Conflicting theories exist regarding the effects of exercise on the body systems of people with Duchenne muscular dystrophy (DMD).1,2,3,4 While the typical response to exercise is an increase in muscle mass and more efficient oxygen and nutrient transportation, exercise may actually increase the degenerative processes associated with DMD.5,6,7 One of the most accepted theories regarding the degenerative process of muscular dystrophy is that the lack of dystrophin in the cytoskeleton decreases the integrity of the muscle fiber leading to increased tearing of the cell membrane in response to contractile forces.8,9
Exercise, especially eccentric exercise, increases tensile forces placed on the cell membrane as myosin and actin pull to initiate or maintain contraction.10,11,12 While muscles typically regenerate and hypertrophy in response to exercise-induced stress, the muscle cells of people with DMD degrade and are not able to regenerate appropriately.5,6,7 For this reason, exercise may actually accelerate disease progression. Alternatively, certain forms of low to moderate intensity exercise may induce a transformation of certain muscle fibers from a fast-twitch glycolytic profile toward a slower, oxidative preference.13 Slow twitch fiber composition may be beneficial to the person with DMD in the later stages of disease.
Minimal research has explored the cellular response of dystrophic muscle fibers in humans in response to exercise. Typically, animal models are used to explore effects of training prior to exploration in human subjects. The most common animal model in use for DMD is the mdx mouse model. Mdx mice display a lesser phenotype than humans despite a similar genetic mutation14, thus exercise studies on mdx mice do not accurately represent the severity of changes seen in humans with exercise. With this in mind, results of exercise responses in animal studies must be carefully assessed prior to initiating a training program in the human model. Currently no exercise protocol exists for children affected by the genetic mutation causing DMD.
Table of Contents
Figure 1. Satellite Cells
There are many causes of muscle damage including trauma, disease, and even some types of exercise. The most common exercise-induced damage is in response to eccentric exercise.10,11,15 In a normal muscle, there is a specific healing process the muscle goes through after the microtrauma caused by eccentric contraction during exercise. The phases of skeletal muscle healing are necrosis/degeneration, inflammation, repair/regeneration, and scar tissue formation (fibrosis).15
In the first phase, characterized by necrosis/degeneration of the myofibers, the plasma membrane and basal lamina are disrupted resulting in an ingress of extracellular calcium and subsequent autogenic degradation of the damaged muscle fiber unit.15 Within the first day of injury, the inflammation phase kicks in to help decrease the degradation of the muscle cells and the area becomes saturated by inflammatory cells such as mononuclear cells, activated by macrophages and T-lymphocytes.15 At the same time, there is a secretion of growth factors and cytokines that help promote increased blood flow to the area to enhance the inflammatory response. Only once the phagocytic inflammatory cells have cleared away the necrotic tissue can the muscle begin to regenerate. If inflammatory cells do not leave the area, muscle regeneration is blocked.15 Therefore, even though there is an initial benefit of the inflammatory response and cells of the inhibition of myofiber degeneration, this benefit can be short lived if they do not clear out of the muscle to let regeneration occur.15
As stated before, for regeneration to occur the inflammatory cells need to clear out of the myofibers. After the inflammatory cells exit, for regeneration to commence it is necessary for satellite cells to be activated in the damaged skeletal muscle fibers.15 Satellite cells are unspecialized, mononucleated cells that are found to be in a dormant state between the muscle fiber and the basal lamina in an uninjured state. In response to the injury satellite cells are activated by cytokines and growth factors (IGF-1) and are then considered to be myoblasts.15 As a myoblast, it works to assemble and fuse together to form a new myofiber.15 They also contribute a small amount of cytoplasm and a new nucleus to the myofiber structure. An important feature of the satellite cell is that they are self-renewing, allowing them to create a residual pool of satellite cells for future use during another muscle microtrauma. It has been found that satellite cells account for approximately half of the force restoration after eccentric contraction-induced injury.15 This shows that they are a key player in the repair/regeneration phase of the muscle healing process. Regeneration of the muscle typically occurs within 7-10 days from the time of the injury, peaks at 2 weeks, and gradually declines, ending 3-4 weeks later.15
Individuals with DMD experience an increased rate of degeneration of skeletal muscle fibers as a result of the lack of dystrophin. These people are unable to continuously replenish their pools of satellite cells, thus hindering the regeneration process.15 This inability to self-renew may be due to the fact that fibroblasts found in dystrophic muscle have been shown to secrete increased levels of IGF-1 binding proteins, thereby diminishing the IGF-1 supply available for satellite cell activation. Therefore, the dystrophic muscle fibers become inflamed, necrotic, and fibrotic resulting in muscle wasting, weakness, and fatigue.15
As stated above, eccentric contraction-mediated muscle damage is a major contributor to progressive fiber damage, degeneration, muscle wasting, and decreased force production seen in boys with DMD.10,11 Fast-twitch Type II fibers are more susceptible to damage caused by eccentric contractions, exacerbating the overall decrease in gross force production commonly observed.11,12,13,16 Another mechanism leading to decreased force production involves branching of muscle fibers seen with advancing age in DMD due to decreased levels of IGF-1 and other enzymes which promote muscle cell regeneration following mechanical damage. Branched fibers are inherently fragile, as shown by research demonstrating fiber segments containing branch points were more liable to rupture during whole muscle contraction compared with normal fibers. Additionally, branched segments were more likely to rupture than unbranched fibers within the same muscle during stimulation.17
