PROBLEMS WITH EXISTING SKI BOOTS


As a segue to my post on Turntable Power and how it cantilevers ground reaction force acting along the running surface of the inside edge of the outside ski, I have decided to post the discussion on the problems with existing ski boots from my US Patent 5,265,350 with associated international patents. The patent was issued on November 30, 1993 (24 years ago) to me as the sole inventor and assigned to MACPOD Enterprises Ltd. (Toronto).

The objective of US Patent 5,265,350 and subsequent patents filed and granted to MACPOD was to identify problems with existing ski boots and offer solutions and a functional criteria for advancing the state-of the art going forward. Some of the problems noted and solutions offered, apply to footwear in general.

The final paragraph raises the issue of the limitations of conventional ski boots in terms of accommodating and enabling biomechanically generated forces such as torque from the mechanical force transfer points of the foot to the structure of the ski boot.

The following material is verbatim from the text of US Patent 5,265,350.


Problems with Existing Ski Boots

Existing footwear (ski boot design) does not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg. 

Although somewhat vaguely stated, a generally accepted theme has arisen over the years, one of indiscriminate envelopment and “overall restraint” applied to the foot and leg within the footwear. The stated position of various authorities skilled in the art of the design and fabrication of footwear for skiing is that the foot functions best when movement about its articulations is substantially prevented or restricted.

To serve this end, inner ski boot liners are usually formed around inanimate lasts or, alternatively, the foot and leg are inserted into an inner liner within the ski boot shell and foam is introduced into a bladder in the liner so as to totally occupy any free space between the foot and leg and the outer ski boot shell. The outer shell of the footwear is closed around this inner envelopment forming an encasement with which to secure and substantially immobilize the foot and leg. This is considered the optimum and, therefore, ideal form of envelopment. The perspective is that the physiologic structures of the foot are inherently weak and thus, unsuited for skiing. Enveloping the foot within an enclosure which makes it more rigid is thought to add the necessary strength with which to suitably adapt it for skiing. The reasoning being, that the foot and leg now having being suitably strengthened, can form a solid connection with the ski while the leg, now made more rigid, can better serve as a lever with which to apply edging force to the ski.

To some degree, the prior art (existing ski boot design) has acknowledged a need for the ankle joint to articulate in flexion. However, the prior art has not differentiated exactly how articulation of the ankle joint might be separated from the object of generalized and indiscriminate envelopment and thus made possible. Therefore, the theme of prior art (existing ski boot design) is inconsistent and lacks continuity.

The only disclosure known of a process wherein the separation of envelopment of the foot from articulation of the ankle joint is contained in U.S. Pat. No. 4,534,122, of which the present applicant is also the inventor. This material discloses a supportive structure (i.e Dorthotic) wherein restrictions to flexion of the ankle joint are essentially removed, support being provided from below the hinge of the ankle joint.

In keeping with the theme of indiscriminate envelopment and overall restraint, the following structures are generally common to all footwear for skiing disclosed by prior art (existing ski boot design):

(a) a continuous counter system which surrounds the foot and provides for the process of envelopment;

(b) an arrangement of pads or padding with which to envelope the foot;

(c) a substantially rigid outer shell which encases the structures employed for envelopment;

(d) an articulation of the ski boot lower outer shell and the cuff or cuffs which envelope the leg of the user, usually accomplished through a common axis or journal;

(e) a structure to brace and support the leg since prior art considers the ankle joint to be inherently weak and in need of support; and

(f) some form of resistance to movement of the cuff (shaft of the ski boot).

The prior art (existing boot design and boot fitting procedures) refers to the importance of a “neutral sub-talar joint”. The sub-talar joint is a joint with rotational capability which underlies and supports the ankle joint. The sub-talar joint is substantially “neutral” in bipedal function. That is to say that the foot is neither rolled inward or rolled outward.

If the foot can be substantially maintained in a neutral position with the arch supported and with a broad area of the inner aspect of the foot well padded, there will exist a good degree of comfort. Such a state of comfort exists because the foot is not able to roll inward (pronate) to a degree where significant mechanical forces can be set up which would allow it to bear against the inner surface of the boot shell. In effect, this means that initiation of the transition from a state of bipedal to a state of monopedal function, is prevented. This transition would normally be precipitated by an attempt to balance on one foot. If the foot is contained in a neutral position, traditional supportive footbeds (arch supports) are quite compatible with the mechanisms and philosophies of the prior art.

Problems arise when the foot is attempting a transition from a state of bipedal stance to monopedal stance. If the transition to monopedal stance or function can be completed without interference from the structures of the ski boot, all is fine and well. However, if the transition is allowed to proceed to a point where the mechanics associated with the monopedal function can establish significant horizontal forces, and the further movement of the foot is blocked before the transition can be completed, the skier will experience pain and discomfort at the points where the inner aspect of the foot bears against the structures of the footwear. This is the situation experienced by a majority of the skiers with prior art footwear. It is at this point where arch supports, if employed, also begin to cause discomfort. It should be noted that it is the normal tendency of the foot to pronate when weight bearing on one foot.

Footbeds (arch supports) may work in conventional boots (which traditionally do not allow natural biomechanics or movement of the foot to occur), but in a boot which accommodates and supports natural leg and foot articulation and function, arch supports can be detrimental.

