ABSTRACT
Objective To study the myoelectric activity and physiological function of lumbar dorsal and abdominal trunk muscles during the lateral flexion with or without loading. Methods Trunk angle was measured between C7-S1 and vertically with a protractor. Electromyographic activity was recorded unilaterally from 8 trunk muscles using intramuscular fine-wire electrodes inserted under the guidance of ultrasound. Results The electromyographic data showed that all muscles on the contralateral side to the load, except rectus abdominis, had their highest activity while loaded in the most lateral flexion to the loaded side. The degree of bilateral co-activation was greater for the abdominal than for the lumbar dorsal muscles. The activation of transversus abdominis showed a difference from traditional consideration. Conclusion The myoelectric responses of most lumbar trunk muscles to static lateral flexion were dependent on trunk position and loading. The abdominal muscles demonstrated more co-activation than the other trunk muscles and thus appeared to contribute more to trunk stabilization in lateral bending and loaded trunk positions.