Contributions of the inside and outside leg to maintenance of curvilinear motion on a natural turf surface.

Little is understood of the mechanisms of locomotion if human subjects are not moving in a straight path. The identification of contributory variables to curved motion would also underpin other non-linear actions such as cutting and turning. The performance of such tasks has relevance to both success in sports and exercise, and accident avoidance in an occupational setting. Comparison of ground reaction force values in successive footstrikes would allow an understanding of the contribution of each limb's movement to motion in a curved path. For ecological validity to field games, two natural-turf covered force platforms were located outdoors in a field. Six males (age 25+/-4.73 years; mass 79.7+/-7.17 kg) wearing standard six-stud soccer boots performed straight and curved trials (radius 5m) at velocities of 4.5 and 5.5 ms(-1). Ground reaction force measures were collected on successive footstrikes at 500 Hz, whilst kinematics of the lower extremity were measured at 50 Hz. Results for two successive footfalls showed greater average total force in straight motion (3.53BW versus 3.08BW), with the outside leg contributing most to the movement pattern in curvilinear motion. Ballistic airtime was reduced from straight to curvilinear motion, creating a greater proportional foot contact time during curved running. This, with lowered total force values, suggested a lower centre of gravity during curved motion to minimise drift towards the tangent of the curve. In curved motion, all vertical force measures were greater for the outside leg, with anterior-posterior forces showing the outside leg provided greater propulsion forces and impulse. Improvement in performance in curvilinear motion should therefore be focused at the outside limb.