Human bipedal running has been predominantly barefoot or in minimalist footwear for millions of years. Cushioned running shoes developed/appeared around the 1970s (Lieberman et al., 2010). Between 37 and 56% of runners suffer a musculoskeletal injury annually (van Mechelen, 1992), and despite shoe design advances, injury incidence has remained similar over the last 40 years (van Gent et al., 2007). These stable injury risks have influenced a trend back to shoes designed to mimic barefoot running with claims of improved performance and reduced injuries (Jenkins and Cauthon, 2011; Rothschild, 2012).Vibram Fivefingers (VFF) is a minimalist shoe that mimics barefoot running while providing a layer of protection. To date, VFF and Nike Free 3.0 are the only minimalist shoe designs that have undergone biomechanical evaluation (Bonacci et al., 2013; Squadrone and Gallozzi, 2009).
Athletes have recorded shorter step and stride lengths, and stride frequencies (De Wit et al., 2000; Divert et al., 2008; Kerrigan et al., 2009; Squadrone and Gallozzi, 2009) when running barefoot. In addition, mean contact time was reduced when running barefoot (Divert et al., 2005; Braunstein et al., 2010). However, Squadrone and Gallozzi (2009) found no differences in contact time between barefoot and shod running; and speculated that the degree of protection from the VFF minimalist shoes allowed athletes to push-off more vigorously, resulting in spatiotemporal variables being more aligned with shod running.
The gait cycle begins with contact, and occurs as a rear foot strike (RFS); a mid foot strike (MFS); or a fore foot strike (FFS), depending on which part of the foot contacts the ground first (Lieberman et al., 2010). A RFS occurs when the heel contacts the ground first, a MFS occurs when the ball of the foot and heel land simultaneously, and a FFS occurs when the ball of the foot lands before the heel (Lieberman et al., 2010).
Biomechanical research evaluating barefoot running has focused on ankle joint kinematics and, subsequently, cited foot-strike patterns (FSP) as possible reasons for observed differences (Bishop et al., 2006; Braunstein et al., 2010; De Wit et al., 2000; McNair and Marshall, 1994; Squadrone and Gallozzi, 2009). However, only one study has directly compared FSP between barefoot and shod running (Lieberman et al., 2010), and no study has examined FSP running in minimalist shoes.
Most analyses of barefoot and shod running have reported increased ankle plantarflexion at initial contact when barefoot which may be due to, or result in, changes in FSP (De Wit et al., 2000; Bishop et al., 2006; McNair and Marshall, 1994; Squadrone and Gallozzi, 2009). However, Lieberman et al. (2010) investigated habitually shod and habitually barefoot athletes and theorised that barefoot runners adopted a flatter foot placement at initial contact. De Wit (2000) reported that this flatter foot placement was brought about by significantly larger planterflexion and a significantly more vertical position of the shank at initial contact; the latter effect being brought about by increased knee flexion. In the study by Lieberman et al. (2010), habitually shod athletes mostly used a RFS pattern, with 100% of participants using a RFS when shod, and 83% adopting a RFS when barefoot. In contrast, athletes who habitually ran barefoot, mostly used a FFS pattern (75%) when barefoot, but changed to a RFS pattern when shod (50%). The majority of runners who grew up running barefoot, but subsequently switched to shod running, ran with a FFS pattern (91% barefoot vs. 51% shod). The authors hypothesised that differences in the FSPs were due to the cushioned shoes absorbing some of the "impact transient" and allowed a more comfortable RFS when running. However, statistical analysis of these data was not reported.
Hasegawa et al. (2007) documented that conventional running shoes facilitated a RFS. When examining the type of FSP adopted by shod runners at the 15 km point of a half-marathon, most athletes (74.9%) adopted a RFS pattern. But, more of the faster runners used a MFS pattern (36% of the top 50 runners vs. 19% for the last 50 runners; significance not reported). However, as running velocity was not controlled in this scenario, one can only speculate whether velocity or individual differences in running style contributed to this finding.
This study compared acute spatiotemporal variable changes in stride length and frequency, ground contact and flight times; when running barefoot, and in minimalist and conventional running shoes. In addition, kinematic determinations of FSP for each of the three conditions were made and the effects of condition and running velocity on FSPs were assessed.
This study used a repeated measures design to investigate differences in FSP and spatiotemporal variables during sub-maximal running in three different conditions, barefoot (BF), minimalist shoes (Vibram FiveFingers, VFF) and shod (participant's own running shoes); and at two individualised velocities. The velocities (V1 and V2) were nominated percentages (70 and 85%) of experienced runners' best race times from 5-km competition, the group mean velocities equated to 13.0 [+ or -] 1.0 and 16.1 [+ or -] 1.3 km x [h.sup.-1], for V1 and V2, respectively.
Fourteen (n = 14) competitive, habitually shod male athletes (age 25 [+ or -] 6 yr; height 1.78 [+ or -] 0.06 m; mass 67.6 [+ or -] 5.8 kg, competitive running experience 8 [+ or -] 3 yr) were enlisted as participants. Participants were trained middle- (n = 8) or long- (n =6) distance runners, running at least 30 to 50 km x [week.sup.-1], aged between 18-35 yr and free from any lower limb injuries in the six months prior to study commencement. Ethical approval was obtained from the Faculty of Health Science Research Ethics Committee, Trinity College Dublin.
A Cartesian Optoelectronic Dynamic Anthropometric CX-1 (CODA) motion analysis system (Codamotion, Charnwood Dynamics, Rothley, UK) was used to capture real-time 3 dimensional (3-D) joint kinematics at 100Hz. Miniature infra-red light-emitting diodes (LEDs), each identifiable to indicate location, were placed on specific anatomical landmarks. Signals from the infra-red LEDs were picked up by two Codamotion sensor units. Two separate CX-1 measurement units were placed equidistant (3.5 m) and orthogonally to the left and right of the centre point of a motorised treadmill. Masked linear arrays in each sensor unit combined to measure the X, Y and Z coordinates of each infra-red LED. A Proform 700 ZLT treadmill (Icon Health and Fitness, Utah, USA) was positioned with the left posterior leg placed on an embedded AMTI force...