Saturday, June 14, 2008
What makes a perfect sport shoe?
Today’s athletes prefer lightweight shoes styled to their particular sport. Improved lasting and construction techniques have guaranteed better comfort and support suffice 'running in', is not required. Shoes are still walking machines and require a matched heel pitch and toe spring for efficient propulsion. The durometer of rearfoot/ forefoot midsole and components need to be appropriate in stiffness and thickness for extreme use. The heel of the sport shoe needs to support and stabilise the anatomical heel throughout the side to side pendulum action and protect it from surreptitious injury during stance phase. Outsoles that provide proper traction for playing surfaces improve stability and lacing systems appropriate to comfort and serviceability enhance fit. The inclusion of biotechnologies, such as sock-liners, improve comfort and make for a more serviceable shoe. We have come a long way from the early sandals of the ancient games. Most sport involves modified locomotion determined by the gait cycle. This describes a repetitive cycle of events, which involves stance phase and swing. During stance phase the heel hits the ground first, then as the foot adjusts to the ground surface, the centre of mass passes over the pedestal as weight transfers from one leg to the other. Swing phase completes the cycle when the weight-bearing limb propels through space to return to stance phase again. The gait cycle consists of one stance phase and one swing phase and in normal walking would take approximately one second to complete. Stance phase consists of 60% of the cycle. Cadence, or the speed of walking determine the length of stance phase and swing phase. Up to middle distance running the normal gait cycle, with heel contact occurs but at a certain point heel strike is omitted and faster track events depend on forefoot contact, only e.g. sprinting. The sport of speed walking depends on the presence of heel contact and its absence would disqualify competitors. Ground reaction forces during contact cause feet to change shape as the joints move to lock and unlock the foot. This subtle movement is hardly visible but essential to reduce damaging forces, which pass upward through the leg to vulnerable weight bearing joints. Heel strike during normal walking generates one and a half times body weight, running at medium speed increase 'G force' to two to three times bodyweight. When a basketball player lands on his heel there may be eight times body weight. Accelerated and exaggerated activities consistent with professional sport increases wear and tear on the body and can cause severe damage to the musculoskeletal system. For this reason sport shoes have taken a more protective role of late and are unlikely, by themselves to improve the athlete’s performance. Sport shoes primarily require to respond to the natural function of the foot. They cope with dimensional change of the foot during critical contact phases with the ground and or external forces (as in contact sports). Unlocking the forefoot during stance phase can change the foot size by as much as one and half times its size. Comfort is considered critical and this is often seen to equate with a tighter fitting shoe, made from a soft upper, which adjusts to the changing foot anatomy. Propulsion is necessary and relies on static friction in the form of traction. Hence the basic anatomy of a sport shoe describes a soft upper combined with a sole tread appropriate to the ground surface the athlete competes upon. Early sports shoes were made with canvas tops and vulcanized rubber soles. Kangaroo skin was a popular upper material from the late 19th century with quality made sport shoes such as croquet and cricket shoes. Metal spikes were used in early leather sprinting shoes. The polymer revolution has meant new materials, some of which are out of this world, are now regularly employed to further assist the athlete in their endeavours. However no shoe yet, has won a gold medal. Not for competing, anyway. At best sports footwear enhances foot performance during the critical points where the lower leg and foot may be compromised and become less efficient as a pedestal or lever. Shoes which enhance foot performance need to be durable and capable of absorbing tremendous sheer and stress. The heel and forefoot experience different loads and forces. Much use is made of 'intelligent polymers' such as visco-elastics (viscos) and elastic foams with good elastic memory. Viscos plastics in the heel absorb impact forces and highly elasticated polymers in the forefoot area minimize the loss of energy transmitted to the shoes during the running movement. Sport surfaces and shoes are deliberately constructed with the intension to reduce excessive loading and optimise performances. The elastic response of the sport surface /shoe system is a critical variable for determining mechanical behaviour. The mechanical behaviour includes energy storage and return, frequency of energy return and cushioning. Original tracks and shoes were less efficient and had limited capacity to store and return energy. New combinations are considered to be 14 times better and thought to covert about 3% of the average energy expenditure per step during middle distance running. Scientists believe energy return and cushioning are closely associated and hence much emphasis has been placed on developing dual density mid soles which are soft on the lateral side and hard on the inside of the foot. This is thought to give to stability when the foot rolls over. The outsoles of the shoe are made from different materials which are combined in a complex way to provide the physical response required in each part of the sole as the foot progresses through stance phase. The sole of an athletic shoe now resembles a grid of squares to allow greater flexibility and most can be rolled from end to end into a coil. There are competing ideas of what makes an ideal sports shoe. Some designers use the principles of energy return (spring). Heel strike or forefoot contact in fast running or landing and take off, exerts physical forces which meet with ground reaction, equal and opposite to them. Placing material with high modulus of crush and elasticity may help dampen these forces which transfer through the weight bearing joints through the spine and to the head. New polymers have helped reduce reaction times by fractions of milliseconds and these are thought to aid in landing and take off. In track and field footwear, shoes which advantage a rigid lever at take off are considered to give the athlete a mechanical advantage. Middle distance runners run on the ball of the foot and have no heel contact in their cycle. Sprinting shoes rely less of cushioning with more emphasis placed on reinforced shanks and spikes combined to advantage propulsion with increase ground traction. To become an efficient lever when the heel is off the ground, the subtalar joint requires to become supinated. This is a complex action caused by turning moments caused by reactive forces coming through the ball of the foot. Extension of the metatarsophalangeal joints tightens the facial bands in a Windlass action helping the calcaneum to reinvert. Rigidity of the midfoot during heel off responds to cradling of the foot inside the shoe at this time. Popular sprinting shoes now include a full length, full width thin shoe plate. The plate has sufficiently flexibile and stiff to optimise propulsion without interfering with natural foot compensations e.g. adidas Beijing Sprint. Lightweight microcellular polyurethanes are used for midsoles. The polymer properties can be adjusted to offer the best balance between damping shock and elasticity. According to Stefanyshyn and Fusco (2004) there is a direct relationship between the shoe bending stiffness and sprint performance. The authors speculated individual differences in calf muscle performance was a better predictor of shoe stiffness and performance than the athletes mass, height, shoe size or skill level. More sophisticated biomechnaical analysis has facilitated better assessment of midsole stiffness and now new polymers are revolutionizing midsole construction. Thermoplastic and visco-elastic insocks further help reduce peak pressures arising during contact phases whilst promoting customized comfort. New formulation polyurethanes are also used in outsole construction through injection to enhance abrasion resistance and ground traction, especially in wet conditions. Injection techniques allow multilayered soles which help with colour, density and hardness of the sole. Running tracks are now made from synthetic materials e.g. Mondo which give a uniform surface and good traction. The traditional running spike gives no advantage and now reinforced plastic spikes have replaced them. These are more compatible with the running tracks. In 2000, adidas introduced Z shaped spikes which were designed not to penetrate (damage) the running surface and the same traction is now achieved with three spikes was previously attributed to seven or eight conventional spikes. A brief review of modern spikes is given at flyingfeet.com (Flying Feet Sport Shoes).