Journal of Applied Biomechanics, 2014, 30, 521-528 http://dx.doi.org/10.1123/jab.2012-0224 © 2014 Human Kinetics, Inc.

An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH

Midsole Thickness Affects Running Patterns in Habitual Rearfoot Strikers During a Sustained Run Trampas M. TenBroek,1,2 Pedro A. Rodrigues,1,2 Edward C. Frederick,3 and Joseph Hamill2 1New

Balance Sports Research Laboratory; 2University of Massachusetts-Amherst; 3Exeter Research, Inc.

The purpose of this study was to: (1) investigate how kinematic patterns are adjusted while running in footwear with THIN, MEDIUM, and THICK midsole thicknesses and (2) determine if these patterns are adjusted over time during a sustained run in footwear of different thicknesses. Ten male heel-toe runners performed treadmill runs in specially constructed footwear (THIN, MEDIUM, and THICK midsoles) on separate days. Standard lower extremity kinematics and acceleration at the tibia and head were captured. Time epochs were created using data from every 5 minutes of the run. Repeated-measures ANOVA was used (P < .05) to determine differences across footwear and time. At touchdown, kinematics were similar for the THIN and MEDIUM conditions distal to the knee, whereas only the THIN condition was isolated above the knee. No runners displayed midfoot or forefoot strike patterns in any condition. Peak accelerations were slightly increased with THIN and MEDIUM footwear as was eversion, as well as tibial and thigh internal rotation. It appears that participants may have been anticipating, very early in their run, a suitable kinematic pattern based on both the length of the run and the footwear condition. Keywords: shoes, barefoot, foot strike kinematics, acceleration, shock attenuation Minimal footwear can be defined as a shoe with a thin, flexible midsole and outsole and a light, basic upper with little or no heel counter. These shoes are typically built with minimal shock attenuating material and do not provide any significant impact protection. However, the amount of underfoot material can vary substantially between minimal footwear models. While the popularity of minimal footwear has increased, published research on how this type of footwear can affect running biomechanics is limited and oftentimes contradictory. Squadrone and Gallozzi1 found that impact forces were reduced with a minimal shoe compared with typical training footwear (TTF), with differences likely a result of kinematic alterations such as significantly greater plantar flexion at touchdown (TD), a shortened stride length, and an increased stride frequency. Conversely, other research has shown impact peaks to be consistent between minimal footwear and TTF, observing change only when a runner was barefoot.2 Hamill et al2 did not find that rearfoot strikers adopted a midfoot or forefoot strike pattern when running in minimal footwear. For rearfoot strikers, because there was a lack of shock attenuating material, there would be no reason to expect that impact characteristics would be reduced if no gross foot strike changes were observed. When comparing the kinematics of running barefoot to running in TTF, it has been shown that at TD the barefoot runner’s foot is in a more horizontal position relative to the ground (frontal Trampas M. TenBroek and Pedro A. Rodrigues are with the New Balance Sports Research Laboratory at New Balance Athletic Shoe, Inc., Lawrence, MA, and the Department of Kinesiology at the University of Massachusetts-Amherst, Amherst, MA. Edward C. Frederick is with Exeter Research, Inc., in Brentwood, NH. Joseph Hamill is with the Department of Kinesiology at the University of Massachusetts-Amherst, Amherst, MA. Address author correspondence to Trampas M. TenBroek at trampas. [email protected].

and sagittal planes).3,4 This could be an adaptation to manage local pressures underfoot and/or impact forces. In the sagittal plane, this change in foot position appears to be due to a more plantar flexed ankle joint complex (AJC) and a more flexed knee.4,5 However, despite the knee being more flexed at TD when barefoot, the leg is stiffer because the knee undergoes less flexion through midstance, as does the hip.4,6,7 In addition, while peak eversion is generally unchanged, there has been a lack of consistency in tibial internal rotation findings.8–10 At terminal stance as a runner pushes off, most differences between barefoot and shod conditions are no longer present.4 Finally, stride lengths are generally smaller when barefoot, resulting in greater stride frequencies for a given velocity.4,11,12 Lieberman et al13 reported that a forefoot strike converts a portion of translational energy to rotational energy at the ankle joint, which could reduce the tibial acceleration and effective mass compared with a rearfoot strike pattern. TenBroek et al3 found an increase in tibial acceleration with no foot strike change for minimal footwear compared with TTF. This increased acceleration could be due to: (1) a reduction in effective mass, which would make the leg easier to accelerate and reduce impact characteristics;14 or (2) a reduction in shock attenuating material, causing the increase in acceleration of the tibia. Valiant15 reported alterations made in the frontal plane at the AJC could also reduce effective mass through a more inverted AJC at TD. The methodologies used in many footwear-related studies may not accurately portray the responses made by runners while adapting to new footwear. For example, many experiments use runways to gather kinematic and kinetic data, which limits the number of consecutive steps taken in each footwear condition. Divert et al11 believed this might allow runners to maintain large impact characteristics, which may not be representative of what would occur during longer runs. In addition, the short runs used in many research studies may not capture the longer term adaptations made by a runner during more realistic training runs.1,4,6–8,11,13,16 Additional research 521

