Electromyographic correlation of behavior of motor units with histochemical (HC) type of their muscle fibers is difficult in normal human muscle, because muscle fibers of different HC type belonging to different motor units are closely interspersed. An open-biopsy electromyography technique was developed to record from extensive, nearly homogeneous populations of muscle fibers of either major HC type in normal strength limb muscles in patients with chronic low-grade ordinary motor neuropathies and from a patient with rod neuromuscular disease.
Motor-unit potentials recorded from large, nearly uniform fields of muscle fibers low in pH 9.4 myofibrillar ATPase activity (type 1 muscle fibers) have been activated on mild voluntary contraction as relatively rhythmic, low-frequency discharges capable of sustained firing. Potentials recorded from large, nearly homogeneous fields of muscle fibers high in pH 9.4 myofibrillar ATPase activity (type 2 muscle fibers) have been activated only by vigorous or sudden contraction as brief bursts, or as single or double discharges.
Warmolts JR, Engel WK. Open-Biopsy Electromyography: I. Correlation of Motor Unit Behavior With Histochemical Muscle Fiber Type in Human Limb Muscle. Arch Neurol. 1972;27(6):512–517. doi:10.1001/archneur.1972.00490180048011
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