Integration of Surface EMG, US Imaging and 3D Kinematic: New Frontiers for Muscle Function Investigation

The integration of grids of electrodes, US imaging and 3D kinematic can allow investigating how muscle architectural changes influence surface EMGs: this innovative approach can provide novel spatio-temporal information regarding electromechanical function of muscle, relevant both for improving basic knowledge and for clinical applications. The aims of the present study were i) to verify whether movements of the US probe lead to artefacts in surface EMGs detected with a grid of electrodes transparent to US; ii) to analyse how much muscle architectural changes, induced by changes in joint positions, can influence the amplitude distribution of surface EMGs. A young healthy participant performed contractions of the right tibialis anterior (TA) at two different ankle positions: A) neutral and B) full plantarflexion. While provided with EMG visual feedback, the participant was asked to recruit a single motor unit: contractions lasted similar to 50 s. Surface EMGs, kinematic data and ultrasound images were acquired. The spatial distribution of the root mean square amplitude of motor unit action potentials was assessed from collected EMGs. Ankle angles were obtained from 3D kinematics. Muscle volume was obtained through the segmentation of US images reconstructed in 3D space: width and thickness variations between the two positions were measured. Movements of the US probe over the grid of electrodes did not result in artefacts in the surface EMGs. Results indicate a marked effect of TA architecture on the amplitude distribution of action potentials of a single TA motor unit. When moving the foot from plantarflexion to neutral position, TA width increased of 11.6%. Interestingly, a corresponding increase in the RMS spatial distribution was observed for the ankle neutral position. Presuming the same motor unit was recruited for both ankle positions, the changes in EMG amplitude reported here were predominantly due to TA architectural changes.