522  TenBroek et al.

on longer, more ecological runs would be a beneficial supplement to all the excellent work done previously. Therefore, the purpose of this study was to: (1) investigate how kinematic patterns are adjusted while running in footwear with THIN, MEDIUM, and THICK midsole thicknesses; and (2) determine if these patterns are adjusted over time during a sustained run in footwear of different thicknesses. It was hypothesized that runners would use impact and pressure modulating behaviors at the AJC and the knee when wearing the thinnest, most minimal footwear. These behaviors were expected to be consistent with previous literature and include greater plantar flexion, less inversion of the AJC, and greater knee flexion at TD, as well as greater eversion at midstance. Second, it was hypothesized that when wearing more minimal footwear, these adaptations would become more pronounced as the run progressed and the number of impacts accumulated.

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Methodology Participants Data from the literature were used to estimate sample size for a minimum statistical power of 80% with an alpha level of .05.4 Sagittal plane dependent variables used in the power analysis included AJC angle, foot segment angle, lower leg angle, and knee angle at TD. Therefore, for this study, 10 injury free, recreational male runners with treadmill running experience aged 18 to 55 years were recruited. All participants used a rearfoot footfall pattern and were comfortable running for 30 minutes without serious fatigue. Each participant gave their informed consent, which was approved by the University of Massachusetts Institutional Review Board.

Experimental Set-up Three pairs of shoes were constructed with the same lightweight upper, pliable heel counter and slabs of ethylene-vinyl acetate (EVA) midsole with an average hardness of 61 Asker C (Table 1). The hardness of the EVA foams was measured using an Asker C type durometer in accordance with the methods described in JIS Standard K7312.17 The primary feature of the footwear of interest

for this study was the midsole thickness. One shoe had a typical thickness often used in TTF (THICK); one simulated a very minimal, barefoot inspired shoe (THIN); and one fell between the previous two midsole thicknesses (MEDIUM). On the bottom of the footwear, the lateral heel and the medial forefoot had a single basic layer of rubber outsole material attached. Rearfoot shock attenuating capacity was evaluated using a peak g score obtained with a gravity driven impact tester (Exeter Research, Inc., Brentwood, New Hampshire) using ASTM F1614-99.2006 (procedure A)18 and ASTM F1976-06.19 These three midsole thicknesses created additional differences between the footwear conditions which included mass, heel to forefoot thickness difference, and different midsole flares. The THIN shoe had no heel flare, but the MEDIUM and the THICK shoes did. To create more ecological footwear conditions, these additional differences were not controlled. Participants performed all runs on a motorized Woodway treadmill (Woodway, Waukesha, Wisconsin) at 3.0 m/s for 30 minutes in each of the three footwear conditions following a standard treadmill warm up in their own footwear. The pace was chosen to accommodate all participants and to minimize possible fatigue effects. For each participant, data collections were accomplished at least 1 day after the previous collection to ensure sufficient rest from fatigue and impact. The three footwear conditions were administered in a balanced design to minimize order effects. A key aspect of this study was to evaluate a novice’s response over time to minimal footwear; therefore, participants received little information regarding each footwear condition and were not allowed to walk or run in any footwear condition before the test started. Running kinematics were obtained at 200 Hz using an eightcamera motion capture system (Oqus 500; Qualisys AB, Gothenburg, Sweden) and acceleration signals were captured at 1000 Hz using accelerometers (Delsys Incorporated, Boston, Massachusetts). The orientation of the segmental coordinate systems were defined using retro-reflective markers attached to the greater trochanters, the medial and lateral knee joint, medial and lateral malleolus, and first and fifth metatarsal heads. Tracking markers were attached via rigid shells to the heel of the footwear, the lower leg, and the thigh. One accelerometer (±11g max) was attached to the left distal, anteromedial tibia and another (±3.6g max) was attached to the anterior aspect of the forehead. The accelerometers were securely

