Section: Original Research. Article Title: Effect of Heel Construction on Muscular Control Potential of the Ankle Joint in Running

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1 Note. This article will be published in a forthcoming issue of the. The article appears here in its accepted, peer-reviewed form, as it was provided by the submitting author. It has not been copyedited, proofread, or formatted by the publisher. Section: Original Research Article Title: Effect of Heel Construction on Muscular Control Potential of the Ankle Joint in Running Authors: Steffen Willwacher 1, Wolfgang Potthast 1,2, Markus Konrad 1, Gert Peter Brüggemann 1 Affiliations: 1 Institute of Biomechanics and Orthopaedics, German Sport University, Cologne, Germany. 2 Institute of Sport and Sport Sciences, Karlsruhe Institute of Technology (KIT), Germany. Journal: Acceptance Date: February 1, Human Kinetics, Inc.

2 Effect of heel construction on muscular control potential of the ankle joint in running Steffen Willwacher 1, Wolfgang Potthast 1,2, Markus Konrad 1, Gert Peter Brüggemann 1 1 Institute of Biomechanics and Orthopaedics, German Sport University, Cologne, Germany 2 Institute of Sport and Sport Sciences, Karlsruhe Institute of Technology (KIT), Germany Funding: There were no third party funding sources for this study. Conflict of Interest Disclosure: There are no conflicts of interest. Correspondence Adress: Steffen Willwacher Institut für Biomechanik und Orthopädie Deutsche Sporthochschule Köln Am Sportpark Müngersdorf Cologne, Germany Phone: Fax: s.willwacher@dshs-koeln.de Word count: 3359

3 Abstract The purpose of this study was to investigate the effect of heel construction on ankle joint mechanics during the early stance phase of running. Kinematic and kinetic parameters (ankle joint angles, angular velocities and joint moments, lever arms of ground reaction force, triceps surae muscle tendon unit lengths and rates of muscle tendon unit length change) were calculated from 19 male subjects running at 3.3 m/s in shoes with different heel constructions. Increasing heel height and posterior wedging amplified initial plantarflexion velocity and range. The potential for a muscle to control the movement of a joint depends upon its ability to produce joint moments. Runners in this study showed decreased external eversion moments and an increase in eversion range. Maximum eversion angles were not significantly affected by shoe conditions. Without considerable tendon pre-stretch, joint moment generation potentials of triceps surae and deep plantar flexors might be inhibited due to rapid plantar flexion based on the force velocity relationship. It could be speculated that increasing ankle inversion at heel strike could be a strategy to keep maximum eversion angles inside an adequate range, if joint moment generation potentials of deep plantar flexors are inhibited due to rapid plantarflexion. Keywords: footwear, biomechanics, initial plantar flexion, heel strike

4 Introduction Heel toe running is the most common human running style adopted by 75% to 80% of all runners during shod running. 1,2 A typical feature of heel - toe running is an initial plantar - flexion movement of the foot, caused by an external plantar flexion moment acting during the first 10% to 15% of stance. 3 The introduction of cushioning systems in the 1980s led to an increase in heel height in most running shoes. Mechanical consequences of different heel constructions have been frequently investigated with regards to the frontal plane of movement. 4-7 Only a few studies examined mechanical differences due to heel elevation in the sagittal plane in running. 3,8,9 Both, the geometry and stiffness of the heel construction might have an influence on the mechanical loading of biological structures around the ankle by creating an increased lever arm of the ground reaction force (GRF) during stance. 10 Reinschmidt and Nigg 3 examined the effects of different heel heights on resultant ankle joint moments during running at 4.6 ms -1. Their results indicate that the amplitude of the initial external plantar flexion moment seems to be systematically influenced by heel height. Joint motion is actuated by force generating muscle tendon units (MTUs) using the skeleton as a lever system to create joint moments. The control potential of a MTU depends on its ability to create joint moments. Therefore, the control potential is dependent upon the size of the lever arm and the force production of the MTU. Force production is mainly governed by the force length and force velocity relationships. 11 The lines of action of the triceps surae MTU and the MTUs of deep plantar flexing muscles (e.g. m. tibialis posterior, flexor hallucis longus) pass the ankle joint posterior to its joint center. 12 A rapid plantar - flexion movement forces these MTUs to operate at high shortening velocities where force is diminished, which limits their potential to keep their tendons tensed. Slackening of these tendons would result in a short time loss of muscular control potential of the ankle joint in the sagittal as well as in the frontal plane of movement. Evidence for this theory comes from in vivo force measurements performed by Komi and co - workers showing a sudden drop in Achilles tendon force immediately after heel strike in walking and heel toe running. 13,14 Loss of muscular control potential

