While most sports medicine specialists rec­ognize the need for high quality athletic equip­ment (footwear included), it should be noted that defects in running shoes (i.e. crooked heel counters, loosely glued midsoles, under-inflated shock absorbing pockets etc.) are not unusual. These defects have been overlooked by the gen­eral population and have the potential to cause an injury, or aggravate an already existing injury.

Shoe design and wear patterns are routine­ly examined by clinicians to ensure that proper support is being provided for the athlete’s foot. A natural extension of this routine procedure is to check the quality of the shoe’s construction for any possible defects which may relate to the patient’s musculoskeletal condition.

This paper will describe how running shoes with manufacturing defects or excessive mileage can contribute to, or be potentially responsible for, a variety of musculoskeletal complaints. We will also describe how running shoe design can influence the prevention and treatment of lower limb overuse running injuries(1). In order to prevent recurring injury or further injury, recommendations will be made regarding how to check existing shoes as well as new shoes, for defects prior to pur­chase.

Typical Runners’ Injuries
Running shoes are usually selected to pro­vide support, and counteract biomechanical deformities or deficiencies in the foot. Despite this, injuries such as shin splints, patellar ten­donitis, and iliotibial band friction syndrome commonly plague runners. The shoe itself may often be the cause of the runner’s problem. For instance, during the stance phase, a shoe that tilts medially due to uneven wear will have a ten­dency to cause the foot to pronate excessively. Conversely, if a shoe tilts laterally, it may pre­vent pronation and prolong supination. This may lead to stress fractures in the foot or leg as well as anterior knee pain.

In order to demonstrate how defective shoe construction can cause running injuries, the patient’s running mechanics, lower limb musculoskeletal alignment, and shoe design and construction must be evaluated.

Biomechanics of Running

The gait cycle during running consists of a stance phase and a swing phase. The stance phase constitutes 60% of the gait cycle. Running is distinguished from walking by the flight phase: the period when both feet are off the ground. During running, the lower limbs absorbs 1.6 to 2.3 times the body weight as speed increases from an 8:56 minute mile to a 5:22 minute mile(2). Cavanagh, and coworkers, found that as running speed increases, peak forces of 2.5 to 3 times body weight are generat­ed at heel strike(3). During a marathon, the body experiences over 25,000 heel strike impacts(4). This amounts to a tremendous load on the lower limbs. As a result most, if not all, running injuries occur during the stance phase(5).

The stance phase consists of heel strike, mid-stance, and push off. At heel strike the foot initially contacts the ground in a supinated posi­tion. As the foot continues to make contact with the ground during mid-stance, it pronates to absorb shock; minimizing ground reaction forces, The flattening of the foot that occurs during pronation consists of subtalar joint aversion, forefoot abduction, and talocrural dorsi­flexion(6). This allows the foot to adapt to the ground’s contour and become a mobile adapter. During running, each foot goes through these motions about 600 times per mile. When these motions are excessive, a torsional force is creat­ed which stretches the plamar fascia, resulting in inflammation and pain; the syndrome known as plantar fascitis.
 
A Typical Case
Plantar fascitis is characterized by inflam­mation or degeneration of the plantar fascia. particularly at the calcaneal attachment(7). It has been mostly attributed to anatomical or bio­mechanical abnormalities such as excessive pronation of the subtalar joint beyond the nor­mal range of approximately 9.4 degrees(8). It has also been attributed to training error: rea­soning that is well supported by mans’ related studies(9).

Other (anatomical) causes of abnormal pronation include congenital pes planus, acquired deformities, and abnormalities secondary to neuromuscular disease(10). Frequently, excessive pronation is associated with ankle joint equinus, most commonly caused by limited flexibility of the triceps surae. resulting in a shortened Achilles tendon(11). The cavus foot, which actually has a tight plantar fascia, conversely has a tendency toward exces­sive supination.

Shoe defects are now proving to be an unexpected new cause for this common condi­tion: one that cannot be overlooked. Relating the effects of various types of shoes to plantar fascitis, Gross, and others(12,13), have indicat­ed that musculoskeletal pathologies caused by external factors (e.g., an overpronator wearing a shoe designed for shock absorption rather than motion control), can also be exacerbated by lower limb malalignments or biomechanical imbalances. This conclusion is supported by clinical observations of changes in the patients symptoms with interventions such as training modifications, corrections in running form or style, use of foot orthoses, or replacement of shoes.

Stacoff, and colleagues(14), investigated relationships between peak impact, pronation. and forces at the subtalar joint, and on muscles (under tension during pronation) at heel strike in the rear foot during running. Stacoff concluded that shoe design should concentrate more on controlling rear foot movement, and less on shock attenuation.

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