Trunk antagonist co-activation is associated with impaired neuromuscular performance

The goal of this paper was to determine if trunk antagonist activation is associated with impaired neuromuscular performance. To test this theory, we used two methods to impair neuromuscular control: strenuous exertions and fatigue. Force variability (standard deviation of force signal) was assessed for graded isometric trunk exertions (10, 20, 40, 60, 80% of max) in flexion and extension, and at the start and end of a trunk extensor fatiguing trial. Normalized EMG signals for five trunk muscle pairs (RA rectus abdominis, EO external oblique, IO internal oblique, TE thoracic erector spinae, and LE lumbar erector spinae) were collected for each graded exertion, and at the start and end of a trunk extensor fatiguing trial. Force variability increased for more strenuous exertions in both flexion (P < 0.001) and extension (P < 0.001), and after extensor fatigue (P < 0.012). In the flexion direction, both antagonist muscles (TE and LE) increased activation for more strenuous exertions (P < 0.001). In the extension direction, all antagonist muscles except RA increased activation for more strenuous exertions (P < 0.05) and following fatigue (P < 0.01). These data demonstrate a strong relationship between force variability and antagonistic muscle activation, irrespective of where this variability comes from. Such antagonistic co-activation increases trunk stiffness with the possible objective of limiting kinematic disturbances due to greater force variability.