Dominic Stone
Muscle hypertrophy, or muscle growth, refers to an increase in muscle mass. There are two types of muscle hypertrophy: myofibrillar and sarcoplasmic. Myofibrillar hypertrophy refers to an increase in myofibrils, while sarcoplasmic hypertrophy refers to an increase in muscle glycogen storage. Muscle damage is often thought to be a significant contributor to muscle hypertrophy, with the idea that muscle fibers are stimulated to grow by being damaged and then growing back larger. However, recent research suggests that muscle damage might actually be detrimental to muscle growth, as the repair process for muscle damage and muscle growth both utilize the same cellular resources.
| Characteristics | Values |
|---|---|
| Definition | Muscle hypertrophy refers to an increase in muscle mass. |
| Types | Myofibrillar hypertrophy and sarcoplasmic hypertrophy |
| Myofibrillar hypertrophy | Increase in myofibrils |
| Sarcoplasmic hypertrophy | Increase in muscle glycogen storage |
| Cause | High physical activity or anabolic hormones/drugs |
| Role of muscle damage | Muscle damage is not necessary for hypertrophy. However, it is widely believed in the fitness industry that muscle fibres grow by being damaged and then growing back larger. |
| Exercise-induced muscle damage | Eccentric training causes more muscle damage than other types of muscular contractions. |
| Muscle repair | The repair process involves oxidative stress, an inflammatory response, and anabolic signalling in the mTOR pathway. |
| Muscle growth | Muscle growth involves an increase in the protein content of a muscle fibre. |
| Mechanical tension | Mechanical tension is the 'load' or force placed on the muscle during resistance training. It is considered the primary driver of muscle hypertrophy. |
| Muscle damage and growth | Muscle damage might be detrimental to muscle growth as the repair process for muscle damage and muscle growth utilise the same cellular resources. |
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What You'll Learn
Muscle damage and hypertrophy are separate processes
Muscle damage and hypertrophy are indeed separate processes. Hypertrophy refers to an increase in muscle mass, size, and strength. This is usually achieved through strength training such as weightlifting. The strain placed on muscles during strength training causes damage to muscle fibres, which the body repairs. Repeatedly challenging muscles in this way causes them to adapt by growing in size and strength.
However, muscle damage is not necessary for hypertrophy to occur. For example, concentric-only strength training and isometric training at short muscle lengths produce little to no damage but still result in a robust amount of muscle growth. Additionally, muscle damage can be caused by impacts leading to contusion injuries, but hypertrophy does not occur after such injuries, even though there is an anabolic signalling response that produces repair.
The belief that muscle damage is required for hypertrophy stems from the observation that there is an increase in muscle protein turnover after damaging exercise, as both muscle protein synthesis and breakdown rates are increased. Eccentric training, which produces more muscle damage than other types of muscular contraction, also appears to cause greater hypertrophy. However, this does not mean that muscle damage causes hypertrophy, as muscle growth and repair are separate processes, although both require an increase in muscle protein synthesis rates.
Furthermore, while satellite cell activity is often elevated when muscles are damaged, this can be explained as a response to exercise or solely directed towards muscle fibre repair, rather than a process that increases the number of nuclei in each muscle fibre. Research has shown that the increase in satellite cell activation at the start of strength training programs does not convert into increased nuclei inside the muscle fibres.
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Exercise-induced muscle damage
EIMD can have a detrimental impact on athletic performance, making it challenging to adhere to an exercise training program. The main effects of EIMD typically arise 24 to 48 hours after the initial muscle damage, with symptoms lasting for up to 14 days. These symptoms include loss of skeletal muscle function, soreness, inflammation, oxidative stress, and reduced range of motion in the affected limb.
To mitigate the negative consequences of EIMD, various interventions have been explored, including nutritional, pharmacological, electrical, and manual therapies, as well as exercise strategies. Nutritional interventions, such as omega-3 fatty acid supplementation and astragalosides supplementation, have shown potential in reducing signs and symptoms of EIMD and accelerating recovery. Additionally, long-term supplementation with antioxidants or beta-hydroxy-beta-methylbutyrate has been found to reduce EIMD, as does consuming protein before and after exercise.
While EIMD is a well-researched topic in exercise and sports science, it is important to note that muscle damage is not a prerequisite for muscle hypertrophy. Hypertrophy refers to the increase in muscle fibre size induced by high physical activity or anabolic hormones/drugs. Although EIMD can trigger similar activation and proliferation of muscle satellite cells (MuSCs), it is not necessary for muscle growth. Concentric-only strength training and isometric training, for example, produce minimal or no damage yet still result in significant muscle growth.
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Muscle repair after damage
Muscle repair is a highly coordinated and complex process involving cell-cell, cell matrix, and extracellular matrix interactions. The repair process can be divided into three main phases: destruction, regeneration, and remodelling.
The destruction phase is characterised by the rupture and necrosis of the myofibers, formation of a hematoma, and an inflammatory reaction. This is followed by the regeneration phase, where phagocytosis of damaged tissue occurs, leading to the activation and proliferation of satellite cells, which are tissue-resident muscle stem cells essential for muscle repair and growth. During this phase, immune, fibrotic, vascular, and myogenic cells appear with distinct temporal and spatial kinetics. The remodelling phase involves the maturation of regenerated myofibers, with the recovery of muscle function, and the formation of scar tissue. This phase is crucial for restoring muscle strength, as the scar tissue initially formed is the weakest point of the affected muscle.
