Introduction
Humans have walked and run barefoot for millions of years.1 Indirect evidence suggests that footwear emerged as recently as ~30 000 years ago.2 The majority of time since then, humans have worn minimalist footwear designed to protect the sole of the foot.3 The first indication of fashionable footwear beginning to alter the shape of the foot emerged a little over 100 years ago4 and the invention of the mass market cushioned running shoe is as recent as ~1970.5 Therefore, large changes in footwear have occurred in a very short space of time relative to human evolutionary history. When rapid changes in physical or nutritional environments occur, it is described as a mismatch between biological evolution (occurring slowly over millions of years) and cultural evolution (occurring rapidly due to events such as the industrial or technological revolution). This mismatch has been proposed as a hypothesis, mainly by evolutionary biologists, to explain some of the current burden of chronic disease.6 To provide an example, diseases which did not previously exist or were rare such as type 2 diabetes,7 osteoporosis8 and plantar fasciitis,9 are thought to be contributed to by rapid changes in energy availability, physical activity and modern footwear use.
Using an evolutionary lens to understand the current prevalence and incidence of running injuries has sparked great debate among scientists.10 11 Unlike other primates, the human foot has an arch capable of both stiffness and deformation12 to varying degrees dependent on the terrain. It can store and release energy via springs (ligaments, aponeurosis, tendons) in order to make efficient use of muscle work during running.6 13 14 In fact, it is for this reason that the Achilles tendon is ~10 times longer in humans compared with other primates.6 Coordination between the spring system and the muscular system during running is thought to be optimised via using a mid to forefoot strike which allows the foot to stretch and recoil as the muscle moves from eccentric to concentric work.13 15 On this basis, it is suggested that humans likely ran with a mid to forefoot strike prior to the invention of running shoes.1 An estimated ~75% of runners (recreational,>5 km) now use a rearfoot strike (RFS)16 and some researchers suggest that this increases the risk of certain types of stress pathology (ie, plantar fasciitis, medial tibial stress syndrome) in some runners.17–19 The loading rate or instantaneous loading impulse on ground contact may be greater during a RFS which does not allow for a gradual deceleration,19 as would be the case with a mid to forefoot strike.20
Although, that is the essence of the debate, the aetiology of running injury is multifactorial.19 21 The incidence of running injury is high relative to many team sports22 and other aerobic exercises23 but there are many runners who do not get injured, including those that use a RFS. In fact, most world championship marathon runners use a RFS.24 Even habitually barefoot modern hunter-gatherers who farm more than hunt and therefore, do not require running as part of activities of daily living, predominately use a RFS up to speeds of ~5.0 m/s.25 A tendency to RFS is also seen in habitually barefoot children,26 perhaps due to lower body mass; women who do not run27 and in runners running on soft surfaces.28 It is clear that body mass, maturation, sex, running experience, speed and surface influence foot strike patterns in addition to footwear. That being said, we speculate that adult runners, regularly required to run at moderate speeds, without shoes and on firm surfaces would likely have used a mid to forefoot strike pattern prior to the invention of shoes.1 29 This strike pattern would allow runners to capitalise on innate impact moderating mechanisms (safety) and to maximise running speed for a given energy cost (efficiency).30 31
The link between footwear, foot strike patterns and running injury may be better discussed in the context of certain types of runners and or certain types of injuries. For example, given that taller and heavier runners have a higher risk of injury21 and women with a history of plantar fasciitis19 experience greater loading rates, footwear and foot strike may become more important in certain groups of runners. The aim of this review is to discuss how footwear alters human innate impact moderating mechanisms and how that may contribute to running injury. Towards this aim, the paper is divided into three sections:
The innate capacity of humans to moderate impact during dynamic activities.
The impact of footwear on innate impact moderating behaviour and injury risk.
The use of barefoot training to reduce the incidence of running injury.