We have all heard about the classic rocker mechanisms of the foot and ankle: the heel rocker, ankle rocker and forefoot rocker. Dananberg gave them special prominence in his article “Sagittal Plane Biomechanics”1. Here is a case of being blinded by familiarity so as to miss the point. Rockers, so what? Well, to begin with, what if we stop talking about them as if they are phenomena in gait that are somehow independent of the cycle of pronation and supination that defines foot function. If we look more closely at ideal foot posture in gait, the significance becomes more apparent.

We talk often of the medial longitudinal arch and how its loss can have such enormous consequences in foot function such as an unstable first ray. But the MLA is just a 2D slice of the total 3D architecture of the foot called the “plantar vault”2. This is the architecture defined by a series of intersecting arches that all share at least 2 of the 3 key weight-bearing foci of the plantar foot: the inferolateral calcaneus, the fifth metatarsal head and the first metatarsal head. As in most areas of the body, form follows function and vice versa. So what is the functional significance of both the arch/vault structure and the 3 weight-bearing foci that contact the ground?

The arch/vault structure defines the supinated or stabilized posture of the foot that is so important at heel strike in delaying the premature onset of pronation and, most significantly, defining the ideally re-supinated foot after heel lift that insures first ray functionality during propulsion. It is the normal structure of the foot that naturally resists flattening in standing and in gait. The 3 key contact points at the base of this vault enable efficient rocker function when they can act as stable pivot points. That stability depends on the amount of supination maintained against body weight and ground reaction forces.

If we consider the biomechanical goals in an ideal gait, these rocker pivots begin to make sense. First of all, in a general sense, a goal is to conserve forward momentum. This is done through the conversion of groundward momentum developed by the moving body at heel strike into an efficient forward roll. Ground reaction forces can either work to stop this forward roll or act as a fulcrum/pivot that enhances it. The difference is determined by the posture of the foot throughout stance phase. If, at heel strike, the calcaneus veers immediately into too much eversion and plantarflexion with concomitant over-pronation, the center of pressure progression on the plantar foot shifts medially off course in early stance phase. If proper supinated foot posture is maintained, however, the calcaneus rolls sagittally and efficiently along its lateral side. When the heel and fifth met maintain stability against the ground, the lateral arch resists flattening and the forefoot resists lateral splay, avoiding a dampening of forward momentum. Ideally the foot continues along a lateral load trajectory before moving quickly medially towards the next pivot point: the first metatarsal head.

It was during a day that we spent with Tom Sgarlatto about a year ago that I first began to appreciate the value of efficient lateral to medial forefoot pressure shifts late in stance phase. Tom showed us videos he had taken of runners coming toward him so he could see, in slow motion, what was happening at the forefeet in the frontal plane during propulsion. He discovered that the best runners demonstrated more loading of the medial forefoot during propulsion. He saw it on the video as a slight forefoot eversion. This needs further critical analysis and research, but it seems logical to conclude that the efficiency and relative presence of this shift, appreciated more acutely in the athlete, is an important element of overall fluid forward motion in gait.

The final pivot point rocker is around the first met head as mentioned above. As this series has argued consistently, the stability of this pivot is related to the degree of re-supination a foot can achieve. An over-pronated tarsus will create an “unlocked” first ray that can profoundly disable this rocker and thwart efficient, forward-rolling momentum. In the over-pronated foot, the plantarfascia acts as a splint across the first MTP, restricting dorsiflexion.

From a functional point of view, then, it makes sense to look at all the rocker mechanisms as belonging to one of two categories: intrinsic or extrinsic to foot posture. The intrinsic rockers are the calcaneal and first met heads because they are made or broken by foot posture, i.e. a well-formed plantar vault. The ankle rocker stands alone as an extrinsic rocker because barriers to its function are things usually other than foot posture, such as a tight gastroc/soleus or limited range of motion at the talocrural joint. They all affect the quality of gait but must be treated differently.

In conclusion, while we have in past articles repeatedly emphasized the importance of restoring the medial longitudinal arch for normal first ray function, the big picture in stance phase gait must include the functional enabling of the intrinsic rockers by posture that contribute to efficient forward momentum. MASS position casting not only captures the optimal MLA, it also captures the optimal plantar vault that will enable the pivots for the intrinsic rockers. In order for such enabling to persist through stance phase, however, only minimal pronation must be allowed to occur.

Reference List:

(1) Dananberg, Howard. Sagittal Plane Biomechanics. JAPMA January; 90(1):47-50
(2) From Kapandji, IA, The Physiology of the Joints, Vol.II, p.199