The main symptom of DMD, a progressive neuromuscular disorder, is muscle weakness associated with muscle wasting with the voluntary muscles being first affected, especially those of the hips, pelvic area, thighs, shoulders, and calves. Muscle weakness also occurs later, in the arms, neck, and other areas. Calves are often enlarged. Symptoms usually appear before age six and may appear in early infancy.

Other physical symptoms are:

According to Lewis P. Rowland, in the anthology "Gene Expression In Muscle," if a boy is affected with DMD, the condition can be observed clinically from the moment he takes his first steps. It becomes harder and harder for the boy to walk; his ability to walk usually completely disintegrates between the time the boy is 9 to 12 years of age. Most men affected with DMD become essentially “paralyzed from the neck down” by the age of 21. Muscle wasting begins in the legs and pelvis, then progresses to the muscles of the shoulders and neck, followed by loss of arm muscles and respiratory muscles. Calf muscle enlargement (pseudohypertrophy) is quite obvious. Cardiomyopathy particularly (dilated cardiomyopathy) is common, but the development of congestive heart failure or arrhythmia (irregular heartbeat) is only occasional.

Some symptoms consistent with Becker muscular dystrophy are:

Individuals with this disorder typically experience progressive muscle weakness of the leg and pelvis muscles, which is associated with a loss of muscle mass (wasting). Muscle weakness also occurs in the arms, neck, and other areas, but not as noticeably severe as in the lower half of the body.Calf muscles initially enlarge during the ages of 5-15 (an attempt by the body to compensate for loss of muscle strength), but the enlarged muscle tissue is eventually replaced by fat and connective tissue (pseudohypertrophy) as the legs become less used (with use of wheelchair).

Possible complications associated with MD are cardiac arrhythmias.(BMD) Becker muscular dystrophy also demonstrates the following:

- Mental impairment (less common in BMD than it is in DMD.)

- Pulmonary failure

- Pneumonia

Duchenne muscular dystrophy (DMD) is a severe type of muscular dystrophy. The symptom of muscle weakness usually begins around the age of four in boys and worsens quickly. Typically muscle loss occurs first in the upper legs and pelvis followed by those of the upper arms. This can result in trouble standing up. Most are unable to walk by the age of 12. Affected muscles may look larger due to increased fat content. Scoliosis is also common. Some may have intellectual disability. Females with a single copy of the defective gene may show mild symptoms.

The disorder is X-linked recessive. About two thirds of cases are inherited from a person's parents, while one third of cases are due to a new mutation. It is caused by a mutation in the gene for the protein dystrophin. Dystrophin is important to maintain the muscle fiber's cell membrane. Genetic testing can often make the diagnosis at birth. Those affected also have a high level of creatine kinase in their blood.

No cure for muscular dystrophy is known. Physical therapy, braces, and corrective surgery may help with some symptoms. Assisted ventilation may be required in those with weakness of breathing muscles. Medications used include steroids to slow muscle degeneration, anticonvulsants to control seizures and some muscle activity, and immunosuppressants to delay damage to dying muscle cells.

DMD affects about one in 5,000 males at birth. It is the most common type of muscular dystrophy. The average life expectancy is 26; however, with excellent care, some may live into their 30s or 40s. Gene therapy, as a treatment, is in the early stages of study in humans.

Muscular dystrophy (MD) is a group of muscle diseases that results in increasing weakening and breakdown of skeletal muscles over time. The disorders differ in which muscles are primarily affected, the degree of weakness, how fast they worsen, and when symptoms begin. Many people will eventually become unable to walk. Some types are also associated with problems in other organs.

There are nine main categories of muscular dystrophy that contain more than thirty specific types. The most common type is Duchenne muscular dystrophy (DMD) which typically affects males beginning around the age of four. Other types include Becker muscular dystrophy, facioscapulohumeral muscular dystrophy, and myotonic dystrophy. They are due to mutations in genes that are involved in making muscle proteins. This can occur due to either inheriting the defect from one's parents or the mutation occurring during early development. Disorders may be X-linked recessive, autosomal recessive, or autosomal dominant. Diagnosis often involves blood tests and genetic testing.

There is no cure for muscular dystrophy. Physical therapy, braces, and corrective surgery may help with some symptoms. Assisted ventilation may be required in those with weakness of breathing muscles. Medications used include steroids to slow muscle degeneration, anticonvulsants to control seizures and some muscle activity, and immunosuppressants to delay damage to dying muscle cells. Outcomes depend on the specific type of disorder.

