Safety, feasibility, and efficacy of strengthening exercise in Duchenne muscular dystrophy

Abstract

Background: This two-part study explored the safety, feasibility, and efficacy of a mild-moderate resistance isometric leg exercise program in ambulatory boys with Duchenne muscular dystrophy (DMD).

Methods: First, we used a dose escalation paradigm with varying intensity and frequency of leg isometric exercise to determine the dose response and safety in 10 boys. Second, we examined safety and feasibility of a 12-wk in-home, remotely supervised, mild-moderate intensity strengthening program in eight boys. Safety measures included T₂ MRI, creatine kinase levels, and pain. Peak strength and function (time to ascend/descend four stairs) were also measured.

Results: Dose-escalation revealed no signs of muscle damage. Seven of the eight boys completed the 12-wk in-home program with a compliance of 84.9%, no signs of muscle damage, and improvements in strength (knee extensors P < .01; knee flexors P < .05) and function (descending steps P < .05).

Conclusions: An in-home, mild-moderate intensity leg exercise program is safe with potential to positively impact both strength and function in ambulatory boys with DMD.

Keywords: Duchenne muscular dystrophy; T2 MRI; functional ability; in-home exercise program; isometric exercise.

Figure 1.

The six participants from Study 1 who performed single or repeated bouts of acute moderate intensity strengthening exercise did not demonstrate any clinically meaningful increases in creatine kinase (CK) levels. Values are shown before (Baseline) and after a single bout (Subjects 1-4 from Experiment 2) or after 3 bouts (Subjects 5 and 6 from Experiment 3) of exercise. Each bout consisted of four sets of six repetitions of isometric knee extensor and knee flexor exercise at both 30 and 60 degrees of knee flexion at an intensity of 50% maximal volitional contraction (MVC).

Figure 2.

Baseline (BL) measures compared to post-training (PT) of the knee extensors (KE) and flexors (KF) after 12 weeks of moderate intensity isometric exercise: A) No increase in the primary safety measure of MRI T₂ (a construct of muscle damage) was noted [non-significant increase in mean T₂ from baseline to 12 weeks: KE = 2.3 (SD 3.6)% and KF = 0.4 (SD 4.6)%]; B) Significant improvements occurred for both outcome measures assessing strength (sum of KE or KF at both leg positions tested) and functional ability. Peak torque increased by 20.6% for KE and 14.3% for KF, and the time to ascend (13.5%) and descend (22.7%) four steps improved after 12 weeks of leg strengthening. Differences noted between BL and PT (*p < 0.05, **p < 0.01).