LAUSR

Elbow flexor force steadiness is less with the forearm pronated (PRO) compared with neutral (NEU) or supinated (SUP), and may relate to neural excitability. Although not tested in a force steadiness paradigm, lower spinal and cortical excitability were observed separately for biceps brachii in PRO, possibly dependent on contractile status at the time of assessment. This study aimed to investigate position-dependent changes in force steadiness as well as spinal and cortical excitability at a variety of contraction intensities. Thirteen males (26±7 years; mean±SD) performed 3 blocks (PRO, NEU, SUP) of 24 brief (~6s) isometric elbow flexor contractions (5, 10, 25 or 50% of maximal force). During each contraction, transcranial magnetic stimulation or transmastoid stimulation was delivered to elicit a motor evoked potential (MEP) or cervicomedullary motor evoked potential (CMEP), respectively. Force steadiness was lower in PRO compared to NEU and SUP (p≤0.001), with no difference between NEU and SUP. Similarly, spinal excitability (CMEP/maximal M-wave) was lower in PRO than NEU (25 and 50% maximal force; p≤0.010) and SUP (all force levels; p≤0.004), with no difference between NEU and SUP. Cortical excitability (MEP/CMEP), did not change with forearm position (p=0.055); however, a priori post hoc testing for position showed excitability was 39.8±38.3% lower for PRO than NEU at 25% maximal force (p=0.006). The data suggest that contraction intensity influences the effect of forearm position on neural excitability and that reduced spinal and, to a lesser extent, cortical excitability could contribute to lower force steadiness in PRO compared to NEU and SUP.