Comparison of the ballistic contractile responses generated during microstimulation of single human motor axons with brief irregular and regular stimuli

Introduction: Ballistic contractions are induced by brief, high frequency (60-100 Hz) trains of action potentials in motor axons. During ramp voluntary contractions, human motoneurones exhibit significant discharge variability of ∼20% and have shown to be advantageous to the neuromuscular system. We hypothesized that ballistic contractions incorporating discharge variability generate greater isometric forces than regular trains with zero variability. Methods: High impedance tungsten microelectrodes were inserted into the fibular nerve, and single motor axons were stimulated with both irregular and constant-frequency stimuli at mean frequencies ranging from 57.8-68.9 Hz. Results: Irregular trains generated significantly greater isometric peak forces than regular trains over identical mean frequencies. Conclusions: That the high forces generated by ballistic contractions are not solely based on high frequencies, but rather a combination of high firing rates and discharge irregularity. It appears irregular ballistic trains take advantage of the "catch-like property" of muscle, allowing augmentation of force.