Motor units come in a variety of sizes. Small ones have single, thin nerve fibers that innervate several muscle fibers, whereas the lone, thicker nerve fiber in a larger motor unit may innervate several thousand of them. Normally, when motion is initiated, it takes stronger stimulation to "fire" thick nerve fibers than thin ones. Thus, the smaller, so-called "slow-twitch" muscle fibers start contracting before larger "fast-twitch" fibers.
Fast-twitch fibers are essential for powerful athletic motions such as running, but fatigue quickly as they burn through finite stores of their primary fuel, glycogen. Their more diminutive slow-twitch counterparts, which burn their fuel slowly, are crucial to delicate movements such as those involved in sewing or drawing, as well as for fine-tuning coarser, more powerful movements. Activities relying mainly on small slow-twitch fibers can proceed for long periods of time, while larger but more-fatigable fast-twitch fibers are reserved for brief bursts of high-powered activity.
Previous attempts to restore lost motor function using programmed sequences of electrical impulses, delivered via a cuff placed around a nerve, have enabled paralyzed people to walk, if only for a few minutes. Unfortunately, large nerve fibers are more responsive than smaller ones to electrical stimulation, so muscles contract in the wrong order - large, fast-twitch fibers first, then small, slow-twitch ones; this results in jerky motion and, soon thereafter, fatigue.
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|Contact: Bruce Goldman|
Stanford University Medical Center