Physiological Changes in Skeletal Muscle and Resistance Training in Sarcopenia

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The natural process of ageing makes every individual experience some degree of muscle loss and strength as they undergo different life stages, which, according to scientists, is an inevitable process. Hence, the definition and characterization of the term Sarcopenia are significantly related to the natural ageing process (Santilli et al., 2014). Sarcopenia is a medical condition branded by progressive and generalized skeletal muscle mass loss, function, and power. Many research studies suggest that Sarcopenia is majorly a disease affecting the elderly. It should be noted that for Sarcopenia to occur there must be a decrease in both the muscle fibre size and number, which is quite different from the typical ageing process. The standard ageing process involves only a reduction in muscle fibre size and not quantity. However, younger individuals diagnosed with inflammatory diseases have a high risk of developing the disease. The rate of Sarcopenia increases by 14% to individuals aged above 65 years and its prevalence ranges from 11% to 50% to adults aged 80 years and above (Santilli et al., 2014). The most common significant factor contributing to the development of Sarcopenia is the level of physical activity. It has a substantial effect on the physiological changes observed primarily in the skeletal muscles, impacting an individual’s functional ability. This essay will focus on the physiological changes in skeletal muscles resulting from Sarcopenia and the significance of resistance training as a key strategy in preventing the development of the disease.
The exact mechanism behind the cause of Sarcopenia remains unidentified. However, it has a great relationship with hormonal changes, physical inactivity, poor nutrition, and inflammatory and chronic diseases. These risk factors to Sarcopenia result in several adverse outcomes such as physical disability, decreased quality of life, and even mortality. The most commonly accepted cause of Sarcopenia is the decline in the number of satellite cells (SC). Satellite cells generate myoblasts during embryonic development that facilitate the production of multi-nucleic muscle fibres. The presence of SC is quite vital in the regeneration of skeletal muscles, especially after an injury as they have the ability of self-renewal. Hence, SC becomes activated and regenerate and re-innervate muscle fibres to keep up with their power, quality, and function in the human body (Waltson, 2012). The differentiation and proliferation of SC occur in the presence of an intact basal lamina which causes the fusion of myoblasts into myotubes, which then mature to a new muscle fibre. However, in old age, the satellite cells decrease in number and fail to keep up with the process of reinnervation of muscle fibres, eventually contributing to the loss of motor units. The pathophysiology behind the decline of SC in older adults is linked to a decline in Notch activation due to a decrease in protein kinase activity; hence, reduction in satellite cells activation. With reduced satellite cell activation, then regenerative actions after an injury fail to occur, resulting in the observed skeletal muscle mass loss, power, and function among the elderly population.
Every individual’s skeletal muscles are made up of type 1 and type 2 fibres, which provide the muscle with the necessary strength required in daily life activities (Waltson, 2012). Primarily, type 2 fibres respond much faster than type 1 muscle fibres due to their more enormous glycolytic potential. On the other hand, Type 1 fibres aid in resisting fatigue due to their high mitochondrial and myoglobin content (Waltson, 2012). Both type 1 and type 2 skeletal muscle fibres provide the body muscles with the required strength. However, as an individual, age muscle loss and power primarily affect the type 2 fibres.
According to Ogawa et al. (2016), one of the significant causes of Sarcopenia is neuromuscular ageing. It relates to a progressive loss of the neurons that have a damaging consequence on muscle mass and strength. Secondly, Sarcopenia can be linked to an imbalance in the secretion of hormones, especially the growth hormone, insulin, androgens, and estrogen. Older adults experience a decrease in their growth hormone levels and insulin-like growth factor, which have significant impacts on body composition. Thus, it results in a reduction of lean body mass and bone mineral density that contribute to the overall structure of the strength of the body muscle (Ogawa et al., 2016). The loss of skeletal muscle mass in the older population has increased resistance to insulin action that has predisposed them to various metabolic diseases (Dhillon et al., 2017). The most prevalent metabolic syndrome is type 2 diabetes, which contributes to the rapid loss of muscle strength and mass.
Environmental factors, including a lifestyle lacking exercise and a poor nutritional diet, are the primary risk factors for Sarcopenia. Individuals who have an inadequate intake of calories and protein decrease their body’s ability to synthesize proteins that sustain muscle mass. Lack of sufficient protein leads to a buildup of non-contractile dysfunction that severely reduces muscle strength, as evidenced in Sarcopenia (Dhillon et al., 2017). Individuals who practice a sedentary lifestyle have a higher risk of muscle fibre and strength decline than individuals who engage in various exercises.
