The activity of muscles in the trunk and lower limbs during sidestepping was recorded in nine healthy young men (control group) and three young men with groin pain syndrome. Muscle synergies among subjects were compared. Non-negative matrix factorization was used to extract muscle synergies from electromyographic data. Thereafter, to compare muscle synergies, a scalar product evaluating synergy coincidence was calculated. Three muscle synergies were extracted in both groups from non-negative matrix factorization analysis. In both groups, two out of three synergies were found to be the same by scalar product analysis. In one of these synergies, the oblique muscle, rectus femoris, and adductor muscle were activated before landing in mid-stance during the sidestep motion in the control group. Therefore, this synergy is thought to suppress excessive hip abduction. However, the peak timing of this synergy in the groin pain group was at mid-stance. This delay may cause hip instability because muscles must be activated before landing to enhance joint stability. Risk factors for groin pain are dysfunctional coordination between trunk and lower-limb muscles and decreased hip stability. Even though the number of subjects in this study was small, it is possible that delayed activation of this synergy may be related to the mechanism of injury in groin pain.
