The incidence of lower extremity injury in traditional running shoes (TRS) is as high as 79.3% (van Gent et al., 2007). Of these injuries, the knee and ankle are the most commonly injured (van Gent et al., 2007). Such injuries include patellofemoral pain syndrome (Clement et al., 1981; Taunton et al., 2002; Tiberio, 1987), Achilles tendinopathy (Clement et al., 1981; Smart et al., 1980; Taunton et al., 2002), and medial tibial stress syndrome (Clement et al., 1981; Taunton et al., 2002; Vtasalo and Kvist, 1983). In an effort to avert the injuries associated with TRS, many people have adopted the use of minimalist running shoes (MRS). MRS arguably simulate barefoot running by replicating barefoot biomechanics (Goss and Gross, 2012; Robbins and Hanna, 1987; Vormittag et al., 2009). Some studies, however, suggest that MRS still cause injuries, just different ones from those caused by TRS (Goss and Gross, 2012; Guiliani et al., 2011). Thus, the data are equivocal on whether MRS decreases injury risk or are "better" than TRS. Short of prospective studies examining injury rates between different shoes (Lieberman et al., 2010), one approach to this question is to identify biomechanical factors contributing to injury risk in order to avoid injury altogether.
The effect of footwear on biomechanics and injury rates/types is complicated by at least two factors. First, foot strike pattern may be related to injury incidence (Daoud et al., 2012; Goss and Gross, 2012) and foot strike may change with footwear (Bonacci et al., 2013; Lieberman et al., 2010; but see McCallion et al., 2014). If MRS cause increased forefoot strike, the increased plantarflexion would result in reduced instability of the ankle mortise (Wright et al., 2000), and predispose MRS runners to ankle sprain. Second, speed is another factor complicating comparisons between TRS and MRS (Goss and Gross, 2012; Lieberman et al., 2010). Increasing speed in TRS causes greater midfoot and forefoot strikes (Keller et al., 1996), but whether this same relationship is true when running in MRS or barefoot is untested. One recent MRS study has collected data at two categories of speed (e.g. fast and slow) (McCallion et al., 2014), but others have constrained analyses to a single speed (e.g. Cheung and Rainbow, 2014; Divert et al., 2008; Shih et al., 2013; Sinclair, 2014; Sinclair et al., 2013). Thus, how foot strike changes as speed increases is not well understood (Tam et al., 2014).
Speed affects knee and ankle biomechanics in traditional shoes (Arampatzis et al. 1999; Bishop et al. 2006; Lohman III et al. 2011). Prior works on kinematic differences between running footwear have been conducted at single speeds and the results vary depending on the speed utilized. For example, at 4.0 m x [sec.sup.-1], knee angles are not affected by footwear, whereas ankle angles are affected (Sinclair et al., 2013). In contrast, at 4.48 m x [sec.sup.-1], knee angles are affected by footwear (Bonacci et al., 2013). Foot strike style also may be implicated in such differences (Perl et al., 2012). Therefore, the relationships between knee and ankle joint kinematics and speed, foot strike style and speed in various footwear conditions, and the interaction between speed and footwear, deserve further investigation.
This study investigates the effects of speed on running biomechanics, foot strike and step length with various footwear conditions through several questions. 1) Does increasing speed affect foot strike pattern, lower limb joint kinematics, or relative step length? 2) Does changing footwear, but not footwear type, affect foot strike pattern, lower limb joint kinematics, or relative step length? 3) Does changing footwear type affect foot strike pattern, lower limb joint kinematics, or relative step length? 4) Do kinematics or relative step length differ among shod conditions within a foot strike style?
Twenty six recreational runners were recruited and completed this study. Recreational runners were the target group so the results will be applicable to the general population, rather than trained/elite athletes. In this way, the results are more clinically relevant to family practice physicians who may be asked to advise patients regarding running shoes. Recreational runners are defined as in Gehring et al. (1997), where recreational runners are those who train at a speed of slower than 3.33 m x [sec.sup.-1] and run at between 24 and 40 km x [week.sup.-1]. Thirteen subjects were male and thirteen were female. Subjects were healthy individuals who ran at least 30 minutes twice weekly and were naive to barefoot or minimalist running. Each subject completed a survey estimating average running speed and distance per week (Table 1). Typical running surface was recorded as either hard (concrete, sidewalk), medium (asphalt, road, track, treadmill), or soft (grass, gravel, trail). Finally, personal running shoe brand was recorded.
Kinematic data were collected during running on a Cybex[c] 770T-CT treadmill (Cybex International, Inc.; Medway, MA). Five reflective markers were placed on the left lower limb: 1) 3 cm proximal to the lateral femoral epicondyle,(It is typical to define the thigh segment using the greater trochanter, however, this landmark was often obscured by the treadmill in our study. Thus, we created a point along the line created by the greater trochanter and the lateral femoral epicondyle. This point was created to ensure the marker would not be obscured by the subjects ' chosen clothing or the arm of the treadmill.) 2) the lateral femoral epicondyle, 3) the lateral malleolus, 4) the calcaneal tuberosity (or the appropriate point on the shoe), and 5) the fifth metatarsal head (Figure 1). Data collection and analysis of the left limb, only, ensured independence of data for each stride. Video data were collected at 100 Hz using two digital video cameras (Basler A601f[R]; Basler AG, Ahrensburg, Germany), 70 degrees from each other, placed lateral to the treadmill. Digital cameras interfaced with a personal computer using Streampix[R] software (NorPix, Inc.; Montreal, Quebec, Canada). Video data were synchronized with an Innovision Systems, Inc. setup (Columbiaville, MI). The cameras were calibrated using a static, calibrated 8-point calibration cube (Hedrick 2008). Lower extremity reflectors provided data for video digitizing, kinematic analysis, and spatiotemporal calculations, completed with MaxTraq3D[R] and MaxMate[R] systems (Innovision systems, Inc.; Columbiaville, MI).
Prior to subject participation, the study was described to each subject along with potential risks and benefits of participation, after which informed consent was obtained as approved by the WVSOM Institutional Review Board, in accordance with the Belmont report. Subject age, height, body mass, and leg length, measured with calipers (Paleo-Tech linear spreading calipers; Paleo-Tech Concepts, Crystal Lake, IL), were recorded at the initial visit. The subject then completed the survey described above.
Shod conditions: Data were collected for a single, different footwear condition at each visit. Visits were on consecutive days to avoid fatigue. Footwear conditions included: 1) personal traditional running shoe (personal), 2) standardized traditional running shoe (Nike[c] Air [Pegasus.sup.+] 27; Nike, Inc., Beaverton, OR) (standard) to determine if results were due to changing footwear, 3) minimalist shoe (Ryan et al. 2014) (Vibram FiveFinger[c] KSO; Concord, MA) (minimalist), and 4) barefoot without shoe (barefoot). The initial visit was always in the personal shoe and the remaining shod condition visits were randomized to minimize effect of order. While there are a variety of choices for minimalist shoes, this particular model (Vibram FiveFinger[c]; Concord, MA) is arguably one of the closest to barefoot running. The KSO has a maximum sole thickness of
Data collection: At each...