Abstract
Background. Physical training might delay the functional deterioration caused by disuse in boys with Duchenne muscular dystrophy (DMD). The “No Use Is Disuse” study is the first explorative, randomized controlled trial in boys with DMD to examine whether assisted bicycle training is feasible, safe, and beneficial. Methods. Ambulatory and recently wheelchair-dependent boys with DMD were allocated to the intervention or control group. The intervention group received assisted bicycle training of the legs and arms during 24 weeks. The control group received the same training after a waiting period of 24 weeks. The primary study outcomes were the Motor Function Measure (MFM) and the Assisted 6-Minute Cycling Test (A6MCT). Group differences were examined by an analysis of covariance. Results. Thirty boys (mean age 10.5 ± 2.6 years, 18 ambulant and 12 wheelchair-dependent) were allocated to the intervention (n = 17) or the control (n = 13) group. All boys in the intervention group (except one) completed the training. After 24 weeks, the total MFM score remained stable in the intervention group, whereas it had significantly decreased in the control group (Δ = 4.9, 95% confidence interval = 2.2-7.6). No significant group differences were found for the A6MCT. No serious adverse events were observed. Conclusions. Our results suggest that assisted bicycle training of the legs and arms is feasible and safe for both ambulant and wheelchair-dependent children and may decline the deterioration due to disuse. Progressive deterioration, however, may compromise the design of trials for DMD.
Keywords
Introduction
Duchenne muscular dystrophy (DMD) is the most common debilitating neuromuscular disorder affecting 1/3500 live-born boys. 1 Although symptomatic treatments, such as corticosteroids and respiratory ventilation, have extended the life expectancy of boys with DMD into adulthood, boys still become wheelchair-dependent around the age of 10 years and also lose their upper-limb functions.2,3 Therefore an important aim in the management of DMD is to delay the loss of functional abilities, which is relevant for both activities of daily living and physical aspects of health-related quality of life.
The loss of functional abilities is primarily the result of a progressive decrease in muscle strength and muscle endurance during the course of DMD.4(pp269-321),5 However, increasingly limited physical and social activities gradually cause a discrepancy between a boy’s potential capacities and his actual performance. For example, the increasing amount of energy needed to ambulate and the increasing risk of falling will make boys move less and make them opt for wheelchair mobility. Confinement to a wheelchair will lead to secondary deterioration of the musculoskeletal and cardiorespiratory systems. 5 Another example is the loss of arm function that is accelerated after the onset of wheelchair dependency by forcing the boys to function within the confines of the wheelchair. 6 This leads to secondary physical deterioration or “disuse.”
Physical training might delay the secondary functional deterioration as a result of disuse as long as eccentric exercises and exhausting high-resistance exercises are avoided.7,8 Current international guidelines therefore recommend regular submaximal activities for boys with DMD. 9 However, the number of randomized controlled trials (RCTs) that have examined the effects of physical training in muscle diseases is limited to 3 trials conducted in adults. 10 Based on these trials, no specific training advice can be given. Even fewer studies have examined the effects of physical training in boys with DMD and none of these were conducted using a controlled design. These studies mainly focused on resistance exercises in ambulatory boys and concluded that although submaximal resistance exercises did not cause any harm, they had only limited beneficial effects.11-13 In contrast, previous studies in mdx mice (an animal model for DMD) concluded that dynamic exercises (such as voluntary wheel-running) had beneficial effects on muscle strength and fatigue resistance.14,15 Dynamic exercises (bicycle training) also improved endurance and strength in patients with Becker muscular dystrophy, a milder variant of DMD. 16 However, these patients were all adults. Therefore, no RCTs have been published concerning the effects of dynamic, active-assisted training in children with DMD.
Based on previous studies in adults and mdx mice and on clinical experience, it was hypothesized that a dynamic physical training would be feasible and safe and would counteract the secondary functional decline caused by disuse in boys with DMD. An RCT was therefore conducted that compared assisted bicycle training with no intervention (usual care) in children who were at the end of their ambulatory phase or who had recently been confined to a wheelchair: the No Use is Disuse (NUD) study. 17 We chose assisted bicycle training instead of bicycle training without support, because even the smallest resistance can be too hard to sustain for patients in whom large muscle groups are affected.4(pp343-365) This is also the first study to use assisted training in DMD, since previous (uncontrolled) trials all examined the effect of resistance training.
Methods
An extensive description of the NUD study has previously been published. 17 No deviations from the original protocol were made.