5 of frontal plane ankle joint movement and vibrations of slack MTUs are considered to be risk factors for the development of running related overuse injuries like plantar fasciitis, medial tibial stress syndrome, patello femoral pain syndrome and achillo tendinitis Therefore, the purpose of this study was to investigate the influence of different heel constructions on the initial plantar flexion movement and joint kinetics of the ankle joint. It was hypothesized that a more rigid, higher and flared construction of the heel would increase the posterior lever arm of the GRF and external joint moments in the initial phase of stance, causing a more rapid plantar flexion motion of the foot. It was further hypothesized that differences in frontal plane kinematics and kinetics would occur as a consequence of altered joint moment generation capacities of deep plantar flexing muscles in response to rapid initial ankle plantarflexion. Methods A cross sectional study design was used to quantify the effect of different heel constructional designs on ankle joint lever arms, kinematics and kinetics. The experimental intervention consisted of two controlled variations in footwear design and a barefoot condition. Subjects The study was conducted in the biomechanics lab located in the track and field indoor track at the German Sports University in Cologne. 19 male subjects (age: 34.7 ± 11.9 years; height: 1.84 ± 0.05 m; mass: 79.2 ± 7.0 kg) participated in the study. All subjects were performing running exercise on a regular basis, with at least three training sessions per week and were free of running related pain and injury for at least half a year prior to the experiment. Also, informed written consent was obtained by each subject before participation in the study. Technical setup and experimental protocol Subjects were asked to run with an average speed of 3.3 m/s ± 0.2 m/s (photocells 1 m before and behind the force plate) along a 25 m long track incorporating a force platform (1250 Hz, 0.9 m x 0.6 m, Kistler AG, Winterthur, Switzerland) embedded in the middle of the track. The running track and the top of the force platform were covered with a one centimeter thick tartan layer. Lower

6 extremity joint kinematics and kinetics were calculated by means of a custom made mathematical Matlab (R2010a, The MathWorks, Natick, USA) model. Running kinematics were recorded using a 10 camera Vicon Nexus system (Vicon Motion Systems, Oxford, UK) at 250 Hz. One barefoot condition and two shod conditions were investigated; an orthopaedic shoemaker modified two commercially available neutral running shoes (Nike Pegasus, Nike Inc., Beaverton, USA) for the shod conditions. In the first condition, the midsole was removed over the entire length of the shoe (flat condition), while in the second shod condition an elevated posterior heel flare, made of stiff EVA foam (shore A 85) was added (wedged condition) (see fig.1). Heel heights were 22 mm for the flat condition and 37 mm for the wedged shoe condition, respectively. Each subject contributed five trials to each condition. A trial was judged valid if running speed was inside the predetermined range and the force platform was hit with the entire right foot without obvious changes in running technique. Running conditions were assigned to subjects in randomized order to avoid a carryover effect. Mathematical model and data analysis All kinematic and kinetic analysis was performed for the stance phase of running (vertical GRF threshold = 10 N). The movements of retro-reflective markers attached to the right lower extremity were tracked with the Vicon Nexus system. Markers were placed on the following anatomical landmarks: Right and left anterior and posterior iliac spines; right trochanter major; right medial and lateral femoral condyles; right tibia (approx. on the midpoint between ankle and knee joint); right medial and lateral malleoli; right medial, lateral and posterior aspects of the calcaneus; slightly distal to the 1 st and 5 th metatarsal phalangeal joints and on top of the second toe. Heel markers were attached directly on the skin through holes cut into the heel cap to avoid overestimation of rearfoot movement due to the use of shoe mounted markers. 7 GRF data and marker trajectories were implemented into a 5 segment inverse dynamics model of the lower extremity. 18 The model consists of a pelvis, thigh, shank, rearfoot and forefoot segment. The Hip joint center was estimated using regression equations given by Seidel. 19 Knee and ankle joint centers were defined as the midpoint between femoral markers and malleoli markers respectively. The foot was modeled incorporating two

