Medical specialists routinely treat running injuries and are familiar with the etiology of their pathologies. They regularly assess running technique, musculoskeletal alignment, and shoe wear when evaluating an injured runner. However, we have noticed that further inspection of the running shoes revealed an alarming finding. We have found an increasing incidence of manufacturing defects that correlate directly as causative factors-in patients’ injuries. These findings demonstrate a need for clinicians to become aware of the possibility that the patient’s shoes may be an underlying cause of injury, in conjunction with other more typically recognized biomechanical malalignment issues.
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
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.
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.