Participants
An RCT with multiple baseline measurements was conducted between January 2009 and January 2011 among boys with a DNA-established diagnosis of DMD. Boys were eligible if they were in their late ambulatory phase and showed a labored gait and/or had difficulties with rising from the floor. Boys were included if they needed more than 5 seconds to rise from the floor, were not able to rise from the floor, were not able to cycle without electric assistance, or needed a wheelchair to move over a long (>500 m) distance. Wheelchair-dependent boys were eligible if they were able to touch the top of their head with both hands or were able to use a hand-operated wheelchair. Exclusion criteria were as follows: age <6 years, a clinical cardiomyopathy, and other disabling diseases influencing mobility. Approval was obtained from the regional medical ethics committee. Parents and participants who were older than 12 years provided informed consent. The trial was registered with the Netherlands National Trial Register, number 1631.
Randomization and Masking
Stratified randomization (ambulant vs wheelchair-dependent) was used to allocate participants to either the intervention or the control group in a 2:1 ratio. After a baseline period of 8 weeks, the intervention group received assisted bicycle training, whereas the control group received usual care during 24 weeks. Usual care means that boys were allowed to continue their activities, including physiotherapy, but they did not receive any specific intervention. The control group received the same training after the 24-week waiting period. The intervention and control groups were followed up until 56 and 60 weeks after study entry, respectively. Participants and outcome assessor were not blinded to treatment allocation but had no information about previous test results at each assessment.
Sample Size
No historical data were available at the start of this study in 2008. 18 The sample size was therefore not based on statistical analysis. We arbitrarily chose to include 20 to 30 participants. This seemed a realistic number of participants to recruit in the Netherlands in view of an estimated number of ~100 eligible boys with DMD aged 7 to 13 years. Boys were recruited from the Dutch Duchenne Parent Project database and by advertisements on the websites of the Dutch Association for Neuromuscular Diseases and Duchenne Parent Project between January 2009 and January 2010.
Procedures
Intervention
The intervention consisted of an assisted bicycle training program of 5 days per week during 24 weeks. In each session, participants cycled 15 minutes with both their legs and arms using a mobility trainer with electrical motor support (KPT Cycla, Kinetic, France). The mobility trainer could be used with a participant’s own (electric) wheelchair. We chose assisted bicycle training instead of bicycle training without support, because even the smallest resistance can be too hard to sustain for patients in whom large muscle groups are affected (assistance was at least 6 rounds per minute if no active movement).4(pp343-365) Participants were instructed to cycle at a constant speed (~65 rpm) and to keep this up for 15 minutes without getting overexerted as assessed with the OMNI scale for perceived exertion (OMNI > 6). 19 This means that participants had to be able to cycle at an exertion ranging from “a little tired” to “getting more tired.” The mobility trainer recorded the time and number of revolutions. Starting positions for both leg-cycling and arm-cranking were standardized, 20 that is, for leg-cycling the hip and knee of the bended leg were held in 90° flexion, while the knee of the other leg was submaximally extended. The feet were placed in a heel cup with straps. For arm cycling, the pedal axis was a few centimeters (with a maximum of 5 cm) below shoulder level when the pedals were horizontal. The distance from the chair to the bicycle was decided by allowing the participants to move their legs and arms over the submaximal range of motion, which was allowed to produce a feeling of stretch but not pain. Participants were seated in a comfortable position with the back supported and they were recommended to watch television while cycling to make training more pleasurable. Participants trained at home or at school (depending on their preferences). Parents and/or teachers were instructed to assist the boys. Training intensity and posture were monitored and if necessary adjusted by the primary investigator (MJ).
Assessments
Figure 1 shows the subsequent clinical assessments (T0-T8) with the primary endpoint at T4. Multiple baseline measurements (T0-T2) were performed to control for initial severity and disease progression. All assessments were performed by the primary investigator (MJ) at a university hospital (T0, T2, T4, T6), and also at home (other assessments), to reduce the burden for the participants and families.
Clinical assessments during the study period.
Outcome measures
The primary outcome measures were the Motor Function Measure (MFM) 21 and the Assisted 6-Minute Cycling Test (A6MCT). 20 The MFM assesses the functional abilities and consists of 32 items in 3 dimensions: dimension 1 = standing position and transfers (D1), dimension 2 = axial and proximal motor functions (D2), and dimension 3 = distal motor function (D3). We calculated the percentage of the maximum score for the total score and for the 3 dimensions separately, with a higher score indicating a better motor function. The A6MCT for both the legs and the arms was used to assess endurance (at T0, T2, T4, and T6), in which participants were instructed to make as many revolutions as possible in 6 minutes.
