Influence of fatigue on the simulated relation between the amplitude of the surface electromyogram and muscle force

Abstract

A linear relation between surface electromyogram (EMG) amplitude and muscle force is often assumed and used to estimate the contributions of selected muscles to various tasks. In the presence of muscle fatigue, however, changes in the properties of muscle fibre action potentials and motor unit twitch forces can alter the relation between surface EMG amplitude and force. A novel integrative model of motor neuron control and the generation of muscle fibre action potentials was used to simulate surface EMG signals and muscle force during three fatigue protocols. The change in the simulated relation between surface EMG amplitude and force depended on both the level of fatigue and the details of the fatiguing contraction. In general, surface EMG amplitude overestimated muscle force when fatigue was present. For example, surface EMG amplitudes corresponding to 60 per cent of the amplitude obtained at maximal force without fatigue corresponded to forces in the range 10-40% of the maximal force across three representative fatigue protocols. The results indicate that the surface EMG amplitude cannot be used to predict either the level of muscle activation or the magnitude of muscle force when the muscle exhibits any fatigue.

Muscle fatigue – from motor units to clinical symptoms

“…In contrast, the rate of decline in maximal torque during 5 sets of 30 repetitions with maximal lengthening contractions with the ankle dorsiflexor muscles was greater for old adults (Baudry et al, 2007). (Dideriksen et al 2010b). One protocol (top) involved simulating contractions at five submaximal target forces (20,35,50,65, and 85% of MVC force) beyond the point in time when the muscle can achieve the target force and both EMG amplitude and muscle force began to decline.…”

“…(1) repeated ramp contractions to 100% of the maximal voluntary contraction (MVC) force; (2) contractions sustained at two target forces (30 and 60% MVC force) for a similar force-time integral and then followed by ramp contractions; and (3) contractions sustained at five target forces (20-80% MVC force) for longer than the time to task failure (Dideriksen et al, 2010b). The association between EMG amplitude and muscle force varied across the protocols being bounded by the relation in the absence of the fatigue and the adjustments observed when the simulated contractions were sustained longer than task failure (Fig.…”

Electromyographic models to assess muscle fatigue

“…In recent studies, Dideriksen et al (2010aDideriksen et al ( , 2010b) tested a computational model that simulated changes in sEMG amplitude and muscle force during different fatigue protocols. They showed that during three different isometric protocols at different percentages of the maximal voluntary contraction (MVC), the relations between the amplitude and force were different depending on the fatigue protocol of the given study.…”

Unraveling the neurophysiology of muscle fatigue

“…The thin solid lines represent the relations obtained from the various protocols. Reproduced with permission fromDideriksen et al (2010b).…”