Muscle morphology during varying ages of mdx mice have been studied, finding 17% of overall muscle fibers branched in 1-2 month old mdx mice, while 6-7 month old mdx mice exhibited 89% of fibers branched. In addition to greater numbers of branched fibers, branching patterns seen in older mdx mice were more complex than those found in younger mdx mice.18 Muscles from both mdx age groups were also subjected to an in situ eccentric protocol consisting of 15% strain on the Extensor Digitorum Longus muscle (EDL) via pinning to a force transducer and 3 trials of stimulation using an electrical current with 5 minute rest intervals. Force production was measured before and following completion of the procedure. Younger mdx mice displayed a 7% force deficit following eccentric contractions, while older mdx mice exhibited a 58% force deficit, which was significantly higher than young mdx mice and control mice force levels.18 Research by Head19 displayed a 40+8% drop in maximal force production following a moderate in situ eccentric protocol, using mounting to a force transducer and stimulation via electrical stimulation, in older mdx muscles with >90% branched fibers compared to no observed force loss in either control muscles or mdx muscles with <10% branched fibers. This data shows a correlation between age of muscle fibers and extent of muscular branching and amount of contraction-induced damage leading to overall loss in force production capacity in the mdx mouse model.19 While these results are convincing, one should maintain caution in generalizing them to the human phenotype of DMD. As stated before, the mdx mouse expresses a milder form of DMD than humans, and the nature of in situ muscle dissection experiments do not lend themselves to realistic comparison to the types of contractions experienced by boys with DMD. In the above studies, 3 trials of stimulation, at most, were performed. Lengthening contractions during exercise or general activity by boys with DMD will typically consist of more than 3 consecutive episodes of muscle fiber lengthening. When combining this fact with the reported force loss in mdx mice and their known milder phenotype, one could assume that eccentric contractions have the potential to be even more damaging in humans.
The absence of the protein dystrophin has widely been linked to the underlying cause of muscular damage following lengthening contractions due to its hypothesized role in stabilizing the sarcolemmal membrane. However, taking into account the above results, it seems that the extent and amount of muscle branching is also a primary determining factor, since younger mdx mice displayed less force deficit than older, more branched mdx muscle fibers even though both ages lack dystrophin.18 Therefore, exercising during earlier stages of DMD, when branching has not progressed to a damaging extent, may have benefit as a protective means to lengthen the time before muscle damage occurs. Also, Zanou et al.20 identified the abnormal activation of TRPV2 as partly responsible for resultant force deficits following eccentric contractions, not sarcolemmal rupture.20 TRPV2 is a stretch-sensitive transient membrane channel which is pathologically present in the sarcolemma of mdx fibers and allows an abnormal influx of calcium into the muscle cell, preceding myotubule damage.20
The Role of Calcium
The absence of dystrophin in the muscles of individuals with DMD results in mechanical weakness in the myofiber membranes.21 Stresses put on the membranes, such as eccentric contractions during exercise, can cause a breakdown in the cell’s structure.21 The microtrauma caused by muscle contraction during exercise causes disruption of the plasma membrane and basal lamina of muscle cells.15 In dystrophin deficient muscles, there is excessive calcium entry into the intracellular space and calcium homeostasis is altered so that there is an unusually high resting level of calcium.22,23 This increased permeability to calcium can impair many different cellular processes within the cell causing widespread cellular damage. This imbalance impairs the excitation-contraction (EC) couple of the muscle.22 In a normal muscle, this EC coupling helps propagate an action potential along the sarcolemma, activate myofilament contraction by releasing calcium from the sarcoplasmic reticulum, and, finally, relax the muscle by calcium re-uptake in sarcoplasmic reticulum pumps.24 The excess calcium in a dystrophin-deficient muscle activates proteases that damage the sarcoplasmic reticulum, hindering this EC coupling process.22 The calcium-overloaded and impaired sarcoplasmic reticulum release of the excess calcium causes muscles to fatigue quickly and causes a decline in force production of the muscle.25 This also impairs oxidative phosphorylation or alters ATPase activity in these individuals.22 As this cycle continues, the muscles become more susceptible to injury, especially during high intensity exercise.21 One study conducted in mdx mice has found that as this process continues to escalate, later on in life, not only will eccentric contractions of muscles cause damage and necrosis, but simple shear stresses of isometric muscle contraction will be just as damaging as eccentric stresses.19 The ultimate ending to the excess calcium is apoptosis and necrosis and increase muscle protein degradation which will eventually lead to significant decreases in strength and a decreased tolerance for activity.3,22
Burdi et al.26 have discovered that cAMP and the phosphodiesterase inhibitor pentoxifylline help decrease the increased calcium influx.26 This discovery has helped scientists focus their research on these mechanisms to help stop the overwhelming influx of calcium into the cells.26 Kaczor et al.3 have found that low intensity aerobic exercise can actually help improve calcium homeostasis.3 This is discussed more in depth below.
Figure 2. Representative light micrographs of left ventricles from mdx mice after aortic band operation to simulate increased pressure similar to exercise induced increase in systolic pressure. Figures show the sham operated mdx mice (A), aortic banded mdx mice on day 2 (D) and on day 14 (G), and aortic banded control mice on day 14 (J). B and C, E and F, H and I, and K and L are light micrographs of the areas surrounded by arrows in A, D, G, and J, respectively, at higher magnification. Arrowheads show lesions. Scale bar in J represents 0.4 mm and in K and L 20 μm. HE, hematoxylin–eosin stain; Azan, Azan stain.
