Using a modified vector coding technique to describe the calcaneus-shank coupling relationship during unanticipated changes of direction: theoretical implications for prophylactic ACL strategies

Shank rotation is associated with increased risk in lower limb injuries of weight-bearing sport activity. Straight-line running injury prevention research proposes a 'bottom-up' approach to minimising injury risk to the knee. This prophylactic recommendation is due to the observed distal-proximal coupling relationship between rearfoot and shank rotations. However, the coupling relationship between the calcaneus and shank is unknown in sports with high intensity decelerations, frequent changes of direction, associated with increased anterior cruciate ligament (ACL) injury risk. The aim of this study was to determine whether the movement of the calcaneus has a potential effect on the internal and external rotational movement of the shank, associated with ACL injury risk. We implemented a modified vector coding technique using segmental velocities in a local, anatomical reference frame to quantify the coupling relationship between the individual tri-planar calcaneus rotations and transverse plane shank rotations. During the loading phase, a distal-proximal coupling relationship between calcaneus eversion deceleration, abduction acceleration, and shank internal rotation deceleration was observed amongst most subjects. The distal-proximal coupling found between the calcaneus and shank justifies exploring interventions geared towards manipulating calcaneus motion to affect shank rotational movements during unanticipated change of direction tasks associated with ACL injury risk.