In the 1970s, manufacturers began to produce different types of running shoes aimed at reducing mechanical overload or altering foot-ground interaction using motion control mechanisms, with a view to lowering the risk of sports-related lower extremity injuries (Lieberman et al., 2010). Running, currently one of the most popular sports, has a 20 to 79% lower extremity injury rate (van Gent et al., 2007), with one of the risk factors being rearfoot kinematics (Chang et al., 2014; Cheung et al., 2010; Morley et al., 2010; Rabbito et al., 2011; Willems et al., 2006). More specifically, over pronation has been associated with stress fractures, plantar fasciitis, and lower limb pain in runners, resulting from greater peak rearfoot eversion, increased excursion eversion and maximal eversion velocity (Chang et al., 2014; Morley et al., 2010; Rabbito et al., 2011; Willems et al., 2006), while over supination has been linked to leg injuries, resulting from increased leg stiffness and greater impact force (Williams et al., 2004).
In an effort to mitigate the rise of leg injuries, in the 1960s, shoe manufacturers started developing running shoes with rearfoot motion control that would supposedly alter the mechanics of the foot-ground interaction and neutralize excessive pronation or supination (Willy and Davis, 2014). However, the relationship between abnormal RM, injury incidence rates and choosing the right running shoes has yet to be elucidated (van Mechelen, 1992; Morley et al., 2010; Taunton et al., 2003).
The rearfoot angle (RFA) is a biomechanical variable widely used to determine the rearfoot motion, namely pronation, neutral or supination (Song et al., 1996). Specifically, pronation, which is the main focus of the current study, constitutes a complex combination of movements such as ankle dorsiflexion, forefoot abduction, and subtalar eversion. The movement of the subtalar joint at the rearfoot is deemed independent from the one at the forefoot (Perry and Lafortune, 1995; Stacoff et al., 1990). In a study that investigated foot kinematics during running using Principal Component Analysis, it was shown that different joints and regions of the foot should be assessed as separate variables to represent RM, as they were not inter-correlated (Behling et al. 2019). During running, supination occurs at initial contact in the stance phase and is immediately followed by pronation, which might absorb the impact forces. Without pronation, these forces would have to be absorbed suddenly and directly by the support structures, causing problems related to excessive stress. However, there has been a discussion that the medial peak occurs after the lateral peak and before maximum eversion (Morley et al, 2010), thus maybe the pronation is not capable of absorbing the impact forces, since the peak pronation occurs later than the impact (Behling et al. 2019). Finally, the rearfoot begins to supinate again and the foot becomes more rigid and stable (Dugan and Bhat, 2005).
As such, the study intended to answer three questions: (i) Do the shoes runners wear correspond to their respective barefoot rearfoot motion? (ii) Does the eversion angle change during shod running, regardless of the shoes worn? (iii) Can footwear designed for a specific rearfoot motion correct or neutralize the eversion angle of runners? The research hypotheses are that (i) runners choose footwear compatible with their rearfoot motion based on subjective perception, (ii) shoes alter the kinematics of the eversion angle in relation to barefoot running, and (iii) choosing footwear specially designed for certain types of rearfoot motion can neutralize excessive eversion.
Participants and study design
This was a cross-sectional study with 111 recreational runners (81 men and 30 women) aged 38.6 [+ or -] 9.7 years (74.9 [+ or -] 12.0kg, 1.74 [+ or -] 0.08 m), who ran an average of 3.4 [+ or -] 1.0 times a week and 31.8 [+ or -] 16.6 km a week, and could comfortably run at 10km/h on a treadmill ergometer. Inclusion criteria were no musculoskeletal injuries for at least 6 months prior to the tests, orthopedic leg surgery or degenerative conditions such as osteoarthrosis and chondromalacia (Runner's knee). The study protocol was approved by the local institutional Ethics committee granted full ethical approval (CAAE: 41171215.7.0000.0065). All participants were asked to read and sign a consent form.
Instruments and procedures
Running kinematics was assessed with participants running barefoot and shod on a treadmill ergometer (HPX 40, Total Health, Brazil) surrounded by 6 infrared cameras at 120Hz (Vicon Motion System Ltd., Oxford Metrics, UK). In order to minimize the variability of foot-ankle segmental motion in running, the subjects kept their self-selected speed during all the assessments (Queen et al., 2006). The self-selected speeds for each participant were the same speed in both conditions (mean speed 9.86km/h [95%CI: 9.75 to 9.96]). Barefoot data acquisition aimed to classify the natural rearfoot motion of the runner without the potential effects of shoes on foot mechanics (Altman and Davis, 2012). At the beginning of the study, all runners were randomly assigned to their first set of measurements--barefoot or shod condition. Both conditions were recorded in the same day.
A marker set consisting on eight reflexive-passive markers (14mm diameter) placed on both subject's foot and shoes (Cheung et al., 2007; 2011; Clermont et al., 2017; Kernozek et al., 1990; Kong et al., 2011; Kosonen et al., 2017; McClay and Manal, 1998; Morley et al., 2010), respectively for the barefoot and shod conditions. Four markers were placed on each lower extremity: one on the Achilles tendon between the malleoli (TG); one 15 cm above TG at the center of the leg (AG), immediately below the gastrocnemius muscle; a third and fourth markers on the upper (CP) and lower posterior (CD) surfaces of the calcaneus, respectively, when the subject was barefoot, or at the same height but on the shoe when they were shod (Reinschmidt et al., 1997) (Figure 1).
Each participant ran for 5 minutes on a treadmill at the self-selected speed for habituation, and the last minute was recorded for each condition (barefoot and shod) to analyze both feet. The footwear used in the shod condition was the habitual running shoes of the subjects, with fewer than 6 months use and less than 500 km run (Wang et al., 2010). The footwear worn included cushioned, motion control and stability shoes (Richards et al., 2009), produced by Adidas, Asics, Mizuno, Nike, Olympikus, Saucony among others (supplementary material 1). The running shoes were covered with strips of surgical tape to make the markers more easily visible.
Supplementary material 1. Type of shoes and manufactures. Manufacturer Specif Model Quantitative Adidas Adidas Durano 2 Adidas Boost 5 Adidas Adiprene+ 1 Asics GEL-Sendai 1 Asics Noosa 8 Asics GEL - Nimbus 14 Asics GEL - Kayano 15 Asics Asics GEL Kinsei 4 Asics GT 4 Asics GEL - Cumulus 3 Asics GEL - Pulse 3 Asics GEL - Quantum 1 Asics GEL - Kinetic 1 Brooks Brooks Glycerin 2 Merrel Merrell 1 Mizuno Mizuno Wave 13 Mizuno...