantly greater during the last four training sessions compared with the first four sessions (pre = 0.30 ± 0.04 m/s; post = 0.54 ± 0.03 m/s, p < .001). This represented an 80% increase in the training speed over the course of the program. The effect size was large (Cohen’s d = 0.80). In a similar fashion, the dimensionless walking speeds were also significantly different (p < .01). The 33% improvement in preferred walking speed was accompanied by a significantly faster cadence (p = .04). The effect size was large (Cohen’s d = 0.94). The mean pre–post change was not statistically significant for GMFM Section E scores (p = .15; Cohen’s d = 0.09), SWAPS scores (p = .28; Cohen’s d = 0.36) and OGA scores (p = .16; Cohen’s d = 0.28). Youth (ages 16–21 years) had a 45% change in their walking speed after the training, whereas younger children (ages 9—10 years) had a 16% change in their walking speed. The change in the cadence paralleled the walking speeds, where youth had a 15% faster cadence and younger children had a 9% faster cadence. Although youth improved by 11% on the SWAP, the younger children had SWAP scores that were 9% lower. Similar outcomes were seen for the OGS where youth had a 13% improvement in their scores, whereas younger children had a 12% decrease in their scores. Lastly, both youth and younger children had a 2% improvement in Section E of the GMFM.

DISCUSSION The 8 children and youth with motor impairments and intellectual disabilities made on average a 33% improvement in their preferred walking speed after a 12-week BWS overground gait training. These faster walking speeds are well above the minimum clinically important difference thresholds calculated by Oeffinger and colleagues (2008) (5.7%–9.1%), which indicates that the changes reported here are large and grossly observable. Moreover, the improvements in walking speed were within the range of what has been previously reported for children with CP who have participated in BWS treadmill training (13%–67%) (Cherng et al., 2007; Dodd & Foley, 2007; Kurz et al., 2011a; Kurz et al., 2011b; Kurz et al., 2012). Although the improved walking speed is most likely attributed to the overground gait-training protocol, we suspect that the improvements may be partially related to the fact that the therapy was performed in the school. For example, it is likely that the school staff may have spent more time working with the participant in the classroom on their walking since they were seeing improvements in the walking abilities of the participants. An enhancement in the child’s walking ability would have direct benefits in the school setting because it would result in less reliance on the school staff during the day-to-day activities. Although this inference is speculative,

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Pre

0.19 0.61 0.71 0.71 0.65 0.34 0.27 0.55 0.51 0.21 0.33 0.68

Subject

1 2 3 4 5 6 7 8 Mean SD CI lower bound CI upper bound

0.18 0.76 0.81 0.82 0.88 0.69 0.29 0.97 0.67 0.28 0.43 0.91

Post

Speed (m/s)

0.07 0.24 0.26 0.26 0.22 0.12 0.09 0.18 0.19 0.07 0.12 0.25

Pre 0.07 0.30 0.29 0.31 0.31 0.25 0.10 0.33 0.25 0.10 0.16 0.33

Post

Speed (dimensionless)

30.6 39.6 46.2 47.4 35.4 40.2 24.6 40.2 37.8 7.2 31.8 44.4

Pre 39.6 42.6 45.6 40.2 45.0 45.0 28.2 58.2 43.2 8.4 36.0 49.8

Post

Cadence (steps/min)

60 86.7 73.3 73.3 73.3 63.3 50 73.3 69.1 11.1 59.9 78.4

Pre 33.3 93.3 73.3 76.7 73.3 73.3 66.6 80 71.2 17.2 56.8 85.6

Post

SWAP (%)

4.5 15.5 13 10.5 10.5 10 14 13 11.4 3.4 8.5 14.2

Pre

OGS

4 15 10 12 13.5 12 14 14 11.8 3.5 8.9 14.8

Post

15 28 35 8 15 0 5 39 18.1 14.3 6.1 30.1

Pre

15 32 33 8 15 0 5 40 18.5 14.7 6.2 30.8

Post

GMFM Section E (%)

TABLE 2. Individual and Group Results for Walking Speed, Cadence, Supported Walker Ambulation Performance (SWAP), Observational Gait Scale (OGS), and Section E of the Gross Motor Function Measure (GMFM). SD Represents the Standard Deviation of the Data, and CI is the 95% Confidence Interval of the Data




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it should be considered for future studies that evaluate therapy that is performed in the school setting. Faster walking speeds in children with typical development and adolescents occur by uniformly scaling the step length and cadence of the gait pattern (Abel & Damiano, 1996; Schwartz, Rozumalski, & Trost, 2008). Contrary to this, children with CP have been reported to achieve faster walking speeds primarily by increasing the cadence (Abel & Damiano, 1996). Our results suggest that faster walking speeds may have been accomplished after the overground BWS training by increasing cadence. Although it is alternatively probable that the faster walking speeds may have also been related to a longer step length, we cannot make this conclusion base on our data. However, previous studies that have employed BWS gait training with children with CP have noted that faster walking speed after training was also accomplished by an increased step length (Kurz et al., 2011a; Kurz et al., 2011b). Therefore, it is likely that the participants in this study may have had similar results. Qualitative evaluation of the percent change in the older and youngest participants suggested that youth who were 16 years and older might have responded better to the overground BWS gait training. The youth tended to walk faster, relied less on support for walking, and had better gait biomechanics after the training. Our findings cannot be generalized since the sample size was small and impairment levels were not similar. In spite of this limitation, we speculate that the youth may have had a larger percent change in their walking performance because they may have been stronger, had more walking experience, and were more motivated to improve their walking abilities. Further research is recommended to identify at what ages overground gait training is most effective. Although overground gait training has been an integral part of rehabilitation for many years, surprisingly few studies have evaluated its efficacy or sought to identify optimal training parameters. Given the recent studies on BWSTT, with generally positive results (Damiano & DeJong, 2009), relative effectiveness of the various gait-training methods has come into question, particularly by therapists seeking evidence to guide their decision making. Investigators have identified numerous factors that are thought to be important for successful BWSTT, such as continuous rhythmic stepping at a speed that approaches normal values and use of manual assistance to approximate typical gait kinematics (Damiano & DeJong, 2009). However, the effect of these factors has not been determined for overground training. Exploration of the outcomes after various gait-training protocols will help distinguish which features are crucial for success in terms of walking speed, gross motor function, and/or improved gait patterns and kinematic profiles. Previous research has shown that faster gait-training speeds tend to translate to larger improvements in walking speed after gait training (Sullivan et al., 2002). We suspect that our participants were capable of achieving faster walking speeds during the overground training sessions because their body weight was supported. Hence, BWS may be an important therapeutic strategy because it enables the participant to achieve training speeds that they cannot attain while supporting their full body weight. Likewise, the BWS may have also allowed for greater massed practice, which is necessary for the rewiring of the neural connections that govern the stepping pattern (Cha et al., 2007).


