The midsole must combine a capacity for shock absorption with capacities to control ankle pronation and provide adequate flexibility. Yet to some extent, two of these characteristics are mutually exclusive; an EVA that has good shock absorption will be soft and therefore have good flexibility but very poor pronation control, whereas EVA that provides good control of pronation will be hard and inflexible and have poor shock-absorbing characteristics.
In an attempt to compensate for these mutually exclusive characteristics, shoe manufacturers have used midsoles of different hardness in different areas: a soft, shock-absorbing material along the outer heel border and under the ball of the foot to increase shock absorption and flexibility plus a firmer material along the inner border of the shoe, extending from heel to midfoot, to control pronation.
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These techniques have largely been successful. The only problem that has not been effectively solved concerns the midsole underneath the ball of the foot. This area does not absorb the highest forces during landingthe heel does thisbut this area is exposed to moderately high pressure for a much longer time; thus, it will tend to compress even more than the heel. Yet it must be soft enough to allow flexibility.
Exercises 6.2 The thumb compression test. A hard or firm midsole compresses very little (top); a softer midsole compresses more, providing greater shock absorption.
One attempt to solve this problem resulted in the Nike Tailwind, first released in 1979. The midsole of this shoe contained a series of five polyurethane tubes extending from heel to forefoot into which Freon gas was injected at a pressure of about three atmospheres (Cavanagh, 1980). Although this shoe ultimately proved unsatisfactory because it had poor rearfoot control, its offspring have
Clearly shown that the air sole does not become compressed in the same way as does conventional EVA. However, in not all of these shoes does the air sole extend to the forefoot. If the air sole is present only in the heel, the EVA under the forefoot will still be prone to compression for those runners who land heavily on the forefoot (see Exercisess 14.3 and 14.4). In 1987, Asics running shoes introduced a gel-containing midsole, which, like the air midsole, resists compression yet likely has equivalent if not superior shock-absorbing capacity than EVA. More recently, other major shoe manufacturers including Reebock, Converse, Hi Tec, Saucony, and Tumtec have introduced so-called “energy return systems” to their midsoles. These are discussed in post 14.
In summary, the features of the midsole that require consideration are its hardness and whether or not it is made of mixed material. Those who require shock absorption in their running shoes because they have “rigid” lower-limb structure must look for shoes with soft midsoles; those with “mobile” feet need firmer shoes (see post 14).