Table 1  Footwear condition characteristics Footwear Condition THIN

MEDIUM

THICK (TTF)

Forefoot thickness (mm)

3

9

12

Rearfoot thickness (mm)

3

14

24

Heel-forefoot difference (mm)

0

5

12

Midsole width (mm) Mass (kg) Rearfoot impact score (g)

66

75

82

0.164

0.200

0.237

40.1 (0.20)

16.8 (0.30)

14.3 (0.20)

Abbreviation: TTF, typical training footwear. Note. Impact results were taken from an average of 3 tests.

Midsole Thickness Affects Running Kinematics  523

attached to the participant’s tolerance using two-sided tape and athletic prewrap.

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Data Processing Individual marker trajectories were filtered using a dual pass, second order low-pass Butterworth filter with a cut-off frequency of 12 Hz.20 From kinematic data, local right hand coordinate systems and segment end-points were derived for lower extremity segments. Segment and joint angles were calculated using an Xyz Cardan rotation sequence.21 For kinematic data, TD was determined using maximum forward position of the heel markers while for toe-off, knee extension maximum was used.22 Touchdown was defined to be four frames after these maxima through visual inspection. Angles were calculated for the foot, lower leg, and thigh segments, as well as the AJC and the knee. Raw acceleration data were low pass filtered using a dual pass, second order low-pass Butterworth filter with a cut-off frequency of 50 Hz.23 Touchdown and toe-off were determined in acceleration signals through visual inspection using recurring spikes in the tibial acceleration plots. For each stance phase, the mean and linear trends were removed from the acceleration signals.24 Power spectral densities (PSD) were calculated on each stance phase’s acceleration signal using a Fourier transformation. The ratio of PSD for the head to PSD for the tibia was calculated for each frequency within the range of 0 Hz to 20 Hz. Ratios were averaged across these frequencies to describe shock attenuation, with greater ratios indicating more impact shock attenuation.24–26 To investigate the effect of time on running patterns, time epochs were created using data from every 5 minutes of the treadmill run. The initial time epoch (time epoch 1) included the first 10 steps once the treadmill was up to speed. The remaining time epochs were created using 10 steps at the beginning of every 5-minute epoch on the treadmill. A repeated-measures ANOVA was used to determine statistical differences (P < .05) for footwear condition and time. Dependent variables consisted of three-dimensional segment and joint angles at key instances during the support phase, as well as peak accelerations and impact attenuation. When differences were found between conditions, a Tukey multiple comparison test was employed to determine the locus of the differences. Effect sizes (ES) were calculated between the largest and smallest means across footwear conditions to interpret the estimated biological relevance of the differences.27,28 Effect sizes at 0.2 were considered small, those at 0.5 were considered medium, and those more than 0.8 were considered large.

Results No significant footwear condition × time interactions were present study-wide for any dependent variables (P > .05). Thus, all time epochs were averaged when comparing footwear conditions and all footwear conditions were averaged when investigating time epochs. There was a significant main effect for footwear conditions for several dependent variables (Table 2). In addition, there was also a significant main effect of time for seven dependent variables (Figure 1). Footwear had a significant effect on both the lower extremity 3D kinematics at landing and later in stance (Table 2). At TD, the AJC was more plantar flexed (P < .001, ES = 0.42) in the THIN and MEDIUM conditions. This joint angle was a result of a more horizontal foot segment at landing (P < .001, ES = 0.37). At TD the knee was also found to be more extended (P = .038, ES =

