Importance Of Muscle Regeneration In Sports Biology Essay

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Diagnosing a muscle injury is based on the clinical impairment they bring. Initiation of muscle regeneration is prompt, but the formation of scar tissue hinders this process, which can increase the tendency for injury to recur. This is why it is important to focus on a muscle regeneration therapy that can decrease muscle healing time, to ensure that sports players are as active as possible during their prime. Muscle regeneration proceeds through three phases: destruction phase, repair phase and remodelling phase. Knowing the duration of each healing phase and what exactly occurs at each stage will aid researchers in figuring out a therapy to reduce healing time. This should aid sports players greatly as it can both reduce the severity of the injury and therefore not as much time will be taken in their recovery period.

Muscle injuries can occur for a variety of reasons. Diagnosing a muscle injury is based on the clinical impairment they bring (Järvinen et. al 2005.) Järvinen also states that an injury which causes a tear to a few muscle fibres with minimal swelling and restricted movement is classified as a minor/first-degree injury. An injury with greater damage to the muscle, causing an inability to contract is defined as a moderate/second-degree muscle fibre, whereas a tear across the entire cross section of a muscle fibre, resulting in a complete loss of muscle function is known as a sever/third-degree muscle injury (Järvinen et. al 2005.) An injury can be so severe that it can cause a premature end to a sports players career. As age increases, susceptibility to muscle damage increases and the ability to repair skeletal muscle after injury decreases (Pietrzak, Vacanti 2008.) This may be one of the reasons why the peak age for many competitive sports may be low as muscle may not be able to keep up with the demands of the physical activity as a person ages. This is why it is important to focus on a muscle regeneration therapy that can decrease muscle healing time, to ensure that sports players are as active as possible during their prime.

Regeneration in injured muscles can be initiated promptly, but is hindered by the formation of scar tissue, which can increase the tendency for injury to recur (Huard et. Al 2002.) Sports injuries can occur as a result to muscle insult, for example, muscle contusion occurs as a result of a sudden, heavy compressive force (Järvinen et. al 2005.) This is likely to occur in contact sports, where sportsmen are subject to heavy blows, but unlikely to occur in sports requiring a lot of running and jumping (Järvinen et. al 2005.) Degeneration in such cases differs in causation to degeneration as a result of genetic disorders such as muscular dystrophy. Genetic disorders which cause muscle degeneration result from a mutation which gives rise to an absence or abnormal integral muscle protein (Emery 2008.) In Duchenne's and Becker's types of muscular dystrophy, mutations in the dystrophin gene cause a complete lack of dystrophin (Duchenne's) or a partially functioning dystrophin protein (Becker's.) (Emery 2008.) Dystrophin is a protein that anchors the sarcolemma to the actin cytoskeleton in the sarcoplasm, therefore dystrophin has an important role in both muscle contraction and stretch (Davies and Nowak 2006.)

Even though there are various types of muscle injury, the phases of healing that have been identified and characterised are common to all (Huard et. al 2002.) The healing of an injured muscle proceeds through three phases: destruction phase, repair phase and remodelling phase, of which the latter two are closely related (Järvinen et. al 2005):

Destruction phase: There are cytoskeletal proteins that act as a system of "fire doors" which limit and localise the necrosis of myofibres (Järvinen et. al 2005.) In addition, blood vessels that supply the muscle are also damaged, which results in the leakage of blood borne inflammatory cells entering the site of injury (Järvinen et. al 2005.) The formation of a haematoma further promotes muscle degeneration (Huard et. al 2002.) T-lymphocytes that invade the injured area, also secrete cytokines and growth factors which play a number of roles in the inflammation stage of the destruction phase, such as influencing local blood flow, vascular permeability and promoting regeneration by regulating myoblast proliferation (Huard et. al 2002.) Huard also states that the satellite cells, the key cells in muscle regeneration and are activated by both the destruction of the basal lamina and by growth factors.

Repair and Remodelling phase: Following degeneration of muscle, repair and remodelling processes occur, which are highly regulated to ensure the optimal recovery of the contractile function of the muscle (Järvinen et. al 2005.)

Satellite cells, which are involved in the repair phase, are required for the regeneration of myofibres and this occurs in a series of stages. They are first activated in response to injury. Satellite cells firstly proliferate and then differentiate into myoblasts, which then differentiate further to form immature muscle fibres called myotubes. These then become mature to form myofibres and begin linking the damaged muscle fibres together (Huard et. al 2002.)

(Fig. 1 (Huard et. al 2002.): A: Endomysium is a membrane that surrounds the individual muscle fibres. B: satellite cells play a key role in muscle regeneration. Found in the basal lamina of a myofibre (Zammit et. al 2006.) C: myoblasts are a result of satellite cell proliferation and differentiation, and continue to differentiate. D: myotubes originate from myoblasts, and a way of identifying a myotube is by the line of central nuclei. These go on to form E: a mature myofibre which links the damaged fibres together to form muscle. The nuclei have now moved to the periphery of the muscle fibres.)

