Part One - Background
The term "rocker soles" is often misused, in my view. There are many shoes on the market that call their sole design a rocker sole, that are both misleading, and potentially harmful to most people who either do not need a rocker sole, or who use a design that will actually cause them severe problems down the line. Remember Earth shoes and Root shoes? These two brands used what they termed a "negative heel" design, where the heel of the shoe was in fact lower than the ball of the shoe, or the forefoot. This actually made the wearer's knees hyper-extend. The companies used the spurious marketing claim that since this was the way we walk on the sand, it was somehow the "natural" and the "healthy" way to walk. Of course, they failed to mention two critical issues related to walking on the sand - one - after a while walking on sand would actually make your knees and back hurt, and two - how many hours of the day do we actually walk on a forgiving surface like sand? The results were numerous lawsuits, and the eventual demise of the negative heel concept. But don't just forget about this - we'll be coming back to it later.
A properly designed rocker sole, when applied correctly, that is, to the specific sort of problems that call for a rocker sole in the first place, is designed to do three things simultaneously. First, they prevent motion that is actually harmful to certain foot issues. Secondly, they provide the necessary motion for a more "normal" gait. And third, they protect certain foot issues from repetitive trauma, especially in patients with diabetic neuropathy. I will deal with each of these issues in depth over the next few posts.
For now, lets start with what a rocker sole design should be, and what it should NOT be.
If one Googles the term "rocker sole," they will find many medical articles either damning or praising them. Part of the problem is that, as with all forms of research, you can get what you want to find merely by how you design a study, and by how you ask the question itself. Rocker soles are no different in this regard. If the design used for the study is based on a specific patient population, but the rocker design used for the study is not a) consistently applied to all study participants, and b) not the appropriate design to address the actual pathomechanical issue at hand, then the results will have little value. Because few people have spent much time actually applying rocker soles to a wide array of pathomechanical issues and an equally broad array of foot types, not to mention using many different shoe styles to apply the rocker soles on in the first place, it comes as little surprise that the results are all over the map. But I am one of those few people who has actually done this kind of work with real patients who present with many different types of pathology. So what follows is based mostly on that experience, but with reference to the work others have done in the area of applied rocker sole therapy.
A rocker sole needs to do several things: they must permit the foot to proceed through the various phases of the gait cycle while simultaneously preventing painful or mechanically unsound motions; they must dramatically reduce the need for all joints above the foot to take on motion that is not in keeping with the healthy and natural motions commensurate with each of those joints; they must allow for a stable stance when the body is not in forward motion; and they must aid in promoting as smooth as possible over-all gait. This sometimes means the rocker sole must be rigid, and sometimes flexible, as far as the over-all impact on the foot is concerned. I will address each of these issues as we go forward here. (A little gait joke - sorry:-)
Some of the earliest work on the idea of rocker soles was done at the Carville, Louisiana site of the National Hansen's Disease Program. Hansen's disease is the official name for leprosy. One of the hallmarks of Hansen's disease is severe peripheral neuropathy, as is also found in some diabetic patients. Peripheral neuropathy means the complete, or near-complete, loss of sensory nerves. Without sensory neural feedback, the body's tissues can break down very rapidly, and become ulcerated. In extreme cases, those ulcers can become gangrenous, and the result is, too often, amputation. It is the mechanical pressures and motions that in a "normal" foot would be accounted for cause this tissue breakdown. So the need in such patients is to significantly reduce both vertical pressure and horizontal "shear" in order to slow or completely stop the onset of ulcers that might lead to amputation. It was this need that led to the development of the rocker sole.
The need was to allow reasonably normal ambulation while severely reducing the vertical and horizontal forces that occurred during weight-bearing gait. No mean trick, it turns out. To stop motion, you have to stop the flexion and extension motions - that is - plantar (bottom of the foot) flexion, dorsi (top of the foot) flexion, but also, in- and eversion, ad-and abduction, and thus, lose supination and pronation, the motions provided to a significant degree by the actions of the subtalar joint. Now, all of these motions can be stopped simply through the use of a rigid sole - think steel plate, or plywood. Yes, that rigid. But what do you do, then, to permit somewhat normal ambulation? This can only be provided by applying those ancient principals passed down to us by the great Archimedes himself. You must provide a lever. Actually, you must provide several levers.
If you break down the human weight-bearing portion of the gait cycle, you see there are three primary moments required - heel strike (when the heel hits the ground,) full foot loading (when all of the foot is on the ground, and toe-off, or the propulsive phase of gait. But it makes better sense to break each of those three moments down a little further. There is heel strike, followed by an adduction moment, when the subtalar joint is working to internally rotate the leg on the ground. This is followed by full foot loading, and then by the subtalar joint reversing direction to allow the leg to externally rotate through a supination moment. This sets up the foot to enter the propulsive phase of gate. This is then followed by heel-off, when the knee above begins to flex in preparation for the limb to leave the ground and enter the swing phase while the opposite limb begins its weight-bearing phase of gait. There is then a momentary portion of the gait cycle where a part of both feet are on the ground - the toes of the limb now entering the propulsive phase, and the heel of the opposite limb as it begins it's weight-bearing portion of the gait cycle. There - you just got your first class in biomechanics!
And this is why its helpful to understand this with respect to rocker soles. The design of the rocker sole must permit heel strike, full foot loading, and toe-off in a smooth and efficient manner. The pronational and supinational moments are provided by a specific element in the proper design of the rocker sole that we will get to later. For now, let's stay on the first three elements. If you look at a typical shoe, the posterior edge of the heel represents a right angle to the ground. That is, from where the back edge of the heel strikes the ground to when the heel is fully on the ground is the result of a single point of contact. This causes the foot to have a very abrupt heel strike and what can be described as a "slap" into full foot loading. This is where the greatest vertical force occurs for the foot with the use of a traditional heel design. So, we need to rethink that part of the design first. We need to both soften the heel strike, and slow the foots advance into full foot loading. And here comes good old Archimedes, right on time. We want a ramp, essentially. Most athletic footwear today has figured this out, by "rolling up" the heel of the shoe at some varying angle to the rest of the sole of the shoe. And since the foot is at an angle to the ground as it descends in preparation for heel strike, the effect of this rear rocker angle is to allow heel strike to begin a slight amount before it normally would, and arrive at full foot loading slightly later than it usually would. This lowers the vertical shock at heel strike, and reduces the horizontal, or shear forces, that occur in the later portion of the heel strike phase of gait. Essentially, it reduces, and in some instances, eliminates "slap."
Now, there are many so-called rocker soles out there that have a very minor rear rocker angle, which do not act to reduce this "slap" into full foot loading. But there are also some that use such an extreme angle that it forces the mid-foot to "climb a hill" to arrive at full foot loading. Essentially, they represent a negative-heeled shoe by virtue of this design choice. Remember the knee? Remember hyper-extension? Of course you do! And these extreme rocker angles do in fact drive the knee to hyper extension, and in people who have tight ligaments, that hyper-extension can be very damaging. In people with more normal or loose ligament tone, there is still potential for damage, but it might take a longer period to manifest. And some of these extreme rear rocker angle designs seem as though they are competing with each other for being the most extreme. As I said before, just because the marketing folks can spin a good rationale for any type of design does not mean it works, or is right for any particular person. There must be logical biomechanical reasoning behind the design.
OK, that's all for Part One. Next, I will go further into the design issues, and provide some diagrams to aid in understanding the rationale for these designs. Stay tuned!
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F. De Maio, F. Mancini, D. Bellini, and A. Orefice