Introduction

There are multiple forms of exercise and repetition that can produce muscle damage. However, this review is focused on endurance-type activities such as weight training, running, and specific types of sport. Weight training, whether it is circuit resistance training or intense bodybuilding-type workouts, is a popular form of exercise for altering body composition and increasing or maintaining muscle mass. It is also a form of exercise that is used to rehabilitate muscle wasting or other types of injury that have occurred in the musculature. Exercise prescription involves manipulation of intensity, duration, and type of activity with specific goal orientation, and for the purpose of this article, we are looking at different effects of weight training on the skeletal muscle for healthy individuals. High force eccentric (muscle lengthening) contractions associated with weight training have the greatest potential to cause muscle damage and elevated enzymes in blood, often seen 1-5 days following exercise, in comparison to muscle damage incurred by other forms of exercise. We damage muscle during exercise in hope that it will adapt and be stronger in the future, and the understanding of the process and degree of damage that has occurred is useful in assessing how different types of exercise can be beneficial to the musculature.

Skeletal muscle is quantitatively the most abundant tissue in the human body, accounting for approximately 45% of body mass. It is therefore an organ system, with an estimated 640 skeletal muscles all having different tasks and in some instances multiple tasks. Muscles are the motors that your body uses to complete the majority of tasks and physical activities ranging from everyday activities such as walking, lifting and posture maintenance to high intensity activities related to specific sport or work requirements. The dynamic nature of the musculature is observed in the continuum of changing muscle protein synthesis and degradation throughout the day and night. It is this dynamic nature of skeletal muscle that adapts to various forms of physical activity and this is particularly relevant to the readers of this article. Specifically, the maintenance and repair of muscle tissue is a vital component in the adaptability of the musculature and unless activity-induced muscle damage and the recovery process are to a certain extent understood, optimal adaptation to different forms of physical training is not likely to occur.

Importance of Muscle Health

Health eating alongside regular physical activity is also required for good muscle health. Muscle is comprised mainly of protein so it is vital to ensure an adequate protein intake. Protein in the diet can be sourced from animal foods (meat, fish, and dairy products), soy products, and a mixture of plant proteins. Other nutrients which contribute to maintaining and building muscle are adequate energy intake, vitamins and minerals. There are many fad diets which suggest cutting out certain food groups or eating very low energy diets for weight loss, these can be detrimental to muscle health and should be avoided. Muscles also serve as an important store of protein in the body so during an illness where there is increased muscle protein breakdown, a balanced diet is required to provide the protein and nutrients needed for convalescence and recovery.

Good muscle health is important as muscles are involved in every physical activity we do. Not everyone is involved in athletic activities, but we all need to maintain at least some muscle tone if we are to remain independent throughout our lives. Muscle health is achieved by a combination of regular physical activity and healthy eating. Using muscles through physical activity helps maintain muscle mass, strength, and function. This can be achieved through a gym session, digging in the garden, vigorous housework, circuit training, a sport or by walking to work. As we get older we need to maintain strength training to make up for the muscle loss associated with aging and to maintain independence. We have to keep active as any reduction in regular physical activity will be detrimental to our health. Bed rest, for example, in which there is little muscle activity leads to rapid muscle atrophy.

Understanding Muscle Damage in Weight Training

You cannot understand the process of muscle recovery without understanding the process of muscle damage. It has been proved that muscle strength and mass increase as a result of resistance exercise, but it is also commonly associated with muscle damage. Muscle damage has been reported following different types of resistance exercise and can be presented as disruption of muscle ultrastructure or increase in muscle proteins in the plasma. There are a few ways in which muscle damage can occur. The first happens during the eccentric part of resistance exercise. This, in turn, increases tension on the muscle, which can cause greater damage to the muscle fibers. Secondly, high force eccentric exercise can cause damage to the connective tissue, which is related to the increased torque placed on the joint. Another way in which muscle damage can occur is by adapting to different exercises and as strength level increases. This is because damage increases when the exercise is unfamiliar and as strength increases, the amount of load applied to the muscles increases, therefore causing more damage. Activity-induced muscle damage is defined as the "prolonged loss of muscle function" and can sometimes cause swelling of the muscle, which also increases the pain. Development of muscle damage is not always a negative process as it can depend on the goal of the exercise. Up-to-date research has said that it is impossible to totally avoid muscle damage during resistance exercise; it is dependent on the force applied to the muscle. It should be noted that frequent occurrence of high-level muscle damage is associated with overtraining and has adverse effects on muscle health.

