Tuesday, April 28, 2009

The Great Running Shoe Debate

Up until middle distance running, sports shoes are designed for normal heel-to-toe walking. The gait cycle changes at middle distance speeds and the heel contact is lost and so running shoes have no heel. The mechanics of walking and running are complex and involve tri-plane motion at the junction between the end of the leg and the foot. Triplane motion of supination and pronation arises at the sub talar joint which is made up of the talus and the calcaneum bones. The talus is a small bone which can either be part of the foot (independent of the leg) in motion such as dorsiflexion and plantarflexion of the ankle; or part of the leg (independent of the foot) in inversion and eversion of the foot against the leg. During supination and pronation the talus a calcanium cause movement in three planes which leads to a very complex and co-oridinated set of compensations which can be simply referred to as torque conversion. When these co-ordinated movements are out of sequence then this may lead to repetitive stress injury both at site as well as proximal and distal to the tri plane joint. Supination of the sub talar joint causes the foot to lock, subsequently the arch heightens, and the foot adapts to the function of a rigid lever. This makes the foot ideal for propulsion. When the sub talar joint pronates this unlocks the forefoot, arch drops, and the foot becomes a mobile adapter capable of shock attenuation during weightbearing from heel strike to the propulsive phase and take off. This sequence of events is brought about by the position of the calcaneum as determined by ground reaction forces (GRF). The talus has no muscles inserted into it or originates from it and hence is classified as a gravitational bone. The position of the talus is determined by the bones which surround it. During heel strike the calcaneum works as a pendulum and depending on its position determines the movement which takes place at the subtalar joint. At heel strikes the calcaneum is inverted and the subtler joint urinated, GRF cause the heel to Evert causing the joint to probate. Just prior to the propulsion phase when the heel lifts form the ground, the subtler joint starts to resupinate caused by moments (turning forces) generated from toes contact. The function of all shoes is to support the foot through these changes and allow ‘normal’ walking to take place. Problems can arise when there is a delay in resupination of the sub talar joint and the mobile foot requires to speedily change position to lever itself off the ground. This action needs energy which may eventually cause fatigue and subsequent the grinding of adjacent bones result in subluxation (partial dislocation) and traumatic osteoarthosis (wear and tear). These chronic changes are associated with aging but can become acute due to accelerated activity such as sport. A preoccupation with sport shoe designers has been to produce shoes which prevent abnormal pronation (excessive rolling movement of the foot). To do this the trend has been to wedge the heel on the inner side (next to the long arch) which physically prevents the full range of pronation motion from occurring at the subtalar joint. Sometimes this is included into the shoe design or a heel tilt can be incorporate into foot orthoses (shoe inlays). Prior to the 70s the arch support inlay was used to support the “collapsing foot.” The main disadvantage of arch supports was GRFs were brought into contact with the midfoot and this caused asymptomatic chronic traumatic arthritis. Many people became dependent upon their shoe inserts not aware their feet were stiffening as a result of wearing them. In the 70s a new theory emerged from podiatrists working in California and was based on an assumption there was a standard gait cycle with a critical change over point from a pronated foot to a rigid lever. This occurred at a point when the foot was about to enter propulsive phase (just after heel lift during stance phase). A criterion for the normal foot was defined at the point in time when the heel (leg) was perpendicular to the ground and the fore foot and heel plains were parallel to the ground. Referred to as the sub talar neural theory the model gave a universal reference position which provided a useful means to define abnormal (out of synchronicity) movements and their sequester (pathomechanics). Three dimensional motion is complex and advances in gait analysis have however subsequently shown the subtalar joint theory to be imprecise. Some attempts were made in the late 70s to include heel wedging into the design of shoes for the football codes but this was unsuccessful. More recently as sport shoe design has seen footwear with independent heel and forefoot sections then the principles of the sub talar neutral hypothetical model have become apparent. A new debate has revolved around a call for a more natural shoe or one which allows the kinetic foot to behave unhindered. Cries for barefoot running has caused a small but significant number of serious athletes to consider their footwear options. Recent reports suggesting the absence of scientific justification for sports shoes has added considerably to the debate. What has further fuelled discussion is the presence of designed footwear which gives added spring in marathon and sprinting shoes. Currently this type of footwear is classified as performance enhancing and not able to be worn in professional competitions. The main thrust of the barefoot running debate comes from commercial sectors that would in the interests of breaking records wish to see the new designs accepted. Reference Trimble T 2009 The Running Shoe Debate: How Barefoot Runners are Shaping the Shoe Industry Popular Mechanics April.

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