Action Potentials of Single Motor Units During Fatigue.

Abstract

The electromyogram is frequently used to estimate neural activation or localized fatigue in muscle. The purpose of this study was to determine how individual motor unit action potentials (MUAPs) change with fatigue. Single motor units in cat medial gastrocnemius were studied by stimulating isolated ventral roots. Isometric tension was recorded from the muscle tendon. MUAPs were recorded using a monopolar needle electrode. The extracellular action potential of a single muscle fiber (FAP) was simultaneously recorded by positioning a microelectrode near an active fiber. The motor unit was fatigued by stimulating at 10 Hz, 80 Hz, or with 40 Hz trains lasting 0.3 sec and delivered every 1.0 sec. The stimulus was continued until tension fell to 20% of the maximum value. The motor unit was allowed to rest and its recovery monitored by periodically stimulating with brief test trains. Different stimulation patterns produced different types of fatigue. Ten and 40 Hz stimulation produced primarily low frequency fatigue. This fatigue was characterized by slow recovery and a greater percentage reduction in tension in response to low frequency test trains compared to the percentage reduction from high frequency test trains. Eighty Hz stimulation produced primarily high frequency fatigue, characterized by fast recovery and a greater percentage reduction in tension at high test frequencies. During 10 and 40 Hz stimulation, MUAPs from all motor unit types exhibited increased duration, and in general, transient increases then decreases in amplitude. FAPs showed steady decreases in amplitude and increases in duration. There was no evidence of blocking (fiber dropout). Changes in MUAPs were variable and showed poor correlation with tension throughout the stimulation. During rest, MUAPs recovered within minutes yet tension showed little recovery, even after 30 minutes. In response to 80 Hz stimulation, MUAPs exhibited large increases in duration and decreases in amplitude, which correlated well with the fall in tension. Blocking was observed. With rest both tension and MUAPs recovered within minutes. Volitional MUAP activation produces primarily low frequency fatigue. Changes in MUAP configuration may not reflect changes in tension and therefore are an unreliable measure of fatigue.