Effects on the Skeletal Muscles due to Sarcopenia and Resistance Training as a Healing Technique
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Introduction
Sarcopenia is a medical term used to characterize the loss of body functions due to the loss of skeletal muscle mass. It is a Greek word which means “poverty of flesh,” which leads to physical disability, functional impairment, and mortality. Although Sarcopenia is associated with old persons, that is, 40 years and above, its development in some cases is related to medical conditions that are not strictly observed in older people. According to a study conducted by Safonova and Zotkin, the main risk factors of Sarcopenia are level of workouts or physical activities, gender, and age (2020, p. 149). Some of the effects of Sarcopenia that lead to reduced skeletal muscle mass include altered protein and caloric intake, poor absorption of minerals such as calcium, altered peripheral and central nervous system, and inflammatory effects. Sarcopenia leads to several physiological changes in skeletal muscles such as reduced muscle cross-section, reduction in muscle mass, satellite cell degradation, autophagy, and fibrosis; research has shown that resistance training is an effective remedy for Sarcopenia since it can be used to reduce or reverse Sarcopenia condition because it aims at increasing muscle mass and strength., as discussed in this research paper.
Causes of Sarcopenia and Physiological Changes on Skeletal Muscles as a Result of the Condition
Sarcopenia is the leading cause of health issues among the old due to fall injuries, disabilities, and hospitalization. Li, Han, Cousin, and Conboy showed that if a person begins a healthy training program at an early age, the effects of Sarcopenia are reduced by more than 70% regardless of the age group (2015, p. 943). Sarcopenia is predominantly a multifunctional and geriatric disease with few of its contributing factors being reduced physical activities, muscle decline, alterations in muscle metabolism, continuous increase in fibrosis, chronic inflammation condition, and neuromuscular junction degradation.
Muscle decline due to Sarcopenia begins at 40 years, and approximately 50 to 55% of the muscle fibers are lost by the age of eighty years. Varying hormonal growth patterns such as the thyroid hormone, testosterone, growth, and insulin hormones lead to muscle mass loss, which are considerably associated with tumor necrosis factor and interleukin-6. These chemical compounds in the body lead to an imbalance in the transmission of anabolic signals. Tieland, Trouwborst, and Clark (2017, p. 17) concluded that ‘As humans grow old, the FGC (force-generating capacity) or strength significantly reduces due to reduced muscle mass accompanied by a gain in fats.’ Studies have also shown the loss of muscle strength and mass is primarily the result of alteration in muscle atrophy and contractile tissues rather than the deficit in Motor Unit (MU), which is responsible for the recruitment of new muscle cells.
Low nutrient intake and reduced protein synthesis also lead to Sarcopenia condition. From biological research, the Sarcopenia condition affects the type 2 muscle fibers due to the low utilization of protein nutrients or low rate of synthesis. This decreases their size, amount, and the number of mitochondria. In their research, Tieland, Trouwborst, and Clark also showed protein consumption among adults reduce by more than 25%, with the quality of food being severely compromised (2017, p. 19). Protein is the main building block of muscles; the low rate of absorption or synthesis can be used to explain why there is a rapid reduction in skeletal muscle mass among the elderly.
Muscle CSA (cross-section area) is the area of a muscle at its largest point perpendicular to its fibers. The CSA is used to determine or describe contraction properties and strength of skeletal muscles. The CSA of the skeletal muscles usually decreases with an increase in age. Sarcopenia is, therefore, a product of old age because it is associated with reduced muscle fibers, muscle mass, or a combination of the two. The most used methods to evaluate the effect of aging on muscle CSA are the biopsies and imaging techniques. Alchin (2014, p 263) observed that each technique studies the change in muscle growth in a cross-sectional manner, which means the techniques are limited to a small group of muscles.
Biopsy techniques were used to study changes in growth pattern and strength of the vastus lateralis muscles in men with no impairments in 1985 and 1986. The main aim was to find the effect of aging on skeletal muscles for more than a decade. Twelve years later, microscopic result analysis suggested that a small reduction of the muscle size had occurred. The decline in the total number of muscle fibers was significant, hence the primary cause of Sarcopenia. Additionally, Alchin (2014, p. 264) concluded that ‘Apart from a decrease in muscle fibers in older persons (63 – 85 years), their muscles also contained limited contractile tissues and a significant increase in non-contractile tissues when compared to people in the age bracket of 25 to 42 years.’ Much of the non-contractile tissues were the connective tissues and fat that significantly hinders force and range of motion capabilities. Changes in tissue composition among the aging people indicate there is a drastic reduction in muscle mass; hence CSA alone can provide reliable results.