Potential Reasons for Exasperating Muscle Damage

Muscle damage can have negative implications for an athlete as it can result in a decrease in force production, decreased endurance in the affected muscles, and an increase in intramuscular enzymes. In severe cases, muscle disruption can result in the inability to participate in future events. Since muscle damage can have such a detrimental effect, it is important to know what things can cause it to increase in hopes to avoid them. There are many reasons muscle damage can be increased during resistance training; one of the most common ways is by increasing the intensity of a certain activity. This can be beneficial for an athlete in hopes of improving to a higher level of fitness, yet it can also be detrimental. Intensity can be increased by performing the same activities as usual with an increased resistance, performing more repetitions of a certain activity, or increasing the volume of work done in a certain time frame. All of these things can result in delayed onset muscle soreness (DOMS), which is a prolonged sensation of pain/discomfort felt 24-48 hours after the activity has been performed. DOMS is caused by microtrauma to the muscle and can result in loss of strength, reduced range of motion, and muscle function. Another common way to increase muscle damage is by performing an eccentric activity. Eccentric muscle activity is where the muscle contracts while lengthening and is the force applied to a muscle group of a weight greater than it can eccentrically absorb. This can be very detrimental if the muscle is not prepared to perform an activity of this type, as is commonly seen in a clinical setting when a patient undergoes an injury due to a trip or a fall. It makes weak muscles relatively strong but at great cost it injures the strong. Finally, one of the most important things to consider for an athlete is the nutritional intake before and after a certain activity. A poor nutritional status can result in an increased possibility of muscle weakness and fatigue. If the muscles are already in a weakened state and vulnerable to fatigue, performing a certain activity may result in increased muscle damage. It is important to ensure that the muscles will have enough energy to perform a given activity with minimal damage and also enough for adequate recovery. After an activity is done, the muscle will have a heightened need for certain nutrients to repair the damage done; a meal or nutritional snack can be consumed within 15-60 minutes post-exercise to optimize recovery. High-quality carbohydrate and protein-rich foods are the most important for recovery of muscle glycogen stores and to provide the amino acids for rebuilding of muscle protein.

Remedies for Muscle Damage

There are a number of remedies for muscle damage which intervene in the process of muscle recovery or focus on boosting the rate and extent of this recovery. These techniques thus can accelerate the return to normal functional capacity of muscle. The regeneration of an effective contractile web is important in the recovery process and is the key to the return of a previous level of muscle performance. This recovery process occurs in the absence of further insult and provided the damaged muscle is protected from loading to the point of additional trauma. Therefore, techniques for reducing muscle damage itself during exercise are also important in the overall process of muscle rehabilitation. These may involve modification of an exercise program or specific technique strategies. By avoiding muscle damage in the first place, the rehabilitation process is avoided thus minimizing the effects of detraining.

Recovery Techniques for Muscle Health

Cryotherapy includes ice massage and ice pack application directly onto the skin over the injured muscle. Cooling therapy has long been used as a muscle damage repair technique; however, the evidence is somewhat conflicting, and a recent review has recognized that it is difficult to draw conclusions as to its precise effects. Cold therapy inhibits metabolic reactions by reducing tissue temperature, which in theory will reduce the extent of secondary damage, decrease oedema stemming from vasodilation, and reduce the permeability of the vascular wall. This, in turn, will reduce the blood content of the damaged muscle and the extent of swelling that occurs. A reduction in the metabolism of the cells can cause ischemia. In this case, it is possible that there is potential for further damage due to reduced oxygen availability. This may illustrate that there is an optimal window of opportunity to apply cooling therapy post-damage.

Muscle damage prevention and repair methods act in various ways and at many levels. Specifically targeted exercise-related muscle damage (muscle damage) has become a considerable concern of exercise professionals because of its well-documented effect on causing muscle soreness and the potential for long-term symptoms, primarily loss of muscle function and extended recovery.

Nutrition for Optimal Muscle Recovery

High-temperature stores such as Thailand often have poor food hygiene, causing an increased risk of gastrointestinal problems that may disrupt muscle recovery. In such places, a salt and sugar mixture with water can be used to make an oral rehydration solution. This not only replaces fluid and electrolytes but also has the added benefit of increasing water absorption in the small intestine.

