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The effects of CrossFit and minimalist footwear on Achilles tendon kinetics during running

by Jonathan Sinclair1, and Benjamin Sant1pdflrg

The Foot and Ankle Online Journal 9 (4): 2

The aim of the current investigation was to comparatively assess the influence of barefoot, CrossFit, minimalist and conventional footwear on the loads experienced by the Achilles tendon during running. Twelve male runners (27.81 ± 7.02 years, height 1.77 ± 0.11 cm and body mass 76.22 ± 7.04 kg) ran at 4.0 m·s-1 in each of the four footwear conditions. Achilles tendon forces were calculated using a musculoskeletal modelling approach allowing the magnitudinal and temporal aspects of the Achilles tendon force to be quantified. Differences between footwear were examined using one-way repeated measures ANOVA. The results showed the peak Achilles tendon force was significantly larger when running barefoot (5.81 ± 1.21) and in minimalist footwear (5.64 ± 1.03 BW) compared to conventional footwear (5.15 ± 1.05 BW). In addition it was revealed that Achilles tendon impulse was significantly larger when running barefoot (0.77 ± 0.22 BW.s) and in minimalist footwear (0.72 ± 0.16 BW.s) in comparison to both conventional footwear (0.64 ± 0.15 BW.s). Given the proposed association between high Achilles tendon forces and tendon degradation, the outcomes from the current investigation indicate that CrossFit athletes who select barefoot and minimalist footwear for their running training may be at increased risk from Achilles tendon pathology in comparison to conventional footwear conditions.

Keywords: Footwear, Achilles tendon, running, CrossFit

ISSN 1941-6806
doi: 10.3827/faoj.2016.0904.0002

1 – Centre for Applied Sport and Exercise Sciences, School of Sport and Wellbeing, College of Health & Wellbeing, University of Central Lancashire, Lancashire, UK.
* – Corresponding author: jksinclair@uclan.ac.uk


CrossFit represents a relatively new activity associated with aerobic exercises, calisthenics, and Olympic weightlifting [1]. CrossFit as a discipline has expanded to become an international sport which has been linked to significant gains in aerobic and anaerobic fitness [1]. Given the novelty of CrossFit in relation to more established sports it has received a paucity of published attention in the sports science and strength and conditioning literature.

A key feature of CrossFit training is aerobic conditioning and the manner in which this is examined during competition is via distance running events. Engagement in distance running mediates numerous physiological benefits but it is known to be associated with a high rate of chronic pathologies, with around 70 % of runners experiencing an injury injured during the course of a year [2,3]. Shorten proposes that athletic footwear with suitable mechanical features may be able to manage the incidence of chronic running related injuries [4].

CrossFit athletes are able to select from a wide range of different footwear conditions with distinct design characteristics. There has been no peer reviewed research which has examined the biomechanical influence of different footwear available to CrossFit athletes. CrossFit specific footwear represents a hybrid footwear designed to incorporate the stability characteristics of a weightlifting shoe with the cushioning and flexibility of a running trainer. Currently, there is a trend for CrossFit athletes to opt to train and compete either barefoot or minimalist footwear in lieu of traditional footwear options, although the efficacy of barefoot and minimalist footwear is not yet fully established.

The effects of different footwear on the loads experienced by the Achilles tendon have been examined previously. Sinclair examined the effect running barefoot had on minimalist and conventional footwear on Achilles tendon kinetics during the stance phase of running [5]. The findings showed that peak Achilles tendon kinetics were significantly larger when running barefoot and in minimalist footwear. Similarly Sinclair et al, examined the effects of minimalist, maximalist and conventional footwear on the loads borne by the Achilles tendon during running[6] . Their findings confirmed that peak Achilles tendon force and Achilles tendon impulse were significantly larger in minimalist footwear in relation to the conventional and maximalist conditions. Currently there are no published scientific investigations regarding the effects of barefoot, CrossFit, minimalist and conventional footwear on the loads experienced by the Achilles tendon.    

Therefore the aim of the current study was to comparatively examine the influence of barefoot, CrossFit, minimalist and conventional footwear on the loads experienced by the Achilles tendon during the stance phase of running. Given that running activities are associated with a high incidence of chronic Achilles tendon pathologies, the current investigation may deliver key information to CrossFit athletes concerning the selection of suitable footwear.