Loss in muscle mass is also related to obesity. An increase in fats results in increased body mass, accompanied by lean muscle loss. According to McCormick and Vasilaki (2018, p. 521), obesity is defined by the WHO (World Health Organization) as the BMI (body mass index) equal to or above 30kg per square meter. Excess energy intakes, insulin resistance, and low-grade inflammation and lack of physical activities are associated with the hormonal milieu, which leads to sarcopenic obesity. Initially, it was believed that only the reduction in muscle mass was the culprit of age-related muscle weakness. However, muscle quality and composition are predominant in determining the effect of Sarcopenia among the elderly.
Longitudinal medical studies show fat mass increases at a later age cohort picking at around 60 years and above. In contrast, muscle strength, flexibility, and strength progressively decline at the onset of age 40 years. According to Pantanowitz, intramuscular and Visceral fats tend to increase rapidly, while the subcutaneous fat was reducing in other parts of the body (2001, p. 253). Fat infiltration into muscle tissues is made by a reduction in the number of muscle fibers and strength and reduced leg performance or capacity.
Sarcopenia and obesity complement each other in the development of sarcopenic obesity. The decline in the total energy a person may use is reduced due to a decline in physical activities as a result of old age. On the other hand, excess intake of caloric accompanied by little or no physical activity leads to a gain in fats. The fats infiltrate the muscle tissues and replace them. The result is a heavier person with low muscle mass. Obesity and low muscle mass or strength were thought to exist in older people by chance; however, there is evidence Sarcopenia can be directly linked to obesity and weak muscle strength.
Also, Sarcopenia is associated with the degradation of the physiological properties of skeletal muscles and the skeleton. The skeleton frame is responsible for providing a support structure for the body organs and flesh. On the other hand, skeletal muscles are responsible for holding the flesh and organs into position, movements, and the provision of strength and reflexive movements. Coordination in living beings is overseen by the brain but is affected by changes in joints and muscles. Neme Ide (2012, p. 9) stated the primary type of muscle affected by aging is the type 2 muscle group. The physiology changes that affect the type 2 muscle include a change in the functioning of the satellite cells within the muscles, change in protein synthesis, rapid protein and proteasomal degradation, alteration of antioxidant defense cells, fibrous and fat infiltration, and dysfunction of mitochondria cells during Sarcopenia.
Satellite cells are available in type 2 muscles and are responsible for the healing and regeneration of muscle cells in case of an injury. A study done on mice showed the number of satellite cells decline with age in specific types of muscle fibers. In human beings, the decrease in satellite stem cells was observed to occur in type 2 muscle fibers with no decline in type 1 muscle being seen. Glass et al. emphasize the drop in satellite cells of the extensor digitorum longue (EDL), which is the type 2 muscle fiber in mice, occurs in the first one year (2012, 1). The degradation in satellite cells in soleus muscles (type 1) in mice start later when the mice are 2.5 years. A study conducted by Glass D. et al. concluded that the changes in satellite cells are specific to the muscles under investigation or the species being used (2012, 3).
Satellite cells are self-replenishing, which ensures the availability of a quiescent pool of the cells. During aging, that is, 40 years and above, the ability of these stem cells to renew themselves is reduced to a rapid increase in proliferation, which is associated with senescence and apoptosis (John, 2005). The result is the development of Sarcopenia, which is related to poor regeneration of the skeletal muscles of aged human beings and animals. Furthermore, the degeneration of satellite cells is significantly associated with the loss of neuromuscular coordination at an older age.
Some of the major pieces of evidence that associate satellite cell degradation with physiological changes that occur in the skeletal muscles include changes in Wnt and Notch signaling. Notch signaling is responsible for the proliferation of the cells in satellite stem cells, whereas the canonical Wnt signals are used to differentiate muscle groups. During the Sarcopenia condition, the Notch signaling is reduced, and the canonical Wnt signals change into non-canonical signals. Lim and Yoon Lee (2012, p. 629) concluded that this phenomenon hinders the self-renewal ability of injured or old muscles. Essential genes that are responsible for skeletal muscle growth, that is, MyoD and Myf5, also experience stunted growth.