Often overlooked in the recovery process is rehydration through fluid replacement. Dehydration is a common problem due to excessive sweating and inadequate fluid replacement during intense training. Water is important for many physiological functions, and maintenance of intracellular volume is vital for the anabolic processes involved in muscle recovery. Sodium replacement is important for restoring fluid balance and should be consumed if a meal is likely to be more than a few hours away. The sodium content of the meal can also replace the salt lost through sweating during the workout.

A study conducted by Tipton et al found that 6 g of essential amino acids mixed with a carbohydrate solution enhanced protein balance after resistance exercise. This has important implications for athletes wishing to recover as quickly as possible before returning to exercise. This can be achieved by consuming foods high in protein and essential amino acids such as milk, eggs, meat, and fish. Regular protein intake should continue over the 24-hour recovery period.

Utilization of nutrition for optimal muscle recovery generally starts with consuming a meal within an hour of the workout and continues for the next 24 hours. The purpose of consuming this meal is to increase muscle glycogen storage and reduce protein breakdown in the muscle. This will ensure the initiation of the muscle recovery process. Carbohydrate consumption is vital for the immediate recovery period, and glycogen resynthesis is most rapid during the first few hours immediately after exercise. Thus, carbohydrate consumption at this time is of utmost importance. High carbohydrate foods also stimulate the release of insulin, a hormone that aids in the transport of glucose and amino acids into the muscle cells. This hormone is particularly important in regards to muscle recovery because protein breakdown in muscle is reduced when glucose and insulin are present in the blood. High glycogen storage and protein availability thus create an environment within the muscle that is primed for recovery and subsequent exercise.

Tips and Tricks for Promoting Muscle Healing

Sleep is when the body does the majority of its muscle recovery. The earlier sleep is scheduled before 12 am, the more recovery you will get due to your circadian rhythm and the release of melatonin. Melatonin is also shown to increase growth hormone secretion, which correlates to faster recovery. Speeding up the rate of muscle recovery will allow for more continuous stored exercise and strength. Studies have shown massage can increase muscle recovery and diminish Delayed Onset Muscle Soreness (DOMS). Massage will increase blood flow and help remove toxins in the muscles. Static stretching has shown to help increase blood flow to muscles and increase their flexibility. Although the effect on muscle recovery is questionable, it will help overall muscle health.

Hydration is an extremely important part of recovery, overlooked by many. Water is vital to many bodily functions and being dehydrated can result in a decrease in strength, muscle atrophy, and a decrease in muscle recovery. Electrolyte imbalances are common in athletes and those who regularly workout. Electrolytes such as sodium and potassium have many key roles in the body, one of which being muscle contraction. When you workout, you use up your body's stores of these electrolytes and if not replenished, they can lead to muscle cramps and slower muscle recovery. The best source of hydration is water, with sports drinks being used to supplement electrolytes that may have been lost during a high-intensity workout. Caffeine and alcohol are both diuretics and can hinder muscle recovery, so it's best to stay clear of excessive amounts of these. A good indication of hydration is thirst and the colour of your urine; the closer to clear it is, the more hydrated you are.

Slow and Steady Programme to 100% Muscle Health

With muscle and tendon injury, however, the healing process can be a source of frustration for athletes, depending on the severity of the injury. Although the healing process can be broken down into three phases, for the sake of returning to full fitness, we can consider two phases. The first phase is the inflammatory phase and the repair phase. Following this is the remodelling phase. The time it will take for complete healing of an injury is highly variable, but with the correct treatment, healing and re-strengthening can be done simultaneously during the remodelling phase, and to a high percentage of the pre-injury strength. For muscle and tendon injuries, treatment will primarily involve decreasing the activity level of the injured area to allow an optimal environment for tissue repair. This can be achieved through immobilization of the injured part with a cast, boot, or sling, or through modifying activity and movement patterns. Step one for treatment and recovery while maintaining general conditioning is to temporarily replace any activities that use the injured area with a non- or low-impact alternative. This may involve different modes of cardiovascular training or modifying lower body activities to upper body activities. The next and final step is to re-strengthen the injured area with a progression back to the normal use of the body around the healed injury. The time frame of when to progress from step one to step two is highly variable depending on the severity of the injury. A stress fracture may take 3-4 months to progress to step two, whereas a more minor muscle strain may take a few weeks. The progression back into normal activity is the more simple relative to the treatment of the injury itself, but with whatever the injury, the concept of gradually increasing stress and load to the injured area cannot be forgotten. This concept can be related to the reverse of the principles of strength and conditioning discussed in part one of the article "Dangerous use of an effective tool."