Sarcopenia represents a variety of consequences that change the entire life of the affected individual. It consists of both primary and secondary outcomes. According to Clark and Manini (2010), the fundamental issues associated with this condition include loss of muscle strength. In contrast, frequent falls, increased dependence, increased cardiovascular risk, and mortality can be categorized as secondary outcomes. These consequences further result in increased hospitalization rates and depression among the older population due to impaired functionality. Hence, it is crucial to formulate prevention strategies that may conserve muscle mass and strength in an individual as they age. There is no current approved pharmacological therapy in the management of Sarcopenia.
The skeletal muscles are essential and involved in critical functions in human life, such as body movements, facilitating the production of heat, and maintaining a body posture. In a healthy individual, the skeletal muscle comprises of sarcomeres which are responsible for their contraction properties. Muscle contraction and relaxation is a cycle which requires sufficient energy to produce their effect. The primary source of energy utilized by skeletal muscles is adenosine triphosphate (ATP). However, the absence of ATP leads to the accumulation of lactic acid and depletion of glycogen that causes muscle fatigue, as seen in individuals with Sarcopenia (Lang et al., 2010). Sarcopenia results in a variety of physiological changes in the skeletal muscles that alter their structure and function. Some of these physiological changes include reduced muscle cross-section area (CSA), decreased contractile tissue volume within the muscle CSA, a decrease in muscle mass, satellite cell degeneration, and autophagy.
As an individual age, so does significant changes that occur within their skeletal muscle CSA. Muscle CSA is the area found on the skeletal muscle that is directly perpendicular to its muscle fibres. It relies on an individual’s muscle strength, hence the more significant the force of the muscle, the larger the muscle CSA and vice versa. In Sarcopenia, both the fibre size and number significantly reduce affecting the muscle CSA, which is the primary cause of its development (Williams et al., 2010). The skeletal muscle relies on the CSA for contractile effectiveness; hence with reduced muscle CSA, the percentage of contractile tissue decreases, which affects the force-generating capacity of skeletal muscles. Thus, it implies that the older population has more non-contractile tissues in their muscles, resulting in the overall decrease in force production (Williams et al., 2010). The reduction in muscle CSA reduces the muscle mass to a greater extent, which predisposes the older adult to falls and fall-related injuries, loss of independence, and lack of gait.
Another significant physiological change that occurs in skeletal mass due to Sarcopenia is the decrease in muscle mass. Ageing is a process that carries along with complex alterations in the entire body composition due to age-related chronic inflammatory illness, hormonal imbalances, and decreased physical activity. These changes contribute significantly to a decrease in lean body mass. Averagely older adults aged over 80 years old experience a 30-50% decline in the total skeletal muscle mass and function (Siparsky et al., 2014). The percentage in an overall reduction of skeletal muscle mass may worsen in the elderly individuals who do not engage in any physical activity. Siparsky et al. (2014) state that regular disuse of muscles due to a lack of exercise causes muscle weakness, which leads to reduced muscle strength and eventually loss of muscle mass. Besides, loss of muscle mass is linked to a decreased CSA of the skeletal muscle fibres. It thus makes muscle reduce in size and fail to perform their function in body movement and posture (Larsson et al., 2019). Aside from body movement, loss of skeletal muscle mass affects the regulation of blood glucose levels as they are target tissues for their action. Thus, it predisposes the individual to contract type 2 diabetes due to resistance to insulin action from inadequate skeletal tissue.
Sarcopenia causes changes in satellite cells, which have a critical role in muscle regeneration after an injury. Skeletal muscles have the powerful capability of rapid re-establishment and reconstruction despite severe damage in the presence of satellite cells, which produce myoblasts for this purpose. Myoblasts fuse with damaged myofibers to replace the lost myonuclei during damage to the skeletal muscle. This process of regeneration continues even with repeated injury as long as the satellite cells are present. Satellite cells can renew themselves and facilitate muscle growth hence the name stem cell (Williams et al., 2010). However, individuals suffering from Sarcopenia compromise the function of satellite cells as they get depleted. Therefore, muscle regeneration becomes defective as notch signalling is inhibited by the inactivation of its target genes, which are responsible for their self-renewal and regenerating abilities. Instead, the satellite cells in Sarcopenia differentiate; a process that does not support skeletal muscle regeneration. The loss of satellite cells also occurs with ageing due to neuromuscular degeneration resulting in the loss of muscle fibres, principally the type 2 muscle fibres responsible for the generation of satellite cells.
