Estimation of voluntary elicited motor neuron discharge using a peripheral nerve collision technique at different contraction strengths.

Objective: To estimate non-invasively the amount, recruitment pattern and discharge frequency of spinal motor neurons (MN) at contraction strength >20% of maximal voluntary contraction (MVC) of small hand muscles.

Methods: A peripheral collision technique was used and consisted of supramaximal electrical stimuli at Erb's point and at the wrist, synchronizing descending volleys of action potential during voluntary isometric contractions of the abductor digiti minimi muscle at 20-80% of MVC strength and 1-8 s of contraction duration. Responses of 13 healthy volunteers were quantified and analysed using a recently described model of MN behaviour.

Results: A linear relationship between MN discharge and force generation was noticed with R2 = 0.996, and was confirmed using the simulation results (R2 = 0.997) for contraction durations up to 8 s. For each investigated force level, discharge frequency and recruitment pattern were calculated for individual MN.

Conclusions: Using this method, MN discharge properties during voluntary activity can be estimated non-invasively.

Significance: This method provides new opportunities for the non-invasive study of MN behaviour, and could be expanded to patients with conduction failure and during fatigue.