Discuss the physiological changes that occur in skeletal muscle as a consequence of sarcopenia and explain why resistance training is a potential strategy for its prevention
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Introduction
One of the basic stages of human life is growing up and getting old. As one gets old, the human body gradually becomes weak as a result of weakened organs. This significantly affects the older population’s ability to function and operate with speed and precision like the younger generation. As human age, their force-generating capacity, widely known as endurance of their skeletal muscles, is significantly reduced (SCHOENFELD et al., 2019). As a result, most of the senior population may find it challenging to perform the basic activities and chores that they used to perform effectively and efficiently at a young age. Current studies indicate that the observed loss of the ability to carry out basic activities is fundamentally attributed to muscle decay and modifications in the degree of contractile tissue inside the muscle rather than shortfalls in muscle enactment. One of the health conditions that is mainly associated with the changes in skeletal muscles is sarcopenia (Lopez et al., 2017). The current literature defines it as a condition that leads to the loss of skeletal muscle and the ability to function at the highest level possible. Even though it is primarily associated with old age, its development and manifestation in an individual’s life are linked to conditions that are not exclusively associated with old age. Therefore, studies suggest that although aging is the leading cause of this disease, living a sedentary lifestyle, getting little or no physical activity regularly increases the risk of developing the disease, particularly in old age (Holeček, 2017). Given the condition surrounding this health condition’s development, this study will identify and discuss the physiological alterations that occur in skeletal muscle as a consequence of sarcopenia and explain why resistance training is a potential strategy for its prevention.

Sarcopenia
Sarcopenia rises from 15% to those above 65 years of age but below 70 years, to 36% among those above 80. The current literature indicates that the sarcopenia prevalence in the current society is estimated to be more than 960 million. Demographic statistics by World Bank data indicate that by the year 2019, there were more than 960 million who are above 60 years of age and are at higher risk of developing sarcopenia (Robinson et al., 2018). This is more than double the number of people aged above 60 years in 1980, 382 million. Further statistics indicate that the majority of this population is in Asia and Europe. Notably, the number of people suffering from sarcopenia is expected to rise in the next few years, reaching over a billion by 2050 Vitale et al., 2016). Therefore, there will be an expectation that there will be an increase in physical therapy among the aging population as they seek to fight the adverse impact of this condition.
As people age, they are expected to lose some muscle mass, which is biologically normal for an older adult. However, sarcopenia enhances the severity of this muscle loss that it strays from the normal loss and adversely affect the ability of an individual to perform daily chores. This is the main reason why most of the senior population may find it difficult to live on their own and have to live in nursing homes (Tomlinson et al., 2015). Notably, the disease often affects the patient’s gait and balance, thus limiting the individual ability to perform as anticipated. For years, most scholars on this subject believed that the deterioration associated with sarcopenia was inevitable. However, with technological improvement and more research, they are looking into treatments that might slow down or reduce the deterioration (Naranjo et al., 2017). The most common signs and symptoms of this condition include body weakness and loss of stamina. Unfortunately, a reduction of physical activity resulting from loss of stamina or body weakness further leads to muscle mass loss (Naranjo et al., 2017).
The physiological changes that occur in skeletal muscle
Although sarcopenia itself is an adverse health outcome, it is also a significant risk factor for other health conditions. The existing literature suggests that sarcopenia increases the risk of physical limitation and subsequent disability. Current studies indicate that this condition raises the risk of comorbid conditions (Lixandrão et al., 2017). Some of the physiological alterations that take place in skeletal muscle include:
A reduction of the muscle fiber number and size
Increased severity of the sarcopenia adversely affects the skeletal muscle cross-sectional area (CSA). According to Reginster et al., (2016) there is a significant relationship between this decreased CSA and the reduction of the size of the muscle fiber, the number of fibers, or the combination of these two. Researchers who have invested their money and time on infinitesimal assessment of cross-areas from entire human vastus lateralis muscle found that, although there is no notable difference in the size of the fiber among the senior population, there is a notable difference in the number of fibers within a muscle, which aggravates the severity of sarcopenia (Williams et al., 2017). Further studies suggest that apart from reducing the cross-sectional area of the CSA, the muscles of the older population, especially those between 65 and 80 years old have less contractile tissue and more non-contractile tissue when contrasted with the skeletal muscle of the young population, especially those between 26 and 44 years. Notably, the rise in the number of the non-contractile tissue, which is often characterized by fat and connective tissue, leads to reduced muscle strength, affecting the ability of a patient to perform simple and basic activities (Tinsley et al., 2019). This significant change in tissue among the senior population is an indication that their muscle weight may decrease to a higher length than what can be measured or determined through microscopic evaluation of the CSA muscle alone.
Further research by Hidayat et al., (2017) on muscle morphology suggest that the capacity of type 1 (slow) fibers is rarely affected by age, but type 2 fibers (fast) undergo selective atrophy. This suggests that type 2 fibers are substantially affected in size as a result of old age, particularly in those without impairments. In the same breath, the number of type 1 and type 2 fibers is greatly affected by an individual’s age. This means that sarcopenia substantially affects the number of type 1 and type 2 fibers in the human body, irrespective of whether an individual has been diagnosed with a form of impairment at one point in their lives or not (Hidayat et al., 2017).
