a) Field of the Invention
This invention relates to a sole construction for a shoe and also to a shoe incorporating the sole construction of this invention.
b) Description of the Prior Art
A useful function of conventional shoes is to protect the foot of a wearer from sharp objects such as sharp stones, broken glass, drawing pins, nails, thorns and so on. The puncture resistance of the sole of a shoe is achieved by providing a sufficiently thick sole made from a material of adequate strength and hardness. For example, leather or natural or synthetic rubbers may be used, the material being selected to have the required strength, thickness and wear resistance. However, a conventional shoe has an adverse and potentially serious effect on the human locomotive system and can contribute to poor posture. Also, several harmful effects are associated with the use of a relatively hard and inflexible shoe sole.
In the case of a hard and relatively inflexible shoe sole, the following effects may be noted, which are even worse with a truly rigid sole:
a) The sole limits tactile perception through the foot, which can lead to overloading of the locomotive system since high foot impacts may not be detected.
b) The sole reduces the capacity of the foot for tactile sensation, leading to a reduction in foot comfort as the sense of touch becomes deadened.
c) A relatively inflexible sole restricts articulation of the joints in a foot. For example, when walking a foot should flex through about 550 at the point of step push-off, but with a typical shoe the foot flexes through only about 250. As a consequence, the foot has to work harder creating undue strain and fatigue.
d) A relatively inflexible or rigid sole usually is generally planar in the transverse direction, with a sharp corner at the outer edge of the sole. This can give a false sense of stability to a wearer of the shoe, as it can give resistance to lateral ankle rolling up to a particular point but once this limit is exceeded, then support is instantaneously lost. The common result is a sprained or is twisted ankle.
e) A relatively stiff and inflexible sole prevents the foot rolling over the ground with a smooth shock-absorbing action. Instead, the profile of the sole tends to cause a wearer to assume a ‘slapping’ jarring gait which can lead to various overloading-related injuries.
In the case of a relatively thick sole, the following effects may be noted:
f) With wear, a relatively thick sole can distort to take up a bow-shape (i.e. concave in the lengthwise direction, when viewed internally of the shoe), which will cause the metatarsal heads to lose their natural flat plane. In turn, this can contribute to the possibility of a fallen metatarsal arch.
g) A relatively thick sole can localise the weight distribution on the foot. With a barefoot print, the weight is shown to be distributed over the whole area of the foot—a broad heel, a lateral border, the ball region of the foot and toe imprints. By contrast, tread area of a thick sole on the ground may be only 65% to 85% of the weight bearing area of a bare foot, and with a thick and inflexible sole, this increases the local loading on the foot.
h) A thick sole tends to increase ankle instability because it increases the lever arm of the heel and causes unnaturally high stresses on the ankle. The thicker the sole, the greater the chance of a wearer suffering a twisted ankle.
g) A relatively thick sole will, in general, weigh significantly more than a thinner sole. Most shoes are too heavy for proper foot comfort and a thick sole will make a shoe even heavier. It has been shown that every added 100 g on the weight of a shoe may add another tonne to the daily foot lift-load. This foot lift-load imposes an energy drain not only on the foot but on the whole body. This is a common but little recognised source of foot and body fatigue.