Did you know…
●  Feet can expand as much as a size and a half during the course of the day. Buy shoes in the afternoon.
●  After a year, the rubber soles harden, even if they have not been used. This will hinder the mobility of the shoe.
●  The life of a running shoe is typically 500 miles. The shoe may not appear to be “worn out,” but the ability to absorb shock is significantly reduced.
●  The yearly incidence of running injuries is estimated to be between 37% and 56%.

Shoes are meant to help normalize deficiencies. If there is any deviation from normal biomechanics, you are at risk for injury. The shoe protects the body from injury absorbing a significant amount of the forces.

To understand the potential link between running shoe selection and injury risk, it is essential to review the normal gait pattern and biomechanical principles. Compared to walking, the running gait cycle has an increased step rate, with increased joint range of motion, a shorter stance phase, and the addition of a float phase. The body must absorb a tremendous amount of force, with nearly three times the athlete’s body weight experienced at each foot strike.

Biomechanics
When you walk, the heel is the initial point of contact and the foot is in a supinated position (the arch of the foot is maintained, or up). The foot moves quickly into pronation (the arch flattens) as it moves into the stance part (the foot is even with the ground) of the gait cycle. After it reaches maximum pronation, it begins to supinate again preparing for push off. Pronation allows for shock absorption and adjustment for terrain changes, whereas during supination, the foot is more rigid and functions as a solid lever to propel the body forward.

Arch type and injury patterns
Arch type has been shown to be associated with specific patterns in ground force and muscle activation. High-arched runners tend to supinate more, with increased leg stiffness and a higher vertical loading rate, whereas low-arched runners tend to have increased pronation and eversion excursion. A rigid flat foot has been associated with overpronation and can lead to medial lower extremity soft-tissue and bone overload injuries. Pronation and low arch index have been shown to be good predictors of anterior knee pain. A pes cavus (high arch) foot has been associated with increased ground force and can be associated with lateral lower extremity injuries and stress fractures. The modern running shoe has, therefore, been designed to take these factors into account.

Running shoe replacement and care
Independent of running shoe selection, proper running shoe replacement and care can be important factors in injury prevention. Running shoe mileage in excess of 400 to 500 miles has been shown to reduce shock absorption capacity, which may lead to increased injury rates. Increased running shoe mileage has been associated with a greater risk of anterior knee pain. One study found that after 250 to 500 miles of running, less than 60% of shock absorption remains in a pair of running shoes, and at 500 miles, only 45% to 60% of shock absorption remains. Another study found that peak plantar pressure increases 100% after 300 miles of running due to the loss of shock absorption capacity. Structural damage to the midsole material is evident on scanning electron microscopy after 450 miles. Wet running shoes absorb less shock than dry shoes, while cold ambient temperatures also significantly reduce the shock attenuation of commonly used running shoes.

Three-point testing
The American Academy of Podiatric Sports Medicine utilizes a basic three-point testing methodology for evaluating athletic shoes. This evaluation process was developed by Mark Reeves, DPM, of Seattle and adopted by the academy board of directors more than 10 years ago. The academy teaches its members to perform this test and, in turn, instruct their staff and patients in it as well.

The focus of the testing is primarily on whether the shoe has a firm heel counter, whether the shoe has a firm resistance to torque when the heel and toe are twisted in opposite directions, and whether the most distal third of the shoe flexes easily while the middle third resists flexion. These techniques are all performed manually and can be done easily in shoe stores or in the office. The three-part test is quick and easy and provides objective metrics because it is repeatable and results can be compared from shoe to shoe. A certain amount of subjectivity is built into any type of manual evaluation, however, as different testers may have differing strength capabilities affecting their ability to flex, squeeze, or twist a shoe. The academy appreciates this, but at this time there is no other basic hands-on testing methodology available that provides a metric for comparing one athletic shoe to another, especially running shoes.

Stacoff et al, in a study of running shoes that did have heel counters, found that movement of the heel while running barefoot was similar but not identical to movement of the heel counter of a shoe. Torsion control in a shoe may play a more important role in sports in which lateral cutting is of primary importance. In court shoes and trail running shoes, for example, the foot is placed in very different positions due to varying terrain or lateral movements while running, jumping, and landing. The shoe should be able to allow the foot to pronate and supinate as needed in response to these positioning stresses. In running shoes, an increase in motion-controlling properties built into a shoe often leads to a shoe that will be more resistant to twisting torque forces.
This may be acceptable for most road-running on flat surfaces, but it won’t work in general for off road running or other lateral cutting sports.

Finally, we come to the flexion properties of the sole of the shoe. This is probably the most important factor in evaluating any type of athletic shoe. The AAPSM test involves evaluating the ease of flexion of the most distal third of the shoe as well as the flexion resistance of the middle third of the shoe. This component of the evaluation is the most significant departure from other methods of evaluating running shoes. Biomechanics labs and some magazines tend to evaluate only the flexibility of the front third portion of a running shoe, while choosing to ignore the potential for flexibility at the middle third of the shoe. Instead, labs evaluate the compression properties of the midsole of a shoe. While compression is a valuable thing to measure, many studies show that cushioning in running shoes is not necessarily beneficial. These papers and studies show that cushioning properties of running shoes decreased consistently in less than 400 miles of running, and that very compliant or cushioned shoes may possibly increase the risk of injury because there is a correlation between cushioning compliance and excessive subtalar joint motion. In defense of cushioning, some studies have shown a decrease in shock in high-arched runners that may help to protect from bone injuries often suffered by this patient population.

Barefoot running
Interestingly, a recent study has called into question whether shoes are necessary for running at all. Habitually barefoot runners tend to land on the forefoot or midfoot, while the habitually shod runner is more likely to land on the rearfoot due to the elevated heel and cushioning of the running shoe. Some authors claim that barefoot running may prevent the impact-related lower limb injuries experienced by many runners today. They cite evidence from gait studies showing that barefoot runners who land on the forefoot produce smaller peak impact forces than rearfoot strikers wearing running shoes. Barefoot running has also been shown to decrease joint torques at the hip, knee, and ankle compared to shod running.

Conclusion
The selection and care of running shoes represents a potentially modifiable injury risk factor among runners. In spite of differing opinions on how running shoes may affect biomechanics, there is evidence that running biomechanics are altered based on shoe design. There is also debate as to how specific arch types may interact with running shoe design. Newer research has led to the introduction of minimally supportive shoes to simulate running barefoot, and barefoot running itself has become a topic of interest.
Although it can be difficult to determine whether one athletic shoe is better than another, there are several things to keep in mind. The athletic shoe industry is market driven. Many shoe tests touted by running shoe magazines and catalogs are not backed by scientific literature and are too subjective in their recommendations. The AAPSM suggests evaluating athletic shoes utilizing a basic three-point, hands-on assessment based on scientifically researched principles. Of particular importance is the flexibility of an athletic shoe in its distal third and the resistance of flexibility in the middle third. Shoes that meet these guidelines will, in general, work much better for athletic patients and can potentially minimize negative mechanical compensations in the feet and at higher levels of the kinetic chain.