Offering proper feedback to control for out-of-plane lumbar moments influences the activity of trunk muscles during unidirectional isometric trunk exertions

The assessment of trunk muscle activation and coordination using dynamometric measurements made in one anatomical plane has hardly minimized the production of out-of-plane coupled moments (CMs). This absence of control may add much variability in moment component partition as well as in recorded muscle activation. The aim of the study was to assess whether providing these CMs as visual feedback efficiently reduces them and whether this feedback influences trunk muscle activation. Twenty men performed five 5-s static ramp submaximal contractions, ranging from 0% to 55% of the maximal voluntary contraction (MVC), in six different directions while standing in a static dynamometer measuring L5/S1 moments. For each direction, four feedback conditions were offered, ranging from simple 1D-feedback in the primary plane of exertion, to full 3D-feedback. Surface electromyographic signals were collected for eight back and six abdominal muscles. Muscle activation amplitudes and CMs were extracted at each 10% force level from 10% to 50% maximum voluntary contraction (MVC). Providing 3D-feedback significantly reduced the CMs, at 50% MVC, by about 1–6%, 1–8% and 2–10% MVC in the sagittal, frontal and transverse planes, respectively. Providing 3D-feedback produced relatively small systematic effects (2–7%) on trunk muscle activation. However, the subjects responded differently to adequately control the coupled moments, leading in some cases to relatively high inter-individual differences in muscle activation. Interestingly, the statistical differences, and size of the effects, were mainly observed when the primary exertions were performed in the frontal and transverse planes. The implications of these findings are discussed.