Power spectrum and turns analysis of EMG at different voluntary efforts in normal subjects

The power spectrum and the number of turns of the needle EMG were analysed at different degrees of voluntary contraction in the brachial biceps muscle or the first dorsal interosseus muscle of 23 normal subjects. During a stepwise increase in force HF/LF amplitude ratio showed a slight decrease with increasing force from 10% to 80% maximal voluntary contraction (MVC). At moderate to high force (i.e., 30-80% MVC) the HF/LF amplitude ratio increased with increasing turns at the individual site of the muscle. This relation probably reflects that both the HF/LF amplitude ratio and the number of turns are influenced by the degree of summation in the electrical activity. There was no correlation between HF/LF amplitude ratio and mean amplitude. The finding of a relation between HF/LF amplitude ratio and turns suggests that power spectrum analysis may differentiate between controls and patients with neuromuscular diseases.