One of the Most Important Types of Stress, in Terms of Its Effect on the Human Body, Is Impact

One of the most important types of stress, in terms of its effect on the human body, is impact force. Impact force has been defined as a force resulting from the collision of two bodies over a relatively short time period (29). (845)

Impact forces during running vary in magnitude from approximately 1.5 to 5 body weights and last from about 10-30 ms (31). (845)

A number of variables have an effect on impact forces including the foot and center of mass velocity at contact, the effective mass of the body at contact, the area of contact, and the material properties of the damping elements such as soft tissue, shoes, and the surface of contact (30). (846)

Impact and Overuse Injuries in Runners Med. Sci. Sports Exerc., Vol. 36, No. 5, pp. 845-849, 2004. ALAN HRELJAC Kinesiology and Health Science Department, California State University, Sacramento, Sacramento, CA

Pronation indeed forms an integral part of the normal foot unroll. It seems wise to "correct" these variables in a gentle, individualized way. The correcting strategies could consist of tape, inserts, or orthotics in the shoe or specially designed, adequate, antipronation shoes. (338)

Gait-Related Risk Factors for Exercise-Related Lower-Leg Pain during Shod Running Vol. 39, No. 2, pp. 330-339, 2007 TINE MARIEKE WILLEMS', ERIK WITVROUWl, ANNELEEN DE COCK 2, and DIRK DE CLERCQ 2 'Departments of Rehabilitation Sciences and Physiotherapy and 2Movement and Sport Sciences, Ghent University, Ghent, BELGIUM

Risk Factors and Mechanisms of Knee Injury in Runners Med. Sci. Sporis Exerc., Vol. 40, No. 11, pp. 1873-1879, 2008.

STEPHEN P. MESSIER', CLAUDINE LEGAULT2, CASEY R. SCHOENLANK', JOVITA JOLLA NEWMAN', DAVID F. MARTIN3, and PAUL DEVITA4 J.JB. Snow Biomechanics Laboratory, Department of Health and Exercise Science, Wake Forest University, Winston-

Studies on lower extremity loads and overuse injury have shown that impact forces at commonly injured sites are extremely high (41), (1874) Although these data do not provide unequivocal support, it appears that high forces applied to the lower extremity tissues during running are associated with running-related injuries and therefore may be mechanisms of knee injuries. (1874)

Mercer et al. (27) found that overstriding by 15% of freely chosen stride length increased peak leg accelerations compared with normal stride length and understriding by 15%. Over thousands of strides, the increased shock to the lower extremity may result in overuse injury. (1874)

Increased muscle strength should increase the shock absorbing capabilities of the muscles surrounding the knee joint, resulting in lower knee joint loads. (1877) poor flexibility could affect changes in joint angle and the knee extensor moment, increasing joint stiffness and knee joint forces. Consequently, poor hamstring flexibility would be detrimental to joint function by increasing joint stiffness and reducing the contribution of the knee to shock attenuation after heel strike. (1878)

We found that larger knee joint loads were related to poor hamstring flexibility, higher BW, greater weekly mileage, and greater muscular strength. Most of these risk factors could potentially be modified to reduce knee joint loads to lower the risk of injury. (1878)

A well-designed shoe can assist in reducing the number of lower limb injuries arising from sport and training activities. (584) … properly made footwear may attenuate the heel strike-initiated shock waves and reduce the degree of injuries. (584) The magnitude of a shock wave initiated at the heel strike is activity dependent, subject to the particular characteristics and mechanical properties of the footwear and the ground surface. (583) Since grass surface is intrinsically more uneven . . . one may hypothesize that this unevenness may be a reason for overall increase in dynamic loading. (583) Injury Prevention in Distance Running

Distance running involves racing over distances from 1500 meters to marathon, even ultra-marathons. While suggestions for injury prevention in this paper can be applied to all of these distances, it will be focused on the application to races of 1600 meters, 3200 meters, and 3 miles. Distance running, like all sports requires great physical strain on the body, and overstepping the limits of the human machine often results in injury.