0.15) in the THIN condition due to a change in the thigh segment orientation (P < .001, ES = 0.36). No statistical differences were found in the frontal plane position of the AJC at TD (P = .38, ES = 0.16). Although the knee was in a similar position at TD for both the MEDIUM and THICK conditions, at midstance the knee was more flexed in the THICK condition (P < .001, ES = 0.36). As a result, the THICK condition resulted in greater knee excursion than both the THIN and MEDIUM conditions (P < .001, ES = 0.38). In the frontal plane, eversion variables tended to be greater with less underfoot material (ie, THIN and MEDIUM conditions). Transverse motion of the lower leg and thigh exhibited similar behavior, as there was more internal rotation with the THIN and MEDIUM footwear. Finally, stance times were similar for the THIN and MEDIUM conditions but greater in the THICK condition (P < .001, ES = 0.20). Acceleration peaks showed consistent changes as footwear became thinner (Table 3). Peak acceleration values at the head and leg were greater as underfoot material was reduced (P < .001, ES = 0.36 and P = .007, ES = 0.28). The transfer function describing impact shock attenuation failed to exhibit statistical differences (P = .22, ES = 0.11). Statistical differences in select kinematic variables were also found across time epochs for seven dependent variables (Figure 1), but no such differences were found for any acceleration variables. Figure 1 shows all of the dependent variables that were generally increasing or decreasing as the run progressed. The AJC’s sagittal joint angle at TD, the peak eversion angle, and eversion excursion were all different during the initial time epoch compared with the rest of the time epochs. The AJC also was more everted but not statistically different at the last time epoch compared with time epoch 2 (P = .052). The sagittal plane position of the knee and thigh at TD resulted in significant changes throughout the run as the knee became more flexed and the thigh became less vertical. Maximum knee flexion and stance time also exhibited this behavior. There was also a trend for foot eversion to continue to increase although statistical significance was not quite reached (P = .055).

Discussion The purpose of this study was to: (1) investigate how kinematic patterns are adjusted while running in footwear with THIN, MEDIUM, and THICK midsole thicknesses; and (2) determine if these patterns are adjusted over time during a sustained run in footwear of different thicknesses. The first hypothesis was partially supported as there was greater AJC plantar flexion and eversion for the THIN and MEDIUM conditions. However, the second hypothesis was rejected, as runners adjusted their running pattern in the same way throughout the 30-minute run regardless of the footwear condition. While not all hypotheses were accepted, using the same footwear conditions, TenBroek et al3 reported some similar results for a 6-minute run. These authors also found the AJC to be more plantar flexed at TD and show greater eversion excursion when wearing THIN footwear over a 6-minute run. In addition, no differences were observed in the frontal plane position of the AJC between the footwear conditions in either study. It was also hypothesized that with a longer run, the alterations made due to thinner midsole footwear would be exacerbated. However, this was not the case, as runners made the same kinematic adjustments over time regardless of footwear condition. At TD, distal segment and joint positions were similar for the THIN and MEDIUM conditions (sagittal foot angle, sagittal AJC angle, and nearly frontal AJC angle). However, at the more proximal joints and segments (knee and thigh), the MEDIUM condition

524  TenBroek et al.

Table 2  Kinematic means (SD), probability values, and effect size Footwear Condition THIN

MEDIUM

THICK

P Value

Effect Size

Sagittal knee at TD (+ ext)

–9.5b (5.5)

–10.2a (5.5)

–10.3a (5.5)

.038

0.15

Sagittal thigh at TD (+ fle)

18.7b (3.1)

19.3a (3.1)

19.8a (3.1)

< .001

0.36

Sagittal leg at TD (+ ext)

9.3 (4.1)

9.2 (4.1)

9.5 (4.1)

.12

0.07

Sagittal AJC at TD (+ dorsi)

8.7b (3.1)

9.1b (3.1)

10.0a (3.1)

< .001

0.42

Sagittal foot at TD (+ dorsi)

18.8b (4.3)

19.1b (4.3)

20.4a (4.3)

< .001

0.37

Frontal AJC at TD (+ evers)

–5.7 (2.5)

–6.0 (2.5)

–6.1 (2.5)

.38

0.16

Frontal leg at TD (+ abd)

–3.8 (1.3)

–3.8 (1.4)

–3.6 (1.3)

.056

0.15

Frontal foot at TD (+ evers)

–8.7 (2.1)

–8.9 (2.1)

–8.6 (2.1)

.58

0.14

Peak knee flexion (+ ext)

–37.2c (6.4)

–38.0b (6.4)

–39.5a (6.4)

< .001

0.36

Sagittal knee excursion

27.7b (4.0)

27.8b (4.0)

29.2a (4.0)

< .001

0.38

Peak AJC eversion

9.1b (2.6)

9.2b (2.6)

8.4a (2.6)

.007

0.31

14.8a,b (2.8)

15.2a (2.8)

14.6b (2.8)

.04

0.21

Peak foot eversion

2.2a (1.7)

1.8b (1.7)

1.5b (1.7)

.001

0.41

Peak thigh internal rotation

–4.3a (5.7)