A balance between protein degradation and synthesis in an animal body is critical in the maintenance of muscle mass and strength. Studies and research on basal protein synthesis have provided contradicting results as far as protein synthesis is concerned across different age groups. A survey by Hasten et al. showed protein synthesis in mice was higher compared to protein synthesis in older humans. Another study by McCormick and Vasilaki (2018, p. 531) showed no difference in protein synthesis in older adults when compared to young adults. The inconsistent result prompted the research to focus on postprandial protein synthesis and how well older people utilized the consumed protein.
Under new muscle studies, results showed older people develop anabolic resistance, which is related to reduced mTOR activation even after the protein has been synthesized into the body of the older persons. Appropriate protein intake is vital to ensure the synthesis and utilization of the protein for the development of muscles in older adults. Additionally, protein degradation tends to increase in people above 40 years. The imbalance in protein synthesis and increased protein loss are detrimental and can lead to mortality. Two common protein degradation pathways result in physiological changes in the skeletal muscles, Autophagy, and Proteasomal pathways
Autophagy is a self-eating process which is necessary for renewal or turnover of cells during normal conditions or cellular stress phase such as starvation. Unlike the ubiquitin-proteasome system (UPS), which degrades only protein fibers, the lysosomal system, which is associated with Autophagy, degrades the whole organelles, macromolecules, and incorporates protein aggregates. Experiments on Drosophila by Breusing and Grune (2018, p. 205) showed an increase in protein aggregates in skeletal muscles, which is related to impaired muscle functions. It is evident Autophagy dysregulation process plays a part in the development of Sarcopenia conditions.
UPS system maintains protein homeostasis while regulating protein degradation. Proteins labeled with ubiquitin are safe for degradation where they are passed of the body as water matter. UPS system is capable of conducting safe muscle protein degradation, but Atrogin-1 and Murfl or the RING-finger protein-1 have been known to play a part in the development of Sarcopenia. Tests done on rats showed the upregulation of both the RING finger protein-1 and Atrogin in the degraded muscles of the rodents.
Fibrosis is a condition that leads to the accumulation of excess cellular matrix and fats hence obesity. During Sarcopenia, both fat and fibrosis infiltration into the skeletal muscle is observed. This infiltration process is thought to cause or contribute to impairments that come with old age, that is, reduced force generation, especially in the lateral transfer of force in the skeletal muscle fibers. The accumulation of collagen, which is an extracellular matrix, results in the incomplete repair of the skeletal muscle following an injury or damage (Sorokin, 2010). When muscle damage occurs, a local condition is created to speed up the regeneration process. However, with the onset of Sarcopenia, the remodeling of the local condition becomes impossible. As stated by Pantanowitz, collagen accumulation leads to an increase in AGE (advanced glycation end) products (2001, p. 253). The myogenic progenitor cells responsible for healing are transformed into adipogenic or fibrotic fate.
ROS or reactive oxygen species are incredibly reactive chemicals that play a useful role in cell signaling and metabolism. In his journal, John (2005, p. 361), stated that when in excess, ROS can damage the macromolecules such as DNA, lipids, and protein due to oxidation, leading to cell death. Studies show there is a reduced amount of antioxidant defense cells in older subjects, which contributes to increased ROS. The increase in ROS is detrimental to the skeletal muscle as it is reflected with an increase in oxidative damage markers such as malonaldehyde, protein carbonyl, and oxidation of the protein and DNA.
Mitochondrial dysfunction is the inability of a mitochondrion to burn completely burn oxygen and nutrients to produce energy. This condition makes the older adults feel tired most of the time because their energy need is not met. The reduced mitochondrial performance changes the redox status of the patent cell, which in turn causes mutation of the genes in the mitochondrial DNA (mtDNA). mtDNA is associated with the production of an impaired electronic transport chain (ETC). Impaired ETC results in deficient quality oxidative phosphorylation, which leads to a further rise in ROS. The vicious cycle is responsible for aggravating the aging phenotype. Studies show during aging, there is an increase in mtDNA, ROS, and mitochondrial dysfunction, which are all associated with physiological changes and atrophy of the skeletal muscles.
