Running Title: Heel lifts lower Achilles tendon loading

The effect of an in-shoe orthotic heel lift on loading of the Achilles tendon during shod walking

Mathias Wulf1,2, Scott C. Wearing2,3, Sue L. Hooper4, Simon Bartold5, Lloyd Reed2 and Torsten

Brauner1

1Faculty of Sports and Health Sciences, Technische Universität München, Munich, Germany.

2Institute of Health & Biomedical Innovation, Queensland University of Technology, Brisbane, Australia.

3Division of Neurophysiology in the Center of Rare Diseases, Ulm University, Ulm, Germany.

4Office of Health and Medical Research, Queensland Health, Brisbane, Australia.

5University of Melbourne, Melbourne, Australia.

Financial Disclosure

This research received financial support from the Queensland Government.

Conflict of Interest Statement

The authors declare no conflicts of interest, financial or otherwise.

Ethics Statement

This study received approval from the University Human Research Ethics Committee.

* Correspondence, proof reading, and reprint requests to:

Scott C. Wearing

Institute of Health and Biomedical Innovation

Queensland University of Technology

60 Musk Avenue,

Kelvin Grove, Qld 4059

Australia

t: +61 7 3138 6444

f: +61 7 3138 60330

e: s.wearing@qut.edu.au

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The effect of an in-shoe orthotic heel lift on loading of the Achilles tendon during shod walking

ABSTRACT

Study Design: Controlled laboratory study 1

Objective: To investigate the effect of a 12–mm in–shoe orthotic heel lift on Achilles tendon 2

loading during shod walking using transmission–mode ultrasonography. 3

Background: Orthotic heel lifts are thought to lower tension in the Achilles tendon but evidence 4

for this effect is equivocal. 5

Methods: The propagation speed of ultrasound, which is governed by the elastic modulus and 6

density of tendon and is proportional to the tensile load to which it is exposed, was measured in 7

the right Achilles tendon of twelve recreationally–active males during shod treadmill walking 8

at matched speeds (3.4±0.7 km/h), with and without addition of a heel lift. Vertical ground 9

reaction force and spatiotemporal gait parameters were simultaneously recorded. Data were 10

acquired at 100Hz during 10s of steady–state walking. Statistical comparisons were made 11

using paired t–tests (α=.05). 12

Results: Ultrasound transmission speed in the Achilles tendon was characterized by two maxima 13

(P1, P2) and minima (M1, M2) during walking. Addition of a heel lift to footwear resulted in a 14

2% increase and 2% decrease in the first vertical ground reaction force peak and the local 15

minimum, respectively (P<.05). Peak ultrasonic velocity in the Achilles tendon (P1, P2, M2) was 16

significantly lower with addition of an orthotic heel lift (P<.05). 17

Conclusions: Peak ultrasound transmission speed in the Achilles tendon was lower with the 18

addition of a 12–mm orthotic heel lift, indicating the heel lift reduced tensile load in the 19

Achilles tendon, thereby counteracting the effect of footwear. These findings support the 20

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addition of orthotic heel lifts to footwear in the rehabilitation of Achilles tendon disorders 21

where management aims to lower tension within the tendon. 22

Level of Evidence: Therapy, level 2a 23

24

Key Terms: Soft tissue, Quantitative ultrasound, Speed of sound, Footwear, Orthoses 25

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27 Dz, Cohen’s effect size statistic for repeated measures 28

EVA, Ethylene–Vinyl Acetate 29

F1–F3, Vertical ground reaction force maxima (F1 & F3) and local minimum (F2) 30

FTI, Total–foot impulse (force–time integral) 31

M1–M2, Minimum axial transmission speed of ultrasound in the Achilles tendon 32

P1–P2, Peak axial transmission speed of ultrasound in the Achilles tendon 33

PFLR, Peak force loading rate 34

TF1–TF3, Timing of vertical ground reaction force maxima and local minimum35

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INTRODUCTION 36

In–shoe orthotic heel lifts are often used as a key therapeutic intervention in rehabilitation of the 37