7 segments (rear- and forefoot). All marker trajectories underwent an optimization procedure 20 to comply better with rigid body assumptions prior to their implementation in the model. An anatomical coordinate system was created for each segment, with the z axis representing the upwards pointing axis of each segment, the y axis representing the flexion extension axis (pointing medial) and the x axis being the abduction - adduction axis (pointing anterior). Joint angles were expressed using the Cardan angle convention (sequence of rotation: y, x, z), with the distal segment moving in relation to the proximal segment. The orientation of each segment was related to a standing reference measurement conducted prior to the running trials for each condition. Ankle joint angles were determined from the orientation of the rearfoot segment with respect to the shank segment. Lengths of Gastrocnemii and soleus MTUs were determined using regression equations determined by Hawkins and Hull: 21 M. soleus MTU length: α (1) M. gastrocnemius lateralis MTU length: α β (2) M. gastrocnemius medialis MTU length: α β (3) Where α represents the ankle joint plantarflexion / dorsalflexion angle and β the flexion / extension angle of the knee joint, respectively. Rate of change of MTU lengths were calculated by numerical differentiation of MTU lengths. Anthropometric data of Zatsiorsky 22 corrected by de Leva 23 were used for inverse dynamic calculations. All joint moments are expressed in the proximal segments as external moments in the course of this study. Joint powers are calculated for each anatomical plane separately by multiplying the corresponding internal joint moments and angular velocities. All model calculations were made using custom - made Matlab (R2010a, The Mathworks, Natick, USA) code. Due to the inaccuracy in CoP determination at low force intensities, all kinetic values are presented between 5 % and 95 % of stance only.

8 For each trial, several kinematic and kinetic parameters describing the ankle joint behavior were extracted for statistical analysis. Additionally, each data curve was averaged over 10 % intervals, to allow for statistical comparison at distinct periods of the stance phase. Statistics Data curves and discrete parameter values were averaged for each subject and condition. Afterwards group means and standard deviations were calculated for every shoe condition. A one factor ANOVA (general linear model with repeated measures) was utilized to identify any statistical influence of shoe condition on the variance of observed parameters. To quantify statistical significant differences between individual conditions a post hoc test using Bonferroni correction was applied to the dataset. The level of significance was set to 5 %. Results Heel construction had a significant effect on ankle joint kinematics in the first half of stance (see fig. 2 and table 1). Subjects struck the ground more dorsiflexed, when running in the wedged shoe condition, compared to the barefoot and flat condition. The initial plantar - flexion movement was about 7 higher for the wedged condition and occurred at a 200% higher angular velocity. Subsequent dorsi flexion was performed over a greater range and with increased angular velocity. Only minor differences were observed between the flat shoe and barefoot condition. In the frontal plane runners hit the ground in a more inverted position in both shod conditions (see fig. 2). No significant differences in maximum eversion angle could be observed, but total eversion range (maximum eversion minus inversion angle at touchdown) was increased for both shod conditions. Eversion velocity was enlarged in the wedged shoe condition between 10 % and 30 % of stance (fig. 2). Inversion velocity and range of motion were clearly enlarged for the barefoot condition in the last 20% of stance. MTU lengths and shortening rates are presented in figure 3. Due to the high dependency of MTU length to ankle joint angle configuration in the sagittal plane, similar curve patterns to angle angles and angular velocities can be observed. Initial triceps surae shortening range was increased and shortening occurred at a higher velocity in the wedged condition compared to the

