The influences of muscle fibre proportions and areas upon EMG during maximal dynamic knee extensions

This study is an investigation of the relationship between muscle morpholog y and surface electromyographic (EMG) parameters [mean frequency of the pow er spectrum (MNF), signal amplitude (root mean square, RMS) and the signal amplitude ratio (SAR; i.e. the ratio between the RMS level during the passi ve part of the contraction cycle and the RMS level during the active part o f the contraction cycle)] during 100 maximal dynamic knee extensions at 90 degrees . s(-1). Each contraction cycle comprised of 1 s of active knee ext ension and 1 s of passive knee flexion. The surface EMG was recorded from t he vastus lateralis muscle. Twenty clinically healthy subjects participated in the study, and muscle biopsy samples of the vastus lateralis were obtai ned from 19 of those subjects. The relationships between muscle morphology and EMG were investigated at three stages of the test: initially, during th e fatigue phase (initial 40 contractions), and at the endurance level (the final 50 contractions). Major findings on correlations are that SAR and MNF tended to correlate positively with the proportion of type 1 fibres, and R MS correlated positively with the proportion of type 2 muscle fibres. The m uscle fibre areas showed little correlation with the EMG variables under in vestigation. The results of the present study showed that the three EMC var iables of a dynamic endurance test that were investigated (RMS, MNF and SAR ) were clearly correlated with the proportions of the different fibre types , but only to a small extent with fibre areas. These findings contradict so me of the theoretical models of the EMG, especially for parameters in the f requency domain.