Rocker Soles, Part 2

OK, I promised some drawings, and I deliver now on that promise. Today, I want to start with two images, that will help to illustrate two things - the general shape of a rocker sole profile (side view), and some of the considerations that have to go into shaping the rockers for a specific person. Please be clear - I am not trying to describe a generic rocker design. I am illustrating some of the considerations that must go into the making of the proper design for a given individual only. This is because, as I have already stated, each person's biomechanics are different than that of another person. Yes, there are some general guidelines, but to apply some general design to rocker soles for every person would be to cause pathology in some significant percentage of those people. And we do not want to do that!

So, this first image is the profile of a general shape for a rocker sole (general here is used to mean average only):

OK, not the best drawing, but you try to do a graphic while a poodle is flicking your arm into the air every five minutes in a demand for attention - go on - try it!

The critical elements in this view are the rear rocker shape and angle, and the forward rocker shape and angle, both relative to the ground plane. The space between the two points needs to be flat - this is critical. Many commercially made so-called rocker soles fail on this point - they tend to have some element of a continuous roll heel to toe. The problem with that is it presupposes you never intend on standing still! Midfoot stability is important for both static stance, and the stance phase of gait. Note the "terminus" of both rocker angles. The rear rocker terminus should be across the center of the heel, and not forward of that center line. Some generic designs keep the rear rocker angle going much further forward of this point, and subsequently cause the knee to hyperextend - not a good thing.

The rear rocker provides several critical element to what makes the rocker sole effective:
1. It serves to aid the leg in deceleration on heel strike. In other words, it reduces heel strike shock by effectively increasing the percentage of the heel to initially contact the ground, which immediately begins to spread out the weight-bearing on heel strike.

2. It reduces, and in some people eliminates what can be called "slap" - slap occurs with most footwear, as the sharp corner of a heel, acting as a lever arm, propels the rest of the foot forward and down simultaneously. That is, there is no ability for the shoe to move gradually into full foot loading, or the stance phase of gait. The rocker angle works to slow the forward and downward momentum, essentially extending the moment between heel strike and stance phase in time. This slowing reduces shock, reduces the amount of motion in the joints of the foot during these phases of gait, and thereby permits the leg to move over the foot efficiently while the foot is required to do less work. In people who have joint fusions, there is little normal motion available due to those fusions. The rear rocker (and the forward rocker) provide motion to the gait cycle that is largely lost the the foot proper.

The forward rocker does some of these same things - reduces motion in the foot while restoring motion necessary for gait to the limb as a whole. Properly angled, and when the rocker sole is sufficiently high enough, nearly all motion the limb requires for effective gait can be provided through the rocker sole. The Hansen's Disease Center in Carville, Louisiana pioneered much of the design and understanding of how a rocker does what it does. Where any motion the foot performs - both vertical and horizontal forces (shear) will cause the Hansen's foot or the diabetic foot to very rapidly break down soft tissues and lead to potential ulceration, these forces and motions need to be minimized to a high a degree as possible. So the higher the overall sole, the more extreme the forward rocker angle can be, and the more of that motion it provides to gait without allowing damaging motions and forces to be applied to the foot itself.

The forward rocker angle (properly, in the side view, this should be called the pitch, or slope of the forward rocker,) can be designed either gradual or abrupt. A gradual pitch/shape will slow the stance-to-toe off phase of gait, whereas a more abrupt shape will speed the foot forward in this phase. In people where there may be a knee component to their gait issues, the more gradual slope should be used. People with a reasonable amount of joint motion in the knee and the forefoot can tolerate the more abrupt slope. Note that when I say it "slows" the phase of gait, this is a relative idea. Nothing prevents someone from running or walking fast in rocker soles - in fact, there is a tendency to feel better equipped to move at a more normal cadence than without the rocker soles. The slowing refers to a very brief moment of motion being extends for a small fraction more of time. Again, this slowing aids in significantly reducing motion in the foot itself while providing that motion required for gait.

It can help to think of it in this manner - imagine a stick with a one foot long piece of wood attached to one end. Picture using this object as a cane. Now, its clear that it would be very awkward - you would be forced to lift it fully from the ground and them swing it forward before planting it again flat on the ground.This would be a) loud, b) silly looking, and c) extremely inefficient and a high expenditure of energy.

Now, let's reshape that piece of wood by giving it a rocker shape. Now, you would be able to lift, swing, plant, roll, and easily lift for the next time the "cane" hits the ground. This requires much less energy, and won't result in your cane either breaking apart from the uneven distribution of forces, or getting thrown away as more a hindrance than a help. This is what a rocker sole can do for someone who either needs as much motion reduced as possible, or, in the case of us clubbies, provides motion necessary for gait where it is not available to the foot.

Remember, fusions are not the only thing that reduces available motion for the foot in various aspects of the gait cycle. Painful motion, such as with osteoarthritis-affected joints, benefit by this reduction of motion, as well.

Now, there is one problematic issue with rocker soles, and I would be remiss not to be clear about it. Because the shoe must be more elevated in order to permit the rocker angles to be properly incorporated (remember, this is a geometry problem - foot length, distance between the knee and the ankle, stride length - all of these and more have to be considered in order for the design for a given person to be effective). So this increased height (while simply loved by shorter folk!) also means that walking on uneven ground is more difficult, and takes some time to get used to. If a person is prone to falls, rocker soles are not recommended. But most people with severe post-club foot syndrome don't go "off-roading" very often, anyway.

OK, next time, we will look at the rocker sole from the bottom. It is not enough that the profile have the right slope front and back. We also have to take into consideration other issues, in order for the design to work properly. So more on that later.

Remember - if you have any questions about rocker soles, please post them in the Comments.