Agonist/Antagonist Trunk Muscle Co-Activation During Fatiguing Isometric Contractions

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0817 PURPOSE: To examine agonist/antagonist trunk muscle co-activation and perceived exertion (RPE) during fatiguing isometric contractions. METHODS: Eighteen healthy young adults (8 men, 10 women) were evaluated for peak isometric trunk flexion and extension torque (PT), in a seated position. Subjects subsequently performed separate, sustained trunk flexion and extension isometric contractions at 80% of PT to failure. Electromyographic (EMG) activity was measured over the external oblique (EO), internal oblique (IO), rectus abdominis (RA), and the lumbar paraspinal (L1, L2, and L3) muscles. The EMG signal during the 80% contraction was normalized to the EMG of the PT. RPE's were sampled with a modified Borg CR-10 scale every 5 seconds during each sustained contraction. RESULTS: Subjects maintained the isometric contraction significantly longer in trunk extension than flexion (36.3 ± 17.7 vs. 25.5 ± 12.9 seconds, p = 0.012). A significant increase (p = 0.02) in lumbar paraspinal EMG activity was observed during the 80% trunk extension, in which overall activity at the L1 level was greater than the L2 and L3 levels (p<0.001). Abdominal muscle EMG activity increased significantly (p<0.001) during the trunk flexion test, in which no differences were observed between muscles. A significant increase in abdominal muscle activity was observed during the trunk extension test (p = 0.004), where IO muscle EMG was significantly greater than the EO and RA muscles (p = 0.004). Lumbar paraspinal EMG increased significantly during the trunk flexion test (p<0.001), while no differences were observed between the muscles. The exponent of the RPE power function was not significantly different (p = 0.163) between extension (0.28 ± 0.13) and flexion (0.36 ± 0.15) contractions. CONCLUSION: The major findings suggest that antagonist muscle recruitment during a fatiguing contraction parallels the EMG increase of the agonists, albeit at a lower amplitude. Despite findings of a greater endurance capacity of the trunk extensors, the perceived exertion response during fatigue development exhibits a similar nonlinear behavior between the two motions.