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

Abstract

This study is an investigation of the relationship between muscle morphology and surface electromyographic (EMG) parameters [mean frequency of the power spectrum (MNF), signal amplitude (root mean square, RMS) and the signal amplitude ratio (SAR; i.e. the ratio between the RMS level during the passive part of the contraction cycle and the RMS level during the active part of the contraction cycle)] during 100 maximal dynamic knee extensions at 90 degrees. s(-1). Each contraction cycle comprised of 1 s of active knee extension and 1 s of passive knee flexion. The surface EMG was recorded from the vastus lateralis muscle. Twenty clinically healthy subjects participated in the study, and muscle biopsy samples of the vastus lateralis were obtained from 19 of those subjects. The relationships between muscle morphology and EMG were investigated at three stages of the test: initially, during the 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 RMS correlated positively with the proportion of type 2 muscle fibres. The muscle fibre areas showed little correlation with the EMG variables under investigation. The results of the present study showed that the three EMG variables 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 some of the theoretical models of the EMG, especially for parameters in the frequency domain.

Motor unit recruitment patterns 2: the influence of myoelectric intensity and muscle fascicle strain rate

“…By contrast, the electrical properties of the sarcolemma have been shown to vary between fast and slow fibre types within mammals (Luff and Atwood, 1972). As these properties will determine the conduction velocity of an action potential (Buchtal et al, 1955), it can be predicted that faster motor units will have faster conduction velocities and hence generate higher myoelectric frequencies (Gerdle et al, 2000;. We therefore interpret that a higher value of , representing relatively more low frequency signal content, can be associated with the recruitment of slower motor units.…”

Recruitment of faster motor units is associated with greater rates of fascicle strain and rapid changes in muscle force during locomotion

“…This evidence is based, in large part, on analyzing the time-varying frequency spectra of electromyographic (EMG) signals (see reviews by Hodson-Tole and Wakeling, 2009;Raez et al, 2006). Different motor unit types have muscle fibers with different electrical membrane properties (Luff and Atwood, 1972), and there is an intrinsic speed dependence of the EMG frequency such that faster fibers generate higher-frequency signals (Gerdle et al, 2000;Hodson-Tole and Wakeling, 2008b;Lee et al, 2011). For example, when human subjects cycle at increased cadences, EMG signals from the gastrocnemii shift to higher frequencies (Wakeling et al, 2006), consistent with the recruitment of faster motor units (Citterio and Agostoni, 1984).…”

A Study of the Influence of Muscle Type and Muscle Force Level on Individual Frequency Bands of the EMG Power Spectrum

“…The same authors [15] also concluded that the EMG power spectrum might contain information about the features of muscle fibres. These suggestions were confirmed by Gerdle, Karlsson, Crenshaw, et al's studies [21], which aimed to evaluate the relationship between the proportion of type I and type II muscle fibres and the parameters of the EMG signal such as MF and amplitude (RMS) in the vastus lateralis muscle. This research demonstrated that there were positive correlations between the value of MF and the proportion of type I muscle fibres as well as positive correlations between the value of RMS and the proportion of type II muscle fibres.…”

“…Those parameters are described as global parameters and their values change over time due to the fatigue processes within the muscle [10,11]. However, changes in the distribution of frequencies of the EMG power spectrum are also affected by changes in force level [12,13,14,15], recruitment of new, not fatigued, motor units [16,17,18,19,20], the proportion of muscle fibres [21], muscle type [22] and muscle length [15].…”

“…The results of these studies indicated that the content of muscle fibres of a determined type as well as the proportion regarding activation of type I and type II fibres influenced the values of the EMG parameters. The quotient of the proportion of type I (slow) to type II (fast) fibres affected the amplitude of surface EMG [21] as well as the values of global parameters of the EMG power spectrum such as MPF and MF [16,17,18]. Gerdle, Eriksson and Brundin [26] even suggested that the EMG power spectrum might be a tool for evaluating the features of muscles regarding, e.g., the content of muscle fibres of a specific type.…”