Secondary outcome measures were the Pediatric Evaluation of Disability Inventory (PEDI), 22 timed tests (rise from floor, 10-m run, 9-hole peg test), muscle strength, passive joint range of motion (ROM), and quantitative muscle ultrasound (QMUS). Muscle strength of the hip extensors, knee extensors, ankle dorsiflexors, shoulder abductors, and elbow extensors was scored bilaterally with the Medical Research Council (MRC) scale. 23 We calculated a total sum score, a sum score for the lower limb, and a sum score for the upper limb. ROM was recorded bilaterally for knee extension, ankle dorsiflexion, and elbow extension, and we calculated a sum score for each joint. 24 QMUS was used to calculate the echo intensity (EI) of the biceps brachii muscle, the forearm flexors, the rectus femoris muscle, and the tibialis anterior muscle. 25 The EI is a gray value quantified by a histogram-based analysis that shows the infiltration of connective tissue and fat. In boys with DMD, muscle EI increases with age and disease severity. 26 EIs were expressed as z-scores (ie, the number of SD from the mean of the reference group) and were then integrated in a total sum score, and a sum score for the lower and upper limb. Age (years), co-interventions (eg, physiotherapy), and corticosteroid use (in the Netherlands: 10 days on 10 days off schedule of 0.75 mg/kg) were recorded as well.
Adverse events
During the intervention period, participants who received the assisted bicycle training were monitored for signs of overexertion by means of a postal questionnaire once every 2 weeks and home visits by the primary investigator (MJ). The questionnaire focused on excessive muscle pain, any severely uncomfortable feeling during or after the training, and (extreme) fatigue. Parents and their boys were informed by telephone when compliance was insufficient or when other problems (such as signs of overstrain) were identified. Parents were also instructed to call or e-mail the investigator if their boys had significant pain or fatigue. In that case, the training intensity was adjusted. The study terminated prematurely for a participant if training remained excessive after a reduction of the training intensity.
Statistical analysis
Participants were analyzed as treated. Group differences for the primary outcome measures were examined by an analysis of covariance, using group (intervention vs control) as the independent factor, and the multiple baseline assessments (T0-T2), corticosteroid use, and ambulatory status as covariates. Multiple baseline assessments (T0-T2) were added as covariates, as paired t tests showed small changes in total MFM score over time already between T0 and T1 (−2.7 [4.3], 95% confidence interval [CI] = 0.09-5.30), and T0 and T2 (−2.9 [3.6], 95% CI = 0.74-5.11) for the control group, and also between T1 and T2 (1.7 [2.3], 95% CI = 0.39-3.04), and T0 and T2 (−2.5 [2.0], 95% CI = 1.41-3.66) for the intervention group. However, a high degree of correlation was found between T0 and T2 for both the control (intraclass correlation coefficient = 0.96, 95% CI = 0.88-0.99) and intervention (intraclass correlation coefficient = 0.99, 95% CI = 0.97-0.99) groups.
The same analysis of covariance statistics was performed to establish effects for the individual dimensions of the MFM, the primary outcomes at T3, and the secondary outcomes at T3 and T4. Effect sizes (ES) were calculated to investigate the clinical relevance of the observed differences by dividing the group differences between the time effect (T2-T4) by the pooled SD at T2. Interpretation of the ES is as follows 0.20 to 0.50 = small, 0.50 to 0.80 = moderate, and ≥0.80 = large. 27 Nonparametric Wilcoxon rank-sum tests were used to evaluate the effect of training on the time to rise from floor and 10-m run at T4, as these data did not show a normal distribution and groups were small because only a limited number of boys were able to perform the tests. Boys who were unable to perform the tests were coded with an unrealistically high score (100 seconds). In addition, 2 boys lost the ability to walk 10 m and to rise from the floor in the control group, and 1 boy lost the ability to walk 10 m and to rise from the floor in the intervention group during the study period.
Changes during the follow-up period for the intervention group were determined by comparing means at the end of the training with those 4 weeks (short term) and 24 weeks later (long term) using paired t tests. Paired t tests were also used to calculate the changes during the training period of the control group. All statistical analyses were performed using SPSS version 16·0 for Windows (SPSS Inc, Chicago, IL) with a significance level set at P < .05.