In the cardiovascular system, cardiomyocytes of mdx mice that have been experimentally stressed show increased cardiac necrosis and fibrosis similar to that found in DMD patients.27 Mdx mice that participated in moderate voluntary exercise had significantly thinner lateral ventricular walls and larger septolateral diameters than sedentary mdx mice.27 Additionally, fibrotic lesions can sometimes be seen in the myocardium of the left lateral ventricular wall. Pronounced fibrotic lesions and cardiomyopathy usually do not appear in mdx mice until one year of age; therefore, the presence of both cardiomyopathy and fibrotic lesions in young, seven-week-old mice in a study by Costas et al.27 indicate that increased mechanical stress from exercise can accelerate the dystrophic process.27 Lesions in the myocardium consist of injured myocardiocytes characterized by disappearance of muscle fiber, mononuclear inflammatory cells, fibrosis, and increased collagen fibers.28 While some studies have seen an increase in fibrotic lesions following exercise, others have only noted a significant increase in TGF-β1 without a morphological expression of fibrosis.29
Dystrophin deficient myocardium in mdx mice is more vulnerable to overpressure and the degree of injury correlates with the level of systolic pressure (see Figure 2).28 Pressure overload is hypothesized to induce apoptosis in cardiomyocytes in mdx mice because apoptotic cardiomyocytes have been linked to various cardiac diseases and apoptotic cells are found in the skeletal muscle of mdx mice.28 Due to the possible harmful effects of pressure overload on cardiomyocytes, it is recommended to avoid stresses that elevate blood pressure to reduce damage in the hearts of patients with DMD.28 In addition to increased apoptosis in exercised mdx mice, there is a higher infiltration of inflammatory cells and an increase in fibrosis and adipose tissue in cardiac muscle.30 There are also elevated levels of phosphorylated p38 MAPK (p-p38 MAPK), phosphorylated extracellular signal regulated kinase 1/2 (p-ERK1/2) and calcineurin.30 Calcineurin and MAPK are involved in the activation of growth factors such as TGF-B and IGF that are part of the process of fibrotic proliferation. MAPK and calcineurin are also directly involved in the regulation of apoptosis with calcineurin inducing apoptosis through dephosphorylation of Bad.30
As mentioned in the cellular biology section, with advances in pulmonary interventions for patients with DMD, there is an increase in the incidence of deaths from cardiomyopathy.27,31,32 Additionally, advances in technology to repair skeletal muscle in DMD could have devastating consequences on the hearts of patients with DMD. Repairing skeletal muscle and improving the ability of the muscle to withstand exercise leads to higher participation of individuals with DMD in physical activity and an increased vulnerability of cardiac muscle to sustain damage from contractions.31,33 A mouse model, termed Tg-mdx, has been used to determine the effects of a modified dystrophin molecule or mini-dystrophin to repair skeletal muscles, including the diaphragm, and the resulting implications for the heart.31,33 Results showed reduced ejection fraction, a significant increase in left ventricular diastolic volume, and cellular necrosis indicated by a significant, fivefold increase of intracellular accumulation of immunoglobulin in the heart.31,33 This indicates that future therapies need to be mindful of the cardiac involvement in patients with DMD when addressing the skeletal muscle deficits.
Respiratory failure has been cited as the leading cause of death in patients with DMD.31,34,35 Similar to skeletal muscle breakdown, the respiratory muscles, including the intercostal muscles, diaphragm and shoulder girdle muscles, decline in function and are likely responsible for the decreased muscle force and respiratory dysfunction in patients with DMD.35,36 The diaphragm and paraspinals have been shown to have heterogeneity in cell size, a high incidence of centrally nucleated fibers (indicating a cycle of degeneration-regeneration), inflammatory cell infiltration, and fibrous tissue.35,37 Both the diaphragm and the intercostal muscles show multifocal endomysial inflammation and the diaphragm demonstrates marked fiber loss and replacement with intersistial collagen.37,38 Weakness in inspiratory and expiratory muscles can result in an ineffective cough and decreased ventilation leading to pneumonia, atelectases, respiratory insufficiency, alveolar hypoventilation, and/or resting hypercapnia (high levels of CO2 in the blood).36 Additionally, weakness of the paraspinal muscles can lead to severe scoliosis further compromising respiratory function.36,37
Repeated low intensity treadmill exercise has been shown to improve the contractile strength of the diaphragm in mdx mice.39 Additionally, inspiratory training using a resistive breathing device displayed a significant increase in the strength and endurance of inspiratory muscles in humans with DMD which were maintained for six months after completion of the intervention.34 Patients who had vital capacity levels less than twenty-five percent predicted and/or a PaCO2 level greater than 45mmHg, both criteria indicating severely impaired ventilator function, did not experience a benefit from inspiratory muscle training. There were no changes in blood creatine kinase levels across the patients indicating that this training was not deleterious to the muscle.34 Training has been determined to be best when started in the early stages of DMD at a low intensity training load of 30% of the individual’s maximal inspiratory pressure.40
A current program used by patients with DMD involves voluntary hyperventilation for 30 minutes/day for six weeks.41 While the effect of hypercapnia alone on the respiratory muscles is currently not well understood, the response has been an increase in the fatigue-resistant fibers in the diaphragm. Alpha-dystrobrevin is a protein that is partly similar to the COOH terminus of dystrophin, parallels dystrophin in expression, and is significantly reduced in DMD.41 After hyperventilation training, alpha-dystrobrevin expression increases and may help maintain muscle cell integrity in the diaphragm.41 Hypercapnia can also have anti-inflammatory effects by attenuating NF-B activation and therefore TNF-alpha.41 Additionally, CO2 may savenge peroxynitrite and prevent protein nitration and oxidative damage.41