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Overground BWS gait training did not result in improvements in Dimension E of the GMFM (Russell et al., 2002), which evaluates ability to perform tasks such as walk forward, walk backward, kick a ball, and walk up and down stairs. Our finding implies that the improvements in walking speed may not have been accompanied by an improved ability to negotiate challenging and unstable environments that may require more skillful stepping and balancing abilities. However, we are cautious about interpreting these results, given the challenges encountered when administering the GMFM to children and youth with intellectual disabilities. Performing the GMFM requires that the participant either understands and responds to instructions, demonstrates their motor ability during spontaneous behavior, or that the desired motor performance can be elicited by the examiner via a structured testing environment, demonstration, and verbal encouragement. Difficulties encountered when testing some of the participants may have hampered our ability to demonstrate a clinically relevant or statistically significant change. Although 6 of the 8 participants in our investigation required an assistive mobility device for community ambulation, the devices were not incorporated in our overground training protocol. In addition, all of our outcome measures were based on the participants using their assistive mobility device while walking. It is possible that the marginal improvements seen in the SWAPS and OGA scores may be related to a lack of specificity in training of the participants to use the assistive mobility device while walking. Hence, it is possible that by not including training on how to control the assistive mobility device may have hindered the participant’s ability to improve on the motor skills that they typically use for community ambulation. Future investigations that incorporate assistive devices in the overground gait training protocol may enhance the carryover to the day-to-day gait performance of children and adolescents with developmental motor impairments. Another possible explanation is that the quality of the participants’ gait patterns did not change which is more closely linked to gait characteristics assessed by the SWAPS and OGS. Participants walked faster as seen by the increase in cadence; however, the overall walking pattern and level of assistive device used by the participants was only marginally changed by the training protocol. This hypothesis is further supported by the lack of significant change in the GMFM scores. The study design did not include a measure of community participation nor long-term followup on maintenance so the clinical meaningfulness of these large effect sizes for walking speed and cadence require further study and clarification. Our results pertain to children and youth with childhood onset motor impairments and intellectual disabilities. The design used in the study was pre–post intervention, and the long-term effect of the training on walking velocity is not known. The study also did not include a control group and the sample size was small, which limits generalizability of the findings. However, it should be noted that the effect sizes for the walking speed and cadence were large. Based on our findings it is unclear whether the focus of gait training should be on improving the cadence of the preferred walking pattern or modification of the pattern by attempting to increase step length. Future research is recommended to determine if improvements in walking speed following BWS overground gait training carries over to the day-to-day life of children and youth with developmental motor impairments.


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In conclusion, our results indicate that BWS during overground gait training may be an effective therapeutic strategy for improving the walking speed of children and youth with childhood onset motor disability and intellectual disability. On the basis of our reported Cohen’s d value for the preferred walking speed, 12 children would provide greater than 80% power to detect similar outcomes as what have been reported in this study. This pilot work has provided the preliminary support for a clinical trial that will assess the efficacy of overground BWS gait training for youth with motor impairments. Declaration of interest: The authors report no conflict of interest. The authors alone are responsible for the content and writing of this article.

ABOUT THE AUTHORS Max J. Kurz, PhD is an assistant professor in the Department of Physical Therapy at the University of Nebraska Medical Center’s Munroe-Meyer Institute. He is the Director of Biomechanics for the Institute’s Motion Analysis Laboratory that largely serves children and youth with cerebral palsy. Dr. Kurz received his doctorate from the University of Nebraska at Lincoln in neuroscience and behavior. Wayne Stuberg, PT, PhD, PCS is a professor who is the Director of the Department of Physical Therapy at the University of Nebraska Medical Center’s MunroeMeyer Institute. Furthermore, he is the Associate Director for Education and the Director of the Institute’s Motion Analysis Laboratory. He has over 30 years of experience as a pediatric physical therapist, and is a Catherine Worthingham Fellow of the American Physical Therapy Association (APTA). The Section on Pediatrics of the APTA has previously recognized his excellence in research by awarding him with their Research Award. Stacey DeJong, PT, PhD is currently a post-doctoral fellow at the Landon Center on Aging at University of Kansas Medical Center. She received her masters degree from the University of Nebraska Medical Center in medical sciences, and her doctorate from the Washington University in movement science. David J. Arpin, MS is currently a doctoral student in the Department of Physical Therapy at the University of Nebraska Medical Center’s MunroeMeyer Institute. He received his masters degree from the University of Nebraska at Omaha in exercise science.

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