Differences in Motor Unit Firing Characteristics and Motor Unit Potential Parameters in the Extensor Carpi Radialis (ECR) Muscle during Low Level Contractions

Abstract

With preferential use of one limb over another, measurable differences in motor unit potential (MUP) morphological and firing rate characteristics might exist between dominant and non-dominant upper limbs, as limb dominance might lead to changes in recruitment characteristics and/or fiber-type changes. PURPOSE: This study was designed to determine (i) if firing rate behavior and MUP parameters in the ECR muscle are different between dominant and non-dominant ECR muscles in a sample of healthy volunteers, and (ii) if there are detectable changes in motor unit (MU) firing rate and MUP morphology features between recruitment threshold and a low-level contractions in the wrist extensor muscles. METHODS: Changes in mean MU firing rates and MUP parameters were investigated in the ECR muscles of eight healthy volunteers (mean age = 30.9 yr, SD ± 4.5) during isometric contractions held at threshold and 10% MVC. Handedness was assessed using the Edinburgh Handedness Inventory questionnaire. In random order and on each ECR muscle, threshold contractions corresponding to 1–2 detected MUs (3–5% MVC) were recorded followed by three to five repetitions of a sustained wrist extension contraction (30s) at 10% maximum voluntary contraction. Signal decomposition was achieved using DQEMGTM software, resulting in surface (SMUP) and needle detected MUP size (amplitude, duration, area), and MU firing rates. RESULTS: An independent t-test showed no significant differences between dominant and non-dominant ECR muscles for any of the MUP or SMUP size or firing rate statistics. The data from both arms were therefore pooled to compare the parameters at threshold and at 10%MVC. Significant increases with contraction level were found in MU firing rate, MUP amplitude, SMUP amplitude and negative peak area. CONCLUSIONS: No limb-dominance effect was noted in MUP size of MU firing rate parameters. Larger MUP amplitude and area features at 10%MVC as compared to threshold indicate recruitment, while higher firing rates indicate rate-coding. The ability to detect these differences might be of clinical value in the investigation of certain neuromuscular conditions.