–4.0a (5.4)

–1.4b (5.7)

< .001

0.51

Landing Kinematics

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Midstance Kinematics

AJC eversion excursion

Thigh internal rotation excursion

2.0 (2.0)

2.6 (2.0)

2.0 (2.0)

.146

0.30

Peak tibial internal rotation

–4.0a (5.2)

–3.5a,b (5.2)

–2.8b (5.2)

.01

0.23

Tibial internal rotation excursion

10.5a (2.8)

9.4b (2.8)

9.3b (2.8)

< .0001

0.43

0.267b (0.020)

0.268b (0.020)

0.271a (0.020)

< .001

0.20

Stance Time

Abbreviations: SD, standard deviation; TD, touchdown; ext, extension; fle, flexion; dorsi, dorsiflexion; evers, eversion; abd, abduction. Note. Probability values determined from ANOVA averaged across all time epochs. Effect size from across the largest and smallest mean (Cohen’s d). Superscript letters denote statistically homogenous groups within row statement used. For example, any condition with an a superscript is not statistically different than any other with an a, but was statistically different than any without an a. All angles are in units of degrees (°) and time in units of seconds (s).

matched the THICK condition. This finding may indicate that once a certain level of thickness and/or shock attenuation is reached, any adaptations can occur distally and not require proximal compensations, even over the course of a 30-minute run. This would point, however, to a unique coordination strategy for moderately minimal footwear (MEDIUM) where the distal portion of the lower extremity behaves similarly to wearing very THIN footwear, while the proximal portion behaves as though TTF is being worn. Greater tibial accelerations were matched by greater head accelerations for more minimal footwear conditions. The results at the tibia are consistent with the findings of TenBroek et al,3 as well as Hardin and Hamill.25,29–31 Greater tibial accelerations were expected, but no differences were anticipated at the head.32 This finding could be a result of all participants maintaining a rearfoot strike pattern even with the most minimal condition in this experiment. Furthermore, when a rearfoot strike pattern is maintained, interpreting the increased acceleration in the absence of ground reaction force data is difficult. This is because an increase in acceleration at the tibia could be a result of: (1) a reduction in effective mass and correspondingly reduced impact force characteristics; or (2) a reduction in shock attenuating material and therefore increased impact force characteristics. The increase in acceleration also seen at the head might indicate that increased impact force characteristics was more likely, implying any kinematic alterations made were insufficient to protect the head from increased

peak accelerations. While the THIN footwear group peak acceleration increased, contextually this increase was smaller than what can be expected when running velocity is increased.24 Kinematic variables later in stance were also affected by the manipulation of midsole thickness. In the sagittal plane, the knee was found to have greater peak flexion and excursion with more substantial footwear in lieu of already being more flexed at landing. In the frontal and transverse planes we found greater AJC eversion, tibial internal rotation, and thigh internal rotation with more minimal footwear. There is evidence to suggest that eversion and tibial internal rotation are coupled33,34 with eversion, driving tibial internal rotation. In contrast, Bellchamber and van den Bogert35 suggested that tibial internal rotation could be greatly influenced by proximal segments. While this study was not designed to investigate this difference, it does highlight the need for individuals who are sensitive to increases in these motions to use caution when running in thinner midsole footwear. Although participants adjusted running patterns throughout the 30-minute run regardless of the footwear condition, there were main effects present for footwear condition. These results suggest that very early in their runs, the participants may have anticipated a suitable kinematic pattern based on both the length of run and the footwear condition. The running patterns in the MEDIUM condition matched the THIN condition for several dependent variables

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Figure 1 — Plots of mean values for dependent variables with significant main effects of time where individual footwear condition plots are also shown. Statistical differences only apply to mean values averaged across all footwear conditions. Letters denote statistically homogenous groups within plot statement used. The acronym TD refers to touchdown or landing.

Table 3  Acceleration data mean values (SD), probability values, and effect size Footwear Condition THIN

MEDIUM

THICK

P value

Effect Size

Peak head acceleration

1.36a (0.31)

1.29b (0.31)

1.25b (0.31)

< .001

0.36

Peak leg acceleration

6.04a (1.10)

5.85a,b (1.10)

5.73b (1.10)

.007

0.28

Transfer function

–9.19 (2.67)

–9.44 (2.67)

–9.49 (2.67)

.22

0.11

Acceleration Measures

Note. Probability values determined from ANOVA averaged across all time epochs. Effect size from across the largest and smallest mean (Cohen’s d). Superscript letters denote statistically homogenous groups. Peak acceleration values are in units of gravity (g), while transfer function data are units of decibels (dB).

