Compression arises when external forces oppose each other and press the object under load. Similarly, the weight of the body acts as a compressive force on the bones that supports it. Examples in sports products where compressive forces are regularly experienced are: footballs, golf ball, running shoe soles, snooker cue tips, cricket bats, and foam crash mats. In the human body, the bones and joint experience compression loads, such as the hands and knuckles in boxing, the skull when heading a football or the knee joints during running and jumping.

The opposite of compressive force is tensile force, or tension. Tension force is a pulling force that creates tension in the object to which it is applied. There are many examples in sport, such as the strings of a tennis racquet, the hammer wire during rotation, and the string of an archer’s bow. Tendons in the human body, such as the Achilles tendon, are repeatedly in tension during almost all sport activities (Bartlett R. et al, 2006).

A third category of force is termed shear. Whereas compressive and tensile force acts along the longitudinal axis of a bone or other body to which they are applied, shear force acts parallel or tangent to a surface. Shear force tends to cause one portion of the object to slide, displace, or shear with respect to another portion of the object. For example, force acting at the knee joint in a direction parallel to the tibia plateau is a shearing force at the knee.

During the performance of a squat exercise, joint shear at the knee is greatest at the full squat position. The position places a large amount of stress on the ligaments and muscle tendons that prevent the femur from sliding off the tibia plateaus. The shear strength of a material is its ability to withstand offset or transverse loads without fracture occurring. Running shoe soles experience shear loads on impact with a surface. The load arises from a horizontal force component from the foot on impact and a reaction force from the surface due to friction.

Torsion occurs when a structure is caused to twist about its longitudinal axis, typically when one end of the structure is fixed. Torsional factures of the tibia are not uncommon in football and skiing accidents in which the foot is held in a fixed position while the rest of the body undergoes a twist.

Injuries can result from extreme compression, tension and torsion force. For example, each time a foot hits the pavement during running, a force of approximately two to three times body weight is sustained. Although a single force of this magnitude is not likely to result in a fracture of healthy bone, numerous repetitions of such a force may cause a fracture of an otherwise healthy bone somewhere in the lower extremity (Susan J. Hall, Ph.D, 2003).

The work-energy principle can also be used to explain the techniques used in transferring (or absorbing) energy from an object. When you catch a ball, its kinetic energy is reduced (or absorbed) by the negative work you do on it. Similarly, your muscles do negative work on your limbs and absorb their energy when you land from a jump or fall. The average force you must exert to absorb energy in catching a ball or landing from a jump or fall depends on how much energy must be absorbed and over how long a distance you can apply the force. If this force is too great, it may injure you. You attempt to decrease it by “giving” with a ball when you catch it or by flexing at your knees, ankles, and hips when you land from a jump of fall. These actions increase the distance over which the force acts, thus decreasing the average value of the force.

The safety and protective equipment used in many sports utilizes the work-energy principle to reduce potentially damaging impact forces. The landing pads used in gymnastics, high jumping, and pole vaulting all increase the displacement of the athlete during the impact period, as the kinetic energy of the athlete is decrease (absorbed). The impact force is thus decreased because the displacement during the impact is increased. The sand in a long jumping pit does the same thing when you jump into it, as does the water in a pool when you dive into it, the midsole material in your running shoes when you run on them, the padding in a boxing glove when you punch with it, the air bag in a car when you crash into it, and so on. All of these materials may be referred to as shock absorbing (McGinnis and Peter Merton, 2005).

An example of body’s own shock attenuating mechanisms, which are both active (through muscle tone and proprioceptive information about joint position) and passive (through the elasticity of bone and soft tissues). The adult femur can lose 10mm length after impact because of bowing and elastic deformation. Cancellous bone is able to absorb shock although cartilage seems to play little role in this respect, and the heel fat pad can maintain it s capacity to absorb shock over many impacts (Bartlett and Roger, 1999)

Bones are better at resisting compression rather than tension and /or torsion. That is why most fractures occur when a bone is twisted or bent. The bending motion puts tensile stress (pulling apart) on the opposite side of the bone, where the fracture begins. The repetitive tensile stresses on the bone can result in stress fractures. Fractures may be the result of direct trauma. Example, an impact to the leg, or indirect trauma, e.g. when the foot is trapped, causing the athlete to fall awkwardly and break the leg (Peterson and Lars, 2001).

Shock absorption, sports performance, biomechanics.