Methods

Participants

Thirteen male participants took part in this investigation. All uninjured at the time of data collection and written informed consent was obtained. The mean and standard deviation (SD) characteristics of the participants were: age 27.81 ± 7.02 years, height 1.77 ± 0.11 cm and body mass 76.22 ± 7.04 kg. The research design utilized for this investigation was approved by the University of Central Lancashire, Science, Technology, Engineering and Mathematics, ethical committee. 

Procedure

Participants ran at 4.0 m·s-1 (±5%), while striking an embedded piezoelectric force platform (Kistler, Kistler Instruments Ltd., Alton, Hampshire) which sampled at 1000 Hz. Participants struck the platform with their right foot which was used for analysis. Running velocity was monitored using infrared timing gates (Newtest, Oy Koulukatu, Finland). The stance phase was delineated as the duration over which 20 N or greater of vertical force was applied to the force platform. Runners completed five trials in each footwear condition. The order that participants ran in each footwear condition was randomized. Kinematics and ground reaction forces data were synchronously collected. Kinematic data was captured at 250 Hz via an eight camera motion analysis system (Qualisys Medical AB, Goteburg, Sweden). Dynamic calibration of the motion capture system was performed before each data collection session.

Lower extremity segments were modelled in 6 degrees of freedom using the calibrated anatomical systems technique [7]. To define the segment coordinate axes of the foot and shank, retroreflective markers were placed unilaterally onto the 1st metatarsal, 5th metatarsal, calcaneus, medial and lateral malleoli, medial and lateral epicondyles of the femur. A carbon fiber tracking cluster was positioned onto the shank segment and the foot was tracked using the 1st metatarsal, 5th metatarsal and calcaneus markers. The center of the ankle joint was delineated as the midpoint between the malleoli markers[8] . Static calibration trials were obtained allowing for the anatomical markers to be referenced in relation to the tracking markers/ clusters. The Z (transverse) axis was oriented vertically from the distal segment end to the proximal segment end. The Y (coronal) axis was oriented in the segment from posterior to anterior. Finally, the X (sagittal) axis orientation was determined using the right hand rule and was oriented from medial to lateral.

Processing

Dynamic trials were digitized using Qualisys Track Manager in order to identify anatomical and tracking markers then exported as C3D files to Visual 3D (C-Motion, Germantown, MD, USA). Ground reaction force and marker trajectories were smoothed using cut-off frequencies of 50 and 12 Hz using a low-pass Butterworth 4th order zero lag filter. All data were normalized to 100% of the stance phase then processed trials were averaged. Joint kinetics were computed using Newton-Euler inverse-dynamics. To quantify net joint moments anthropometric data, ground reaction forces and angular kinematics were used.  

Achilles tendon force (BW) was determined using a musculoskeletal modelling approach. This model has been used previously to resolve differences in Achilles tendon force between different footwear [5, 6]. Achilles tendon force was quantified as a function of the plantarflexion moment (PFM) divided by the Achilles tendon moment arm (MA). The moment arm was quantified as a function of the ankle sagittal plane angle (ak) using the procedure described by Self and Paine [9]:

Achilles tendon force = PFM / MA

MA = -0.5910 + 0.08297 ak – 0.0002606 ak2

Average Achilles tendon load rate was quantified as the Achilles tendon force divided by the time over which the peak force occurred. Instantaneous Achilles tendon load rate was also determined as the peak increase in Achilles tendon force between adjacent data points. In addition to this Achilles tendon force, impulse  was quantified during running by multiplying the Achilles tendon force estimated during the stance phase by the stance time.

Experimental footwear

The footwear used during this study consisted of conventional footwear (New Balance 1260 v2), minimalist (Vibram five-fingers, ELX) and CrossFit (Reebok CrossFit CR) footwear, (shoe size 8–10 in UK men’s sizes).

Analyses

Means and standard deviations were calculated for all footwear conditions. Differences in Achilles tendon parameters between footwear were examined using one-way repeated measures ANOVAs, with significance accepted at the P≤0.05 level. Effect sizes were calculated using partial eta2 (pη2). Post-hoc pairwise comparisons were conducted on all significant main effects. The data was screened for normality using a Shapiro-Wilk which confirmed that the normality assumption was met. All statistical actions were conducted using SPSS v22.0 (SPSS Inc., Chicago, USA).