Achilles tendon post–injury and have been suggested to be an effective treatment for Achilles 38

tendon disorders based on self–reported clinical outcomes.15, 25 The clinical rationale for use of 39

in–shoe orthotic heel lifts is based on two premises. The first is that heel lifts may attenuate 40

shock associated with heel strike.24 The second is that elevation of the heel results in 41

plantarflexion of the ankle joint and shortens the muscle–tendon unit, thereby decreasing the load 42

in the Achilles tendon during gait.32 Contradictory evidence exists for the use of heel lifts in 43

patients with Achilles tendinopathy5 and supporting evidence for a tension lowering effect in the 44

Achilles tendon remains equivocal. Elevation of the heel, in the order of 15 to 18 mm, has been 45

estimated to either increase,9 decrease,12 or have no effect 4, 10, 32 on peak tensile loading of the 46

Achilles tendon during gait and has been shown to have inconsistent effects on external ground 47

reaction forces, joint kinematics and lower limb muscle activity during walking.19, 20, 21, 24, 25, 42, 43 48

49

To date studies evaluating the effect of heel lifts on Achilles tendon loading have specifically 50

focused on running gait and used inverse dynamic models, to indirectly estimate tendon loads.9, 51

10, 12, 32 Although insightful, indirect estimation of internal tendon loads using the inverse 52

approach is not without limitation. As noted by Gregor et al.14 in comparing direct and indirect 53

estimates of Achilles tendon loading during cycling, inverse dynamic models likely over–54

estimate tendon loads by as much as 50%. Recently, we have shown that ultrasound transmission 55

techniques have the potential to provide direct non–invasive estimates of human tendon loading 56

under dynamic conditions.39 In contrast to indirect measurement techniques, measures of 57

acoustic velocity are neither predicated on estimates of the Achilles tendon moment arm, nor 58

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require assumptions regarding the contribution of agonist and antagonist muscles to the net ankle 59

joint moment, both of which have been shown to induce substantial errors in estimates of tendon 60

loading.14, 37 Rather, the speed of transmission of ultrasound is dependent on the modulus and 61

density of the material through which it propagates, and in tendon is governed by the classic 62

Newtonian–Laplace equation with some adjustment for Poisson’s effects in elastic media.7, 29 63

With application of physiological tensile load, the tangent or instantaneous elastic modulus of 64

tendon increases as does the speed of axial transmission of ultrasound along the tendon. 7, 29, 38 65

Thus, the change in ultrasound transmission speed is related to the magnitude of load applied to 66

the tendon. We went on to use this technique and demonstrate, somewhat unexpectedly, that 67

standard running shoes with an inherent 10–mm heel lift induced changes in ground reaction 68

force and temporospatial gait parameters that increased tensile load in the Achilles tendon in 69

healthy adults.39 The aim of this study, therefore, was to advance this work and investigate 70

whether an orthotic heel lift when used in combination with the same standard running shoes, 71

as typically employed in the management of Achilles tendon disorders,15, 25 had an influence on 72

loading of the Achilles tendon during walking. 73

74

Given that the axial transmission of ultrasound in tendon is governed by its elastic modulus and 75

density and is proportional to the tensile load in tendon,7, 16, 20, 26, 29 we hypothesized that 76

ultrasonic velocity in the Achilles tendon would be lower in standard running shoes with the 77

addition of an in–shoe orthotic heel lift. 78

79

MATERIALS & METHODS 80

Participants 81

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A detailed description of the participants, materials and methods used in this study have been 82

described elsewhere.39 In brief, 12 healthy adult males, recruited from university faculty, 83

participated in the project. The mean (± SD) age, height, body mass, foot length and shoe size 84

(US) of participants was 31 ± 9 years (range, 20 – 47 years), 1.78 ± 0.06 m, 81.0 ± 16.9 kg, 85

