Chapter 2

### INTRODUCTION

#### AGE-RELATED CHANGES IN THE MUSCULOSKELETAL SYSTEM

James R. Creps, PT, DScPT, OCS, CMPT

Aging presents the human body with many challenges as it relates to the musculoskeletal system. There is a progressive loss of skeletal tissue muscle mass and strength with aging. This process is referred to as sarcopenia and invariably leads to decreased functional ability in the aged. 1 The reduction in muscle mass is most prominent in the lower extremities and is characterized by a preferential atrophy of type 2 muscle fibers. 2,3 The loss of muscle mass can be significant and has been reported to be approximately 30% in several studies. 2 In addition, over the course of an individual’s lifespan, the muscle fiber loss noted as a consequence of aging results in a decline of between 25% and 40% of the cross-sectional area of the muscles of the thigh. 4 Multiple mechanisms have been hypothesized as potentially leading to the age-related loss in muscle mass and strength that is characteristically seen.

Increased apoptosis, or the process of programmed cellular death, may be a contributing factor. 5 In apoptosis, biochemical events lead to characteristic changes in cellular morphology. Specifically, in relation to muscle tissue, it appears nuclear fragmentation may lead to cellular death and subsequent tissue atrophy. 6 Apoptosis is different from necrosis, which involves some form of cellular trauma, and the resulting apoptotic bodies are engulfed and removed by phagocytic cells before they can cause damage to surrounding tissue.

Oxidative stress and inflammation in muscle tissue has also been implicated in the loss of muscle power and mass that is characteristically seen with aging. 7 Oxidation is the precursor to inflammation and is simply the interaction between oxygen molecules and human tissue. When oxygen is metabolized, it creates “free radicals” which steal electrons from other molecules, causing cellular damage. Oxidative processes stimulate inflammation and the cascade of biochemical reactions associated with it. The cytological changes seen in muscle tissue represent some of the adverse effects of the body’s inflammatory reaction to oxidation.

Sarcopenia is a syndrome characterized by progressive and generalized loss of skeletal muscle mass and strength. This loss of muscle mass causes risks of poor outcomes including poor quality of life, disability, and early death. 8

Finally, neuromuscular changes in the elderly also contribute to muscle wasting and weakness. 9 Muscle fiber denervation occurs, although it is not clear whether denervation precedes muscle wasting or is a result of it. In addition, reduced numbers of motor units, as well as a decrease in the conduction velocity of the motor axon, are also partly responsible for the decrease in muscle function seen with aging. 10,11 Finally, the excitability of both spinal and cortical tissues decreases with age, and this is complicit in the age-related decline seen in skeletal muscle function. 12 All of the neuromuscular ...

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