In healthy skeletal muscles, a normal response to exercise is microdamage to the muscle fibers followed by an inflammatory immune response which subsides after several hours or days depending upon the intensity of exercise. In the skeletal muscles of boys with DMD, the inflammatory response does not subside and the extracellular environment is exposed to an increase in inflammatory cells such as macrophages and cytokines including TNF-alpha and TGF-beta for an extended period of time leading to further muscle damage and fibrosis.42
Figure 3. Prednisolone Treatment44 - Click to enlarge
Glucocorticoids, such as prednisolone and deflazacort, are the gold standard drug therapy for patients with DMD.43 While the exact mechanism for steroids in DMD is unknown, it is thought that steroids reduce the secondary inflammation that extends muscle fiber necrosis beyond the primary damage caused by membrane disruption that is induced by exercise.43 Deflazacort has also shown to up-regulate calcineurin/NF-AT pathway activity and advance muscle regeneration by increasing the proliferation of muscle precursor cells (satellite cells) and promoting the growth of undamaged fibers and hypertrophy of new fibers formed during treatment.43 Treatment with deflazacort has shown a decrease in myf5 gene expression, a marker of muscle regeneration, in exercised mdx mice indicating a decreased need for regeneration due to less damage to the muscle during exercise.43 Similar results have been found in treatment with prednisolone. After 1 week of treatment, genes associated with cell growth and proliferation, chemokines and cytokines, ion transport, metabolism and proteolysis, and a modulator of calcineurin were up-regulated while interleukin 17 was down-regulated.44 Following 6 weeks of treatment, skeletal muscle genes were upregulated including two responsible for regulating muscle hypertrophy (p85alpha P12 kinase regulatory subunit and insulin-like growth factor binding protein 6) while proteins induced by TNF-α were down-regulated.44 Centralized nuclei are used as a marker of muscle fiber degeneration-regeneration history and they are prominent in mdx mice, accumulating with age.45 Treatment with prednisolone also results in a 20% lower number of centralized nuclei in exercised mice.45
Acute exercise loading has a severe apoptotic effect on the skeletal muscle of mdx mice.46 Steroid administration before acute exercise loading can diminish an apoptotic surge, however, steroid injection in free-living mdx mice has been shown to have a pro-apoptotic effect.46 Steroids are a pro-apoptotic factor in a free-living state but can be anti-apoptotic in a hostile situation such as post-exercise in which cytokines and ROS can trigger additional apoptotic induction via the extrinsic apoptotic pathway through TNF-α or Fas ligand.46 In control mice, heat shock protein (HSP) may protect against apoptosis induced by acute exercise and HSP may be defective or limited in mdx mice allowing the steroid to act as an anti-apoptotic.46
Growth Hormone (IGF)
The use of systematic administration of insulin-like growth factor (IGF-1) has been studied as a method of ameliorating dystrophic symptoms in the mdx mouse model of DMD and has been found to improve muscle function and decrease vulnerability to muscle fatigue.47,48 The mechanism behind this has been found to be a shift towards a more oxidative muscle phenotype and a reduction in the susceptibility of the muscle to contraction-mediated damage.47 Decreased susceptibility to contraction-mediated damage was found in multiple muscles, including the diaphragm, and was independent of changes to markers of muscle oxidative metabolism.47 In the mdx mouse, abnormal excitation-contraction coupling has been found in the dystrophic muscle, which may occur as a result of disruption of the dystrophin-associated glycoprotein complex (DGC).49 However, IGF-1 administration can ameliorate fundamental aspects of this excitation-contraction coupling failure in dystrophic muscle fibers.49 Another study looked at mdx mice that naturally over-express IGF-1 and found a decrease in muscle fiber necrosis in this population.50 Levels of IGF-1 in mdx mice have been shown to decrease following chronic high intensity forced treadmill training51, but Grounds et al.50 found that over-expression of IGF-1 ameliorated the muscle weakness normally induced by exercise.50 The protective effect of over-expression of IGF-1 may be related to increased protein synthesis as well as decreased protein degradation.50 Further studies still need to be done in the mdx mouse models with IGF-1 and varying intensities of exercise, but the results of current research provide promise for the use of IGF-1 in humans with muscular dystrophy.48 Since respiratory function is a key predictor of mortality in DMD, therapies such as IGF-1 administration that have the potential to improve the diaphragm muscle function have relevance to this population and provide direction for future research.47
A trial of idebenone, a synthetic analogue of coenzyme Q10 (CoQ10) was conducted on male mdx mice based on the drug’s potential to improve mitochondrial respiratory chain function, increase cellular energy production and reduce oxidative stress.52 The drug was administered daily to the mice in the treatment group from the age of 4 weeks to 10 months. Idebenone treatment significantly corrected cardiac diastolic dysfunction, reduced cardiac inflammation and fibrosis and prevented mortality caused from cardiac pump failure induced by dobutamine stress testing in vivo with a reduction in mortality rate from 58% to 19%.52 Though not statistically significant, a reduction in serum levels of cardiac Troponin I, a marker of degree of active myocardial degeneration, was noted in the mdx mice treated with idebenone.52 The mice treated with idebenone also showed improvements in voluntary running performance through improvements in running speed.52 These cardioprotective findings suggest a novel potential therapeutic strategy for people with DMD, a disease where associated cardiomyopathy accounts for approximately 40% of premature deaths.52
