Isometric Exercise in DMD Found to Improve Boys’ Strength, Function

Mild to moderate isometric leg exercise — contractions of a particular muscle or group of muscles — is safe and improves muscle strength and functional ability in boys with Duchenne muscular dystrophy (DMD) who are able to walk, a study suggests.

The study, “Safety, feasibility, and efficacy of strengthening exercise in Duchenne muscular dystrophy,” was published in the journal Muscle & Nerve.

Isometric muscle contractions in the legs occur during exercise when force is generated without any change in length or joint angle. Such exercises have been suggested, as compared with more strenuous physical activity, as a possible way to reduce muscle damage in people with DMD.

Programs incorporating such exercise have been supported by findings in a mouse model of the disorder, but tests in boys with DMD have not been conducted.

To fill this knowledge gap, researchers from the University of Florida explored the safety, feasibility, and efficacy of a mild to moderate resistance isometric leg exercise program in a two-part study.

The team began by testing two increasing intensities and frequencies of isometric exercise to determine dose response and its safety in 10 ambulatory boys with DMD (mean age 8.3 years).

Specifically, the scientists performed initial assessments on day one, consisting in part of three safety measures. These were an MRI of the leg muscles, a pain rating, and an assessment of blood creatine kinase, or CK, a marker of muscle damage. The assessment also included a determination of the peak strength of both the knee extensors and knee flexors of the right leg.

On day three, the participants underwent one exercise session, with the safety measures repeated 48 hours later. The exercise sessions were then repeated on days eight and 10, with the safety tests repeated on day 12.

In addition, the researchers examined the safety and feasibility of a 12‐week home strengthening program of mild to moderate intensity. The program was performed by eight of the boys (mean age 9.3 years) at their homes, with remote supervision.

To build a rapport with the boys and their families, three exercise sessions were conducted prior to the start of the in-person program at the University of Florida, in Gainesville. The boys were provided with the same equipment used at the university. If no signs of muscle damage could be found in the initial safety measurements, the equipment was shipped to each participant’s home.

After a 10% increase of exercise intensity midway through the program, the patients returned for a final assessment at the end of 12 weeks.

Each exercise session (of both legs) lasted approximately 90 minutes, three days per week.

Functional ability was assessed by the time it took to ascend or descend four stairs, while muscle strength was measured with a Biodex dynamometer, a computerized robotic measuring device. Specifically, the hip was set at 90 degrees, while the knee was tested at both 60 and 30 degrees, the researchers said. Data were analyzed using appropriate statistical models.

In the first part of the study, exercise sessions at different intensities revealed no muscle damage. No meaningful increase in pain was reported by any of the participants.

In the second part, seven of the eight boys completed the 12‐week in‐home program, with a compliance of 84.9%. One boy withdrew from the study because of the burden of participation coupled with anxiety he was experiencing.

MRI scans did not indicate signs of muscle damage for any of the participants. One boy had an increase in CK blood levels indicative of muscle damage but he reported no pain.

Notably, both strength and functional ability improved significantly after exercise training, with all seven boys who completed the program experiencing strength gains.

“Appropriately dosed strengthening exercise has the potential to improve strength and ability to descend stairs while being safe for boys with DMD,” the researchers wrote.

Several limitations to the study were noted by researchers. First, since there was no random assignment to different regimens nor a control group, the results cannot be generalized. Second, although compliance was excellent, the exercise session length was long for young boys, the investigators said. Third, the scientists did not explore the potential positive changes seen from exercise aside from the timed stair test.

“The results of this work suggest an in-home, mild- to moderate-intensity isometric exercise program done [three times per week] is safe and potentially has a positive effect on strength and function in ambulatory boys with DMD,” they concluded.