Results

Table 1 and Figure 1 present the Achilles tendon loads during the stance phase of running, as a function of the different experimental footwear. The results indicate that the experimental footwear significantly influenced Achilles tendon force parameters.

Barefoot CrossFit Conventional Minimalist
Mean SD Mean SD Mean SD Mean SD
Peak Achilles tendon force (BW) 5.81 1.21 5.50 1.32 5.15 1.05 5.64 1.03
Time to peak Achilles tendon force (s) 0.13 0.02 0.14 0.02 0.15 0.02 0.14 0.02
Achilles tendon average load rate (BW/s) 45.54 12.76 42.37 14.44 35.76 10.49 40.84 9.07
Achilles tendon instantaneous load rate (BW/s) 128.84 42.10 153.23 51.56 115.45 40.08 136.21 25.93
Achilles tendon impulse (BW.s) 0.77 0.22 0.69 0.20 0.64 0.15 0.72 0.16

Table 1 Achilles tendon forces as a function of footwear.

fig1

Figure 1 Achilles tendon forces during the stance phase as a function of footwear (black = barefoot, dash = minimalist, grey = conventional, grey dot = CrossFit).

A main effect (P<0.05, pη2 = 0.21) was shown for the magnitude of peak Achilles tendon load. Post-hoc pairwise comparisons showed that peak Achilles tendon force was significantly larger in the barefoot (P=0.01) and minimalist (P=0.04) conditions in relation to conventional footwear. A main effect (P<0.05, pη2 = 0.43) was shown for the time to peak Achilles tendon load. Post-hoc pairwise comparisons showed that time to peak Achilles tendon force was significantly larger in the barefoot (P=0.001) and minimalist (P=0.007) conditions in relation to conventional footwear. In addition time to peak Achilles tendon force was significantly shorter in the barefoot condition (P=0.007) in relation to the CrossFit footwear.  In addition a main effect (P<0.05, pη2 = 0.29) was evident for average Achilles tendon load rate. Post-hoc analysis showed that average load rate was significantly larger in the barefoot (P=0.004), CrossFit (P=0.04) and minimalist (P=0.02) conditions in relation to the conventional footwear. A main effect (P<0.05, pη2 = 0.25) was found for instantaneous Achilles tendon load rate. Post-hoc pairwise comparisons showed that instantaneous Achilles tendon load rate was significantly larger in the barefoot (P=0.01), CrossFit (P=0.003) and minimalist (P=0.01) conditions in relation to the conventional footwear. Finally, a main effect (P<0.05, pη2 = 0.34) was observed for Achilles tendon impulse. Post-hoc analyses indicated that Achilles tendon impulse was significantly larger in the barefoot (P=0.007) and minimalist (P=0.04) conditions in relation to conventional footwear.

Discussion

The aim of the current investigation was to comparatively examine the influence of barefoot, CrossFit, minimalist and conventional footwear on the loads experienced by the Achilles tendon during running. To the authors knowledge the current study represents the first comparative examination of Achilles tendon kinetics when running in these specific footwear conditions.

The key observation from the current study is that Achilles tendon force parameters were significantly larger in the barefoot and minimalist conditions in relation to the conventional running shoes. This observation is in agreement with those of Sinclair [5] and Sinclair et al. [6] who similarly noted that barefoot and minimalist footwear conditions were associated with significant increases in Achilles tendon kinetics in relation to more substantial running footwear. This observation may provide important clinical information with regards to the etiology of Achilles tendon pathologies as a function of running activities in CrossFit athletes. The initiation and progression of Achilles tendonitis is mediated by excessive tendon loads that are applied without sufficient cessation between activities [10]. Mechanisms of tendon loading that are above the systematic threshold for collagen related synthesis lead ultimately to degradation of the collagen network as the rate of resynthesis is not able to keep pace with the rate of breakdown [11]. Therefore the findings from the current investigation indicate that running barefoot and in minimalist footwear may place CrossFit athletes performing running activities at a greater risk from Achilles tendon pathology.