TNF-alpha is a cytokine that is dramatically increased in DMD skeletal muscle. Research studies have been completed to determine that injection of TNF-alpha inhibitors, such as cV1q, into dystrophic skeletal muscle in mdx mice actually blocks the inflammatory response of TNF-alpha resulting in a protective effect on the damaged muscle fibers and a reduction in the severity of the dystrophic pathology.53,54,55 Studies have taken these results one step further and explored the effects of the cV1q antibody (specific to the mdx mouse) on dystrophic and non-dystrophic muscle fibers with the addition of a voluntary exercise component56 as well as a forced eccentric exercise component.55 In the study by Radley et al.,56 looking at voluntary exercise, the mice were given a metal mouse wheel and revolutions were recorded for 8 weeks. The results of the study demonstrated that with cV1q antibody treatment, the amount of muscle necrosis was reduced in adult mice who voluntarily exercised.56 The mice that were injected with the cV1q antibody also had a significant increase in the amount of completed voluntary exercise, resulting in an overall increase in exercise when considering all dystrophic mice. The outcome indicates possible long term benefits and increases in function in mice that are injected with the cV1q antibody.56 The mice injected with the cV1q antibody had a significant decrease in skeletal muscle fiber necrosis, despite physical activity in the form of voluntary wheel running.56 Pre-treating the mice with cV1q counteracted the significant increases in quadriceps muscle fiber necrosis experienced in adult exercising mice without the injection. Creatine kinase (CK) is generally increased in dystrophic muscle fibers leading to a more rapid inflammatory response.56 However, after 8 weeks of exercise in mice with the cV1q injection, no increase was noted. It is important to note that a 10-fold increase in CK was seen in the mice who didn’t receive the injection following the intervention. Interestingly, the mice in the non-exercise group who received the cV1q injection saw no significant effects on muscle fiber necrosis or CK levels. This finding indicates that cV1q is specific to protecting against muscle fiber damage as a result of exercise.56 The mice who received the cV1q injection demonstrated an increased tolerance to exercise, decreased CK levels leading to a minimization of the inflammatory cascade, decreased muscle fiber necrosis, and increased muscle regeneration.56 Piers et al.55 looked at the blockade of TNF-alpha in the adult male mdx mouse using eccentric exercise by use of a dynamometer. The study found that the mice injected with cV1q showed reduced contractile dysfunction as well as reduced muscle fiber necrosis following eccentric exercise.55 Injecting the mice with cV1q one week prior to exercise not only showed significant decreases in necrosis following exercise, but also in the untested limb as well indicating an effect on the background pathology.55 The successful results observed in these studies provide promise in developing a similar concept to achieve the same benefits in the human model. However, before trials can be done in the human model, an optimal dosing and exercise regimen need to be established.55,56
TYPES OF EXERCISE
It is established that dystrophin deficient skeletal muscle is weak, easily fatigable, and susceptible to injury,1 especially when performing eccentric contractions. As previously stated, boys with DMD exhibit decreased force levels through maximum voluntary contraction (MVC).57 Even though the actual cause of increased muscle fatigue is still unknown, general hypotheses have been established. Oxidative stress may impair the production of ATP in DMD muscle during anaerobic activity.58 Underlying sources, such as reduction of ATP production due to ROS-caused enzyme slowness or a simple reduction of contractile tissue is a possible mechanism of the increased fatigability that is generally observed.58,59 Resistance training is important for patients with normal skeletal muscle to increase ambulatory function and assist with activities of daily living. However, it is unclear whether the same positive effects are seen in patients with DMD. To determine whether resistance exercise is safe for patients with dystrophic skeletal muscle, the mdx mouse model has been utilized. Despite the fact that exercise parameters for resistance training have not been established in the human model,60 the mdx mouse model has been successful in determining appropriate exercise interventions that demonstrate improvements in function that outweigh the amount of damage to the muscle fibers through skeletal muscle adaptation.1 Many studies have been done to show that the mdx mouse develops increases in muscular force as a result of voluntary wheel running,2,61,62 and also builds a greater tolerance to activity.2
A research study by Call et al.1 divided dystrophic mice into three different experimental groups for a 12 week intervention. The first group was given 24 hour a day access to a running wheel that had progressively increasing resistance. All the wheels had a resistance set at 1 gram (which is 6% of body mass). Resistance was increased by 1 gram each week until 7 grams (20% of body mass) was reached in week 11. The second group was given the same access to a running wheel, but no resistance was added. There was also a control group which was sedentary (no wheel was provided). Measurements taken during the experiment included: body mass, running distances, dorsiflexion torque, forelimb grip strength and whole body tension.1 The results found that the mice in the experimental groups had lower body masses when compared to the sedentary group.1 The group without wheel resistance had greater running distances, but the mice that ran with resistance completed more work. One of the most important findings of this study is that the mice that ran with resistance had no evidence of increased skeletal muscle damage when compared to the non-resistance group after 12 weeks of running.1 This supports the hypothesis that skeletal muscle has the ability to adapt to light resistance training to achieve strength benefits, similarly to the effects of low intensity aerobic exercise. There was not a significant difference in the levels of creatine kinase in the soleus muscle between the resistance and non-resistance group following the intervention, which demonstrates that additional muscle injury did not occur as a result of the added resistance.1 Further research needs to be done to determine the potential beneficial effects of resistance training on the human model (and possible detrimental effects), but recent mdx mouse studies give promise to determining the appropriate level of intensity and duration to improve function in patients with DMD.1