26.4 ± 0.9 cm and 10.5 ± 0.7, respectively. Participants were non–smokers, non–medicated and 86

recreationally active based on self–report. None reported a medical history of diabetes, 87

inflammatory joint disease, familial hypercholesterolemia or Achilles tendon pain or pathology. 88

Participant numbers were based on previously published data for human Achilles tendon,35 and 89

were sufficient to detect a 5% difference in the peak axial velocity of ultrasound (α = .05, β = 90

.20). All participants gave written informed consent prior to participation in the research, which 91

was undertaken according to the principles outlined in the Declaration of Helsinki and received 92

approval from the University Human Research Ethics Committee. 93

94

Equipment 95

A flexible strain–gauge electrogoniometer (SG110A, Penny and Giles, Biometrics, Gwent, UK) 96

was used to estimate the active range of non–weightbearing ankle dorsiflexion and plantarflexion 97

using a standard protocol 28 and the change in ankle flexion during quiet bipedal stance with and 98

without the addition of an in–shoe heel raise. The end–blocks of the electrogoniometer were 99

fixed to the skin overlying the lateral calcaneus and the distal aspect of the fibula of the right 100

ankle using double sided adhesive tape,36 and placed to ensure it did not interfere with the collar 101

of the shoe. The electrogoniometer had a resolution of 1° and is reportedly accurate to within 102

1.5% over its measurement range of 100°.35 103

104

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Vertical ground reaction force and temporospatial gait parameters were determined during 105

walking using an instrumented treadmill system, which housed a capacitance–based pressure 106

platform (FDM–THM–S, Zebris Medical GmbH, Isny, Germany). The pressure platform had a 107

sensing area of 108.4 x 47.4 cm and incorporated 7168 sensors, each approximately 0.85 x 0.85 108

cm. The treadmill had a contact surface of 150 x 50 cm and its belt speed could be adjusted 109

between 0.2 and 22 km.hr-1, at intervals of 0.1 km.hr-1. Reported within–subject coefficients of 110

variation for the majority of temporospatial gait parameters are typically below 10% for repeated 111

measures.31 112

113

A custom–built ultrasound device, which incorporated a five element ultrasound probe, was 114

used to synchronously measure axial transmission speed of ultrasound within the right Achilles 115

tendon (FIGURE 1). The probe consisted of a 1MHz broadband pulse emitter and four 116

regularly spaced receivers (range, 7.5 mm), which was maintained in close contact with the 117

skin by means of a coupling medium and elasticized straps. The received ultrasonic signals 118

were digitized at 20 MHz and the time of flight of the first arriving transient signal between 119

receivers was determined using the first zero crossing criterion.3 The average speed of axial 120

transmission of ultrasound waves was subsequently calculated given the known distance 121

between receivers and the measured time of flight. The mean within–subject coefficient of 122

variation for acoustic maxima and minima during steady-state walking is reportedly <1%,39 123

while the accuracy in predicting applied tensile force in tendon from direct measures of 124

ultrasonic velocity typically <2%.7 125

126

< Insert FIGURE 1 around here > 127

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128

Footwear and Heel Lift 129

A standard running shoe (Oregon, Adidas, Herzogenaurach, Germany) ranging in three sizes 130

between US 9.5 and 11.5 (length 29.4–30.9 cm) and mass between 359 and 396 g, with 131

identical, flexible mesh uppers and incorporating a single density EVA midsole (Shore A 132

Durometer, 60 ± 1) and rubber outsole (Shore A Durometer, 88 ± 1) were used (FIGURE 2). 133

All shoes were made by the same manufacturer and incorporate an intrinsic heel offset 134

(elevation) of 10 mm (forefoot height, 17.0 – 19.5 mm; heel height, 27.0 – 29.5 mm). 135

136

< Insert FIGURE 2 around here > 137

138

Two commercially available orthotic heel lifts (Foot Science International, Christchurch, New 139

