MODULE SUPPLEMENT: MUSCULO-SKELETAL SYSTEM
Changes in Muscle with Age
One of the most commonly discussed change that occurs with age is the loss of lean body mass (LBM) which shifts the proportion of lean-adipose tissue in favor of the latter. Loss of LBM is important because it is the most active tissue. Its loss influences basal metabolism, which declines with age, and total body water, which also declines with age (also see the Renal System supplement--note: this link will open in a new browser window). Loss of LBM also contributes to the loss of muscle strength, which is noted to decline by about 8-16% per decade after approximately 50 yrs of age. Muscle power, the product of the force and velocity of muscle shortening, declines faster and to a greater extent than muscle strength. Power, or the ability to generate force rapidly, has been shown to be particularly important for maintaining the ability to perform activities of daily living (ADLs) such as stair climbing, walking and rising from a chair.
Loss of LBM is primarily due to a reduction in the number and size of muscle fibers, a condition termed "sarcopenia". The prevalence of sarcopenia (defined as muscle mass 2 SD less than the mean for a young, healthy reference group) has been shown to increase with age to greater than 50% for those over 80 (Baumgartner et al., 1998).
But why do we lose muscle as we age? Although the ultimate answer to this is still not fully elucidated, several factors appear to be implicated.
When we talk about a motor unit, we are describing a relationship between a motor neuron (found in the spinal cord), the motor axons that reach out to the muscle, and the muscle fibers that are innervated.
With aging, there is loss of motor neurons and a decrease in the numbers of motor axons available to innervate the muscles. There is also a decrease in the speed of transmission of impulses, neurotransmission, and receptor numbers. And there is loss of muscle fibers and a change in their characteristics. At the single fiber level, there is a decrease in fiber diameter, peak force adjusted for fiber size, and maximum shortening velocity with aging. These changes are summarized in the table below.
|
