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The Journal of Neuroscience, September 14, 2005, 25(37):8528-8533; doi:10.1523/JNEUROSCI.0923-05.2005

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Behavioral/Systems/Cognitive
Muscle Fiber and Motor Unit Behavior in the Longest Human Skeletal Muscle

A. John Harris,¹ Marilyn J. Duxson,² Jane E. Butler,^3,4 Paul W. Hodges,^3,4 Janet L. Taylor,^3,4 and Simon C. Gandevia^3,4

Departments of ¹Physiology and ²Anatomy, University of Otago, Dunedin 9001, New Zealand, ³Prince of Wales Medical Research Institute, Randwick, New South Wales, Australia 2031, and ⁴University of New South Wales, Sydney, New South Wales, Australia 2052

The sartorius muscle is the longest muscle in the human body. It is strap-like, up to 600 mm in length, and contains five to seven neurovascular compartments, each with a neuromuscular endplate zone. Some of its fibers terminate intrafascicularly, whereas others may run the full length of the muscle. To assess the location and timing of activation within motor units of this long muscle, we recorded electromyographic potentials from multiple intramuscular electrodes along sartorius muscle during steady voluntary contraction and analyzed their activity with spike-triggered averaging from a needle electrode inserted near the proximal end of the muscle. Approximately 30% of sartorius motor units included muscle fibers that ran the full length of the muscle, conducting action potentials at 3.9 ± 0.1 m/s. Most motor units were innervated within a single muscle endplate zone that was not necessarily near the midpoint of the fiber. As a consequence, action potentials reached the distal end of a unit as late as 100 ms after initiation at an endplate zone. Thus, contractile activity is not synchronized along the length of single sartorius fibers. We postulate that lateral transmission of force from fiber to endomysium and a wide distribution of motor unit endplates along the muscle are critical for the efficient transmission of force from sarcomere to tendon and for the prevention of muscle injury caused by overextension of inactive regions of muscle fibers.

Key words: human muscle; motor unit; sartorius muscle; electromyography; muscle fiber; conduction velocity

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Received March 8, 2005; revised June 29, 2005; accepted July 19, 2005.

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