Duchenne muscular dystrophy, which represents about half of all cases of muscular dystrophy, affects about one in 5,000 males at birth. Muscular dystrophy was first described in the 1830s by Charles Bell. The word "dystrophy" is from the Greek "dys", meaning "difficult" and "troph" meaning "nourish". Gene therapy, as a treatment, is in the early stages of study in humans.

The diagnosis of muscular dystrophy is based on the results of muscle biopsy, increased creatine phosphokinase (CpK3), electromyography, and genetic testing. A physical examination and the patient's medical history will help the doctor determine the type of muscular dystrophy. Specific muscle groups are affected by different types of muscular dystrophy.

Other tests that can be done are chest X-ray, echocardiogram, CT scan, and magnetic resonance image scan, which via a magnetic field can produce images whose detail helps diagnose muscular dystrophy.

The mdx mouse is a popular model for studying Duchenne muscular dystrophy (DMD).

The mdx mouse has a point mutation in its DMD gene, changing the amino acid coding for a glutamine to a threonine. This causes the muscle cells to produce a small, nonfunctional dystrophin protein. As a result, the mouse has a mild form of DMD where there is increased muscle damage and weakness.

A contiguous gene syndrome (CGS), also known as a contiguous gene deletion syndrome is a clinical phenotype caused by a chromosomal abnormality, such as a deletion or duplication that removes several genes lying in close proximity to one another on the chromosome. The combined phenotype of the patient is a combination of what is seen when any individual has disease-causing mutations in any of the individual genes involved in the deletion. While it can be caused by deleted material on a chromosome, it is not, strictly speaking, the same entity as a segmental aneuploidy syndrome. A segmental aneuploidy syndrome is a subtype of CGS that regularly recur, usually due to non-allelic homologous recombination between low copy repeats in the region. Most CGS involve the X chromosome and affect male individuals.

One of the earliest and most famous examples of a CGS involves a male patient with Duchenne muscular dystrophy (DMD), chronic granulomatous disease (CGD), retinitis pigmentosa and intellectual disability. When it was discovered that an X chromosome deletion (specifically Xp21) was the underlying cause of all of these features, researchers were able to use this information to clone the genes responsible for DMD and CGD.

One of those more common CGS involves a deletion on the X chromosome (near Xp21) that encompasses "DMD" (causing Duchenne muscular dystrophy), "NROB1" (causing X-linked adrenal hypoplasia congenita) and "GK" (causing glycerol kinase deficiency). These patients will have all the common features of each individual disease, resulting in a very complex phenotype. Deletions near the distal tip of the p arm of the X chromosome are also a frequent cause of CGS. In addition to the previously described CGS that occur on the X chromosome, two other common syndromes are Langer-Giedion syndrome (caused by deletions of "TRPS1" and "EXT1" on 8q24 and WAGR syndrome (caused by deletions on 11q13 encompassing "PAX6" and "WT1".)

Coxsackieviruses-induced cardiomyopathy are positive-stranded RNA viruses in picornavirus family and the genus enterovirus, acute enterovirus infections such as Coxsackievirus B3 have been identified as the cause of virally induced acute myocarditis, resulting in dilated cardiomyopathy. Dilated cardiomyopathy in humans can be caused by multiple factors including hereditary defects in the cytoskeletal protein dystrophin in Duchenne muscular dystrophy (DMD) patients). A heart that undergoes dilated cardiomyopathy shows unique enlargement of ventricles, and thinning of the ventricular wall that may lead to heart failure. In addition to the genetic defects in dystrophin or other cytoskeletal proteins, a subset of dilated cardiomyopathy is linked to enteroviral infection in the heart, especially coxsackievirus B. Enterovirus infections are responsible for about 30% of the cases of acquired dilated cardiomyopathy in humans.

Gene doping is the hypothetical non-therapeutic use of gene therapy by athletes in order to improve their performance in those sporting events which prohibit such applications of genetic modification technology, and for reasons other than the treatment of disease. , there is no evidence that gene doping has been used for athletic performance-enhancement in any sporting events. Gene doping would involve the use of gene transfer to increase or decrease gene expression and protein biosynthesis of a specific human protein; this could be done by directly injecting the gene carrier into the person, or by taking cells from the person, transfecting the cells, and administering the cells back to the person.

