Biomechanical load sharing in the foot during dynamic, load-bearing activity

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The foot is structurally redundant. It is unlike the thigh or the lower leg where there are only one or two large bones and really only one way for biomechanical loads to be transmitted. The foot is instead a complex structure of 26 bones that each bear biomechanical load. An interesting fact is that about one quarter of the bones in the entire human body are in the two feet. This gives the foot a tremendous flexibility in how it transmits loads between the body and the ground. But how the foot does so is poorly understood. Connecting the ankle joint to the ground are five ‘rays', or columns, of bones that start at the heel (calcaneus) and terminate at each of the toes. The first ray is also known as the medial longitudinal arch, the fifth ray is the lateral longitudinal arch. Each ray transmits a portion of the total biomechanical load from the body to the ground. But what proportion of the total load each ray bears cannot be calculated deterministically due to the foot's structural redundancy. It remains an open question how much load goes through each ray, whether the sharing of load is always the same, whether it changes from step to step, whether it changes during a step and whether it is the same between different people. This research program addresses the question of load sharing in the foot. It uses two complementary approaches, direct measurement on healthy test subjects performing dynamic tasks and finite element (FE) modeling of the foot anatomy, footwear and orthotics. The motion of the bones of the foot in healthy subjects is measured using the technique of fluoroscopic radio-stereometric analysis (fRSA). This system uses x-rays to image the bones while the test subject is performing dynamic tasks such as walking straight ahead and change directions. Within the shoe, the pressure between the footwear and the top and bottom of the foot is measured directly using an in-shoe pressure sensing system. The overall motion of the body is measured using optical motion capture. These three systems have never been combined for the study of the foot before. This program of research will significantly improve our understanding of how the feet keep us upright and mobile, and how footwear and orthotics can facilitate our mobility.**