Determining minimal stimulus intensity for mechanomyographic analysis

Abstract

Introduction: Mechanomyography (MMG) has recently shown promise in monitoring recovery of injured muscles. However, delivering a maximal percutaneous neuromuscular stimulus (PNS) could potentially be painful on severely damaged muscles. The aim of this paper was to determine whether delivering a sub-maximal PNS could still obtain accurate MMG recordings of muscle contraction time (Tc). The effect of muscle architecture on determining the minimal level of current was also investigated.

Methods: Six muscles were investigated; 5 lower limb and the 1st dorsal interosseous. A 'current ramp' procedure was performed to determine minimal stimulus intensity required for accurate Tc recordings. A current ramp entails beginning at a low current (30mA) and increasing in increments of 10mA until a maximal muscle contraction is observed.

Results: For lower limb muscles, 130mA was the largest current required to obtain accurate Tc recordings in at least 95% of the population. This was up to a 50% reduction in the amount of current delivered for some muscles. Fibre type distribution showed the greatest relationship with mean minimum current.

Discussion: Future studies investigating injured or uninjured muscles via MMG, could use these submaximal currents to obtain accurate MMG recordings, whilst improving patient comfort and reducing experiment duration.

Keywords: Contraction time; Injury; Mechanomyography; Muscle; Stimulation.