9 barefoot and flat conditions. Subsequent MTU lengthening velocity was increased in the mid portion of stance for the wedged condition as well. The highest shortening velocities for all analyzed MTUs were found during the push off phase in the last 20% of stance. In the sagittal plane, initial external plantar flexion moments were enlarged for the wedged shoe condition compared to barefoot and flat (fig. 3, table 1). External dorsi - flexion moments were generated later in stance and at a higher rate. The lever arm of the GRF to the ankle joint was more posterior oriented over the entire braking phase for the wedged condition. In the frontal plane, external ankle joint moments were enlarged in the barefoot condition over the entire stance phase. Differences between shod conditions were observed in the first half of stance with lower moments generated in the wedged condition. Changes in GRF lever arms in the frontal plane corresponded to the differences observed for joint moments. Lever arms were increased for the barefoot condition and slight changes could be seen between shod conditions. Discussion The purpose of this study was to investigate the effect of heel construction on ankle joint kinematics and kinetics in the early stance phase of running, with a focus on a possible change in the muscular control potential of MTUs crossing the ankle. In the context of this study, muscular control potential can be understood as the ability of MTUs to control the movement of a joint by creating internal joint moments. It could be shown that initial plantar flexion range and velocity are increased by running in shoes with a higher and posterior flared midsole construction of the heel by creating an increased lever arm of the ground reaction force. This increased leverage is in line with published data from the literature 24 and explains the increased initial external plantarflexion impulse found for the wedged shoe condition, causing the accentuated plantar flexion movement. It was further assumed that a rapid initial plantar flexion movement of the foot might lead to a short time loss of muscular control potential of the ankle joint. High initial plantarflexion velocities would force the triceps surae as well as deep plantar flexor MTUs to work in an elevated region of their force velocity relationship.

10 Shortening velocities of the triceps surae MTU were amplified for the wedged shoe condition in the first 20% of stance. A mean maximum initial plantar flexion velocity of per second was found for the wedged shoe condition. Compared with data of Wickiewicz et al. 25, this value is in a region, were human plantar flexor MTUs are capable to produce less than 10% of their maximum isometric strength. The triceps surae consist of three different muscles, differing in their geometry und functional role. Contrary to the soleus muscle, the two gastrocnemius muscles do not only cross the ankle joint but also the knee joint. In the initial stance phase, the knee joint undergoes a flexion movement and as a consequence gastrocnemii MTUs length shortens. Therefore, shortening velocities evoked by a rapid plantarflexion are higher in gastrocnemii MTUs compared to soleus MTU (see table 1). Simultaneously to the increase in triceps surae shortening velocity, the rate of shortening of deep plantar flexor MTUs, who cross the ankle joint with a posterior moment arm as well, must have been increased for the wedged shoe condition. Additionally to their role as plantar flexors, these muscles are prime regulators of frontal plane ankle joint movement. Thus, it can be speculated, that amplified initial plantarflexion intensity might lead to a reduced muscular regulation capacity at the ankle joint. Nigg 26 proposed in his concept of the preferred motion path that the movement of a joint is controlled by muscle activity with the goal to minimize mechanical resistance against this motion. In the absence of muscular control, the ankle joint complex would be exposed to a higher risk of leaving the preferred motion path and putting biological structures (i.e. bones and adjoining ligaments) under higher stress. The loss of muscular control potential of deep plantarflexing muscles during the initial stance phase reduces the potential to counteract the external eversion movement of the rearfoot by creating sufficiently high muscular inversion moments (see fig. 2). Consequently, overall eversion range of motion was increased for the wedged and the flat shoe condition compared to barefoot. This increase was mostly related to a more inverted position of the foot at touchdown. Maximum eversion angle was not significantly affected by shoe modifications. It can be speculated that increasing inversion of

11 the foot at heel strike might be a feed forward adaptation strategy to keep maximum rearfoot eversion inside a certain range in the presence of a short time loss of muscular control potential. Angular plantar flexion velocities and shortening velocities of triceps surae MTUs were consistently higher during the push off phase than during initial plantar flexion. At first sight this result negates all before made theoretical considerations and interpretations, because there is obviously no slack tendon during the push of phase in running. However, the increment in plantarflexion and triceps surae shortening velocities is well explained by the fact that during the push off phase the series elastic parts of the MTU are rapidly shortening because of the return of stored energy. 27 In the initial part of stance there is only a small amount of energy stored inside the tendon due to some preactivation of the m. triceps surae. On that account, the relative amount of MTU shortening, which results from the shortening of the series elastic component, is remarkably reduced during the initial plantarflexion phase in heel toe running. Musculus tibialis anterior is the prime dorsiflexor of the human ankle joint. Increased force production of this muscle would seem to be the most effective strategy to avoid excessive plantarflexion velocities. Nevertheless, the results of this study indicate that this strategy seems not or only in part to be applied. This might be due to negative effects of increased tibialis anterior muscle activity such as increased co-contraction and subsequent increased ankle joint contact forces. Furthermore, an increased tibialis anterior muscle activity will lead an earlier onset of muscle fatigue with following negative effects with respect to performance or with respect to the development of overuse injuries. Future studies should investigate ankle joint mechanics and tibialis anterior and eventually toe extensors EMG activity during a fatiguing run while wearing different heel construction designs. This will help to clarify the role of tibialis anterior in initial plantarflexion control in heel toe running. A limitation of this study is that the behavior of muscle fascicles and tendinous parts of the MTUs has not been measured directly. Studies in which ultrasound technology was used to measure fascicle length and the length of the Achilles tendon and aponeurosis in vivo, show a distinct drop in Achilles tendon length in the initial part of stance, both for walking and level running. 27,28 In scenarios

