Exercise Guidelines for Persons with DMD
Persons with DMD experience severe muscular weakness and atrophy. Contractures of the hip flexors and plantar flexors may interfere with ambulation. Most children are wheelchair users by the age of 12, and at this stage contractures of the hips, knees, ankles, shoulders, wrists, and fingers set in quickly.
Another major problem is the development of scoliosis. This may progress to severe thoracic distortion that can interfere with respiration. This thoracic s-curve deformity affects two-thirds of boys within two to three years of losing ambulation. Additionally, a loss of ambulation results in reduced caloric expenditure in these individuals.
Obviously, if caloric intake is not reduced, many of these boys will become obese. Obesity along with loss of strength may lead to other problems such as pressure sores, due to the inability to do wheelchair push-ups or move the body for extended periods. Finally, death due to respiratory failure may occur.
Exercise may be beneficial to these patients for many reasons. First, an increase in strength may improve performance of daily activities such as stair climbing, chair rising, and walking. Second, a flexibility program may reduce the progression of contractures, thus allowing persons with DMD to ambulate longer. Third, strengthening of postural muscles may decrease the formation of scoliosis in these persons due to reduced atrophy of the spinal musculature. Additionally, the increased energy expenditure may reduce the prevalence of obesity after ambulation is lost in these persons.
Flexibility should be the primary focus of the exercise program. Stretching should consist of passive and active stretches. Care should be taken not to cause muscle damage; therefore, ballistic stretching should be avoided. Stretches should be performed three to four times daily, held for 10 to 30 seconds, and repeated three times. Stretching should focus on muscle groups prone to contractures, such as the hip, knee, shoulder, wrist, fingers, and plantar flexors.
Based on an animal model of DMD, results showed that mdx mice ran significantly less distance per week and ran at a significantly slower daily speed compared to normal mice. The exercise resulted in improved resistance to fatigue, increased proportion of oxidative fibers, and improvements in force production of the soleus and extensor digitorum longus of exercised mdx when compared to sedentary mdx mice.
Comparing animals to humans can be difficult. However, based on these results, endurance exercise may be recommended for persons with DMD. Spontaneous walking speed should be the determination for the exercise intensity in these persons. Duration of exercise should be based on each individual's tolerance to exercise. Less distance and slower walking speeds will be expected in this population based on animal studies. The frequency of endurance training should be based on exercise tolerance from one to seven days per week. Since there are no studies providing evidence on dose-response relationships for exercise intensity in this population, precise exercise guidelines are uncertain. However, if possible, exercise goals should aim to progressively achieve 150 minutes per week in accordance with American College of Sports Medicine exercise guidelines. Walking surfaces should be level to avoid the performance of downhill walking, which may cause excessive muscle damage due to the increased eccentric component; any activity that consists of changes in directional velocity can cause muscle damage. Activities that are ballistic in nature or are a dynamic-constant application of external resistance are contraindicated. Cycling or swimming may be recommended for this population.
Based on the animal and human models that have looked at resistance exercise and DMD, formulation of clear muscle strengthening guidelines are difficult. It is clear that eccentric exercise increases muscle damage in normal and Dys negative muscles. Therefore, one would suggest that the eccentric component of the exercise should be eliminated, i.e., dynamic constant external resistance exercise, stepping down off a step repeatedly. However, a recent report showed that isokinetic eccentric exercise did not elevate CK in healthy individuals. Additionally, Lateur and Giaconi used submaximal concentric isokinetic exercise on persons with DMD with no negative effects.
It is hypothesized that concentric-only isokinetic exercise or eccentric-only isokinetic exercise, but not both at the same time, may improve muscular strength in individuals with DMD. With dynamic constant external resistance, (ex. machines or free weights) there may be an increase in muscle damage when there is a change in the direction of the velocity at the point where the concentric contraction is followed by an eccentric contraction. This is eliminated when doing concentric-only isokinetic exercise, or eccentric-only isokinetic exercise, therefore possibly reducing or eliminating muscle damage.