Kaczor et al.3 conducted a study to determine whether low intensity exercise training would reduce markers of oxidative stress (malondialdehyde (MDA) and protein carbonyl content) possibly through increases in antioxidant enzymes (cytosolic superoxide dismutase (Cu/ZnSOD), mitochondrial superoxide dismutase (MnSOD), catalase (CAT), and/or glutathione peroxidase (GPx)) in skeletal muscle.3 Due to the lack of dystrophin in DMD skeletal muscle, muscle protein oxidation and the production of free radicals causes injury leading to apoptosis and protein degradation. The low intensity treatment intervention was comprised of 8 weeks of treadmill running in mdx mice. The exercise group performed 30 minutes of running a day, 2 days each week at a speed of 9 meters/minute. This speed is below the maximal intensity where pathology has been observed in mdx mice from previous studies.3 They found that low intensity exercise training decreases oxidative stress markers and there are few indicators of muscle fiber damage following the low intensity exercise indicating that this type of exercise does not have a detrimental effect on the mdx mouse.3 They found that in the first few weeks of low intensity exercise, ROS generation actually increased and markers of oxidative stress were elevated. However, the mice were able to adapt over the duration of training, resulting in a lower production of free radicals and decreased levels of oxidative markers in the muscles.3 They found lower protein carbonyls and MDA levels following low intensity exercise.3 The authors believed that this suggested that several pathways were involved in decreasing ROS levels including: activation of nNOS and up-regulation of NO formation that results in lower generation of peroxynitrite, improved calcium homeostasis that results in decreased ROS generation, acceleration of the regeneration phase and transformation from fast to slow twitch fibers, and/or other unknown factors.3 Further research needs to be done to establish a treatment protocol with the appropriate intensity and duration to promote adaptation to exercise without accelerating muscle protein degeneration.3
Figure 4. Voluntary Exercise Wheel
It is well known that endurance exercise can produce positive adaptations to skeletal muscle in healthy rodents and humans. Beneficial adaptations include increased skeletal muscle capillarity, increased mitochondrial enzyme activity, increased fiber size, and a shift in fiber type from glycolytic type IIx and type IIb fibers to slower, more oxidative type IIa and type I fibers. Such adaptations allow for greater efficiency in oxygen delivery and adenosine triphosphate (ATP) production and utilization.4 Although these beneficial adaptations occur in healthy rodents and humans in response to endurance exercise, it is less clear as to the effect of endurance exercise on skeletal muscle in mdx mice and individuals with DMD. Research has shown that voluntary wheel running (a form of endurance exercise) has either a beneficial effect or no effect in DMD skeletal muscle. Studies have investigated the difference in soleus and extensor digitorum longus (EDL) muscle function in mdx mice who performed voluntary wheel running for short term (4 weeks or 16 weeks) or long term (1 year) compared to mdx mice who did not run.2,39,61,62 Results reported that mdx mice who voluntarily ran demonstrated a greater force-generating capacity in the soleus muscle and greater fatigue resistance in the EDL muscle than those who did not run.2,62 Other studies have showed that similar voluntary wheel running exercise has no effect on these muscle properties in experimental and control mdx mice groups.39,61 Conclusive results from these preliminary studies have found that in mdx mice that performed voluntary wheel running, hindlimb muscle function did not demonstrate detrimental effects from endurance exercise. This is an important finding because it gives insight into a mode of exercise that has potential to benefit dystrophin-deficient skeletal muscle, but at the same time, has yet to be found to cause further muscle damage.4
A research study by Landisch et al.4 investigated whether the effects of voluntary wheel running on cellular adaptations that are expected to occur in skeletal muscle actually occur in skeletal muscle of mdx mice. Results of this study demonstrate that some expected beneficial cellular adaptations do occur in muscle cells of mdx mice, while other adaptations do not. In summary, the mdx mice showed the following beneficial cellular adaptations to endurance-type voluntary wheel running exercise: an increase in soleus muscle mass, no change in EDL and tibialis anterior muscle mass, a decrease in the percentage of large fibers, a shift in fiber type demonstrated by less type IIb fibers and more type IIa fibers, and no change in muscle capillarity, muscle oxidative capacity, fibers with central nuclei, and fiber embryonic myosin heavy chain (eMHC) soleus expression.4
In regards to the muscle composition of mdx mice, the muscle mass, muscle fiber size, and muscle fiber type all demonstrated changes after 8 weeks of voluntary wheel running. An increase in soleus muscle mass relative to body mass is an important adaptation because the weight-bearing soleus muscle would be at a functional disadvantage if it did not adapt proportionally to changes in body mass.4 Skeletal muscle of mdx mice normally demonstrates a variable distribution of fiber size and an abnormally high percentage of large fibers. A decrease in the percentage of large fibers is an atypical, yet beneficial adaptation seen in mdx mice because it shifts the muscle composition to be more similar to the control, non-DMD affected skeletal muscle.4 A shift in fiber type from type IIb to type IIa is considered beneficial because type IIa is normally associated with greater oxidative capacity, but still has the ability to maintain high-velocity contractility.4
