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Learning Objectives
Summarize age-related changes in the skeletal muscle and motor units.
Appraise the evolving concept of sarcopenia and the characteristics of the different definitions proposed in the past two decades.
Use muscle mass and function to diagnose sarcopenia in clinical practice.
Develop a treatment plan for sarcopenic patients.
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Key Clinical Points
Muscle aging involves anatomical and physiological changes in motor units and their regulation.
Sarcopenia is a progressive and generalized disease involving the accelerated loss of muscle mass and function. The concept of sarcopenia has evolved from low muscle mass alone to include muscle function.
Around 10% of the older persons living in the community suffer from sarcopenia, with a higher prevalence in other clinical settings.
Measures of muscle mass, muscle strength, and physical performance are used to diagnose sarcopenia in clinical practice.
Treatment of sarcopenia requires resistance exercise. Nutrition may have a role. No drugs are yet available for this condition.
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Age-related losses of muscle mass and strength are common and can lead to sarcopenia, a condition typically consisting of a combination of loss of strength, physical function, and muscle mass. This chapter will cover concepts related to the process of muscle aging as well as the current status of sarcopenia detection, evaluation, and management.
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The human body is made up of more than 600 skeletal muscles, accounting for around 40% of the total body mass. Excluding water, muscles are composed of about 80% protein, or about 50% of total body protein. The main functions of skeletal muscle are mobility and regulation of proteins. Adults tend to lose muscle mass at a rate of about 8% per decade after age 40. At age 70, an adult will have lost a mean of 24% of the muscle mass present at age 30. The rate of muscle mass loss accelerates and almost doubles after age 70. It is essential to note that adults lose muscle strength much faster; about 3% to 4% per year after age 50. Strength loss renders older persons vulnerable to physical disability.
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MOTOR UNITS AND THEIR REGULATION
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The basic functional unit of skeletal muscles is the motor unit. Each motor unit consists of a neuron, its axon, and the muscle fibers innervated by that neuron. The neuron terminal is connected to the muscle fiber through the neuromuscular junction, where neurons release neurotransmitters that bind to muscle cells receptors. A single motor neuron may innervate from a few to thousands of muscle fibers, depending on the muscle, with neurons responsible for higher force production innervating a higher number of fibers. Human muscles harbor three types of muscle fibers: type I, type IIa, and type IIx (formerly named IIb). Type I fibers (slow-twitch) are well adapted to perform aerobic exercise and are highly resistant to fatigue, having high oxidative capacity and a low capacity to ...