Non-invasive assessment of single motor unit mechanomyographic response and twitch force by spike-triggered averaging

Abstract

A method for non-invasive assessment of single motor unit (MU) properties from electromyographic (EMG), mechanomyographic (MMG) and force signals is proposed. The method is based on the detection and classification of single MU action potentials from interference multichannel surface EMG signals and on the spike-triggered average of the MMG (detected by an accelerometer) and force signals. The first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles were investigated at contraction levels of 2% and 5% of the maximum voluntary contraction (MVC) force. A third contraction was performed by selective activation of a single MU with surface MU action potential visual feedback provided to the subject. At 5% MVC, the mean (+/-standard error) single MU MMG peak-to-peak value was 11.0+/-1.8 mm s(-2) (N= 17) and 32.3+/-6.5 mm s(-2) (N=20) for the FDI and ADM muscles, respectively. The peak of the twitch force was, at the same contraction level, 7.41+/-1.34 mN and 14.42+/-2.92 mN, for the FDI and ADM muscles, respectively. The peak-to-peak value of the MMG was significantly different for the same MU at different contraction levels, indicating a non-linear summation of the single MU contributions. For the FDI muscle, the MMG peak-to-peak value of individual MUs was 21.5+/-7.8 mm s(-2), when such MUs were activated with visual feedback provided to the subject, whereas, for the same MUs, it was 11.8+/-3.8 mm s(-2), when the subject maintained a constant force level of 2% MVC. The method proposed allows the non-invasive assessment of single MU membrane and contractile properties during voluntary contractions.

Longitudinal and transverse propagation of surface mechanomyographic waves generated by single motor unit activity

“…Moreover, the present results provide evidence of a major role of fiber displacement in the origin of the MMG. The dependence of MMG responses on accelerometer location is in agreement with the observations that muscle morphology and architecture affects the MMG signal features [8,19,20,21,29,30].…”

Muscle tissue oxygenation, pressure, electrical, and mechanical responses during dynamic and static voluntary contractions

“…During static contraction the MMG/EMG was larger at 10% than that at 20% MVC. This result may reflect the possible nonlinear summation, or destructive interference, in the MMG signal at low levels of isometric contraction when more MUs are recruited and the firing rate is increased (Orizio et al 1996;Cescon et al 2004). During the eccentric phase in the dynamic contraction at 20%MVC, the MMG/EMG ratio was significantly higher than that in the intermittent static contraction, indicating that a given EMG activity may result in a larger number of the cross bridges cycling, e.g.…”

“…The EMG amplitude has been shown to be affected by the number and firing rate of the active MUs, the shape of the MU action potential, and the crosscorrelation of the MUs discharge (Basmajican and De Luca 1985). With regard to MMG, several papers have demonstrated that the pressure waves due to the dimensional changes of the fibres of each active MU contributes to the muscle surface oscillations detectable as MMG during voluntary contraction in humans (Gordon and Holbourn 1948;Petitjean and Maton 1995;Yoshitake and Moritani 1999) even if, when several MUs are active, the individual motor unit contributions are summed into the MMG in a non-linear fashion (Orizio et al 1996;Cescon et al 2004). On this basis it can be assumed that during voluntary contractions the MMG origin is intrinsic to the muscle (Stokes and Blythe 2001;Søgaard et al 2004).…”

System identification of the mechanomyogram from single motor units during voluntary isometric contraction

“…The latencies and durations of the displacement MMG were revealed. Cescon et al recorded the MMGs from the first dorsal interosseous muscle and abductor digiti minimi muscle with an accelerometer [1]. The MMG peakto-peak value and mean power frequency were reported.…”