12 were high amounts of pre stretching of plantarflexion MTUs exist (like in forefoot running), the shortening of the series elastic part of the MTU might take up the shortening provoked by rapid ankle plantarflexion. This would allow the contractile components of the MTU to operate at a more isometric state. Nonetheless, the majority of distance runners perform heel toe running. 2 Here, in vivo measurements of the Achilles tendon force show a sudden drop in the initial contact phase 14, giving evidence of a loss of force generation capacity of the triceps surae MTUs. Furthermore, reported Achilles tendon force values in running from the end of the swing phase are comparably low compared to values from midstance. 29 Therefore, it is not reasonable to assume that considerable amounts of MTU length change are brought up by tendon shortening in the initial stance phase of running. Notwithstanding, future studies should try to implement ultrasound or optic fiber techniques to analyze the exact effects of amplified plantarflexion velocities in interaction with different pre stretch levels on the distinct length and orientation behavior of contractile and series elastic parts of the MTUs. From this, the region of the force velocity relationship, in which the muscle fascicles are working, could be determined more accurately. Another limitation of the present study design is that all aspects of neural movement control could not be covered. As a result, future studies should extend to existing mechanical approach to the problem by integrating measures of neural control (EMG, reflex contributions, etc.). In summary, the results of this study show that heel construction affects the initial plantarflexion behavior of the ankle joint during the first 20% of stance. Heel constructions, which accentuate the lever arm function of the posterior part of the midsole cause an increased plantarflexion velocity and range of motion during the initial part of the contact phase of heel toe running. Rapid plantarflexion of the foot might force not only the MTUs of the m. triceps surae but also the deep plantar flexor muscles to work at high shortening velocities and therefore limits their ability to produce internal muscular inversion moments. Running shoe constructions should avoid large heel heights, rearfoot offsets and posterior heel flares because they can lead to adaptations in running mechanics that reduce the muscular ability to control ankle joint movement.

13 References 1. Kerr BA, Beauchamp L, Fisher V, Neil R. Foot strike patterns in distance running. In: Kerr BA, ed. Biomechanical Aspects of Sport Shoes and Playing Surfaces: Proceedings of the Interantional Symposium on Biomechanical Aspects of Sport Shoes and Playing Surfaces. Calgary: Alberta University Press; 1983: Hasegawa H, Yamauchi T, Kraemer WJ. Foot strike patterns of runners at the 15-km point during an elite-level half marathon. J Strength Cond Res. Aug 2007;21(3): Reinschmidt C, Nigg BM. Influence of Heel Height on Ankle Joint Moments in Running. Med Sci Sport Exer. Mar 1995;27(3): Clarke TE, Frederick EC, Hamill CL. The Effects of Shoe Design Parameters on Rearfoot Control in Running. Med Sci Sport Exer. 1983;15(5): Nigg BM, Morlock M. The Influence of Lateral Heel Flare of Running Shoes on Pronation and Impact Forces. Med Sci Sport Exer. Jun 1987;19(3): Stacoff A, Reinschmidt C, Nigg BM, et al. Effects of shoe sole construction on skeletal motion during running. Med Sci Sports Exerc. Feb 2001;33(2): Stacoff A, Reinschmidt C, Stussi E. The movement of the heel within a running shoe. Med Sci Sports Exerc. Jun 1992;24(6): Dixon SJ, Kerwin DG. The influence of heal lift manipulation on sagittal plane kinematics in running.. May 1999;15(2): Dixon SJ, Kerwin DG. The influence of heel lift manipulation on Achilles tendon loading in running.. Nov 1998;14(4): Braunstein B, Arampatzis A, Eysel P, Bruggemann GP. Footwear affects the gearing at the ankle and knee joints during running. J Biomech. Aug ;43(11): Zatsiorsky V, Prilutsky B. Biomechanics of skeletal muscles. Champaign, IL: Human Kinetics; 2012.

