EMG CHANGES FOLLOWING A DAY OF NORMAL ACTIVITY

Abstract

Previous studies of EMG recordings from lumbar para-spinal muscles have shown correlations between some EMG variables and low back pain. However there are discrepancies in the literature concerning the usefulness of some of these variables. It has been suggested that ordinary fatigue influences the reproducibility of these measurements, introducing a confounding factor.

In this study we have investigated changes in EMG variables, following a day of normal activity. Forty six subjects participated in this study. EMG recordings were performed at the beginning of their shift (time 1) and at 6 h 20 ±5 min afterwards (time 2) under isometric condition at 60% and 40% of their lean body mass (LBM). Variables studied were initial medial frequency (IMF), median frequency slope (MFS) and half width (HW).

At 60% LBM, IMF measurements at time 1 and time 2 were highly correlated (r2= 0.84, p> 0001) and this was the case for HW measurements (r2=0.84, p> 0001) and MF slope (r2=0.52, p=0> 001). Conducting paired sample t-test also showed no significant change in the IMF from time 1 (M=48.6, SD=8.9) to time 2 (M=49.2, SD=7.3), t(45)=−0.9, p=0.38, or in HW from time 1 (M=47.2, SD=15.5) to time 2 (M=45.9, SD=13.9), t(45)=1.7, p=0.29, or MF slope from time 1 (M=−0.2, SD=0.17) to time 2 (M=−0.24, SD=0.16), t(45)=1.67, p=0.10). The relations observed at 40% LBM almost mirrored those reported at 60 % LBM but with even less significant difference from time1 to time2.

We conclude that IMF, HW and MFS are highly reproducible EMG variables that are not affected by ordinary fatigue and may therefore be valuable in examining differences between subjects or over longer time periods. However they are not useful in assessing changes due to daily exertion.