Runners who CRFS or CRFS when both barefoot and shod have been studied previously. 5, 9, 14, 16 and 17 However, almost half of the runners in the current study shifted their running style between an RFS when shod and an FFS when barefoot. 12 Within a given group or footwear condition, an increase in speed increases both stride length and stride frequency (Table 1).11, 15 and 20 Barefoot runners generally run with shorter stride lengths and higher stride frequencies and

are more likely to FFS than shod runners (Fig. 3).3, 11, 15, 16 and 20 Shorter stride lengths attenuate the shock wave caused by the heel strike at initial contact2 and may also reduce decelerations that occur within running strides, due to more vertical ground reaction forces.26 Interestingly, CFFS and CRFS runners

used similar stride lengths and frequencies at a given speed and footwear condition (Fig. 3). Shod shifters, however, Protein Tyrosine Kinase inhibitor use longer PI3K inhibitor stride lengths and higher stride frequencies and duty cycles than all other groups (Fig. 3). Runners who change their running style also modulate their stride length and stride frequency. Notably, runners with consistent styles, whether FFS or RFS, have stride lengths, stride frequencies and duty cycles similar to each other across groups. The similar stride lengths, frequencies, and duty cycles between CFFS and CRFS runners may relate more to training level than foot strike pattern. Sometimes, training shortens stride lengths27 and 28 but elite first training lengthens stride lengths.29 In our subjects, even though the level of training and mileage did not differ between the three groups, these effects may mask differences in stride length. When training level is controlled (e.g., in the shifters), FFS barefoot runners shorten their stride lengths compared to the RFS shod runners (Fig. 3).11, 16 and 20 Only the shod shifters ran with longer stride lengths (Fig. 3). These shod runners may be using the cushioning of the shoe to attenuate the

increased shock experienced through the leg and decrease energy absorption when running with longer stride lengths.2 Shorter strides during FFS running also correlate with more vertical landing angles (less overstride; Fig. 4).3 FFS runners land with their shank more vertical (2°) compared to RFS runners (8°). Barefoot RFS runners also land with a more vertical shank compared to shod RFS runners.16 This vertical landing angle in FFS runners likely functions to minimize ankle moments.2 FFS runners slightly plantarflex (−12.5°) their ankle joints at impact, RFS runners slightly dorsiflex their ankle joints (1.2°; Fig. 4 and Fig. 5),3, 11, 12, 13, 14 and 19 while shifters alter their kinematics to correspond to the two styles of running. Typically, running involves a quick plantarflexion at heel contact before dorsiflexion,2, 4, 16, 26, 30, 31 and 32 but only for RFS runners (Fig. 4).