525

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526  TenBroek et al.

during the run. This was not the case in a similar study utilizing 6-minute runs.3 The 6-minute run columns illustrate that in TenBroek et al3 the THIN condition was statistically equivalent to the MEDIUM condition for only one dependent variable showing a main effect (Figure 2). Conversely, in that same study the MEDIUM and THICK conditions were statistically equivalent for six of the dependent variables which showed a main effect. Five dependent variables that resulted in no difference between the MEDIUM and THICK conditions for runs of 6 minutes behaved quite differently for runs of 30 minutes. In the 30-minute runs in the current study, the MEDIUM condition was statistically different than the THICK condition and often statistically equivalent to the THIN condition. Participants may have realized very early in the run that the appropriate pattern for the MEDIUM condition in a run of this length resembled the solution for the THIN condition. The results of this study have implications for minimal (ie, THIN midsole) footwear consumers and manufacturers. Both should be aware that runners may maintain a rearfoot strike pattern even with a run up to 30 minutes in duration on a relatively firm surface while wearing footwear that provides very little shock attenuation. Eversion as well as tibial and thigh internal rotation all increased when wearing thinner midsole footwear; thus, runners sensitive to these motions should be cautious. In addition, we found kinematic adaptations in the MEDIUM midsole thickness moving toward those used in the THIN midsole condition while running for 30 minutes. Therefore, it does seem runners may be able to anticipate an appropriate kinematic solution for a given run length based on information obtained early in that run. However, much more

research is necessary to confirm this notion. Particularly, a distance run at a faster running speed could result in different interpretations of the kinematic behaviors of runners. Finally, footwear companies should (as many have) continue to alert customers that thinner midsole footwear can require very different kinematic patterns and that runners should gradually increase mileage to avoid potential tissue overload. This study had several limitations. First, there were differences between the footwear conditions other than midsole thickness that we chose not to control (eg, mass, midsole width, heel-forefoot difference). We felt that many of these differences are necessary to accurately portray the characteristics of footwear of these types, but they certainly limit our ability to determine differences in running patterns only as a result of midsole thickness. Second, utilizing a treadmill was ideal for data collection, but may not elicit the same response an overground run would produce. As mentioned, our participants did not switch to a midfoot or forefoot landing pattern even in the most minimal footwear. The relatively slow running speed may have contributed to this phenomenon to some extent. In addition, although the treadmill used is stiffer than most other treadmills, it still lacks the stiffness of a road or sidewalk, which could also influence the choice of strike pattern. Furthermore, most of the adaptations revealed were relatively small and often less than 2°, making the determination of practical meaningfulness difficult. Third, as stated in TenBroek et al,3 we believe experience with barefoot or minimal running, amount of continuous running required in the study, stiffness of the running substrate, potential for surface obstacles, and subject knowledge contribute to a lack of consistency in research reporting foot strike pattern. These fac-

Figure 2 — Statistically equivalent groups between footwear conditions for kinematic variables that were measured in both this experiment and in TenBroek et al3 for 6-minute runs.

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Midsole Thickness Affects Running Kinematics  527

tors need to be considered and work needs to be done to minimize their effects. Last, locating the maximum forward position of heel marker then delaying 20 milliseconds to locate TD likely created a virtual TD for kinematic data, which may have occurred before the actual TD for some and slightly later for others. De Witt and colleagues4 found overwhelming differences in kinematics between running barefoot and running with TTF to be present at TD and 30 milliseconds before TD. That being said, the differences in many dependent variables were small and the potential for timing-related issues were present and must be considered. In summary, when running for 30 minutes, several kinematic adjustments are made when running in footwear with different midsole thicknesses. Kinematic measures at TD were similar for the THIN and MEDIUM conditions distal to the knee. The THIN condition was different from the MEDIUM and THICK conditions at and above the knee. Even the THIN midsole condition did not require runners to alter foot strike enough to develop a midfoot or forefoot strike pattern. Therefore, peak accelerations were slightly increased at the tibia and head with the thinner midsole footwear. The thinner midsole conditions also produced greater eversion as well as tibial and thigh internal rotation. Participants may have been anticipating, very early in their run, a suitable kinematic pattern based on both the length of the run and the footwear condition.

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