By my friend, mentor and all around good guy David Tiberio PhD, PT, OCS
GRAVITY, GROUND, and the SWITCH
The internal power sources that drive the body are the hips and trunk: the core of the body. There are many ways to activate that power source, but probably the most important for upright function in our gravitational environment is eversion of the calcaneus.
Calcaneal eversion occurs in the frontal plane. It is one component of the tri-plane motion of pronation of the subtalar joint (STJ). Simultaneous with the eversion, a substantial amount of abduction occurs in the transverse plane at the STJ. The motion in the frontal and transverse planes at the STJ complements the ankle joint motion, which occurs primarily in the sagittal plane, to allow the rearfoot to move in all three planes.
The calcaneal eversion results from the striking the lateral aspect of the heel when we walk. The ground reaction force (GRF) forces the calcaneus to evert. Since the motion is produced by gravity and the GRF (motion given for free), the role of the muscles is to decelerate the motion. In this process, the muscles are lengthened and activated proprioceptively. The strain created in the muscles during the deceleration is transformed into a concentric motion-producing force.
CHAIN REACTION LINKAGE
The calcaneal eversion that creates STJ pronation produces two important reactions in the body: one distal and one proximal. When the subtalar joint is pronated, the bones of the mid-tarsal joint (MTJ) are more mobile. During weightbearing this allows the foot to adapt to uneven surfaces and, more importantly, as the arch lowers certain muscles are lengthened (loaded) in order to become more powerful (e.g. peroneus longus). When the calcaneus begins to invert these processes are reversed in order to create a more stable foot at the time the muscles are “exploding.”
The proximal effects of calcaneal eversion are more significant. Because of the angle of the STJ axis, the frontal plane motion of the calcaneus creates transverse plane motion of the lower leg. The STJ is called a “torque converter” because it converts the frontal plane motion of the foot into transverse plane motion of the leg (and vice versa). This transverse plane motion of the lower leg often transfers to the femur, pelvis, and lumbar vertebrae. Because the STJ is tri-planar and all joints move in three planes, the calcaneal eversion during weightbearing produces tri-planar reactions in the knee, hip, and spinal joints.
During walking, the knee will flex, abduct (valgus), and internally rotate. The hip responding to the calcaneal eversion and ankle motion will flex, adduct, and internally rotate. Since the pelvis is also driven by gravity and GRF, motion will be created in the lumbar spine. Remember that all these motions are “given for free” and muscles must first decelerate these motions prior to creating the opposite motions. It is these motions that turn on the hip and core muscles (both anterior and posterior), all initiated by the calcaneal eversion “switch.”
FUNCTIONAL IMPLICATIONS
Frequently clinicians and trainers evaluate calcaneal eversion to see if the STJ pronation is excessive. A better approach is to apply the “Goldilock’s Assessment” – too much, too little, or about right. Although it is important to be cognizant of the role excessive pronation of the STJ plays in raising tissue stress to a symptomatic level, it is equally important to recognize how the lack of calcaneal eversion can inhibit the normal motion and muscle activation of the entire body. If excessive eversion of the calcaneus is too much of a good thing, then limited eversion is not enough of a good thing. Insufficent calcaneal eversion will inhibit the proximal bone movements. This will minimize the loading of the hip and core muscles. The knee will often exhibit a “varus thrust” and will not be an efficient shock absorber. The ankle will be susceptible to inversion ankle sprains.
All functional assessments should include one or more tests to determine if the client has sufficient eversion available and whether they are using this motion during function. Causes of insufficient calcaneal eversion can be structural or functional. Structural limitations are determined by the amount of eversion when non-weightbearing. Many structural limitations are acquired. Any period of immobilization or protected weightbearing are likely to cause a limitation of calcaneal eversion. This motion must be restored to have a healthy and efficient body.
Functional limitations are present when the joint motion is available, but other structures inhibit the use of that motion. A structural valgus position of the forefoot or a stiff MTJ will block the calcaneal eversion. Limited internal rotation of the hip often dictates to the STJ that calcaneal eversion should not occur. These functional limitations can produce structural changes if they persist over time.
THE PRONATED-NO-AVAILABLE-EVERSION PARADOX
When our clients are standing in a weightbearing position (rather than walking or running) they may demonstrate a lot of calcaneal eversion, but they may not have any additional eversion to load the muscles. For example, a client who wants to be a quicker jumper may stand with the calcaneus everted. This excessive eversion may be caused by a structural varus of the rearfoot or forefoot. If the STJ is at end-range there is no more calcaneal eversion in order to turn on the power sources in the body. The excessively everted calcaneus has insufficient eversion to “load and explode.”
Get Strong! Stay Strong!
Chris
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