2013 年 25 巻 4 号 p. 483-487
[Purpose] To clarify the relationship between spinal alignment and trunk muscle activities in upright sitting without manual intervention. [Subjects] Twenty-three healthy male volunteers with no history of lumbar disorders participated in this study. [Methods] Trunk alignment and surface electromyographic activities of 6 trunk muscles were measured synchronously during the motion from slump to upright sitting position. The amplitude of the muscle activities were normalized to maximal voluntary contraction. Subjects were classified into 3 spinal alignment groups: long lordosis (LL), short lordosis (SL), and kyphosis (K). [Results] The LL group consisted of 9 subjects (39%), the SL group of 9 subjects (39%), and the K group of 5 subjects (22%). The K group had significantly lower muscle activity of the back and abdominal muscles. The SL group had significantly greater muscle activity of the lumbar paraspinals than the LL group, and greater activity of the lumbar multifidus was observed in the SL group than in the other groups. The LL group showed significantly greater muscle activity of the internal oblique muscle than the other groups. [Conclusion] Upright sitting without manual correction leads to various kinds of spinal alignment. Judging from trunk muscle activities, we suggest that the desirable spinal alignment in upright sitting is a neutral lumbar position with no thoracolumbar kyphosis.