Quantitative assessment of co-contraction in cervical musculature.

The co-contraction of cervical musculature is quantitatively assessed. For the modeling procedure, electromyographic (EMG) signal and anatomical data of the neck musculature were used. EMG signals were collected from eight sites of the neck bilaterally using Ag-AgCl surface electrodes from ten adult male subjects. The subjects performed voluntary isometric contractions gradually developing to maximum efforts in flexion, extension, left lateral bending and right lateral bending. The EMG-assisted optimization modeling procedure was used to estimate muscle forces. To quantify co-contraction, muscle forces were decomposed into task subset and co-contraction subset of muscle forces. To show the degree of co-contraction, 'co-contraction ratio' was defined as the proportion of co-contraction muscle forces to total muscle forces. The ranges of co-contraction ratio are 0.08-0.16 during extension, 0.30-0.41 during flexion, 0.27-0.32 during left lateral bending, and 0.30-0.36 during right lateral bending. In all cases, co-contraction ratios increase as external moments increase. The ratios of compressive spinal loads from the co-contraction subset of muscle forces to those from total muscle forces are similar to co-contraction ratios during the whole ramp period. This study provides data demonstrating quantitative measures of the contribution of muscle co-contraction to cervical spinal loads.