EMG fatigue patterns accompanying isometric fatiguing knee-extensions are different in mono- and bi-articular muscles

Isometric, fatiguing knee-extensions at 30%, 50% and 70% maximum voluntary contraction (MVC) were performed by 18 healthy human subjects. Surface electromyographic (SEMG) activity was recorded from the mono-articular vastus medialis (VM) and vastus lateralis (VL) muscles, and the bi-articular rectus femoris muscle (RF). To make the bi-articular muscle work under (1) constant and (2) similar working conditions as the two mono-articulars do, the hip was fixed in a flexed position. The root mean square (RMS) SEMG recorded during fatigue was standardized to the respective values of MVC. The mean coefficients of regression of the RMS and median frequency (MF) changes were then analyzed by multivariate analysis of variance.

The load effect upon the muscle fatigue changes, as measured by increase in RMS EMG, differed between the bi-articular muscle and the two mono-articulars, in that the parameter dropped with maximum load for the bi-articular, whilst it remained stable or even increased for the mono-articulars. This might suggest that the mono- and bi-articular muscles have different roles in fatigue tasks where the bi-articulars function purely as mono-articulars. By contrast, such a clear dichotomy between the bi-articular RF and the two mono-articulars, VM and VL, was lacking for the fatigue parameter of MF.

As these findings were confined to the changes in RMS EMG, different neuronal coding mechanisms for the mono- and bi-articular muscles in the central nervous system may be inferred.