The specific findings from the current study may be explained by taking into account the effects of running barefoot and in minimalist footwear on the sagittal plane mechanics of the ankle joint. When running barefoot and wearing minimalist footwear, runners adopt a more plantarflexed foot position throughout the stance phase in relation to more structured running shoes [12, 13].  Increased ankle plantarflexion serves to reduce the length of moment arm of the Achilles tendon [9]. If the moment arm is shortened, this mediates an increase in the load which must be borne by the tendon when running barefoot and in minimalist footwear.

Research which has examined the influence of different footwear condition on the loads borne by the Achilles tendon during the stance phase of running, habitually examines only the peak forces experienced per footfall. Because running barefoot and in minimalist footwear mediates alterations in stance times and step frequencies, the time integral of loads experienced by the Achilles tendon are not quantified. The current study addresses this by quantifying the impulse experienced by the Achilles tendon during the stance phase which is a reflection of both the load experienced and the time interval of the load. The findings from the current investigation in relation to the Achilles tendon impulse mirror those of Sinclair et al., in that barefoot and minimalist footwear were associated with significantly larger impulse in relation to conventional running shoes [6]. This therefore further supports the notion outlines above that running barefoot and in minimalist footwear may increase the likelihood of experiencing an Achilles tendon injury compared to conventional running shoes.   

In conclusion, although differences in Achilles tendon loading as a function of different footwear conditions has been examined previously, the current knowledge with regards to the effects of minimalist, barefoot, CrossFit and conventional footwear on Achilles tendon forces is limited. As such the present study therefore adds to the current knowledge by providing a comprehensive evaluation of Achilles tendon load parameters when running in minimalist, barefoot, CrossFit and conventional footwear. On the basis Achilles tendon load and impulse parameters were shown to be significantly greater when running barefoot and in minimalist footwear, the outcomes from the current investigation indicate that CrossFit athletes who select barefoot and minimalist footwear for their running training may be at increased risk from Achilles tendon pathology in comparison to conventional footwear conditions.

References

  1. Weisenthal, B. M., Beck, C. A., Maloney, M. D., DeHaven, K. E., & Giordano, B. D. (2014). Injury rate and patterns among CrossFit athletes. Orthopaedic Journal of Sports Medicine (In press).
  2. Taunton, JE, Ryan, MB, Clement, DB, McKenzie, DC, Lloyd-Smith, DR, Zumbo, BD. A retrospective case-control analysis of 2002 running injuries. Br J Sp Med. 2002; 36: 95-101. doi: 10.1136/bjsm.36.2.95  
  3. van Gent, R, Siem DD, van Middelkoop M, van Os TA, Bierma-Zeinstra SS, Koes, BB. Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. British Journal of Sports Medicine 2007: 41: 469-480. http://www.ncbi.nlm.nih.gov/pubmed/17473005
  4. Shorten, MA. Running shoe design: protection and performance. pp 159-169 in Marathon Medicine (Ed. D. Tunstall Pedoe). 2000; London, Royal Society of Medicine.
  5. Sinclair J. Effects of barefoot and barefoot inspired footwear on knee and ankle loading during running. Clinical Biomechanics 2014; 29: 395-399. http://www.ncbi.nlm.nih.gov/pubmed/24636307
  6. Sinclair, J., Richards, J., & Shore, H. (2015). Effects of minimalist and maximalist footwear on Achilles tendon load in recreational runners. Comparative Exercise Physiology, 11(4), 239-244.
  7. Cappozzo A, Catani F, Leardini A, Benedeti MG, Della CU. Position and orientation in space of bones during movement: Anatomical frame definition and determination. Clinical Biomechanics 1995; 10: 171-178. http://www.ncbi.nlm.nih.gov/pubmed/11415549
  8. Graydon, R, Fewtrell, D, Atkins, S, Sinclair, J. The test-retest reliability of different ankle joint center location techniques. Foot Ankle Online J. 2015; 8: 1-11. doi: 10.3827/faoj.2015.0801.0011
  9. Self, BP, Paine, D. Ankle biomechanics during four landing techniques. Medicine & Science in Sports & Exercise 2001; 33: 1338–1344.
  10. Selvanetti, ACM, Puddu, G. Overuse tendon injuries: basic science and classification. Operative Techniques in Sports Medicine 1997; 5: 110–17. http://www.sciencedirect.com/science/article/pii/S1060187297800317
  11. Kirkendall, DT, Garrett W.E. Function and biomechanics of tendons. Scandinavian. Journal of Medicine & Science in Sports 1997; 7: 62–66. http://www.ncbi.nlm.nih.gov/pubmed/9211605
  12. Lieberman, D.E., Venkadesan, M., Werbel, W.A., Daoud, A.I., D’Andrea, S., Davis, I.S., & Pitsiladis, Y. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature, 463, 531-535.
  13. Sinclair, J., Greenhalgh, A., Brooks, D., Edmundson, C. J., & Hobbs, S. J. (2013). The influence of barefoot and barefoot-inspired footwear on the kinetics and kinematics of running in comparison to conventional running shoes. Footwear Science, 5, 45-53.

