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Sub-Talar Picture

Mechanics of the Subtalar Joint

The subtalar joint is the articulation between the talus and calcaneus and is sometimes called the talocalcaneal or lower ankle joint. The subtalar joint permits the foot to bend inward and outward and this motion helps the foot to adapt to uneven surfaces during walking. The motion of the subtalar joint is similar to that of a hinge, and it has long been known that the orientation of the hinge axis varies considerably across subjects. This variation requires that special techniques be used to locate the subtalar joint in individual subjects and we have studied existing techniques in our laboratory and developed our own methods. Our Subtalar Axis Location (SAL) protocol is based on a method developed by podiatrist Kevin Kirby in which a dorsiflexion load is applied and the foot is rocked from side to side. This combination of loads and forces is intended to immobilize the ankle, or tibiotalar, joint and thus permit talocalcaneal motion to be evaluated by measuring the relative rotation of the calcaneus with respect to the tibia. We evaluated SAL using magnetic resonance imaging and determined that tibiotalar motions and the differences between tibiocalcaneal and talocalcaneal helical axes were minimal. We are currently working on using SAL to measure subject-specific subtalar joint moments during standing and walking.

Sprinter's Foot

Musculoskeletal Structure and Locomotor Function

Our muscles' ability to help us move about depends upon several characteristics related to their structure and how their tendons attach near the joints they span. Muscles that pass far from joint axes of rotation have large moment arms (lever arms) that afford them greater leverage for moving limbs or resisting applied loads. We measure these moment arms in living humans and in cadaver specimens by tracking tendon excursion and measuring joint rotation at the same time. We can compute moment arm from excursion and joint angle data because muscles that have larger moment arms will exhibit more tendon excursion for a given joint rotation. Those muscles with large moment arms will also shorten and lengthen more during a given joint rotation, and their rates of length change will also be greater if the joint rotation occurs in the same amount of time. Muscle fiber length and shortening velocity are important determinants of muscle force, so muscle moment arm helps to determine the roles played by muscles during movement in two ways: by determining leverage and by setting operating ranges on the force-length and force-velocity curves. We have found moment arm and fiber length differences between sprinters and non-sprinters that explain how elite sprinters are able to accelerate rapidly at the start of a race. Current work in the lab is directed toward determining how musculoskeletal architecture determines susceptibility to age-related declines in mobility experienced by older adults.

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Identification of Gait Characteristic Biomarkers for Parkinson's Disease

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Total Knee Replacement Image

Simulation-Based Investigations of Total Knee Replacement Mechanics

Assessment of the performance of total knee replacements in vivo requires implantation in actual patients, but getting to that stage takes a great deal of time. Evaluating implant mechanics before implants are approved for use in humans, and even before they have been physically prototyped, would provide critical information early in the design cycle. To this end, we have developed multibody dynamic computer simulations of a benchtop test of isolated implants (the "range of constraint" test"), a cadaver test apparatus (the "Oxford Rig"), and a supine range of motion test. These simulations involve representation of forces caused by muscles, ligaments, and articular contact, and permit the examination of implant kinematics, articular constraint forces, and soft-tissue forces.

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Identifying Strategies for Recovery from Falls

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