Results
Participants
Figure 2 shows the participant flow through the study. Thirty boys (18 ambulant, 12 wheelchair-dependent) were allocated to the intervention (n = 17) or control group (n = 13). Two out of the 12 wheelchair-dependent boys were not randomized, but directly allocated to the intervention group, because the study duration was ending. Another 2 (ambulant) boys were originally allocated to the intervention group, but were replaced to the control group within 2 weeks after trying the intervention. The intervention appeared to be unfeasible for them because of cognitive problems for 1 young boy and motivation problems and a patella luxation for another boy. Of the 17 boys in the intervention group, only 1 wheelchair-dependent participant discontinued the training and the assessments after 12 weeks because of logistic problems imposed on his family, and he was excluded from the analysis. No patients from the control group were lost to follow-up during the waiting period. The mean age in the intervention group was 10.8 years (SD = 2.4) years and in the control group 10.5 years (SD = 2.8), P = .803. At baseline, 8 boys were ambulant and 9 were wheelchair-dependent in the intervention group, compared with 10 and 3 in the control group, respectively (P = .098). In the intervention group, 14 boys used corticosteroids compared with 9 in the control group. The clinical characteristics did not differ substantially between the 2 groups at baseline (T0-T2; Table 1).
Participant flow through the study. Thirty boys were allocated to the intervention (n = 17) or control group (n = 13). Of the 17 boys in the intervention group, only 1 participant discontinued the training and he was excluded from the analysis. No patients from the control group were lost to follow-up during the waiting period.
Between-Group Differences at T3 and T4 (Primary Endpoint).
Abbreviations: MFM, Motor Function Measure; A6MCT, Assisted 6-Minute Cycling Test; NA, not assessed; PEDI, Pediatric Evaluation of Disability Inventory; 9HPT, 9-hole peg test; MRC, Medical Research Council; ROM, passive joint range of motion; ext, extension; dfl, dorsiflexion; CI, confidence interval.
Between-group differences represent differences between the intervention and control group at T4, and these differences are adjusted for the clinical assessments (MFM, A6MCT, PEDI, timed tests, MRC, ROM, or echo intensity) at T0-T2 as well as for corticosteroid use and ambulatory status at T2.
Significant difference between the intervention and control group at the .05 level.
Nonparametric Wilcoxon rank-sum tests were used; no 95% CIs were, therefore, calculated.
Feasibility and Compliance
The 16 participants of the intervention group trained on average 4 (range 3-5) times per week and achieved 550 to 1050 (mean ~800) cycling revolutions with the legs (median OMNI score 3) and 600 to 1100 (mean ~800) revolutions with the arms (median OMNI score 3). One wheelchair-dependent participant (total MFM score 47%) was able to perform the bicycle training with the arms only. Three boys only participated in arm bicycle training because of a lack of time or a limited attention span. One boy cycled only 6 instead of 15 minutes with his arms because of exhaustion (OMNI > 6), even with maximal assistance.
Primary Outcome Measures T2-T4
The functional abilities were 4.9% higher in the intervention than in the control group at T4, after correcting for the ambulatory status and corticosteroid use at T2 and for the total MFM score at T0-T2 (Δ = 4.9, 95% CI = 2.2-7.6). This means that the total MFM score remained stable in the intervention group, whereas it decreased in the control group. In the intervention group, the same results were found for the D1 (Δ = 7.0, 95% CI =2.0-11.9) and D3 (Δ = 5.4, 95% CI = 0.9-9.9), but not for the D2 of the MFM. The D3 already showed a training effect at T3 (Δ = 5.8, 95% CI = 0.8-10.8). Overall, the intervention group showed an ES 0.5 for the total MFM scores, an ES 0.4 for the D1, and an ES 0.8 for the D3. No training effects were found for the A6MCT for the legs (Δ = 5.6, 95% CI = −56.2 to 67.3) and the arms (Δ = 13.8, 95% CI = −60.7 to 88.3; Table 1, Figure 3).
Individual differences in total Motor Function Measure (MFM) scores between T2 and T4 for each participant in the intervention group (black bars) and the control group (gray bars).
Primary Outcome Measures T4-T6
Eleven of the 16 participants who completed the intervention were followed up until T6. Six participants were lost to follow-up because of logistic reasons. The total MFM score had decreased on average by 2.5% (P = .02) from T4 to T5 and by 5.7% (P = .01) from T4 to T6. The D1 (−5.6%, P = .02) and D2 (−6.6%, P = .02) also showed a decline at T6, but the D3 (−4.3%, P = .09) remained relatively stable. No significant changes in A6MCT scores were found for the legs (P = .06) or the arms (P = .71; Table 2.
