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

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.