Intramuscular myoelectric activity and selective coactivation of trunk muscles during lateral flexion with and without load

Study Design. Myoelectric activity of trunk muscles was measured intramuscu larly in six healthy subjects as they maintained static trunk postures at 0 degrees, 15 degrees, and 30 degrees of lateral bending, unloaded or holdin g a 20-kg load in one hand alongside the body. Objective. To determine the position and load dependency of the agonistic a nd antagonistic myoelectric responses of deep and superficial trunk lateral flexor muscles. Summary of Background Data. Loading of the trunk in lateral bending is asso ciated with incidences of low back pain. The neuromotor control of muscles surrounding the spine may be decisive for its vulnerability. Earlier docume ntation of the activation pattern of trunk muscles, particularly those situ ated deeply, is incomplete. Methods. Trunk angle was measured between S1-C7 and the vertical with a pro tractor. Electromyographic activity was recorded unilaterally from eight tr unk muscles using intramuscular fine-wire electrodes inserted under the gui dance of ultrasound. Results. The electromyographic data showed that all muscles on the side con tralateral to the load, except rectus abdominis, had their highest activity while loaded in the position most laterally flexed to the loaded side. The degree of bilateral coactivation was greater for the ventral than for the dorsal muscles. Conclusions. The myoelectric responses of most lumbar trunk muscles to stat ic lateral flexion were dependent on trunk position and loading. The abdomi nal muscles demonstrated more coactivation than the other trunk muscles, an d thus appeared to contribute more to trunk stabilization in laterally bent and loaded trunk positions.