Evaluation of a control paradigm allowing heart rate guided rehabilitative exercise  for boys with Duchenne muscular dystrophy

Background: Aerobic cycle-training counters deconditioning and induces muscle and cardiorespiratory benefits in various neuromuscular disorders. However, its application to Duchenne muscular dystrophy (DMD) is limited due to lack of exercise prescription guidelines, particularly for intensity. A balance between beneficial versus harmful effects of muscle activity must be established given the weakness and concerns of contraction-induced damage inherent to DMD. Previous studies in DMD used motor-assisted cycling applying subjective ratings of perceived exertion to guide exercise intensity, whereas objective parameters such as heart rate (HR) or work performed were not reported. In efforts to develop exercise guidelines for DMD, we designed a motor-assisted cycle-exercise paradigm using closed-loop control of motor effort and individualization of intensity based on HR. Feasibility of this paradigm in DMD was tested in the home setting with remote clinical supervision. Methods: A closed-loop controller was developed with user-defined saturation points for cadence and baseline motor inputs to ensure safety of cycling and adjustments in level of muscle overload (assistive current). The controller allowed remote, interactive adjustment of current based on HR biofeedback, providing cycling assistance when velocity approached a lower-bound and resistance when the upper-bound was approached. A target intensity of 40-50% HR reserve was individualized for each participant and motor effort was adjusted accordingly by the clinician. Force-sensors were embedded in the pedals for quantification of passive and active power. Results: Six ambulatory boys with DMD (aged 7.7 + 0.9 years) completed at least two bouts of cycling exercise (3-10 minutes per bout) with an average 0.53 + 0.15 amps assistive current (range 0.3-0.8 amps). HR increased from rest during passive and active cycling (mean 109.2 + 6.1; 119.2 + 8.5; 149.7 + 4.6 bpm respectively), where boys were actively exercising at 45% of HR reserve at an average cycling power of 5.7 + 1.3 watts (ranging 3-8 watts depending on disease severity). Conclusion: These results show for the first time that boys with DMD can cycle actively to generate power and raise HR to a prescribed intensity, supporting feasibility of this home-based, remotely-supervised control paradigm. They warrant future study to establish clinical exercise prescription parameters and the potential of aerobic cycling as a rehabilitative strategy in DMD.