The repair process can be fully activated without subsequent hypertrophy, as seen in the case of contusion injuries caused by impacts. While eccentric training often leads to hypertrophy, contusions do not, despite the presence of an anabolic signalling response that produces repair. This suggests that muscle damage is not necessary for hypertrophy to occur.
Muscle repair requires the activation of satellite cells, which are skeletal muscle stem cells located between the plasma membrane of myofibers and the basal lamina. After injury, these satellite cells become activated and proliferate, generating a population of myoblasts that can differentiate to repair damaged fibres or self-renew to maintain a reserve for future muscle regeneration. Vascularisation of the injured area is also essential for muscle repair, as new capillaries originate from injured blood vessels and stimulate the vascularisation process.
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Muscle damage and anabolic signalling
The mainstream fitness industry has long propagated the idea that muscle fibres must be damaged to be stimulated to grow, and that the fibres then grow back larger and stronger than before. This theory makes two predictions about strength training: firstly, that muscle damage must occur for hypertrophy to occur, and secondly, that any kind of muscle damage will cause hypertrophy.
However, researchers have disproven this theory by demonstrating that hypertrophy can be produced by concentric-only strength training, which produces little to no damage at low to moderate volumes. Additionally, when muscle damage is caused by an impact leading to a contusion injury, hypertrophy does not occur, despite an anabolic signalling response that produces repair. This repair process involves oxidative stress, an inflammatory response, and anabolic signalling in the mTOR pathway.
Anabolic signalling is a complex intracellular network that influences the regulation of skeletal muscle protein turnover. Exercise and amino acids stimulate anabolic signalling through several intracellular pathways, including the mammalian target of rapamycin complex 1 (mTORC1) and the mitogen-activated protein kinase cell signalling cascades. Mechanical loading through SAC-mediated calcium flux and enhanced intracellular AA concentrations activate the hVps34 kinase, stimulating RHEB-induced mTORC1 activation. Mechanical loading also modulates muscle intracellular membrane permeability to extracellular AA, which may enhance AA uptake and stimulate mTORC1 signalling.
Recent studies have found that canonical anabolic signalling pathways and myofibrillar protein synthesis can be increased in situations of severe muscle damage or atrophy, where an increase in proteolysis outpaces anabolism, resulting in atrophy. Additionally, chronic exposure to resistance-type exercise in rats causes a decrease in anabolic signalling through mTORC1, which can be recovered through detraining. Gene expression data from humans supports the idea that the downregulation or more transient activation of mTORC1 may be a physiological response to chronic training.
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Muscle damage and metabolic stress
Muscle Damage
Exercise-induced muscle damage (EIMD) occurs primarily from performing unaccustomed exercises, with its severity depending on the type, intensity, and duration of training. Muscle damage can be caused in several ways, including eccentric contractions, which involve longitudinal forces that cause muscle fibre strain, and impacts, which involve transverse forces that cause contusions.
Metabolic Stress
Metabolic stress, on the other hand, refers to metabolism, or the chemical processes that occur within a living organism to maintain life. Metabolic stress is a physiological process that occurs during exercise in response to low energy, leading to metabolite accumulation in muscle cells. This accumulation of metabolites, such as lactate, phosphate inorganic (Pi), and ions of hydrogen (H+), places
Traditional resistance training protocols can increase metabolite accumulation, influencing hormonal release, hypoxia, reactive oxygen species (ROS) production, and cell swelling. Changes in acute exercise routines, such as intensity, volume, and rest between sets, can also impact the magnitude of metabolic stress.
Optimising Muscle Growth
To optimise muscle growth, it is important to understand and manipulate these principles in your training routine. For example, mechanical tension involves lifting heavy weights (typically 80-90% of your one-rep max) to create tension in the muscles, while metabolic stress is achieved by doing higher reps with short rest periods, often referred to as "the pump". Muscle damage can be achieved by varying your exercises, implementing slow negatives, and increasing your range of motion to cause micro-tears in the muscle fibres, which then repair and grow stronger.
By combining these three principles in your workouts, you can create a well-rounded approach to maximise muscle growth and avoid excessive stress or damage to the muscles.
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Frequently asked questions
Muscle damage hypertrophy is the idea that muscle fibres must first be damaged and then grow back larger. This idea predicts that muscle damage must happen for hypertrophy to occur and that muscle damage will cause hypertrophy regardless of how it was caused.
There is some evidence to support the idea of muscle damage hypertrophy. Firstly, there is an increase in muscle protein turnover after damaging exercise, with an increase in both muscle protein synthesis and breakdown rates. Secondly, this occurs in conjunction with increased muscular inflammation and elevated intramuscular calcium ions. Thirdly, eccentric training, which produces more muscle damage than other types of muscular contraction, appears to cause greater hypertrophy than isometric or concentric training.
Muscle hypertrophy is primarily driven by three key factors: mechanical tension, muscle damage, and metabolic stress. To maximise hypertrophy, it is recommended to focus on lifting heavier weights with proper form to stimulate anabolic pathways and promote muscle growth. It is also important to get sufficient rest to allow the muscles to recover and grow.