Autophagy is a common physiological adaptation that occurs in every type of cell, including the skeletal muscles, to facilitate self-replication. It refers to the turnover of cell components, especially during cellular stress. Therefore, it allows the self-destruction of damaged cells to pave the way for the formation of newer, healthier cells. However, with the development of Sarcopenia in older adults, autophagy is impaired and dysregulated due to reduced muscle function (Fan et al., 2016). For instance, the mitochondria’s autophagy is the most prevalent in skeletal muscle, leading to the buildup of malfunctioning mitochondria that deprives the muscle of the necessary energy. Dysfunctional mitochondria are unable to completely burn oxygen and nutrients for energy production, which makes the older adults get easily fatigued as their energy demands are not met. The reduced mitochondrial performance changes the redox status of the parent cell, which results in the mutation of the genes in mitochondrial DNA (mtDNA). An increase in mtDNA is observed with ageing, which leads to a rise in the reactive oxygen that results in atrophy of the skeletal muscles (Marzetti et al., 2013). It thus contributes to muscle wasting among older adults leading to a decline in muscle strength that eventually leads to a loss of physical mobility and disability.
Sarcopenia is a preventable medical condition during the early stages of an individual’s life to prevent its undesirable consequences in old age. According to Clark et al. (2016), the best prevention strategy to Sarcopenia is changing from a sedentary kind of lifestyle to a life involved with physical exercise and also a proper nutritional diet. There are different kinds of physical activities that an individual may engage in during their early and middle ages, to prevent the onset of Sarcopenia, including aerobic and resistance training exercises. This essay will major in its discussion on resistance training as a potential preventive strategy in Sarcopenia. Regular and appropriate exercise training is vital for the conservation of muscle function and strength. This is because the more the body engages in exercise, especially with weights, the more the muscle CSA increases (Clark et al., 2016). Hence, it increases the overall force-generating capacity of a muscle as hypertrophy of muscle fibres takes place. It thus strengthens the skeletal muscle, unlike in individuals who lead a sedentary lifestyle and tend to experience loss of muscle mass and strength.
Having no approved pharmacologic agent to manage Sarcopenia effectively, resistance exercise has long been suggested as a treatment and potential preventive strategy to combat Sarcopenia. This is because resistance exercise has been proved to have profound benefits on both the nervous and muscular systems that enhance improvement in physiological and functional adaptations. It is recommended that for resistance exercise training to be practical, it must be regular to facilitate muscle hypertrophy and the consequent increase in muscle CSA (Jadczak et al., 2016). Moreover, the employed strength exercise should increase in intensity and volume to optimize their effect on muscle strength, size, and functional capacity. Resistance training is highly recommended as it has been proved to have significant effects on both the older and the younger population. According to Clark et al. (2016), high-intensity resistance exercise training increases muscle fibre size by approximately 30%. Hence, this implies that older adults’ muscles can regain their strength and function with appropriate and regular resistance exercise training programs. Effective resistance training exercise among older populations result in an increase in independence, a reduction in falls and fall-related injuries, lower mortality rate, and decreased hospitalization.
Resistance exercise training can be grouped into either multi-joint or uni-joint exercises. Uni-joint comprises of only one joint while multi-joint several joints are involved during the exercise. Older adults adapt well in regaining their muscle strength and function with the involvement of multi-joint exercises due to their functional relevance. Some of the activities that they may engage in include but not limited to, lifting dumbbells, press-ups, leg raises, pull-downs, and pull-ups (Vlietstra et al., 2018). The training program often begins slowly, and over time it increases in frequency, duration, and intensity to obtain optimal results (Katsukawa, 2016). However, key considerations should be made in intensity to prevent overstraining of the joints. In older adults, intensities should be carefully evaluated for optimal results; for instance, high intensity of 80% boost faster muscle growth and strength. For the duration, resistance training exercises last approximately thirty minutes to one hour with rest periods of 30 to 60 seconds in between (Johannsen et al., 2016). Rest periods encourage regular exercise among individuals, which contributes to enlargement of their muscle mass and CSA, resulting in greater muscle strength and enhanced functional ability. The duration, frequency, intensity, and type of exercise are essential components that should be evaluated during resistance exercise training programs (Mendes et al., 2016). Moreover, an individual’s overall physical status should also be considered to avoid too many strenuous activities, especially for individuals with chronic illness, such as cardio-vascular-related conditions.