Effects on the motor units attributes
The existing literature suggests that sarcopenia reduces the number of α-motoneurons. As a result, the majority of the senior population has a lower number of motor units. However, as people become older, their remaining muscle fibers are re-introduced by the remaining motor units through secondary growth and development. It is worth noting that a motor unit comprises a motor neuron and skeletal muscle filaments presented by the motor neuron axonal terminals (Gomes et al., 2017). Thus, a collection of motor units works as a group to improve the coordination of body muscle contractions. As a result, all the motor units in a muscle are known as motor pools, improving the ability of the human body to operate and function in a coordinated manner. Unfortunately, as the body gets old, the amount of motor units substantially reduces, while some increases in size (Freitas et al., 2019). This allows the body to re-introduce some of the damaged units
Irrespective of the significant reduction of the motor units, research further suggests that old age adversely affects the firing levels of the motor units. In contrast, some of the units become more variable. This increased variability in the firing motor units can be attributed to the selective denervation of type 2 fibers and the eventual reinnervation through secondary growth and development of the neighboring motor units connected to type 1 fibers (Argilés et al., 2015). Unfortunately, as the body becomes old, and there is an increase in variability in the firing rates remaining motor units, an individual may find it challenging to control motors and force production. The body becomes weaker as it fails to produce enough strength and energy and power to function as anticipated. The reinnervation of denervated type 2 muscle fibers by neighboring type 1 fibers lead to enhanced co-articulation of type 1 and type 2 myosin heavy chain (MHC) isoforms in the skeletal muscle of an aging human body (SCHOENFELD et al., 2019). MHC is the head of the myosin molecule that influences the level and speed of cross-connect response with actin fibers, in this way influencing the speed of the muscle compression. Surprisingly, the muscles an aging human body, especially among individuals above 88 years of age, have a higher co-expression of the MHC isoforms than in the younger population (Tieland et al., 2017). This can be attributed to muscle fiber co-expressing both the isoforms and the denervated original motor unit and the isoforms of the motor unit that re-innervated these fibers.
Apart from reduced muscle mass, the reduced muscle strength among the aging population can be attributed to the modifications in intrinsic muscle function. Considering that muscle strength is decreased to a greater degree than muscle mass in the body, the process through which the aging population’s muscles generate strength is reduced, making it even challenging to move around comfortably (Tinsley et al., 2019). The current literature on the issue suggests that there are mixed results and feelings among researchers regarding why muscle strength reduction is higher than that of muscle mass. However, some of the studies suggest that these changes can be attributed to the modifications in the sarcoplasmic reticulum (SR), which are believed to be the main cause of reduced muscle strength. Therefore, the level of SR Ca2+ released in the body as a reaction to sarcolemmal depolarization is substantially lower among the aging skeletal muscle compared to that of a young human body (Nascimento et al., 2019). Although this has been the main findings in the majority of research on muscle strength, most of the authors feel that reduced muscle alone does not adequately explain the reduced strength production witnessed in aging persons.
Resistance training and its effect on sarcopenia
Nascimento et al., (2019) suggest that before developing and implementing any form of intervention to prevent or reduce the adverse impact of sarcopenia, the care provider, in collaboration with the patient, should start by measuring the severity of the problem. Current research suggests that various strategies at the disposal of the care providers can be used to measure the muscle mass. The common and most appropriate strategies include; measuring and determining the chemical composition of the body, especially the total level of potassium and nitrogen in the body and imaging, particularly computed tomography [CT], magnetic resonance imaging [MRI], and dual-energy x-ray absorptiometry [DXA] (Reginster et al., 2016). Notably, the severity of the condition allows the care provider to design the right workout plan, considering the intensity and the length of the exercise. Wrong exercises can adversely affect the strength and mass of the skeletal muscle, further affecting the ability of an individual to function effectively and efficiently.
Argilés et al. (2015) states that resistance training significantly reduces the risk of developing this disease, while at the same time reducing the severity associated with the disease. In this case, resistance training defines the form of exercise that focuses on improving muscle firmness and endurance. The training is also widely known as strength training or weight lifting, taking into account that those practicing need to move their appendages against obstruction given by their body weight, gravity, bands, and weighted bars (Tomlinson et al., 2015). However, at times the trainers decide to use muscle machines to improve body resistance. The resistance body training, therefore, makes the body stronger, tighter, and leaner.