As mentioned previously, several growth factors and cytokines play a role in muscle regeneration (Huard et. al 2002.) Insulin-like growth factor-1 (IGF-1), hepatocyte growth factor (HGF), epidermal growth factor (EGF), transforming growth factors (TGF-α and TGF-β), and platelet-derived growth factors (PDGF-AA and PDGF-BB) all take part in regulating muscle regeneration (Huard et. al 2002.) Hepatic growth factor and insulin-like growth factor both stimulate cell proliferation and cell differentiation, both transforming growth factors inhibit cell proliferation and differentiation, platelet-derived growth factor (PDGF-AA) inhibits cell proliferation but stimulates cell differentiation and platelet-derived growth factor (PDGF-BB) does the opposite (Huard et. al 2002.)

Knowing the duration of each healing phase and what exactly occurs at each stage will aid researchers in figuring out a therapy to reduce healing time that will benefit sports players. There are a few existing therapies that sports players undergo to either aid muscle healing or reduce the severity of the damage dealt by the injury.

When suffering from a muscle injury in a sport, a therapy called RICE is immediately applied to minimise the amount of bleeding at the site of injury (Järvinen et. al 2005.) It is divided into for stages: rest, ice, compression and elevation and the treatment is normally coupled with anti-inflammatory drugs (Caron and Taunton 2006.) The procedures at each stage are described below:

Rest: Retricting amount of movement prevents the injury from getting worse (Caron and Taunton 2006.) Caron and Taunton also mentions that stimulating blood flow to the injured area is necessary for healing. Resting also reduces size of the haematoma whilst decreasing the size of the scar being formed (Järvinen et. al 2005.)

Ice (cyrotherapy) : Applying ice 20 minutes at a time to the injured area decreases swelling, amount of pain as well as recovery time (Caron and Taunton 2006.)

Compression: Involves the use of a wrap to minimise both swelling and pain (Caron and Taunton 2006.) The jounral continues to mention that the wrap should not be left on longer than 3 hours, making sure it is not left on overnight.

Elevation: At this stage, the injured area is kept above the level of the heart, reduces pooling of blood, hydrostatic pressure and therefore interstitial fluid so swelling around the area is reduced (Caron and Taunton 2006, Järvinen et. al 2005.)

As this is used immediately after injury, it will reduce both the severity of the injury and therefore decrease the patients recovery time.

The use of non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used in the treatment of soft tissue injuries in sports medicine (Eustace et al. 2007.) They work by decreasing the production of the number of inflammatory mediators at the site of injury, which results in the limiting of pain (Eustace et. al 2007.) In an animal model, an inflammatory response, coupled with muscle weakness has been shown within 48 hours of injury, which has also been demonstrated in muscle strains in human studies (Eustace et. al 2007.) A reason why NSAIDs are used quite often when treating sports injuries is because it shows a decrease in inflammatory cell reaction whilst showing no adverse effect on the healing phase of muscle altogether, without affecting the muscles ability to contract (Järvinen et. al 2005.) As mentioned previously, when the used of NSAIDs are coupled with the rice treatment, a maximal response is produced where there is a considerable decrease in the amount of swelling and bleeding at the site of injury (Caron and Taunton 2006.) Even though NSAIDs are said to be useful when recovering from a muscle injury, high and repeated doses are not recommended as they can have unwanted side affects such as gastric ulcers (Challem 2003.)

Growth factors affect the signalling pathways that influence growth and cell differentiation by binding to various membrane receptors (Menetrey et. al 2000.) In vitro, basic fibroblast growth factor, insulin growth factor type 1 and nerve growth factors all stimulate the proliferation and fusion of myoblasts (Menetrey et. al 2000.) The study carried out by Menetrey et. al shows that a direct injection of growth factors into an injured muscle can significantly improve healing. However, whilst being of beneficial use therapeutically, growth factors also have considerable side effects, for example, TGF-β is responsible for the formation of scar tissue and cause also cause the unfavourable differentiation of myogenic cells into myofibroplastic cells (Järvinen et. al 2005.) Although this may be the case, there is evidence that when when a combination of IGF-1 and TGF-β antagonist decorin (which inhibits scar tissue formation in muscle) is applied, it is very successful in regenerating muscle without forming fibrous scar tissue (Järvinen et. al 2005.)

Although there are treatments that are widely practised (RICE therapy), there is not a single clinical trial to prove the effectiveness of it (Järvinen et. al 2005.) Only a few clinical studies exist on recovering from muscle injury, so the treatments used at the moment are based on experimental evidence only (Järvinen et. al 2005.) At the moment, research into improving muscle healing has expanded (Huard et. Al 2002.) The use of stem cell, gene therapy and the use of growth factors seem to be the most promising line of research, however since it is poorly validated, a lot of work has to be done in this area (Järvinen et. al 2005.) The best treatment available when recovering from muscle injury is unclear at the moment (Huard et. Al 2002), but from analysing current methods and research, it seems to be that a treatment that can reduce the damage (swelling and bleeding) at the site of injury coupled with a therapy that reduces recovery time seems to be the best option. This should aid sports players greatly as it can both reduce the severity of the injury and therefore not as much time will be taken in their recovery period.

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