Tendons and ligaments Tendons make movement possible by leveraging bones to muscle force. Aging affects tendon structure and function, and tendon injuries become more frequent in locations such as the rotator cuff, the patellar tendon, and the Achilles tendon (Frontera, 2017). Ligaments provide joint support and stability, and go through changes similar to those the tendons experience. For example, ligaments change in cellular density, collagen and elastic fibers decrease, and cross-links between collagen fibers in the articulated cartilage change (Frontera, 2017). Not only does the connective tissue surrounding a joint change, but the amount and quality of synovial fluid within a joint also decreases. Decreased quality and amount of synovial fluid make movement more difficult by increasing the friction within the joint. The combination of changes in connective tissue and the synovial fluid results in impaired and slower joint movements in older adults. Another well-known change with aging is decreased bone density, which explains some other structural changes seen in the musculoskeletal system (Frontera, 2017). Articular cartilage Aging is associated with a higher prevalence of chondrocytes’ (the cells responsible for cartilage formation) ability to divide (Frontera, 2017). This diminished mitotic activity leads to chondrocyte loss and lessened ability to synthesize collagen (Frontera, 2017). The articular cartilage ends up with stiffness within the collagen network of its tissues, which is even more pronounced after bedrest and which explains the stiff joints older adults often mention in the morning. Fat infiltration and muscle One reason for the decrease in muscle quality seen with aging may be the increase in fatty infiltration found in the muscle. As mentioned previously, aging is associated with changed body fat distribution, away from subcutaneous tissue and toward storage in the more harmful ectopic locations, including muscle. It is currently unknown whether increased intramuscular fat is a product of aging, inactivity, or both—but we do know that increased levels of intramuscular fat are associated with decreased muscle strength, muscle quality, and mobility function in older adults (Addison et al., 2014). Even older adults without current mobility limitations are at an increased risk of future limitations if they have high amounts of intramuscular fat within their thigh muscles (Visser et al., 2005). It is also known that older adults with higher amounts of intramuscular fat may have a decreased ability to activate their muscles to produce a full contraction (Yoshida et al., 2012) and that they may experience a decreased ability to improve their muscle quality with exercise (Marcus et al., 2012). This is an important finding because it suggests that increased intramuscular fat may not only impair the muscles’ ability to function, it may also impair the ability to improve muscle function. Bones It is well known that with advanced age, there is a reduction in bone mineral content and density (Frontera, 2017). Over a lifetime, the human skeleton continuously undergoes a regulated process of bone resorption and formation. This continuous process allows for the repair of microdamage in the bone, the removal of unneeded bone, and the release of calcium from the bones for the maintenance of calcium levels in the body (Schulman et al., 2011). Multiple types of cells are found within the bone, where they work together to
discussion of physiological changes to the musculoskeletal system with aging, “The main challenge associated with advanced age is the relationship between significant alterations in many physiologic functions, the development of multiple impairments, the decline of overall functional capacity, the associated morbidity and mortality, and the resulting loss of independence that most older adults fear and death” (2017, p. 706). If these impairments are left unaddressed, they can lead to life-threatening complications such as falls and bone fractures associated with muscular weakness, thus further increasing mortality. Muscles Age affects all four domains of skeletal muscle’s capacity to generate force power and movement. These domains are structure and architecture, fiber type distribution, excitation– contraction coupling, and energy release. The number of muscle fibers (and therefore muscle mass) is reduced with age, there is in increase in muscle type I fibers (slow twitch) and a decrease in type II fibers (fast twitch), muscle proteins undergo biochemical changes, and there is reduced oxidative capacity as well as impaired excitation–contraction coupling due to the uncoupling and fragmentation of cellular elements (Frontera, 2017). Muscle strength Muscle strength is a strong predictor of mobility limitation, slow gait speed, increased fall risk, risk of hospitalization recidivism, and high mortality (Frontera, 2017). Aging is associated with a significant reduction in muscle strength. One factor that contributes to muscle strength loss is a reduction in the ability to activate motor units (Frontera, 2017). Lean muscle mass strength and size reduce approximately 1% to 1.5% per year, and this is more noticeable in the lower limbs in both men and women (Frontera, 2017). Muscle size Muscle size is another factor to be considered in the aging body, as muscle atrophy occurs with aging, and there are some correlations between muscle strength and muscle size. Though it may seem intuitive that the loss of muscle mass leads to an equal loss of strength, it is now known that the loss of muscle strength actually exceeds the loss of muscle mass. Reduced strength is only partially due to reduced muscle mass (Frontera, 2017), though muscle atrophy is an important determinant of muscle dysfunction in older adults. “Skeletal muscle comprises approximately 40% of the human body weight and contains between 50% and 70% of all proteins in the human body” (Frontera, 2017, p. 707). Sarcopenia is age-related loss of skeletal muscle mass, strength, and function. Muscle atrophy is the loss of muscle tissue. Muscle quality Assessment of the loss of both muscle strength and power at rates that are greater than that seen in the loss of muscle mass has been termed muscle quality (Barbat-Artigas et al., 2012; Russ et al., 2012). Muscle quality is the muscle’s ability to produce strength or power relative to the muscle size (Russ et al., 2012). If the loss of strength and power exceeds the loss of muscle mass, then muscle quality has decreased. Muscle quality may be lost by as much as 2.5% per year in healthy individuals (Goodpaster et al., 2006). This is more than double the loss of lean muscle mass. These findings indicate that neurological changes and/or some change in the intrinsic force-generating capacity of the muscle is/are responsible for the decrease in muscle quality.
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