The skeletal muscle degeneration/regeneration cycle was also influenced by 8 weeks of voluntary wheel running in mdx mice. The presence of fiber central nuclei, fiber expression of eMHC isoform, and the amount of muscle capillarity all reflect the degeneration/regeneration of skeletal muscle. No change occurred in the proportion of central nuclei in skeletal muscle fibers in response to voluntary wheel running in mdx mice. The presence of central nuclei indicates the muscle’s history of regeneration/degeneration. If endurance training, such as voluntary wheel running, was detrimental and injurious to mdx mice skeletal muscle, then an increased proportion of central nuclei would have been evident.4 The presence of eMHC isoform in skeletal muscle indicates ongoing regeneration. In mdx mice, this isoform was 10-fold that of the control mice, which is normal for dystrophin-deficient muscle. In response to voluntary wheel running, the concentration of this isoform did not increase in mdx mice, therefore indicating that voluntary wheel running endurance exercise did not cause muscle fiber injury and associated regeneration.4 Voluntary wheel running did not produce changes in muscle capillarity in mdx mice. However, it has been reported that because mdx mice skeletal muscle consists of larger muscle fibers, the capillarity per fiber cross-sectional area is reduced. This in turn decreases the ability of the body to deliver oxygen to the exercising muscle which may contribute to functional ischemia.4
An undesirable outcome from the Landisch et al.4 study was there was no observed changes in mitochondrial enzyme activity in mdx mice who participated in 8 weeks of voluntary wheel running. The shift in muscle fiber type from IIb to IIa usually occurs in concurrence with improved mitochondrial enzyme activity. Unfortunately, this study revealed no change in mitochondrial enzyme activity (specifically, cytochrome C oxidase, and citrate synthase) in mdx mice, which normally improves oxidative phosphorylation in response to exercise in healthy skeletal muscle. In the control mice group, the improvement in mitochondrial function increased by 25% in the run group compared to the sedentary group. In comparison, the mdx mice run group only improved 2% in the run group compared to the sedentary group.4
Although not all of the expected adaptations occurred in mdx mice in response to 8 weeks of voluntary wheel running, some adaptations did occur, and can provide information for future research on the prescription of endurance exercise for patients with DMD.
High intensity exercise requiring increased levels of exertion from patients with DMD has widely been discouraged as a main mode of exercise due to potentially damaging muscular results via eccentric contractions. In DMD, when the prime movers of a joint weaken, a higher percentage of an individual’s maximal strength level is required to perform relatively low intensity tasks. This ultimately leads to shorter time to fatigue and hypertrophy of remaining functional muscle fibers, which places them at a higher risk for subsequent contraction-mediated damage during future intense actions.11,63 Therefore, exercise which qualifies as high exertion can potentially have serious deleterious effects on the remaining functional muscle cells in DMD patients.
In a study by Okano51, the levels of IGF-1 were decreased following chronic high intensity exercise in mdx mice. IGF-1 has been closely linked to regeneration of muscle following exercise and is responsible for inducing the expression of myogenic factors as well as promoting the proliferation and differentiation of satellite cells.51 Similarly, in mdx mice performing inclined treadmill running (15 meters per minute, 60 minutes per week, 5 weeks), there were significant increases noted in p38 MAPK, p-ERK ½, and JNK2 in the gastrocnemius muscle.64 These results display a correlation between higher exercise exertion levels, increased muscle damage through oxidative stresses, and limited muscle fiber healing and regeneration leading to apoptosis in the mdx model.
However, research involving mdx mice and intense exercise has produced mixed results. Mdx mice displayed increased resistance to fatigue in the EDL and Soleus (SOL) muscles compared to control mice following a 15-week endurance protocol consisting of swimming 2 hours per day for 5 weeks. The exercised mdx mice also exhibited a greater number of Type I (slow oxidiative) muscle fibers in both muscles compared to sedentary mdx mice where SOL was primarily Type I and EDL primarily Type II.63 This change illustrates a potential benefit of muscle fibers converting to Type I fibers specifically as a means to adapt to intense exercise and attenuate fiber damage. Supporting these findings, large, Type II muscle fibers have been shown to be preferentially damaged by high force-producing lengthening contractions often seen in intense exercises.12,13 Normally, an increase in Type I muscle fibers is associated with participation in endurance-based, low intensity exercise, underlining the need for safe, effective human trials in DMD.
"No Use Is Disuse"
A study is currently underway involving exercise training for boys with DMD.65 Research in the mdx mouse suggests that low intensity, non weight-bearing exercises have no detrimental effects; low-stress exercises may actually have beneficial effects on energy efficiency and myofiber contractility through a decrease in oxidative stress and a shift from fast twitch muscle fibers to slow twitch fibers.65 However, there have been no studies demonstrating this effect in the human model. This is the first study designed to examine these effects in humans with DMD.
This study consists of two different exercise interventions targeted at two distinct points during the progression of DMD:
Figure 5. Dynamic Leg and Arm Training
Study 165 involves dynamic leg and arm training in ambulatory or recently wheelchair-bound boys. This portion of the study utilizes bicycle training of both upper and lower extremities with or without electrical motor support. Subjects will exercise at a low to moderate intensity and continuous speed 5 days a week for 24 weeks. Each session will be 30 minutes long with 15 minutes dedicated to upper extremity cycling and 15 minutes dedicated to lower extremity cycling. Termination of the session or an increase in motor support to decrease intensity will occur if the subject is unable to maintain a 60rpm pace during a Six-Minute Bicycle ergometer Test. The control group for this portion of the study will receive usual care.
