Background Hip adductor longus (AL) strains, which could result in missed playing time for athletes in many sports, are encountered in soccer frequently. Although, two of main kicking techniques in soccer, which are side-foot kicks and instep kicks, might be risk factors of AL strains, AL activity was unknown in each kicking technique.

Objective The aim of this study was to compare AL activity of kicking limb between side-foot kicks and instep kicks.

Design Controlled laboratory study.

Setting Participants competitive level was collegiate league level.

Participants 10 male collegiate soccer players with no history of previous significant lower extremity injury were tested.

Interventions Side-foot kicks and instep kicks were recorded in three times for each player. Side-foot kicks approach was 45° and instep kicks approach was straight to the direction of ball movement. Because the approach angle affects pelvic rotation and it will affect adductor muscle activity.

Main outcome measurements Video-based 3-dimensional motion analysis and surface electromyography (EMG) of AL in the kicking limb were conducted. The motion analysis data were used for the phase definition of the kicks. In this study, the phase of kicking was defined as the following: first phase was kicking limbs toe-off to ball impact (pre-impact phase) and second phase was to toe velocity inflection (post-impact phase). EMG data was normalised by maximal voluntary contractions (MVC) and averaged by the phase time.

Results Comparing the pre-impact phase between two type kicks, side-foot kicks demonstrated significantly greater activity than instep kicks (p=0.030). In contrast, during the post-impact phase, instep kicks demonstrated significant greater activity than side-foot kicks (p=0.003).

Conclusions AL activities between side-foot kicks and instep kicks were different. Better definition of AL function during kicking provides a basis for improved insight into soccer player performance, injury prevention, and rehabilitation.