Resistance training exercises is regarded as a potential strategy for combating Sarcopenia as it prevents muscle wasting by maintaining a balance between protein production and degradation. This regular maintenance results in muscle mass hypertrophy, which contributes to the overall increase in the muscle’s force-generating ability. Hence, the generation of force in skeletal muscle increases physical performance and mitigates mobility and disability risk in older persons. Therefore, resistance training can be categorized as a primary prevention strategy to Sarcopenia and its risk comorbidities such as type 2 diabetes (DeFronzo et al., 2016).
Resistance training has been associated with tremendous effects on the older population who are at risk of developing type 2 diabetes due to insulin resistance, which predisposes them to Sarcopenia development. Therefore, appropriate resistance exercise programs in type 2 diabetes provide significant physiological adaptations to the skeletal muscles. Appropriate and regular resistance exercise training paves the way for skeletal muscles to utilize the available glucose, hence improving the body’s metabolic rate (Poblete-Aro et al., 2018). During exercise, an individual’s body metabolic rate surges, thus enhancing glucose passage and consumption in the skeletal muscles. Resistance exercises such as daily push-ups, weightlifting, or leg raise result in muscle contraction, lasting for a duration of 20 to 30 minutes changes the energy levels and increases intracellular calcium concentration. Various glucose signalling mechanisms are activated due to these changes, improving their transport and utilization in the skeletal muscles (Poblete-Aro et al., 2018). Individuals with Sarcopenia and type 2 diabetes usually have a non-functional mitochondrion, and through resistance exercise, their metabolic health is reestablished, thus refining the role of mitochondria. Hence, older adults should have an efficacious follow up of routine exercise training to record normal blood glucose levels and cholesterol levels. Moreover, daily resistance exercise training enhances a healthy life free of severe diabetes complication that occurs due to the resistance of insulin action.
Resistance exercises have significant mechanisms of action at the muscle and neuro-motor level that aid in their effectiveness. At the muscle level, resistance exercise is responsible for neutralizing several mechanisms involved in the development of Sarcopenia. It increases the quality of muscle through an increase in the number of sarcomeres as the higher the number of sarcomeres, the greater the contraction of the muscles, which increased its force generation capabilities. Resistance exercises have an added benefit of inflammation reduction, which is a significant etiological factor in Sarcopenia (Clark et al., 2016). It decreases the chronic status of inflammation experienced in older adults by reducing the pro-inflammatory mediators’ levels produced by macrophages. Thus, it decreases the production of anti-inflammatory cytokines, which are responsible for hormonal imbalance and infiltration of skeletal muscle fat that results in Sarcopenic obesity among the older population.
Strength training exercises lead to an increase in muscle strength than the increase in muscle mass. This relates to the significance of neuromuscular adaptations that get enhanced by resistance training exercises. Neuromuscular adaptations result in improved muscle activation patterns and innervations through an increase of stimulation by the central nervous system (Conlon et al., 2017). Resistance exercise avoids alterations in muscle quantity, contractile quality, and neuronal activation through the maintenance of a robust neuromuscular function.
In conclusion, Sarcopenia is a significant clinical problem that primarily impacts the older population. Its underlying mechanism remains complex, but it is associated with various conditions including hormonal imbalance, increased inflammation, lack of physical activity, and poor nutritional intake, especially the protein element. Sarcopenia may also be aggravated by other medical underlying conditions such as obesity and type 2 diabetes due to increased physical inactivity, insulin resistance, and alterations in hormonal homeostasis. Diagnosis of Sarcopenia in the elderly population results in poor outcomes as it increases the likelihood of physical disability, loss of independence, and mortality in severe cases. It primarily results in significant physiological changes in the skeletal muscle as it is the major affected region. Some of the primary physiological changes observed include reduced muscle CSA, a decrease in contractile tissue volume within the muscle CSA, satellite cell degradation, and autophagy. Despite its impact on human survival and the absence of the most effective pharmacological therapy to clear its effects, evidence suggests that Sarcopenia is a highly preventable condition. Resistance training has been proved to be the most effective primary prevention tool to prevent the onset of Sarcopenia among older adults and reduce their demand for long-term care. Resistance training programs have proved to have a tremendous effect in increased muscle strength and mass; hence they should be adopted at an early age to mitigate challenges in physical mobility and mortality that come with Sarcopenia. The older adults and the younger population can go through an appropriate resistance training exercise program for Sarcopenia prevention.

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