14 12. Schünke, Schulte, Schumacher, Voll, Wesker. Prometheus. Lernatlas der Anatomie. Allgemeine Anatomie und Bewegungssystem. Stuttgart, New York: Georg Thieme Verlag; Komi PV, Fukashiro S, Jarvinen M. Biomechanical Loading of Achilles-Tendon during Normal Locomotion. Clin Sport Med. Jul 1992;11(3): Komi PV. Relevance of Invivo Force Measurements to Human Biomechanics. Journal of Biomechanics. 1990;23: Friesenbichler B, Stirling LM, Federolf P, Nigg BM. Tissue vibration in prolonged running. J Biomech. Jan ;44(1): Moen MH, Tol JL, Weir A, Steunebrink M, De Winter TC. Medial tibial stress syndrome: a critical review. Sports Med. 2009;39(7): Hintermann B, Nigg BM. Pronation in runners. Implications for injuries. Sports Med. Sep 1998;26(3): Bresler B, Frankel JP. The forces and moments in the leg during level walking. Trans. ASME 1950; Seidel GK, Marchinda DM, Dijkers M, Soutas-Little RW. Hip joint center location from palpable bony landmarks--a cadaver study. J Biomech. Aug 1995;28(8): Soderkvist I, Wedin PA. Determining the movements of the skeleton using well-configured markers. J Biomech. Dec 1993;26(12): Hawkins D, Hull ML. A Method for Determining Lower-Extremity Muscle Tendon Lengths during Flexion Extension Movements. Journal of Biomechanics. 1990;23(5): Zatziorsky V, Seluyanov V. The mass and inertia characteristics of the main segments of the human body. In: Matsui H, Kobayashi K, eds. Biomechanics VIII - B. Illinois: Human Kinetics; 1983: de Leva P. Adjustments to Zatsiorsky-Seluyanov's segment inertia parameters. Journal of Biomechanics. Sep 1996;29(9):

15 24. Braunstein B, Arampatzis A, Eysel P, Brüggemann G-P. Footwear affects the gearing at the ankle and knee joints during running. Journal of biomechanics. 2010;43(11): Wickiewicz TL, Roy RR, Powell PL, Perrine JJ, Edgerton VR. Muscle architecture and forcevelocity relationships in humans. J Appl Physiol. August 1, ;57(2): Nigg BM. The role of impact forces and foot pronation: a new paradigm. Clin J Sport Med. 2001;11(1): Ishikawa A, Pakaslahti J, Komi PV. Medial gastrocnemius muscle behavior during human running and walking. Gait & Posture. Mar 2007;25(3): Lichtwark GA, Wilson AM. Interactions between the human gastrocnemius muscle and the Achilles tendon during incline, level and decline locomotion. Journal of Experimental Biology. Nov ;209(21): Kyröläinen H, Finni T, Avela J, Komi PV. Neuromuscular behaviour of the triceps surae muscle-tendon complex during running and jumping. International journal of sports medicine. 2003;24(3):

16 Figure 1: Modified running shoes used for the experiment. A: Wedged shoe condition. B: Flat shoe condition. A [ ] Ankle angle itt l l A kl l f t l l 15 n plantar flexion B bare wedged flat bare wedged flat 10 on C [ /s] Figure 2: Ankle joint kinematics. A: Ankle angle (sagittal plane). B: Ankle angle (frontal plane). C: Angular velocity ankle (sagittal plane). D: Angular velocity ankle (frontal plane). Mean curves are displayed for each shoe condition. Vertical bars indicate ranges of ± 1 standard deviation at the corresponding relative stance phase durations. Curves are time normalized to the stance phase. 1 indicates a statistical significant difference (p < 0.05) between the barefoot and flat condition, 2 between barefoot and wedged and 3 between flat and wedged at the respective 10 % interval.

