Kinematic coupling between the foot and lower limb during gait.

INTRODUCTION: Abnormal kinematic coupling between the foot and lower limb has been associated with chronic overuse injuries of the lower extremity during running. However, the normal coupling relationship between the two segments remains unclear. The equivocal findings in the literature may be due to previous studies concentrating on determining coupling at discrete instances only, along with the failure to include the midtarsal joint in coupling analyses. By including motion across the midtarsal joint and measures of continuous coupling, this research aimed to gain a more complete understanding of the relationship between foot and lower limb kinematics during gait. METHODS: Following the development of a multi-segment foot model, in-vitro and invivo studies were conducted to assess the validity and reliability of determining foot and lower limb segmental kinematics during gait. Three experiments were then undertaken to assess the rigidity of the kinematic coupling between the forefoot, rearfoot and shank by manipulating step width, running speed, foot strike pattern and mode of gait (run versus walk). Kinematic coupling was assessed by determining how well matched the angular displacements of two adjacent segments (e. g rearfoot eversion/inversion with shank intemal/external rotation) were in both spatial and temporal terms using both discrete point and cross correlation analyses. RESULTS: Although the in-vitro study suggested care should be taken when interpreting data obtained from skin mounted markers the modelling and analysis approach used in-vivo was found to have good within- and between-day reliability. In all conditions it was evident that following touchdown, the shank internally rotated, the rearfoot everted and the forefoot dorsiflexed and abducted. This was followed by the reversal of the segmental angular displacements starting with that of the shank, followed by the rearfoot and then the forefoot. During running, coupling between rearfoot eversion/inversion and shank internal/external rotation was consistently high (r > 0.92) regardless of step width, speed or foot strike pattern. In walking, however, this coupling value was low (r = 0.49). Rearfoot eversion/inversion was also highly coupled with both forefoot dorsiflexion/plantarflexion and abduction/adduction in running and walking. However, there was little evidence of any coupling between rearfoot eversion/inversion and forefoot eversion/inversion. CONCLUSION: The consistently high kinematic coupling between the rearfoot and shank during running suggests a robust coupling mechanism that is able to withstand changes in the loading of the subtalar joint. However, lower coupling between these two segments in walking, implies that the relationship is not entirely rigid and some degree of elasticity exists at the subtalar joint. Strong coupling of forefoot sagittal and transverse plane motions with rearfoot frontal plane motion during running and walking suggests the two segments are linked via the action of the midtarsal joint. From the timings of discrete kinematic events it appeared that shank external rotation was driving rearfoot inversion and that this in turn was causing the forefoot to plantarflex and abduct. This implies that a kinetic chain exists with proximal segments driving motion of the distal segments during propulsion. IMPLICATIONS: If the proximal segments drive the motion of the foot then injuries associated with excessive or prolonged pronation should not only be treated using orthoses, but also by using interventions to modify the kinematics of the joints proximal to the ankle-joint-complex. Future work should determine the effects of muscle stiffness on subtalar joint kinematics since this may have important implications in terms of lower extremity injuries.