A comparison of center of pressure variables recorded during running in barefoot, minimalist footwear, and traditional running shoes in the female population

by Andrew Greenhalgh1PhD, Jenny Hampson2Bsc, Peter Thain2 PhD, and Jonathan Sinclair3PhDpdflrg

The Foot and Ankle Online Journal 7 (3): 6

In recent years, barefoot running and running in minimalist footwear as opposed to running in traditional running shoes has increased in popularity. The influence of such footwear choices on center of pressure (COP) displacements and velocity variables linked to injuries is yet to be understood. The aim of this study was to investigate differences between COP variables, linked to injuries measured in barefoot running, a minimalist running shoe, and with traditional running shoes and conditions during running in a healthy female population. Seventeen healthy female participants were examined. Participants performed five footfalls in each footwear condition while running at 12km/h±10% over a pressure plate while COP variables were recorded at 500Hz. The results suggest that minimalist running shoe COP characteristics were similar to those of barefoot runners, with various significant differences reported in both groups compared to runners with the traditional running shoe.

Keywords: footwear, barefoot, running, COP, center of pressure, plantar pressure

ISSN 1941-6806
doi: 10.3827/faoj.2014.0703.0006

Address correspondence to: Andrew Greenhalgh PhD
Email: a.greenhalgh@mdx.ac.uk

1 London Sport Institute, Middlesex University, Hendon, UK
2 School of Life & Medical Sciences, University of Hertfordshire, Hatfield, UK
3 Division of Sport Exercise and Nutritional Sciences, University of Central Lancashire, Preston, UK


Following the introduction of running specific footwear, in recent years barefoot (BF) running as opposed to running in traditional running shoes (TRS) with elevated cushioned heels has increased in popularity among participants and coaches [1]. When running barefoot on roads or pathways the plantar region of the foot may be exposed to cuts and general discomfort from debris and uneven surfaces, therefore running in minimalist footwear that may allow for the change in running kinetics and kinematics observed in barefoot running compared to shod while protecting the plantar region of the feet from injury and discomfort appears to be desirable.

This has led to a rise in the popularity of barefoot inspired footwear amongst running populations and subsequent research [2]. Running barefoot does not appear to restrict athletes from competing at an elite level, with competitors winning Olympic medals in such conditions. In terms of energy cost to the runner, running barefoot appears to reducing angular inertia of the lower extremities. Research suggests minimalist shoes may also decrease oxygen consumption during running [3,4]. However, recent research suggests there is no reduction of metabolic cost when running barefoot compared to lightweight running shoes [5].

Some research suggests that wearing traditional running shoes may restrict freedom of movement and flexibility that can be achieved in comparison to barefoot running [6]. Furthermore, running barefoot compared to shod has been identified as causing adaptation in running mechanics, resulting in a more midfoot footfall in contrast to a favored heel striking movement strategy while running in traditional running shoes [2,7]. Research also suggests that such adaptations occur instantaneously with only minor changes in the lower extremity kinematics  observed in the reported knee angle after two weeks of training in minimalist footwear [8].  Such adaptations observed in barefoot running have been proposed as a mechanism by which the potentially detrimental loading imposed upon the musculoskeletal system during running may be attenuated [9–11]. Conflicting research has however reported such increases in loading of the musculoskeletal system in barefoot running compared to shod, in participants who habitually wore shoes [12,13]. Furthermore, foot injuries including stress fractures most prominently in the metatarsals have been reported in minimalist shoe runners [14]. Currently there appears to be a lack of evidence confirming the influence of barefoot running on movement strategy and injury rates [15,16].

