Well trained endurance athletes spend the majority (~75%) of their training time below the lactate threshold and a smaller (~15 - 20%) proportion of the time far in excess of lactate threshold (Seiler and Kjerland, 2006). Therefore, low intensity-high volume and high intensity-low volume training represents the majority of the training load for the endurance runner and subsequently, represents most (90 - 95%) of the exposure time which contributes to injury incidence. Given the repetitive nature of running, it is preferable to have running kinematics which can balance stress on biological tissue (e.g. muscles, tendons, bone) (Radin, 1986). Runners with a history of plantar fasciitis have greater vertical loading rates and impact peaks compared with healthy control participants during running (~3.7 m x [s.sup.-1]) (Pohl et al., 2009). Rearfoot strike, the predominate (~75%) foot strike pattern used by shod runners when running long distances (Hasegawa et al., 2007) is associated with a more extended lower limb and a more defined impact peak on contact with the surface. Wellenkotter et al. (2014) have reported an increase in cadence (reduction in stride length) to reduce loading to the plantar surface of the foot. Barefoot running appears to be associated with a sub-conscious reduction in stride length and an increase in knee flexion and ankle plantar flexion angles which is suggested to lower the impact peaks and loading rates experienced by the runner (Boyer and Derrick, 2015; De Wit et al., 2000; Derrick et al., 1998; Divert et al., 2005; Schubert et al., 2013). Compared to shod running at ~3.0 - 3.3 m x [s.sup.-1], inexperienced runners are reported to have a ~7 - 8% reduction in stride length when running barefoot overland (Thompson et al., 2014; Thompson et al., 2015), whilst runners with a long history of barefoot running (n = 8), three of whom had run a marathon barefoot, demonstrate a similar (~6.4%) reduction in stride length during barefoot running on a treadmill (Squadrone and Gallozzi, 2009). Whilst many studies have reported a reduction in stride length during barefoot running in comparison to shod, few have investigated well-trained competitive runners (Bonacci et al., 2013; McCallion et al., 2014). Furthermore, there is a need to determine whether differences in lower extremity kinematics and stride length between shod and barefoot conditions are affected by the speed of running. If as has been reported (Schubert et al., 2013), a reduction in stride length favourably alters biomechanical factors associated with running injury, it is important to determine the relative intensity of running where the greatest benefits may reside. This is particularly important in light of research which suggests that the vertical ground reaction forces experienced by the runner are greater during jogging, characterised by a higher centre of gravity, than high speed running characterised by a forward lean (Keller et al., 1996). Furthermore, James (1978) identified 65% of running injuries to occur in runners engaged in repeated low loading (high mileage) on a daily basis and Vleck and Garbutt (1998) who reported the number running injuries to occur in competitive triathletes to be associated with the total distance covered in a week's training. Therefore knowledge of kinematic and stride length changes at low and high speeds when running shod or barefoot may be of value for the runner. Finally, there is a need to investigate changes in lower extremity kinematics and stride length in well-trained competitive runners, with previous exposure to barefoot running but who are not yet chronically trained. This need arises from the fact that acute studies using a short duration of running may observe the period when the runner can tolerate the higher impact of barefoot running without adjusting shod kinematics (Divert et al., 2005). Previous exposure to barefoot running may act as a form of familiarisation and provide a better representation of the difference in lower extremity kinematics between shod and barefoot running in well trained endurance runners. The aim of this study was to compare stride length, hip, knee and ankle angles in well trained distance runners, running in shod and barefoot conditions at speeds which represent low (3.05 m x [s.sup.-1]) and high (4.72 m x [s.sup.-1]) intensity running.
Nine male (22 [+ or -] 3 y; 1.79 [+ or -] 0.04 m; 73 [+ or -] 9 kg) middle distance athletes who were members of the University of Limerick Athletics Club and competing (800m - 5,000m) at national varsity championship level participated in this study. Participants regularly participated in barefoot running as part of warm up or cool down routines but not during formal running sessions of low or high intensity. Participants had a mean 1500m personal best of 3 minutes 59.8 seconds [+ or -] 14.7 seconds and mean training volume of 100 [+ or -] 16km per week. Written informed consent was obtained from participants, and the study was approved by the University of Limerick research ethics committee and conducted according to the Declaration of Helsinki.
Participants ran for 5 minutes at a self-selected speed to warm up and familiarise themselves with the treadmill. Participants were then randomly assigned to undertake the barefoot or shod condition first in order to limit any potential order effects. Participants completed 2 minutes of running at 3.05 m x [s.sup.-1] followed by a 2 minute rest period before undertaking 2 minutes of running at 4.72 m x [s.sup.-1]. Following 2 minutes of stationary rest the same procedure was repeated for the remaining barefoot or shod condition. To standardise the shod condition all participants wore a neutral running shoe from a well-recognised...