Primary Outcome Measures at T4, T5, and T6 for the Intervention Group (n = 11). a
Abbreviations: MFM, Motor Function Measure; A6MCT, Assisted 6-Minute Cycling Test; NA, not assessed; SD, standard deviation.
Data of the participants who were lost to follow-up were excluded from the analysis.
Significant difference at the P < .05 level.
Secondary Outcome Measures
No significant group differences were found for the PEDI, timed tests, muscle strength, ROM, and EI at T4 (Table 1). However, 3 boys lost the ability to walk 10 m and 2 boys lost the ability to rise from the floor in the control group, whereas in the intervention group none of the boys lost the ability to walk 10 m and only 1 boy lost the ability to rise from the floor during the training period. With respect to ROM, knee extension was more limited in the intervention group compared with the control group at baseline (P = .03). However, ROM remained relatively stable in the intervention group during the training period, whereas (although not significantly) it increased from 7.1 to 17.3 for knee extension, from 7.9 to 14.6 for ankle dorsiflexion, and from 5.4 to 13.5 for elbow extension in the control group. This was the same for the EI assessed with QMUS: Although not significantly, the total score decreased with 0.1 z-score in the intervention group whereas it increased with 0.8 z-score in the control group.
Training Effects in the Control Group After the Waiting Period
Nine of the 13 participants in the control group participated in the assisted bicycle training. One wheelchair-dependent participant had to be excluded prior to the start of the training because of a recent diagnosis of symptomatic cardiomyopathy. Three ambulant boys had to be excluded within about 2 weeks after trying the training because of cognitive problems, a luxated patella, and a combination of fatigue and logistic problems. One participant fractured his femur during a fall at home unrelated to the training after 5 months of training. Eight out of 9 participants therefore completed the 24-week training. The data of these remaining 8 control subjects showed an absence of functional decline during the training period comparable to the intervention group as assessed with the MFM, whereas no training effect was found for the A6MCT (Table 3).
Training Effect of the Control Group After the Waiting Period (n = 8). a
Abbreviations: MFM, Motor Function Measure; A6MCT, Assisted 6-Minute Cycling Test; NA, not assessed; SD, standard deviation.
Participants in the control group performed the same assisted bicycle training as the intervention group after a waiting period. Those who did not complete the training were excluded from the analysis.
Significant difference at the P < .05 level.
Adverse Events
No serious adverse events were observed or reported. During the training phase, postural adjustments were made in 3/24 participants who reported pain at the lateral side of the knee or foot due to an external rotation of the hip during training. In addition, 1 ambulant boy in the intervention group had an inversion trauma of his ankle after 12 weeks of training and subsequently stopped walking but he continued cycling. Another, wheelchair-dependent, boy fractured his femur after 12 weeks of training. This boy continued cycling with his arms and he resumed cycling with his legs after 3 weeks of immobilization. Injuries were unrelated to the training activities.
Discussion
The NUD study is the first trial in children with DMD testing the effects of assisted physical training on functional decline using a randomized controlled design. The results indicate that assisted bicycle training of the arms and the legs during 24 weeks significantly delays functional deterioration as tested with the MFM. The results also show that a low-intensity dynamic exercise training is feasible and safe even in boys who are in their late ambulatory phase or are wheelchair-dependent. If anything, the results contradict the current opinion that exercise training accelerates disease progression.
Previous studies have focused on resistance exercises and did not show clear training-induced benefits.11-13 We found that assisted bicycle training prevented a 6.3% functional deterioration from the total MFM score (100%) during the intervention period as observed in the control group over a period of 24 weeks. This decline was greater than the 5.8% decrease of the MFM during 1 year reported previously by Vuillerot et al. 18 In that study, participants were aged 6 to 32 years and also included individuals who were in their early ambulatory and late nonambulatory stage. The present study only included restricted walkers and boys who had recently become wheelchair-dependent who show a faster functional decline than ambulatory boys who are still in a relatively stable phase. Remarkably, the beneficial effects of training appeared to be strongest and most stable for the distal motor functions (D3 of the MFM). This is hardly surprising, as distal muscles are affected later than proximal muscles in DMD. 26 The beneficial effects on standing and transfers (D1 of the MFM) were smaller and disappeared fairly rapidly after cessation of the training. These observations support the notion that the adage “use it or lose it” is certainly applicable to boys with DMD.