Research identifying the influence of footwear conditions should initially focus on areas of greatest injury risk within the musculoskeletal system which research suggests is ankle ligament damage [17]. The ankle joint is unique in that the vast majority of injuries sustained across different populations are of one type;  ligament sprains [17–21]. It is worth noting that such injury rates in females [22] are higher than those of males [23].

The reason for the higher occurrence of ankle sprains while running can only be hypothesized.  Research has suggested that during running the ankle is often placed in a compromised supinated position when the athlete’s center of gravity (COG) is positioned over the lateral border of the weight bearing limb [24,25]. It has been identified that the functionally unstable ankle may be the result of proprioceptive neuromuscular deficits arising from structural damage following an injury [26–29].

Various kinetic and kinematic variables have been investigated to compare differences between barefoot and shod conditions.  However there is a paucity of research investigating the differences in center of pressure (COP) variables between the conditions [16]. Plantar COP velocities and displacements measured during running have been identified as indicators of exercise induced lower leg injuries [30,31]. As such, identifying characteristics of the COP have been identified as suitable reference points for studying the dynamics of the rearfoot and foot function [31,32] and to identify differences in footwear conditions [33]. Studies analyzing the gait of those individuals with functional unstable ankles have identified a tendency for a laterally situated COP on initial foot contact with a greater pressure concentration at the lateral aspect of the heel [26,30].  If the COP is focused to the lateral side of the calcaneus during heel strike, it is possible that the additional force required to place the individual into a compromised position may be minimal [30]. As a result, by examining the location of the COP upon initial contact it may be possible to identify running conditions that could potentially reduce the likelihood of sustaining a lateral ankle sprain by avoiding the COP displacements seen in the unstable ankles.

A commercially available design of minimalist design footwear (huaraches (HU)) have been developed (Figure 1) with minimum cushioning (4mm tread) and string uppers designed to minimally restrict natural foot movement. By comparing COP variables in participants running barefoot and wearing the HU footwear it may be possible to see the different foot mechanics in each. Therefore the aim of this study was to investigate the differences between COP variables, many of which are linked to ankle ligament injuries, measured in barefoot, huaraches and traditional foot wear runners (Figure 1). The differences in kinetics and kinematics measured between genders [19,34–37] demonstrates a need for studies investigating kinetics of locomotion to consider each gender separately and as such this research will focus on conditions during running in a healthy female population.

Methods

Selection and Description of Participants

Seventeen healthy female participants were examined (aged 21.2±2.3 years, height 165.4±5.6 cm, body mass 66.9±9.5 kg, foot size 6.8±1.0 UK). All participants were free from musculoskeletal pathology and provided written informed consent in accordance with the declaration of Helsinki.

Fig1

Figure 1 HU footwear (above) and TRS (below).

Technical Information

Participants were given time to practice running in the minimalist footwear until they felt comfortable, no prior training was undertaken [8]. Participants performed five footfalls in each footwear condition at a controlled speed of 12km/h±10% over a Footscan pressure plate (RsScan International, 1m x 0.4m, 8192 sensors) (Figure 1) collecting COP data at 250Hz positioned in the center of a 28.5m runway. Participants practiced running along the runway and adjusted their starting position to achieve a natural footstrike on the pressure mat to minimize any influence of targeting [38]. They were also instructed to look at a point on the far wall and not slow down until passing the second timing gate.

Various times (Initial Metatarsal contact (IMC), initial forefoot flat contact (IFFC, first instant all the metatarsals heads are in ground contact) and heel off (HO)) during foot to ground contact were identified (Fig.2), anterior-posterior and medial-lateral displacement and velocity data were calculated at these time points [30,39]. COP displacement and velocity values were normalized to a percentage of foot width and length as appropriate and using the same methods as in previous research [30,39]. This method of collecting COP progression data in direct foot contact and under the shoe has been confirmed as reasonable [40,41].