In comparison to muscles and tendons, bones have a fantastic blood supply, resulting in a far superior healing ability. Assuming that the bone is in a healthy state and the severity of the stress is not beyond the repair mechanism of the bone, recovery and treatment of a stress fracture can be relatively simple compared to that of muscular damage. For these reasons, we will discuss the treatment and management of stress fractures in a separate article in the near future.

Gradual Progression in Weight Training

During the early stages of weight training for either a novice or detrained individual, very little overload is required to produce gains in strength. This is an important point to understand, as muscles are highly responsive to strength training in the initial stages, and will need very little stress to produce an adaptation. By knowing this, it can be documented that the old adage of sets and reps for bodybuilders (3 sets of 8-12 reps) comprises a yearly plan starting at the end of the preparatory phase (off-season) and leading to the competitive phase and tapering prior to competition or peaking for strength and power athletes. The mistake of training at too high an intensity can cause an overuse injury by the athlete not allowing the muscle enough time for recovery before the next session. Most strength and power athletes will recognize the feeling of "health" and "100% muscle" when they have fully recovered from a muscle damage inducing session in preparation for supercompensation. This "100% health" can be defined as the muscles being fully recovered and adapted to the previous training session and ready to put forth full effort and produce gains in strength. Failure to achieve "100% health" from a previous training session will result in a decreased effort in the following session and less than optimal results.

Importance of Rest and Active Recovery

Maintenance of muscle health can only occur when exercise is followed by the adequate rest needed for physiological repair of exercise-induced damage. In attempting to promote muscle health for athletic performance, a weight-training program should incorporate both active and passive forms of recovery in order to maximize the specific benefits to muscle health while preventing the occurrence of overtraining. Rest is the most passive form of recovery and is generally characterized by a complete or partial absence of training and competing. Active recovery involves engagement in low intensity exercise and has been shown to be advantageous in promoting blood flow to facilitate the clearance of intramuscular metabolic by-products such as lactate, and promoting psychological recovery; providing a form of diversion from training along with a reduction in general muscle soreness. It is strongly recommended that high intensity or long duration endurance training sessions should be followed by an active recovery day involving low intensity exercise, as this has been shown to facilitate recovery of endurance capacity. This would suggest that in every training program or event that produces some level of muscle fatigue or damage, a recovery process must occur before progress can be achieved. This is vital to the muscle healing process, as attempting to train on fatigued muscles can lead to a decrease in physical performance and predispose the individual to the risk of further muscle damage or soreness.

Incorporating Variety in Training to Prevent Overuse Injuries

Nutrient supply. If you think about the nutrients required to build and sustain muscles, you will think protein and amino acids are the most important. This is only somewhat true. Amino acids and protein are the building blocks of muscles, but the resting muscle in a well-nourished athlete already has a full complement of these substances. The most common nutrient deficit seen in athletes, including bodybuilders, is glycogen. Glycogen is a complex carbohydrate that the body stores mainly in the muscles and liver for energy. High-intensity weight training is more likely to use glucose as the main energy source, and as it depletes the glucose in the blood, the glucose stored as glycogen becomes the alternative energy source. When it approaches a depletion of all glucose, for example, a weight training session that worked the same muscle more than once, the body will start to break down amino acids to convert into more glucose. This is the opposite of what is wanted from weight training. Glycogen supplies can best be restored by an increased consumption of carbohydrates and can be stored beyond normal levels during the first few hours following the weight training with a high glycaemic index, carbohydrate meal. The strategic consumption of a carbohydrate drink during the weight training session should also help to prevent reaching a stage of necessary protein breakdown for energy.