In this study, the training-related preservation of standing, transfers, and distal motor functions was not paralleled by a preservation in axial motor functions (D2 of the MFM). This result was hardly surprising, as the bicycle training (where participants sat in a chair) placed no special demands on the axial muscle groups. We did not find an effect on overall endurance (as assessed with the A6MCT) either, which was most likely because of the low training intensity and the use of external electric support by the mobility trainer. The aim of the assisted bicycle training was to reduce the amount of secondary physical inactivity caused by disuse rather than to improve it by means of an endurance training with increasing demands. This probably also explains why no group differences were found with respect to secondary outcome measures such as strength. We believe that the assisted bicycle training may primarily preserve musculoskeletal flexibility and, thus, functional abilities, which is supported by the unchanged ROM of the ankle and elbow in the intervention group. It may also maintain cerebral representations of motor programs for both the arms and the legs, leading to preservation of functional skills. The training does not impose serious demands on muscle strength or aerobic capacity. Cardiovascular endurance training aims at performing at 70% to 80% of the maximum heart rate, which might not be reached in our study. Aerobic training guided by heart rate is difficult in boys with DMD as their peak heart rate is hard to determine because of limited peripheral capacity. 4 In future studies, the mechanisms underlying the preservation of functional abilities in relation to training need to be further investigated.
This study is the first explorative RCT in children with muscle disease investigating the effects of physical training. It is also the first study that used assisted exercises instead of resistance training, which is often too hard to sustain for these children. We observed a high compliance level in most participants. This is encouraging as it suggests that this type of training can indeed be implemented in everyday life. However, a limitation is the fact that analysis was performed as treated, that is, only those participants who adhered to the study protocol were included. We did not use an intention-to-treat analysis, because of the difficulty in handling missing data. A frequently used method to deal with missing data is carrying the last observation forward. This principle, however, is questionable in progressive muscle diseases. Nevertheless, missing data on the primary endpoints were limited to one dropout in the intervention group. A further limitation of this study is that randomization was incomplete. Two boys were directly allocated to the intervention group, because the study duration was ending. Another 2 boys were replaced to the control group after trying the intervention, because the training was unfeasible for them because of cognitive and motivation problems. A third limitation of this study is that we were not able to perform adequate test–retest reliability procedures on the primary outcome measure that showed a beneficial effect of training, that is, the MFM. We opted for multiple baseline measurements with intervals of 4 weeks to control for initial severity and progression. This time interval might be too long to examine the variability of the MFM, as DMD progresses rapidly. Another limitation of this study is that both participants and the assessor were not blinded, since this was considered impossible. The assessor had to coach the participants and had to monitor for signs of overexertion during the training period. Participants and assessor were, however, uninformed about previous test results at each assessment. Nevertheless, memories from previous assessments cannot be entirely ruled out. However, the assessor was not able to remember the exact test scores of all 30 boys who were assessed with 4- to 12-week intervals. All assessments were performed by the same investigator to reduce interrater unreliability.
In conclusion, this study indicates that assisted bicycle training of the legs and arms is feasible and safe for both ambulant and wheelchair-dependent boys with DMD and that prolonged physical training can delay the secondary functional deterioration due to disuse in these children. Carrying out an intervention study in a progressive disorder such as DMD is, however, difficult because of the phenotypic variability, and further research is required to support the results of this study and to enable clearly defined exercise protocols. Preferably, clinical trials should use stratified randomization to allocate their participants to either the intervention or control group according to the DMD phenotype: the common DMD form (ambulation is lost at a mean age of 9.3 years), the severe DMD form (ambulation is lost at a mean age of 7.1 years), or the milder DMD form (ambulation is lost at a mean age of 12.0 years). 28 If it is not possible to use stratified randomization, then researchers could limit the inclusion to one DMD subtype to reduce the phenotypic variability. To detect a decrease in disease progression, 4 to 6 times fewer boys are needed in a clinical trial, including boys with the severe DMD form compared to a trial including boys with the common DMD form. 28 The cost-effectiveness of training should also be investigated, as it is hypothesized that the incorporation of assisted bicycle training in daily life can reduce the frequency of physiotherapy and can shift the role of the physiotherapist toward a coach and assessor in the management of DMD, at least during the relatively stable stages of the disease. It is expected that boys should start training early in their disease course, as disuse could start at a young age in DMD.
Footnotes
Acknowledgements
The authors would like to thank all the participants and their families for their enthusiastic participation in this trial. The authors would also like to thank Duchenne Parent Project, the Dutch Association for Neuromuscular Diseases (VSN), and the rehabilitation physicians affiliated with the VSN for their help with the recruitment of participants.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article:
This study was financially supported by a grant from the patient organization Duchenne Parent Project.