Statistics

Descriptive statistics including means and standard deviations were calculated for each COP variable in each condition. One way repeated measures ANOVAs were used to determine the differences between footwear conditions with significance accepted at the p<0.05 level. The Shapiro-Wilk statistic for each condition confirmed that the data were normally distributed and where the sphericity assumption was not met, correctional adjustment was made using Greenhouse-Geisser. Effect sizes were calculated using an Eta2 2). Post-hoc analyses were conducted using a Bonferroni correction to control type I error (Table 1). All statistical procedures were conducted using SPSS 19.0 (SPSS Inc., Chicago, IL, USA).

Results

The COP data collected was observed for each trial and various key points in time during the stance phase were identified (Figure 2)

Fig2

Figure 2 Typical BF plantar pressure.

The means were calculated for the COP timing (Table 1), COP medial-lateral (Table 2) and COP anterior-posterior (Table 3) variables.

Time variables

Analysis of the timing variables reported between the footwear conditions is displayed in Table 1 and indicated a significant main effect for the timing of IMC (F(1.41, 22.55)= 57.29, p<0.0005,  η2=0.782) and IFFC (F(2, 32)= 43.69, p<0.001,  η2=0.732) no significant effect was reported for HO (F(1.30, 20.87)= 2.56, p=0.118,  η2=0.138). Post hoc analysis revealed significant differences (p<0.001) between the TRS and both the BF and HU conditions for timing of IMC, This was also the case for the IFFC event timing which additionally reported a significant difference (p=0.04) between the BF and HU conditions.

Tab1

Table 1 Means and standard deviations of center of pressure variables timing variables.=Significantly different (p<0.05) from BF, ¥=significantly different (p<0.05) from HU, *=significantly different (p<0.05) from TRS.

Medial Lateral COP Variables

Analysis of the movement of the COP in the Medial Lateral plane of the foot between footwear conditions are displayed in Table 2 and report that a significant main effects for the position of the COP in terms of medial lateral position (X-comp) were identified at IMC X-comp (F(1.454, 23.268= 5.87, p=0.014,  η2=0.269), IFFC X-comp (F(2, 32)= 18.9, p<0.001,  η2=0.542) and HO X-comp (F(2, 32)= 15.6, p<0.001,  η2=0.494).) No significant main effect was identified for IFCX-comp (F (2, 32) = 3.161, p=0.056, η2=0.165). Post hoc analysis revealed a significant difference for IMC X-comp (p=0.025), IFFC X-comp (p=0.001) and HO X-comp (P=0.003) between BF and TRS conditions, and a significant difference between IFFC X-comp (p<0.001) and HO X-comp (p<0.001) between HU and TRS conditions.

Significant main effects for the position of the medial lateral velocity of the COP in terms of position (VEL-X) were identified for HO VEL-X (F (2, 32) = 32.6, p<0.001, η2=0.671). Post hoc analysis revealed a significant difference for HO VEL-X between BF and TRS (p<0.001) and HU and TRS (p<0.001).  No significant main effect was identified for IMC VEL-X (F (1.46, 23.31= 1.314, p=0.279, η2=0.076) or IFFC VEL-X (F (1.33, 21.24) = 2.073, p=0.161, η2=0.115).

Tab2

Table 2 Means and standard deviations of center of medial-lateral pressure variables.=Significantly different (P<0.05) from BF, ¥=significantly different (p<0.05) from HU, *=significantly different (p<0.05) from TRS, FW%=Percentage of foot width.

Anterior Posterior COP Variables

Analysis of the movement of the COP in the Anterior Posterior plane of the foot between footwear conditions are displayed in Table 2 and report that a significant main effects for the position of the COP in terms of anterior posterior position (Y-comp) were identified at IFCY-comp (F (2, 32) = 5.04, p<0.013, η2=0.239) and HO Y-comp (F (1.09, 17.39) = 30.71, p<0.001, η2=0.657). No significant main effect was identified for IMC Y-comp (F (1.42, 22.66) = 3.28, p=0.07,  η2=0.170) or IFFC Y-comp (F(1.22, 19.58)= 0.88, p=0.38,  η2=0.052). Post hoc analysis revealed a significant difference for HO Y-comp (p<0.001) and IFC Y-comp (p=0.025) between BF and TRS, a significant difference was also identified for HO Y-comp between HU and TRS conditions (p<0.001).