Liguori et al., (2018) suggested that muscle training is largely categorized into two main groups, endurance and strength training. Endurance training focuses on enhancing body stamina, which is characterized by the amount of time that an individual can keep up difficult movement and oxygen consuming limit. Strength training, on the other hand, focuses on improving the strength of the muscles. The author suggests that training positively affects the aging skeletal muscle by enhancing its endurance and strength (Williams et al., 2017). However, the impact of this training on the human body is significantly affected by the intensity of the durations, frequency, duration, and mode of exercise. The most common benefits of resistance training on the skeletal muscle include:
Improving oxidation capacity and muscle capillarization
Care providers, particularly therapists, significantly rely on the level of muscle capillarization oxidation to measure and evaluate muscle adaptation to training. The skeletal muscle of aging people, both the muscle fiber-to-capillary ratio and oxidative capability, is significantly reduced compared to those of a teen or youths (Vitale et al., 2016). Notably, those who engage and participate in endurance training, which is mainly characterized by walking or jogging for three-quarters of an hour per day thrice week for ten months, positively affected their muscle capillarization. Also, such exercises help to adjust maximal heart rate, which ensures that all parts of the human body, particularly the skeletal muscle get enough blood to function accordingly and appropriately. Specifically, the capillary densities of those who participate in endurance training increases by 20%, while the number of capillary per muscle fiber increases by 25% (Fragala et al., 2019). This suggests that new capillaries are generated in the muscle, making it easier for them to participate in more physical activities that the majority of those who fail to exercise find it difficult carrying out.
Lichtenberg et al., (2019) suggest that the oxidation capability on the muscles of individuals who participate in resistance training increases by 125%. This means that such individuals have the strength to carry out basic activities, but they also have the speed to carry them out within a short time. Also, high-capacity training increases capillary per muscle fiber ratio among the older population by 15%. Therefore, high-intensity training and long training duration improve muscle oxidation capacity and capillarization, thus reducing the risk of developing sarcopenia and improving the overall muscle health of the aging population (Fragala et al., 2019).
Improving muscle fiber characteristics
Gomes et al., (2017) suggested that those who participate in endurance and strength training, for ten months, particularly walking or jogging for three-quarters of an hour a day, they are more likely to register increased type 1 fiber CSA of the lateral gastrocnemius muscles. The same exercise schedule and capacity are also likely to increase their type of 2A fiber, widely known as the fast, fatigue-resistant fiber by 6% in males. Still, it is slightly higher in women, who are more likely to register an 18% rise in these muscle fibers. Additionally, the participants are more likely to register an increase in type 2B fiber, mainly known as the fast and fatigable fiber in the CSA by 12% in men and 9% in women (Holeček, 2017). The authors feel that although type 1 fiber remained the same throughout the training and after the training, there is a substantial reduction of type 2B fibers and a rise in type 2A fibers. This is a clear indication that the resistance training helps the body convert type 2B fibers to type 2A fibers, thus allowing the participants to move faster and enhance their fatigue-resistance levels.
Freitas et al., (2019), on the other hand, suggested that quadriceps femoris muscle training, which is characterized by three sets of eight knee extensions with a load of 50% of their 1-RM increases the femoris muscle strength by a mean of 134%. This form of resistance training also increases the contractile tissue of their CSA by 10%. These additional statistics underscore the significance of high-intensity training on muscle strength and mass in older individuals. It also proves that regular training reduces the risk of developing the disease and reduces the severity of the signs and symptoms of the sarcopenia (Lichtenberg et al., 2019).
Improves motor characteristics
According to Fragala et al., (2019), most of the senior population who exercise regularly increase their muscle strengths and the muscle fiber CSA. However, the observed impact in muscle strengths is generally higher than the resulting gains in muscle CSA. The authors believe that this outcome can be attributed to the fact that increased training improves or increases the skeletal muscle CSA and increases muscle adaptations, making it easier for the body to respond differently to different physical activities. Such changes influence the changes in growth and development of motor units and their firing abilities and variability (Liguori et al., 2018). The authors further suggest that neural adaptations are the leading source of muscle strength improvement witnessed in the first weeks of training.
According to Tieland et al., (2017) The increase in muscle CSA, on the other hand, is the leading source of muscle strength gained after the first eight weeks of training. However, the author notes that the generalizability of the existing studies on the effect of resistance training among the older population is adversely affected by the fact that most of the participants in these studies are untrained and often participate in high-intensity training. As a result, the author feels that it is challenging to accurately quantify the impact of neural adaptations produced during the training (Lixandrão et al., 2017). However, based on the findings of the majority of these studies, it is evident that regular training improves motor characteristics by improving muscle strength and muscle mass.
Conclusion
Based on the above-detailed discussion, it is evident that sarcopenia is one of the common old-age health conditions that affect most of the senior population in the world today. The disease is characterized by a reduction in muscle strength and muscle mass, making it challenging for older individuals to perform without assistance effectively and efficiently. This discussion suggests that two main physiological changes are associated with aging skeletal muscle, mainly attributed to sarcopenia. These changes include a significant reduction in the number and size of the muscle fibers in the body and a significant reduction in this population’s motor capacity due to the adverse impact of sarcopenia on motor unit characteristics. However, this population, with the help of therapists and care providers, can reduce the risk of developing the disease by designing and implementing the right resistance training to improve the skeletal muscle endurance and strength. As shown in the discussion, resistance training improves the oxidative capacity and muscle capillarization of the muscles, improves their fiber characteristics to improve muscle strengths and masses, and improves motor unit characteristics to make it easier for them to perform demanding physically.

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