Trends in Ergonomics/Human Factors IV, Proceedings of the Annual International Industrial Ergonomics and Safety Conference, Miami, Florida, 9-12 June, 1987 1987:717-723
An evaluation was made of the single motor unit action potential (SMUAP) discharge rates from the abductor digiti minimi muscle function using fine wire electrodes during a sustained low level isometric contraction held to fatigue. The sustained voluntary isometric contraction was at a 25 percent level of the individual's maximum voluntary isometric contraction. Twenty three subjects between 20 and 40 years of age were studied. Each was seated in a chair with the right forearm and hand in a specially designed splint which limited movements of the hand. The middle of the proximal interphalangeal joint of the right little finger was fixed perpendicular to the load cell. Analysis was made of 24 individual SMUAPs obtained from 14 subjects. The grand mean interspike interval for the 25 percent maximum voluntary contraction isometric contraction in the beginning stage of the required task was 61.6 milliseconds. The time between successive discharges of the same SMUAP became longer as a function of endurance time, such that the grand mean during the end time period was 140.0 milliseconds. The author suggest that the decrease in SMUAP discharge rate as fatigue begins may be explained by central and peripheral sources, including feedback from muscle spindles, inhibition offered by the Renshaw feedback system, increasing duration of after hyperpolarization for small anterior horn cells discharging, and influence of the Golgi tendon organ discharge. Central mechanisms involved may be able to reduce or minimize conduction failures. Oscillatory force readings were noted as subjects neared the endurance time. This oscillatory force may function to limit the effect of fatigue on a particular motor unit by some form of central mechanism. The author concludes that oscillatory force readings exhibited by the muscle may be linked in a causal manner to musculoskeletal injuries as the muscle fatigues.
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