Roach KE, Wang B, Kapron AL, Fiorentino NM, Saltzman CL, Foreman KB. In Vivo Kinematics of the Tibiotalar and Subtalar Joints in Asymptomatic Subjects: A High-Speed Dual Flouroscopy Study. J. Biomechanical Engineering 2016; 138: 091006-1-9.

This study used high-speed dual fluoroscopy in combination with a number of other advanced measurement technologies to document the movements at the tibiotalar (ankle) and subtalar joints during walking on an instrumented treadmill. They also studied a standing heel raise. Their intent was to measure how much rotational motion and translational motion occurred at each joint in each plane. Ten subjects (5 male and 5 female) with a mean age of 30 participated in the study. The authors wanted to establish baseline data for comparison to patients with osteoarthritis of the foot. The walking speeds on the treadmill were .5 m/s and 1.0 m/s. Even the faster speed is below the normal walking speed for humans, but was chosen to match patients that might ambulate much slower than normal.

In general, their results confirmed knowledge gained from the classic anatomical studies on cadavers, as well as more recent investigations on living subjects. They found that the ankle was a tri-plane joint that moved primarily in the sagittal plane, and the subtalar joint provided motion primarily in the frontal and transverse planes. This study did provide information that the tibiotalar joint provides more motion in the frontal and particularly the transverse plane than had been previously thought. Similarly, the subtalar joint appears to contribute more sagittal plane motion especially during the propulsion phase of gait.

The authors discuss the substantial variability between subjects compared to the averages. From a clinical standpoint, one reason for the variability is likely the different foot structure of the subjects. Early anatomical studies showed great variability in the motion axes of these joints. This would produce different degrees of motion in each of the planes. Also, the great benefit of studying intact humans does add to the complexity of the interpretation. Joints and muscle strength above the foot are contributing to the task of walking and will influence how the foot responds to the task of ambulating. This study, along with others, confirms the “magic” of the human machine: consistency combined with variability.

But what specifically about this study matters for function? First, it confirms previous research on cadavers and some subsequent research during actual function that demonstrated the ability of the rearfoot (both tibiotalar and subtalar) to act like a universal 3D joint. The joints act independently even though motion at one joint can influence the other. Our patients / clients often need dorsiflexion at the tibiotalar when the subtalar is everting or everted. During the propulsive phase of gait, dorsiflexion at the tibiotalar with the subtalar inverting / inverted is required. We can assess the two different combinations by selecting the hybrid lunge option detailed in the Performance System of 3DMAPS® (3D Movement Analysis & Performance System). By tweaking the direction of the Anterior Chain Reaction® lunge to right anterior-lateral, or left anterior-lateral, the subtalar joint will be driven into either eversion or inversion with the dorsiflexion. Then part of the intervention program to improve the dorsiflexion can use the support strategy of the Performance System tweaked using the opposite leg as a driver. 

A second, and less obvious, implication of this study relates to the fact of how the frontal and transverse plane motions at the subtalar joint impact function. The subtalar joint acts as a “torque convertor.” It converts frontal plane motion of the foot into transverse plane motion of the leg. The biomechanics also work in reverse. Transverse plane motion of the leg gets converted to frontal plane motion of the foot during weight-bearing activities. If eversion of the subtalar joint is restricted, our training programs can address this with two different strategies. If, for example, there is restricted left subtalar joint eversion, then a left foot opposite side lateral lunge will land on the lateral border creating a large rotatory force to evert the calcaneus through gravity and ground reaction force. However, if we have our patient / client perform a right foot same side rotational lunge, the left femur will internally rotate. The left lower leg will also internally rotate driving the talus internally. The subtalar joint converts this transverse plane rotation driven from above into frontal plane eversion of the calcaneus. Both the bottom-up strategy (left foot lunge) and the top-down strategy (right foot lunge) will create eversion. Both are functional, but one might be more authentic to the function our patient / client desires.