Injury is damage sustained by tissues of the body (Zernicke et al, 514). While injuries during running may have a variety of causes, the most important factor (Zernicke et al, 509), and the one that will be the main focus of this paper, is force applied to the body upon impact by the foot on the ground. Force is the mechanical action applied to an object that tends to produce acceleration (Zernicke et all, 509). Running movement is produced and controlled by both internal forces from the muscles and external forces such as gravity and impact (Zernicke et al,

509). These forces act on tissues in the body of the runner, and when they exceed the tissues ability to withstand the load, an injury occurs (Zernicke et al, 509).

The force that results when a runner’s foot collides with the ground is an example impact force, which is the most important factor when discussing injuries (Hreljac, 845). When a runner’s foot strikes the ground in stride, force is applied to the body. This impact force can be more than five body weights in magnitude (Hreljac, 845). High impact forces and loads on the legs cause many overuse injuries (Messier et al, 1874).

To avoid injury, this force needs to be balanced with the ability of musculoskeletal structures to respond to stress (Mcnitt-Gray, 523). The impact force and pressure on the body in running depends on several variables, including the area of contact as well as the properties of damping elements such as tissue, shoes, and the running surface (Hreljac, 846). The force can be lessened or balanced through simple improvements such as stretching, weight lifting, correct gait and foot pronation, cushioning shoes, and careful choice of running surface. These improvements focus on the physics principles of elasticity and most importantly, force.

According to Newton’s Third Law of Motion, every force has a reaction force equal and magnitude and opposite in direction. Therefore every time a runner strikes the ground, a force is produced downward into the surface, but also back onto the runner’s leg. If this reaction force can be reduced or properly absorbed, injury will be less likely.

Incorporating stretching into the training regimen can help absorb the reaction force that acts on a runner’s legs. Stretching will increase elasticity of muscles and range of motion.

Obviously, the longer a muscle can stretch or the more force it can take before tearing, the less chance of an injury. Conversely, poor hamstring flexibility would be detrimental to joint function by increasing joint stiffness and reducing the contribution of the knee to shock attenuation after heel strike (Messier et al, 1878). Calves are the “shock absorbers” of the body and if a runner’s calves are flexible and elastic, they can absorb some of the reaction force and lessen strain knees, quadriceps, hamstrings, and hip muscles, decreasing chance for injury.

Choosing proper running shoes and a soft running surface can also help reduce the effects of impact force on a runner’s legs because the magnitude of a shock wave initiated at the heel strike is activity dependent, subject to the particular characteristics and mechanical properties of the footwear and the ground surface (Voloshin, 583). A well-designed shoe can assist in reducing the heel strike-initiated shock waves and reduce the degree and number of lower limb injuries

(Voloshin, 584). Also, choosing to run on a grass or rubber track surface will decrease impact force on the legs. By reducing the impact force in running or improving the body’s capability to deal with the produced stress by such force is a key in reducing the chance of injury in distance running. If runners make changes to reduce or deal with these forces, they have a great chance to be more injury-free and successful at running. Bibliography

A. HRELJAC, Med. Sci. Sports Exerc. 36, 845 (2004).

S. P. MESSIER, C. LEGAULT, C. R. SCHOENLANK, J. J. NEWMAN, D. F. MARTIN, and P. DEVITA, Med. Sci. Sports Exerc. 40, 1873 (2008).

J.L. MCNITT-GRAY, in in Biomechanics of Sport, edited by Vladimir M. Zatsiorsky (Blackwell Science, Osney Mead, Oxford, 200), pp. 523-549.

A.S. VOLOSHIN, in Biomechanics of Sport, edited by Vladimir M. Zatsiorsky (Blackwell Science, Osney Mead, Oxford, 200), pp. 577-587.

T. M. WILLEMS, E WITVROUWl, A. DE COCK , and D. DE CLERCQ, Med. Sci. Sports Exerc. 39, 330 (2007).

R.F. ZERNICKE and W.C. WHITING, in Biomechanics of Sport, edited by Vladimir M. Zatsiorsky (Blackwell Science, Osney Mead, Oxford, 200), pp. 507-522.