When the foot attempts to pronate inside the ski boot, it is often the case that the ankle bone will come to bear against the inner surface of the boot shell. When contact of this nature occurs, pain and other related complications usually result. Since the consensus of those skilled in the art of ski boot design and modification is that pronation or the rolling inward of the foot is detrimental, and, thus, undesirable, provision is not made to allow for such movement. Rather, the structure of the footwear is intended to resist or even prevent it.

Thus, the problem with existing footwear arises due to the dynamic nature of the architecture of the foot. When the wearer is standing with the weight equally distributed between left and right feet so that the centre of mass of the wearer is manifesting itself in the centre between the feet, the architecture of the wearer’s foot assumes a specific configuration. As the wearer begins to shift his weight towards one foot so that the other foot bears proportionately less weight, the wearer’s centre of mass moves over the medial aspect of the weighted foot so as to assume a position of balance. In order for this movement of the wearer’s centre of mass to occur, the architecture of the weighted foot must undergo a progressive re-alignment. Existing footwear does not adequately anticipate this re-alignment of the architecture of the foot and thus such footwear inhibits the wearer’s ability to assume a balanced position.

A further problem with existing footwear is the fact that longitudinal relative movement between the foot and the footwear may occur. This happens, for example, when the forefoot/midfoot section of the foot is not adequately restrained under certain conditions, such as when flexion is occurring between the lower leg and the foot. Such longitudinal relative movement contributes to the disruption of biomechanical reference points associated with the dynamics of the ski and, in addition, results in a delay in the transmission of force between the leg and foot and the footwear.

Yet a further problem with existing footwear for skiing, in particular the rear entry type, relates to the obstruction of the leg in forward flexion. A relatively freely flexing gaiter or cuff (i.e. shaft) is necessary in order to permit the posterior muscle groups of the lower leg to modulate external force exerted on the footwear. This requires that the axis of the footwear be allowed to rotate so that small degrees of flexion/extension occur at the foot with the lower leg being relatively passive and that large degrees of flexion/extension occur as coordinated ankle, knee and hip flexion. The construction of the prior art requires flexion/extension to occur primarily at the knee and hip joints which is disadvantageous to the user.

While some types of rear entry boots do disclose gaiters or cuffs which provide a degree of relatively free flexion, there remains numerous problems, the most serious of which is the fact that the device employed to secure the foot of the user exerts, in addition to the downward directed force on the foot, a simultaneous rearward directed force on the leg which acts to resist forward flexion in spite of any free hinging action of the cuff. The result is an interference with the physiologic function of the foot and leg of the user.

Yet another problem resides in buckle or overlap type footwear. In order to provide for entry of the foot of the user and for resistance to flexion, plastic materials are employed for the outer shell which have flexural qualities. This is necessary in order to facilitate the aforementioned requirements. Plastic materials by their very nature tend to resist point loadings by a relaxation of the material at the point where stress is applied. This characteristic creates serious problems for two reasons. First, the teaching of this application is that force must be applied and maintained only to specific areas of the foot and leg of the user while allowing for unrestricted movement of other areas. The application and maintenance of such force by flexible plastic materials in the structures of prior art is necessarily difficult, if it is possible at all.

Second, the plastic materials in relaxing under the application of stress assume a new shape by moving into void areas. Thus, the probability is great that the plastic material will change shape so as to inhabit the very area required for the uninhibited displacement of the structures of the foot and leg. The result of these limitations is interference with the physiologic function of the user.

Top and rear entry footwear for skiing and skating necessarily have interior volumes greater than that required by the wearers foot and leg, particularly in the area over the instep, in order to accommodate entry. This additional volume makes the incorporation of structures designed to provide accurate and consistent support to specific areas necessarily difficult and ineffective. This results in reduced support for the foot and leg.

Another problem with conventional footwear relates to the flexion of the lower leg relative to the foot. It is desirable to provide a degree of resistance to such movement to assist in dampening movement of the mass of the skier relative to the ski resulting from, for example, a velocity change due to terrain changes and to assist the user in transferring energy to the ski. Adjustment of such resistance is desirable in order that the user may compensate for different physical makeup and different operating conditions. In present ski footwear, sources of resistance for such purpose are poorly controlled and often produce resistance curves inappropriate for the operating environment (i.e. temperature) thereby adversely affecting the balance and control of the user and creating a need for additional energy to be expended to provide correction. In many applications, resistance is achieved by deformation of shell structures thereby resulting in reduced support for the user’s foot and leg. If indeed provision is made for adjustment of flex resistance in the instances cited, it is very limited in terms of ability to suitably modify resistance curves.

Torque Transfer and The Turntable Effect

Yet a further problem relates to the efficient transfer of torque from the lower leg and foot to the footwear. When the leg is rotated inwardly relative to the foot by muscular effort, a torsional load is applied to the foot. Present footwear does not adequately provide support or surfaces on and against which the wearer can transfer biomechanically generated forces such as torque to the footwear. Alternatively, the footwear presents sources of resistance which interfere with the movements necessary to initiate such transfer. It is desirable to provide for appropriate movement and such sources of resistance in order to increase the efficiency of this torque transfer and, in so doing, enhance the turning response of the ski. 

In my next post, I will discuss Turntable Power in conjunction with the Over-Centre mechanism.

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