Resistance Training as a Remedy for Sarcopenia
Different efforts have been suggested to help prevent and treat Sarcopenia condition as a result of obesity and old age. Resistance training, which involves lifting of masses, has been used to stimulate muscle cells, which triggers muscle growth. The same criterion used in conventional gyms is employed in resistance training among people suffering from Sarcopenia. Speed, strength, and endurance are the main ingredients for successful physical activities. Resistance training causes skeletal muscles to extend or contract when subjected to an external force or resistance. A study conducted by Evans showed there is an increase in power, strength, hypertrophy, and endurance (2019, p. 10). In the last two decades, evidence has proved that resistance training can be a perfect tool to fight Sarcopenia.
Sarcopenia is mostly associated with older people who are unwilling or unable to perform strenuous physical exercises. Additionally, most of the seniors who participate in resistance training are often on a low dose, which is inadequate for the development or gain of muscles. When coming up with resistance exercise programs to help patients in the Sarcopenia condition, it is essential to consider factors such as duration, frequency, type of exercise, intensity, and progress. According to William (2019), older persons also have chronic health conditions such as cardiovascular diseases and orthopedic limitations, which need to be considered. Resistance training for older adults should be done under constant supervision of a trained exercise physiologist or a physical therapist.
Frequency is the number of training sessions a person should have per week. Concerning the elderly, three to four sessions per week are recommended with the exercises being done on alternating days. A total or whole-body routine should be adopted where all major muscle groups are put under stress in each session. William (2019) also showed that this approach had improved performance in the fight against Sarcopenia as compared to training a particular muscle group in each session. The major body parts that should be exercised include the legs, arms, chest, shoulders, hamstrings, and the back.
Duration is described as the amount of time each session should last. The duration depends on the level of advancement or the stage a person is in. It is recommended, however, that a particular session lasts between thirty minutes and an hour. Antonio (2012, p. 107) concluded that ‘For advanced levels, more time can be added, but the trainer should be keen on the length of the rest period between sets.’ At the beginning of the program, 1 to 2 minutes of rest periods should be provided, and 30 to 60 seconds for advanced levels.
Resistance training exercises are categorized as either unit-joint or multi-joint exercises. Multi-joint involves more than one joint, for example, leg, and chest press. Unit-joint involves only one joint, such as leg extension and bicep curls. Buchan and Kelso (2013), emphasizes ‘multi-joint exercises should be encouraged for the elderly since they are relevant and target more than a single muscle group.’ Examples of activities that can be performed include but not limited to lifting dumbbells, press-ups, leg raises, and squats. The program should start slowly and then gradually increase reps, duration, and intensity at later stages.
Training intensity is a percentage representing the maximum weight that can be or should be lifted. Intensity should be carefully evaluated to prevent straining the joints and the low dose phenomenon. Intensity determines the amount and rate of neuromuscular adaptation induced through resistance training. For older people, high intensity of 80% or more should be encouraged if the person can handle the weight. The intensity in this range has been found to boost faster muscle growth and strength. Russ et al. (2012, p. 102) suggested that intensities ranging from 60% to 75% are mostly associated with increased strength, and intensities below 60% should be adopted at the beginning of the training session to prevent skeletal muscle injuries.
Conclusion
Although Sarcopenia is associated with older people, it is known to affect those in the age groups of 30 to 40 years. Sarcopenia leads to several physiological changes in skeletal muscles such as reduced muscle cross-section, reduction in muscle mass, satellite cell degradation, autophagy, and fibrosis. Resistance training should be adopted at an early stage to mitigate difficulties in mobility, reflex, and mortality that come with Sarcopenia. Well laid progressive resistance training programs have proved to be practical tools used to rectify the muscular and nervous systems. In some instances, the exercises partially reverse the effect of Sarcopenia. Given there are many components that make up the RT (resistance training), it is essential to consult a physical trainer to avoid injuries. Older adults are usually frail; it is the work of the physical trainer to see them through the frailness stage. However, the critical parameters to remember are the number of sets, nature of the exercise, intensity, keeping a strict schedule, and tracking the progress.

 
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