UK-Based References and Further Reading

In addition to the texts in the suggested reading listed above, there are a number of books which will be of interest to the more academically minded athlete and/or coach. Tipton's recent publication is an edited text covering various aspects of exercise, its effect on protein turnover, and general nutritional considerations. There are a number of chapters written by leaders in the field, including a chapter by Rennie on the acute and long-term effects of injury and immobilisation on protein metabolism. This chapter draws comparisons between those circumstances and the changes in protein turnover and muscle damage associated with endurance and resistance exercise. There are also chapters on the role of nutrition in exercise-induced muscle damage and inflammation: these are areas in which research is just beginning and as such there are many practical implications for the field of sports nutrition and physiotherapy. Tipton's book is quite detailed and would be suited to those wishing to obtain a thorough understanding of the biochemistry and physiology of the various processes involved. Baar and King's text is a basic guide providing a practical approach to sports nutrition. It is a comprehensive consideration of the role of diet in the context of training to facilitate various forms of physical improvement. It has its own chapter on protein turnover and muscle growth, which Baar himself described as the only chapter on the basics of the physiology of skeletal muscle in any nutrition book. This text would be an ideal starting point for those not wishing to get bogged down in detail, but who wish to understand better the effects of specific nutrients on tissue turnover and how these may be manipulated to obtain an exercise-specific outcome. Wolf's recent publication is a presentation of current knowledge on the nutrition of the exercising individual. It is a comprehensive review of the topic and like Tipton's text, would be useful for building an in-depth understanding and where topics raised may direct the reader to further investigation. (D'Hulst et al., 2022)

Recommended Books on Muscle Damage and Recovery

An extensive collection of literature exists on muscle soreness and the damage it represents, but much of the earlier work is not easily accessible as it was published before the internet age. The Physiological Society aims to provide both the general public and professionals in sport and health sciences with an understanding of the process, the reasons for it, and the various outcomes. The delegation involved recognises that an executive summary is an important tool in the educational process. This is especially true for an ailment that affects individuals of all ages and fitness levels. To gain the attention of the public, a brief animated video is also being considered as a medium to promote understanding of the exercise.

Alpha Omnimedia is undergoing development of an exercise in existence for every man and woman who has ever endured substantial muscle soreness in the 24-hour period following weight training. This includes soreness experienced after beginning a weight training regimen or after a layoff from exercising. It will be released as a committee statement of The Physiological Society. A delegation formed from the society's executive will use the available scientific data to produce such a document. This is a very important initiative in the understanding of muscle damage and recovery. Alpha Omnimedia supported this endeavour by lending financial aid to the society in 2002.

Websites and Online Resources for Additional Information

Without doubt, websites change and move around every so often, making it difficult - if not impossible - to keep track of what's unique and ongoing. What follows, therefore, is more of an example of how to search rather than a definitive list of resources. Certainly the major search engines, particularly Google, can be of use in tracking down more elusive information, although caution should be exercised concerning the wide variability in the quality of information to be found. Probably the most comprehensive and authoritative website on the Internet that covers the broad topic of exercise and sports science is that of the American College of Sports Medicine "Exercise and physical activity information" from the main menu will take you to a variety of resources including position stands and consensus statements (use the search engine on the site to find these on specific topics), details on the college's annual meeting (with abstracts searchable by year), and the Medicine & Science in Sports & Exercise journal, which is probably the leading journal in the field. The PubMed site is useful for searching the scientific literature in detail. While the abstracts of papers are free, most of the full papers will not be, but some patience and perhaps a university library will usually get you the full text on the topics that interest you. The site is easy to use even for the computer illiterate, as complex search terms can be entered in plain English and there are a variety of free tutorials available. Allow plenty of time because it's easy to get side-tracked into the realms of the obscure and irrelevant when browsing through the seemingly limitless number of abstracts and titles in the search results. (Liguori & College of Sports Medicine, 2020)(College of Sports Medicine, 2022)(Ortega et al.2023)

References:

D'Hulst, G., Masschelein, E., & De Bock, K., 2022. Resistance exercise enhances long-term mTORC1 sensitivity to leucine. Molecular metabolism. sciencedirect.com

Liguori, G. & College of Sports Medicine, A. (2020). ACSM's guidelines for exercise testing and prescription. [HTML]

College of Sports Medicine, A. (2022). ACSM's resources for the group exercise instructor. [HTML]

Ortega, F. B., Leskošek, B., Gil-Cosano, J. J., Mäestu, J., Tomkinson, G. R., Ruiz, J. R., ... & Jurak, G. (2023). European fitness landscape for children and adolescents: updated reference values, fitness maps and country rankings based on nearly 8 million test results from 34 countries gathered by the FitBack network. British journal of sports medicine, 57(5), 299-310. bmj.com