17 A [mm] C [Nm/kg] Lever arm of the ground reaction force sagittal plane Lever arm of the ground reaction force frontal plane bare wedged flat 40 bare wedged flat 1,2,3 2,3 2,3 2,3 2,3 2,3 2,3 1,3 1,2,3 1,2 100 External ankle moment sagittal plane ,3 2,3 2,3 2,3 2,3 1,3 1,3 2,3 3 posterior anterior dorsiflexion plantarflexion [mm] [Nm/kg] B D ,2,3 1,2,3 1,2,3 1,2,3 1,2 1,2,3 1,2,3 1,3 3 External ankle moment frontal plane 2,3 1,2,3 1,2,3 1,2,3 1,2,3 1,2 1,2,3 1,2,3 medial lateral eversion inversion Figure 3: Ankle joint kinetics. A: GRF lever arm ankle (sagittal plane). B: GRF lever arm ankle (frontal plane). C: Resultant external ankle moment (sagittal plane). D: Resultant external ankle moment (frontal plane). Mean curves are displayed for each shoe condition. Vertical bars indicate ranges of ± one standard deviation at the corresponding relative stance phase durations. Curves are time normalized to the stance phase. 1 indicates a statistical significant difference (p < 0.05) between the barefoot and flat condition, 2 between barefoot and wedged and 3 between flat and wedged at the respective 10 % interval.

18 A B C normalized muscle tendon unit length normalized muscle tendon unit velocity Muscle tendon unit length soleus bare wedged flat Muscle tendon unit velocity soleus Muscle tendon unit length gastrocnemius medialis 1 D E F Muscle tendon unit velocity gastrocnemius medialis Muscle tendon unit length gastrocnemius lateralis Muscle tendon unit velocity gastrocnemius lateralis 1.5 Figure 4: MTU lengths and rates of length change for the m. soleus (A,D), m. gastrocnemius medialis (B,E) and lateralis (C,F) MTUs. Mean curves are displayed for each shoe condition. Vertical bars indicate ranges of ± one standard deviation at the corresponding relative stance phase durations. Curves are time normalized to the stance phase. Mean curves are displayed for each shoe condition. Vertical bars indicate ranges of ± one standard deviation at the corresponding relative stance phase durations. MTU lengths and velocities are normalized to shank lengths and shank lengths per second, respectively.

19 Table 1: Main parameters of the study (mean +- one standard deviation). Touchdown angle sagittal [ ] -2.7 ± 3.3 Initial plantarflexion [ ] 1.5 ± 1.8 Maximum initial plantarflexion [ ] -0.6 ± 1.1 Touchdown angle frontal [ ] 2.7 ± 3.8 Eversion range [ ] ± 2.3 barefoot flat wedged w** w** f**, f**, -2.9 ± ± ± ± ± ± ± ± 2.4 b** b**,w** 6.4 ± ± 2.9 Maximum eversion [ ] -8.8 ± ± ± 3.5 Max. init. plantarflexion velocity [ /s] ± Max. dorsiflexion velocity [ /s] ± 59.3 f*, ± ± 64.7 w''' b*,w** ± ± b**, f*** b***,f*** b** b*** b***,f'' b***,f*** b***,f** Init. shortening vel. m.soleus [1/s] ± ± ± 0.35 b***,f*** Init. shortening vel. m.gastroc. Med. [1/s] ± ± ± 0.38 b***,f*** Init. shortening vel. m.gastroc. Lat. [1/s] ± ± ± 0.38 b***,f*** max. lenghtening vel. m.soleus [1/s] 0.44 ± 0.12 f*, 0.51 ± 0.13 b*,w** 0.67 ± 0.21 b***,f** max. lengthening vel. m.gastrocnemii [1/s] 0.35 ± 0.06 w** 0.39 ± 0.09 w* 0.51 ± 0.20 b**,f* Ext. plantarflexion impulse [Nms/kg] ± ± ± b***,f*** Max. ext. plantarflexion moment [Nm/kg] 0.2 ± 0.15 f*, 0.25 ± 0.14 b*, 0.42 ± 0.14 b***,f*** Ext. eversion impulse [Nms/kg] ± f**, ± b**,w** ± b***,f* Max. ext. eversion moment [Nm/kg] ± 0.15 f**, ± 0.11 b**,w** ± 0.12 b***,f** b, f and w indicate a statistical significant difference with respect to the barefoot, flat and wedged shoe condition ( * = p<0.05; ** = p<0.01; *** = p<0.001)