Surface mechanomyogram amplitude is not attenuated by intramuscular pressure

The electromyogram (EMG) and intramuscular pressure (IMP) increase linearly with force during voluntary static contractions, while the surface mechanomyogram (MMG) increases linearly only up to approximately 70% of the maximal voluntary contraction (MVC) and then levels off. The aim of this study was to investigate the possible influence of IMP on the non-linear MMG increase with force and hence on the signal generation process. Seven subjects performed static contractions of the elbow flexors during: (1) ramp contractions from 0 to 60% of the MVC, and (2) steps at 10, 20 and 40% of the MVC. An external pressure of 0 and 50 mmHg for the ramps or 0, 20, 40, 60, 80 and 100 mmHg for the steps was applied by means of a sphygmomanometer cuff in separate trials. The EMG and the MMG were detected in the biceps brachii by means of a pair of surface electrodes and an accelerometer. The IMP was measured using a Millar tipped pressure transducer, and the data was presented as the mean and standard deviation in each case. The IMP was strongly and linearly related to the external pressure and contraction force both during ramps and steps. The EMG(rms) and MMG(rms) were never reduced as a consequence of the IMP increments. In contrast, a steeper MMG(rms) versus %MVC relationship during ramps at 50 mmHg cuff pressure, and an influence of the cuff pressure at 40% of MVC on MMG(rms) were evident. We conclude that IMP per se does not attenuate the MMG generation process during voluntary contraction, suggesting that the previously described MMG(rms) decrease at near maximal static efforts must be attributed to other determinants, such as a fusion-like situation due to the high motor unit firing rate.