Forces in an Athletic Shoe

Power for an athletic movement comes from the body interacting with the ground. We push on the ground, the ground pushes back allowing us to move.   It is a basic law of physics. 

Athletic shoes are the interface between the body and ground.  Thus, the push we apply to the ground and the push-back by the ground is through the shoes.  Like all mechanical systems, the shoes do not perfectly transfer the forces between the body and the ground.  There are loses. 

For example, when we stand in a shoe, we create a vertical force vector that causes the sole of the shoe to compress.  When we lift our foot up, the sole of shoe decompresses.   The compression and decompression of the sole is not perfect; it requires energy (force over a distance).  Thus, the force at which the ground is pushing back on us is less than the force we applied to the ground due to the energy being absorbed by the sole of shoe.          

We lose even more energy in the horizontal direction when we’re making a lateral movement (e.g., making a cut in football, basketball, baseball, tennis, etc.) or when we’re making a rotational movement (e.g., swinging a golf club, a baseball bat, a tennis racket, etc.).  Athletic shoes include two horizontal forces: a friction force between the outsole of the shoe and the ground and a horizontal reaction force between the foot and the inside of the shoe.

Tread or cleats on the outsole of athletic shoes provide the friction force between the outsole of the shoe and the ground.  In many instances, this keeps the shoe from slipping during lateral movements and/or during rotational movements.  When we avoid slipping, we generally say the shoes have good traction.         

Many athletic shoes, however, provide little to no horizontal reaction force, which keeps the foot from sliding horizontally (e.g., medial to lateral) within the shoe.   For instance, lightweight shoes use very lightweight uppers to reduce the overall weight of the shoe.  This, however, reduces the horizontal reaction force of the shoes and allows to foot to slide even more. 

As the foot slides within the shoe, body position is adjusted to maintain balance.  For example, when the foot moves in the shoe (we call this roll out), the body sways in the direction the foot is moving to maintain balance.  The body then has to recover from this sway.  In golf, tennis, and baseball, the sway can cause timing issues and loss of power. 

The slipping of the foot within the shoe increases with better traction.  The shoe stops moving, but the foot doesn’t.  This is like an unsecured object in the car, which keeps moving when the car stops.  To stop the object as the car is stopping, it has to be firmly secured to the car.

Similarly, to keep the foot from sliding in the shoe, it has to be held firmly in the shoe.  This is accomplished by providing a sufficient amount of horizontal reaction force.  For instance, a sufficient amount of horizontal reaction force is created by re-enforcing the lateral wall and by providing optimal athletic positioning (OAP).  OAP includes a conventional heel to toe slope on the lateral side of the shoe and a slightly greater heel to toe slope on the medial side of the shoe.  This allows the foot to be firmly secured in the shoe during lateral movements and/or rotational movements.

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