Significant main effects for the position of the anterior posterior velocity of the COP in terms of position (VEL-Y) were identified for IMC VEL-Y (F(1.41, 22.58)= 13.60, p<0.0005 η2=0.460)  and HO VEL-Y (F(1.17, 18.77)= 13.26, p=0.001,  η2=0.453) No significant main effect was identified for IFFC VEL-Y (F(1.21, 19.33)= 1.710, p=0.209,  η2=0.097). Post hoc analysis revealed a significant difference between BF and TRS for IMC VEL-Y (p=0.005) and HO VEL-Y (p=0.001), significant differences were also identified between HU and TRS for IMC VEL-Y (p=0.002) and HO VEL-Y (P=0.011).

Tab3

Table 3 Means and standard deviations of anterior-posterior center of pressure variables.=significantly different (p<0.05) from BF, ¥=significantly different (p<0.05) from HU, *=significantly different (p<0.05) from TRS,  FL%=Percentage of foot length.

Discussion

The purpose of the current investigation was to compare the COP variables of a healthy female population running in BF, HU and TRS conditions. The first aim was to identify if there existed any differences between the shod and BF conditions, in order to identify whether running in such footwear produced similar kinetics to those found in BF running. The second aim was to determine if there were any significant differences between footwear in the COP variables implicated in the etiology of injury [30].

The significant differences in the IMC and IFFC time parameters (p<0.05) in the TRS compared to the BF and HU conditions, suggest a more plantarflexed foot placement (in BF and HU) at ground contact. This has been reported previously in analyses comparing BF to shod [2,12] and minimalist footwear compared to shod [3] conditions and suggests HU rather than TRS would be the favored footwear to reduce the incidence of injury in runners [10–12]. During running there is often uneven terrain, and as the calcaneus lands, it lends itself to movement in the coronal plane by the very nature of its shape. Furthermore, it has been identified that patients with ankle instability have a longer duration of contact from the initial heel contact to the forefoot landing [42]. Therefore, a quicker loading of the forefoot as observed in the BF and HU conditions, may offer greater support to potentially limit hazardous injury.

During locomotion, as the foot makes contact with the ground, the line of the resulting reaction force is determined by the position of the foot in relation to the athletes COG [24]. Previous research reported that when an increased angle of supination upon touchdown was present, an apparent increase in the number of ankle sprains ensued [43]. With the TRS condition in the current study exhibiting a trend towards a more laterally displaced COP, this may infer that the initial contact of the foot was made whilst being held in slight supination, and therefore similar those suffering from ankle instability which may increase susceptibility to injury.

Previous research identified that an ankle sprain group exhibited a higher loading under the medial border of the foot, and this was identified as an indicator or susceptibility to ankle sprain [30]. The significant difference between the shod and both the BF and HU condition for the IFFC X-comp variable indicated a more medially loaded foot. This may also be a predisposing factor for an inversion ankle sprain.

It appears that the HU shoe minimizes the changes in COP characteristics seen in TRS compared to BF running with only one variable (IFFC time) reporting a significant (p<0.05) difference between HU and BF. Furthermore, this particular minimalist design (HU) may more closely simulate BF running compared to some other footwear designed to simulate BF running [2]. These results suggest that proposed health benefits associated with BF running  [10] may be prevalent in HU footwear conditions.

Conclusions

The data collected in this study provides evidence that the HU design of footwear may be a suitable alternative to running BF for females, by offering protection to the plantar surface of the foot whilst adjusting the running strategy identified through COP variables in a similar way to BF running when compared to running in TRS. From a rehabilitation point of view, it may advantageous to initiate a return to running using minimalist footwear as this appears to have the potential to reduce excessive COP characteristics linked to ankle inversion injury compared to shoes. However potential injury risk reduction benefits of BF running are yet to be conclusively substantiated and any change in habitual running style through footwear choice should be approached with caution.

Future research

This study focused on a population of healthy females. Previous research has demonstrated differences between genders biomechanically and regarding injury rates [19,44] and as such the results cannot be generalized to a male sample. Therefore there is clear need to perform a similar examination using a male population. Previous research has suggested that the thickness of cushioning in running shoes may not have a significant effect on loading characteristics [7] during foot to ground impact. The HU design of shoe is commercially available in different sole thickness. Testing for similar effects of sole thickness that are observed in the HU design of shoe warrant further investigation to identify a move towards the possibility for an optimum design in the general population.

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