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Aging changes of the Musculoskeletal System Age-related reductions in muscle mass are a direct cause of declines in muscle strength with aging. This reduction in muscle strength is a major cause of disability in the older adult since strength and power are major components of gait, balance, and the ability to walk. Age-Related Structural Changes in Skeletal Muscle 1-Reductions in Muscle Mass Aging is associated with decreases in total muscle cross- sectional area, amounting to approximately 40% between the ages of 20 and 80 years. Reductions in leg muscle cross-sectional area have been observed to begin in early adulthood and accelerate beyond 50 years of age. This reduction in muscle cross-sectional area is accompanied by increases in non-contractile structures such as fat and connective tissue. Thus, the girth or volume measurements of muscle, used commonly in the clinic, may not show the actual reduction in the contractile proteins responsible for force generation (actin and myosin). The rates of skeletal muscle protein synthesis decline with age and may also contribute to muscle atrophy and repair process after injury. 2-Reduction in Muscle Fiber Number The total number of muscle fibers is significantly reduced with age, beginning at about 25 years and progressing at an accelerated rate thereafter. The decline in muscle cross-sectional area is most likely due to decreases in total fiber number, especially type II fast- twitch glycolytic fibers. The loss of muscle fibers is followed by a replacement with fat and fibrous tissue and a gradual increase in non-muscle tissue. 3-Changes in Muscle Fiber Size The size of the individual fast-twitch type II fibers decreases with age (vastus lateralis, tibialis anterior, and biceps brachii), whereas the slow-twitch type I fiber size does not change. For example, in the third or fourth decade of life, the mean cross- sectional area of individual fast-twitch type II fibers in the quadriceps femoris muscle exceeds that of slow-twitch type I fibers by approximately 20%. By the age of 85 the area of individual fast-twitch type II fibers is less than 50% of that of slow-twitch type I fibers. In addition, small, angulated fibers and grouped atrophy are commonly seen in muscles of older men and women. These morphological changes are similar to changes that are observed in skeletal muscle diseases. 4-Motor Unit Number and Size There is a decrease in total number of motor units with age. The average motor neuron loss from the second to tenth decade is approximately 25%. The decrease in motor unit number is accompanied by an increase in size or innervation ratio, such that on average, each motor neuron innervates more muscle fibers in the older adult. The increase in size of the motor unit is found primarily in the muscles of the lower limb, particularly in persons 60 years old, and more in distal than in proximal muscles. 5-Axon The loss of motor neurons is accompanied by a reduction in both the numbers and diameters of motor axons. There is an age-related reduction in the numbers of large and intermediate myelinated ventral root fibers but no significant reduction of the small nerve fibers. Quantitative electromyography (EMG) has shown changes in both duration and amplitude of motor unit action potentials with increasing age. Axonal nerve conduction velocities of all motor nerve fibers are slowed with aging. This suggests that with aging, the alterations in conduction velocities could reflect a variety of changes in the nerve fibers, such as segmental demyelination, and a reduced internodal length. In addition, the mean soma size of motor neurons is reduced with age with accumulation of lipofuscin. 6-Altered Motor Unit Remodeling Motor unit remodeling is the natural cycle of turnover of synaptic connections occurring at the neuromuscular junction by the process of denervation, axonal sprouting, and reinnervation of the muscle. In aging, however, it appears that motor unit remodeling is altered. Motor unit remodeling is altered such that type II fibers are selectively denervated and reinnervated by collateral sprouting of axons from fibers of the slow motor units. Type II fibers, which become reinnervated by slow motor unit axons, actually become type I fibers. The fast motor unit axons degenerate when they no longer innervate muscle fibers. Age-Related Changes in Muscle Performance 1-Strength Age-related decreases in strength have been well- documented. Healthy men and women in their seventh and eighth decades of life demonstrate average reduction of 20% to 40% in maximal isometric strength in various muscles. Arm, leg, and back strength decline at an overall rate of 8% per decade, starting in the third decade of life. Loss of muscle strength in leg muscles is greater than loss in arm muscles between the ages of 30 and 80 (40% compared with 30%). Weight-bearing muscles showed greater changes than non- weight-bearing muscle. It is conceivable that age related weakness may be caused by: 1) Decreased central drive and thus a decrease in ability to voluntarily activate a muscle. The threshold of excitability of the corticospinal tract increases progressively with age and is significantly higher in the elderly. 2) Loss of muscle mass and therefore in the number of force generating cross bridges interacting between actin and myosin; a decrease in the force developed by each actin and myosin cross- bridge. 3) Changes in the intrinsic properties of the remaining skeletal muscle fibers. 2- Power The power output is governed by: (1) The velocity of shortening and (2) The force-generating capacity of the muscle. Aging reduces the peak forces that can be generated by muscles at fast contraction speeds, resulting in a decrease in peak power output. Peak power output declines by about 20% with age. The decrease in power output results from motor unit remodeling, which reduces the fast to-slow-fiber ratio. This reduced capacity for rapid-force generation might also limit the ability to respond quickly to a loss of balance and 3-Endurance A decline in muscular endurance is a feature of old age that contributes to functional loss and disability. Alterations in muscle, both contractile and metabolic, with advanced age may contribute to the decrease in muscle endurance. The alterations include : 1-Reduced blood supply and capillary density and 2-Impairment of glucose transport and therefore of substrate availability, 3-Lower mitochondrial density, decreased activity of oxidative enzymes, and decreased rate of phosphocreatine repletion. 4-Blood Flow and Capillarity Sustained muscular performance requires a proper balance between energy supply and demand. Capillary density decreases with age and appears to be due to an actual reduction in the total number of blood capillaries, as both capillary- to-fiber ratio and the number of capillaries in contact with each muscle cell is lower in the aged. The reduction in capillarity has major importance for the ability of muscle to sustain power output over time. In summary, maintenance of muscle mass and strength may be critical for maintaining independent function in the elderly. Muscle mass and muscle strength decline with advancing age, and the underlying mechanisms responsible for the altered muscle performance are unclear but most likely involve multiple levels.
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