A spike triggered averaging technique for high resolution assessment of single motor unit conduction velocity changes during fatiguing voluntary contractions

In this paper we propose an improved spike triggered averaging technique for the assessment of control properties and conduction velocity (CV) of single motor units (MUs) during voluntary sub-maximal muscle contractions. The method is based on the detection of multi-channel surface EMG signals (with linear electrode arrays) and intramuscular recorded single MU action potentials (MUAPs). Intramuscular electrodes are inserted taking into account the MU structural properties (innervation zone and tendon locations, length of the fibers), assessed by the linear array surface EMG detection. A technique for intramuscular EMG signal decomposition is used to identify single MUAP trains. The MUAPs obtained from the intramuscular EMG decomposition algorithm are used to trigger and average the multi-channel EMG signals. CV of single averaged surface MUAPs is estimated by the use of advanced signal processing methods based on multi-channel recordings which allow to consistently reduce the variance of CV estimates with respect to traditional two channel delay estimators. The number of averaged potentials can thus be reduced, improving temporal resolution. The technique proposed is tested with recordings from the tibialis anterior muscle of 11 volunteers. It is shown that the method allows the assessment of CV changes (fatigue) of single MUs as small as 0.1 m/s with a limited number of averages (temporal resolution of 1-2 seconds), leading to a consistent improvement with respect to traditional surface EMG spike triggered averaging techniques. The method has potential usefulness in a number of basic and applied research fields.