The historical development of interest in gene doping by athletes and concern about the risks of gene doping and how to detect it moved in parallel with the development of the field of gene therapy, especially with the publication in 1998 of work on a transgenic mouse overexpressing insulin-like growth factor 1 that was much stronger than normal mice, even in old age, preclinical studies published in 2002 of a way to deliver erythropoietin (EPO) via gene therapy, and publication in 2004 of the creation of a "marathon mouse" with much greater endurance than normal mice, created by delivering the gene expressing PPAR gamma to the mice. The scientists generating these publications were all contacted directly by athletes and coaches seeking access to the technology. The public became aware of that activity in 2006 when such efforts were part of the evidence presented in the trial of a German coach.

Scientists themselves, as well as bodies including the World Anti-Doping Agency (WADA), the International Olympic Committee, and the American Association for the Advancement of Science, started discussing the risk of gene doping in 2001, and by 2003 WADA had added gene doping to the list of banned doping practices, and shortly thereafter began funding research on methods to detect gene doping.

Genetic enhancement includes manipulation of genes or gene transfer by healthy athletes for the purpose of physically improving their performance. Genetic enhancement includes gene doping and has potential for abuse among athletes, all while opening the door to political and ethical controversy.

Coxsackievirus shows a cardiac tropism partly due to the high expression of coxsackievirus and adenoviris receptors (CAR) in cardiomyocytes. Coxsackievirus B genome is approximately 7.4 Kb and translated as a polycistronic polyprotein. Upon translation, the polyprotein is cleaved by two essential viral proteases, 2A and 3C. The viral protease 2A cleaves the proteins in a sequence specific manner. These viral proteases can also act on host proteins exerting negative effects on the residing cell. Enteroviral protease 2A can cleave the cytoskeletal dystrophin protein in cardiomyocytes disrupting the dystrophin glycoprotein (DCG) complex. The cleavage site of dystrophin by protease 2A occurs in the hinge 3 region of the protein resulting a disruption of DCG complex and loss of sarcolemma integrity and increasing myocyte permeability. This eventually results in similar cardiac deformities observed in dilated cardiomyopathy caused by hereditary defects in dystrophin in DMD patients. Additionally, dystrophin deficiency has been shown to increase the severity in dilated cardiomyopathy in a mouse model for DMD. The increased susceptibility of dystrophin deficient heart to coxsackievirus-induced dilated cardiomyopathy is attributed to more efficient release of the virus from infected cells resulting an increased in viral-mediated cytopathic effects.

Viral induced dilated cardiomyopathy can be characterized using different methods. A recent study showed in coxsackievirus infected heart proteome, increased levels of fibrotic extracellular matrix proteins and reduced amounts of energy-producing enzymes can be observed suggesting they could be characteristic in enteroviral cardiomyopathy.

There are notable differences between the hereditary dilated cardiomyopathy in DMD and acute coxsackieviral-mediated cardiomyopathy.

1. The amount of virally infected cardiomyocytes varies in different stages of the disease. In a mouse model, at the acute stage (7 days after infection with coxsackievirus B3) approximately 10% of the myocytes are infected and could affect overall cardiac function. In chronic murine infection, the percentage of infected cardiomyocytes are much lower.

2. Unlike in the DMD, in coxsackievirus induced cardiomyopathy, acute cleavage of dystrophin in cardiomyocytes is unlikely to induce any prompt compensatory mechanism since host cell translation mechanism is defective in the infected cells.

A 2016 review found that about 120 DNA polymorphisms had been identified in the literature related to some aspect of athletic performance, 77 related to endurance and 43 related to power. 11 had been replicated in three or more studies and six were identified in genome-wide association studies, but 29 had not been replicated in at least one study.

The 11 replicated markers were:

- Endurance:

- ACE Alu I/D (rs4646994) (Called ACE I)

- ACTN3 577X

- PPARA rs4253778 G,

- PPARGC1A Gly482;

- power/strength markers:

- ACE Alu I/D (rs4646994) (called ACE D)

- ACTN3 Arg577

- AMPD1 Gln12

- HIF1A 582Ser

- MTHFR rs1801131 C

- NOS3 rs2070744 T

- PPARG 12Ala

The six GWAS markers were:

- CREM rs1531550 A,

- DMD rs939787 T

- GALNT13 rs10196189 G

- NFIA-AS1 rs1572312 C,

- RBFOX1 rs7191721 G

- TSHR rs7144481 C