Evaluation of the lumbar and abdominal muscles behavior in different sagittal plane angles during maximum voluntary isometric extension.

Physical positions and lumbar movements are directly related to lumbar disorders. It is known that the sagittal plane angle affects the person's ability to apply extension torque. However, there is no consensus on whether or not muscle activity and co-contractions change at these angles. This paper aimed to investigate the abdominal and lumbar muscles' behavior at different sagittal plane angles during maximum voluntary isometric extension (MVIE). We have evaluated our findings with the aid of a computational biomechanical model. Fourteen healthy males participated. A total of 16 muscles EMG were recorded during the lumbar MVIE on the Sharif Lumbar Isometric Strength Tester device in 5°, 15°, 30°, and 45° flexion. The torque and muscle activity changes and all co-contraction indexes (CCI) between 120 possible muscle pairs were calculated. Finally, the experimental test conditions were modeled in the AnyBody software, and the MVIE torque, muscle activity, and all CCIs were calculated. Also, muscle torque lever arms were calculated at different angles. Results show that MVIE at four angles is 137.94 ± 36.08, 148.63 ± 47.96, 168.09 ± 50.48, and 171.44 ± 53.95 N · m, respectively. Muscle activity and CCI are similar at all angles. The AnyBody model gives similar findings. Muscles torque lever arms change with angle. In conclusion, to determine the safety mode of lifting in the sagittal plane, it seems that the torque differences are due to changes in the geometrical muscle parameters (including the torque lever arm). Despite the almost constant muscular effort, subjects in the 30°-45° bending positions can apply more MVIE.