Motor neurons and the muscle fibers they command are called motor units. Motor unit restructuring is the product of the inevitable death of neuron cells. These cells translate the signals between the brain and voluntary muscle fibers. When the brain tells the right arm to raise, for example, this command travels through the neurons associated with the muscles associated with raising the right arm. When the motor neurons associated with the bundles of muscle fiber that make up these muscle groups receive the command from the brain, they send electrical impulses which cause the muscle fibers to contract (called innervation). This causes the right arm to raise.
Motor neurons responsible for precise or quick movements, like eye movements or running, are called fast-twitch (FT) neurons. Because they're so specialized, FT neurons usually command a limited number of neurons. Slow-twitch (ST) neurons, on the other hand, command a much higher number of fibers because the muscles they're responsible for innervating require less precision.
Unfortunately, the FT neurons tend to die off first. All cells have a predetermined life span. Once they divide a certain number of times (between 50 and 100, depending on the organism's age), they stop replicating and die. When a motor neuron dies, the muscle fibers it commands can deteriorate -- a process known as atrophy -- and die as well. To prevent atrophy, when a FT neuron dies, a ST neuron situated nearby will attach itself to the now-abandoned muscle fibers the dead neuron commanded, innervating and keeping them alive. This change in command is known as motor unit restructuring.
Restructuring isn't a seamless process: The ST neuron transmits far less precise innervation than a FT neuron. Consider the FT neuron as an agile tightrope walker and the ST neuron akin to a bull in a china shop. Under the command of the ST neuron, once precise movements, like those that allow a person to maintain balance or run, are much less coordinated and quick.
Other muscle groups that atrophy though sarcopenia are even more obvious to the naked eye. Sagging, unresponsive facial skin is the result of lost muscle fiber beneath. The appearance of a sunken ribcage, with deep ruts between rib bones is the result of the loss of the intercostal muscles. The stooped posture many aged people adopt is due to the loss of skeletal muscles, which leads to a loss of support for bones of the spine, shoulders and back that keep a younger person erect. These unmistakable and dramatic signs of age are known as senile sarcopenia.
Between the decline in hormones, proteins and cellular death it seems like the slow march toward sarcopenia is as inevitable as our eventual deaths. Age researchers have found this isn't necessarily the case. There are simple steps that a person can take to ward off sarcopenia.