The outcome measurements for this study include joint mobility, muscle strength, bone density, intra-muscular fibrosis and fatty infiltration, muscle endurance as well as multiple functional outcome measurements.65
Study 265 is an observational study involving functional training with arm support for boys who have been in a wheelchair for a few years. The six month training program includes functional training of the non-dominant arm and hand with mechanical arm support and computer-assisted training. If a boy cannot touch his nose with mechanical arm support, electrical arm support will be supplied. The boys will play 5 rounds of a computer program called Furrballhunt that involves variable reaching motions 5 days a week.
The primary outcome measurement for this study will be the Action Research Arm Test (ARAT).65
The results of this new exercise study will be available in the near future.
SUMMARY OF EXERCISE RECOMMENDATIONS
It is important to note that these recommendations have been established according to animal studies only. Mdx mice show a milder phenotype than humans.14,66 Studies have indicated that while endurance training may improve skeletal muscle function, cardiac remodeling may not occur and mechanical stress may adversely affect the cardiomyocytes by accelerating the dystrophic process.27,28,30,33
Therefore, clinical judgment should be used before prescribing exercise to all patients with DMD.
- Low intensity aerobic and light resistance training1,3,4,67
- Emphasis on low-resistance, repetitive exercises to address Type I fibers instead of Type II fibers which are preferentially damaged in DMD through sarcolemmal membrane leakage and/or breakdown.11,12,13,16,63
- Resistance parameters should not exceed levels 20-40% superior to the patient's maximal muscular strength, durations over 20% longer than maximal patient performance (5-10 seconds), or episodes of 2-5 per day. Exercise intensities trending toward the high end of these ranges are only indicated in certain cases of DMD, such as severe non-use atrophy.63
- Voluntary exercises are more beneficial than non-voluntary exercises in mdx mice because this type of exercise has been shown to maintain muscle strength and it is the most similar to sub-maximal intensity exercise prescribed to humans.67
- Aquatic therapy may be an effective means to control type of muscle contraction with exercise and decrease the load experienced on the moving limb as opposed to ground-based therapy.63
- Breathing exercises
- Patients with a peak cough expiratory flow of <270L/min will benefit from assisted cough techniques.36 While some studies show that respiratory muscle training increases expiratory muscle strength and endurance, other studies have found no effect. Most studies have too small of a sample size to generalize the results, but there is an indication that there is a correlation between quantity of training and improved function.36,40 Hyperventilation training 30 minutes a day for six weeks can also result in beneficial cellular mechanisms and should be further researched.41
Frequency & Duration:
- Determining an appropriate frequency and duration is currently a main focus of research in the mdx mouse, but exercise parameters have not been determined in the mdx mouse at this time.
- Although the dosage isn’t known, it is hypothesized that there is a threshold where exercise does more harm than good.60
- Eccentric, lengthening contractions should be limited when possible. Exercises should start isotonically and finish isometrically in order to avoid muscle stretch during lengthening which has been shown to cause irreversible loss in volitional force production.18,19,63
In dystrophin-deficient muscles, inadequate amounts of satellite cells can lead to impaired muscle regeneration following microtrauma caused by eccentric contraction during exercise. This microtrauma causes calcium to flood the muscle cells leading to further cell damage and necrosis. As more and more muscles become damaged, muscle force production decreases and higher percentages of an individual’s maximal force production are required to perform relatively low intensity tasks. In response, remaining functional fibers hypertrophy and are at an increased risk for further contraction-mediated damage. Decreased IGF-1 levels are seen following high intensity exercise in mdx mice indicating decreased potential for muscle regeneration. Exercise-induced mechanical stress can accelerate the dystrophic process in many different muscles including respiratory muscles and the myocardium. Damage to the myocardium is a serious concern with the increase in physical activity in patients with DMD.
Although the evidence indicates that high intensity exercise is detrimental to individuals with DMD, current research shows promising trends with lower intensity exercise programs. Low intensity aerobic and resistance exercise has been found to be beneficial in mdx mice. ROS production and levels of oxidative stress markers are increased initially, but adaptation occurs as the exercise progresses resulting in a reduction of ROS production and oxidative stress. Other research has found that voluntary non-resistance wheel running can show beneficial effects in DMD skeletal muscle. There is evidence to suggest that respiratory muscles can be trained early in the disease progression to increase endurance and strength with an increase in dystrophin similar protein. Further research needs to be done to determine whether the same effects are seen in the human model and what exercise parameters should be used to achieve the cellular adaptations seen in mdx mice.
Researchers have been trying to find ways to maximize the gains patients with DMD can get from low intensity exercise programs. Steroids have been used in patients with DMD to decrease inflammatory pathways and prevent the degeneration-regeneration cycle in muscle cells. Pharmacological agents such as IGF-I, antioxidants and inhibitors of TNF-alpha are being studied in mdx mice and have shown promise for decreasing negative effects of exercise. Both the blockade of TNF-alpha and IGF-I administration have been shown to decrease muscle fiber damage and CoQ-10 supplementation was shown to have cardioprotective effects in exercise in the mdx mice. Despite these promising pharmacological findings, further research needs to be done to find a way to completely reverse the damaging effects exercise has on an individual with DMD.