We investigated the relationship between lumbar alignment and trunk muscle activities during the underwater streamline position. Twenty-two male collegiate swimmers participated in the study. Firstly, spinal alignments of 22 participants were evaluated during standing and underwater streamline position using image analysis. Thoracic kyphosis angle and lumbar lordosis angle were measured to evaluate the spinal alignment. Secondly, eleven swimmers participated to the continued investigation: 6 participants who had the smallest alteration in lumbar lordosis between the two positions (these became the smallest group) and 5 participants who had the largest alteration (these became the largest group). Their spinal alignments and their trunk muscles activities were measured during two positions in the same manner as was performed in the first experiment. The muscles activities were measured using surface electromyography. As a result, a significant difference between the two groups was observed in the internal oblique/transversus abdominis muscle activities during the underwater streamline position (p < 0.05). Therefore, it was considered that the internal oblique/transversus abdominis muscle activities were related to the magnitude of the lumbar lordosis alteration during the underwater streamline position.
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