A vertical load applied towards the trunk unilaterally increases the bilateral abdominal muscle activities

[Purpose] It is considered that evaluation of the vertical trunk function is important, because humans stand and move with two legs. To evaluate this, a novel method named Trunk Righting Test has been reported. The purpose of this study was to examine the trunk muscle activity during a TRT using electromyogram analysis. [Participants and Methods] This study included 7 healthy volunteer males. The TRT evaluated the supportability of the posture after moving 10 cm laterally from the sitting position using a hand-held dynamometer. The TRT measurements were analyzed separately at the measurement side (ipsilateral side) and at the non-measurement side (contralateral side). The measurements were obtained bilaterally, and the evaluated muscles included the rectus abdominis, internal oblique, external oblique, multifidus, and transversus abdominis. The measured value was expressed as a percentage after comparing with the value at the maximum voluntary contraction (% MVC) for standardization. The changes in the muscle activities in the sitting position and TRT were evaluated. [Results] All the muscle activities significantly increased during the TRT in contrast to that in the sitting posture. [Conclusion] The load support of the trunk on one side during the TRT was significant in all the muscles on both the sides, which increased the muscle activity, in contrast to that in the sitting position.