Zealand) were added to the insole of the rearfoot of each shoe using double–sided adhesive 140

tape. Each wedge–shaped heel lift weighed 5 grams and was made of a single density EVA 141

(Shore A Durometer, 41 ± 1) which tapered over its 8.2 cm length from a height of 7 mm 142

posteriorly to finish flush anteriorly (FIGURE 2). According to the manufacturer, each 143

orthotic heel wedge was 6mm thick at the center of the heel and, since two heel raises were 144

added to each shoe, resulted in an effective heel lift of 12mm at the center of the heel. 145

146

Protocol 147

Participants reported to the gait laboratory having abstained from vigorous physical activity on 148

the day of testing. The skin overlying the posterior Achilles tendon was shaved and cleaned 149

using standard alcohol abrading methods. Following fixation of the probe to the right leg, 150

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participants were afforded a 10–minute treadmill acclimatization session. During the 151

acclimatization session, participants were instructed to adjust the speed of the treadmill to a 152

“comfortable” walking pace using a previously outlined method.31 The defined gait speed was 153

subsequently used to evaluate ultrasonic velocity in the Achilles tendon during two gait 154

conditions; (1) shod with the addition of an in–shoe orthotic heel raise, and (2) shod only. 155

Following five minutes of steady–state walking at the defined speed in each condition, the speed 156

of axial transmission of ultrasound in the right Achilles tendon, vertical ground reaction force 157

and basic temporospatial gait data were synchronously sampled over a 10–second period and at a 158

rate 100 Hz. The order of each gait condition was randomized between participants and each 159

was followed by a rest period in which ankle angle was measured during quiet bipedal stance. 160

161

< Insert FIGURE 3 around here > 162

163

Data Reduction and Statistical Analysis 164

Proprietary software (Zebris Medical GmbH, Isny, Germany) was used to calculate mean 165

temporospatial gait parameters including cadence, step length, step width, stance and swing 166

phase durations and single and double support times (TABLE 1). With the exception of stance 167

phase and swing phase durations, temporal data were expressed as a percentage of the stance 168

phase duration (SPD). Vertical ground reaction force data were exported in ASCII format and 169

custom computer code (Matlab R2012a, MathWorks, Natick, MA) was subsequently used to 170

identify the magnitude (F1–3) and timing (TF1–3) of conventional vertical ground reaction force 171

peaks for each step (FIGURE 3). The relative time to each force maxima and local minimum 172

was expressed as a percentage of the stance phase duration. Peak force loading rate (PFLR), 173

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defined as the greatest rate of force change during the first 20 ms of the gait cycle, was 174

calculated using previously outlined methods,33 while the total–foot impulse (FTI) was estimated 175

by numerical integration of the vertical ground reaction force with respect to time.40 Ultrasound 176

transmission speed in the Achilles tendon is characterized by two maxima (P1, P2) and two 177

minima (M1, M2) during steady–state treadmill walking (FIGURE 4),39 which were analyzed 178

similarly using custom computer code. The range in ultrasound transmission speed in tendon was 179

also calculated as the difference between the maximum and minimum values over the gait cycle. 180

Mean values were calculated for all steps recorded within the 10–second data capture period and 181

only data for the right limb have been presented. 182

183

< Insert FIGURE 4 around here > 184

185

The Statistical Package for the Social Sciences (IBM SPSS Statistics Version 21, Chicago, IL, 186

USA ) was used for all statistical procedures. Shapiro Wilkes tests were used to evaluate data for 187

underlying assumptions of normality. Outcome variables were determined to be normally 188

distributed, so means and standard deviations were used as summary statistics. Paired t–tests 189

were used to evaluate potential differences in ultrasonic, vertical ground reaction force and basic 190

temporospatial gait parameters including cadence, step length, step width, double support and 191

stance duration. Effect sizes were estimated using Cohen’s D for repeated measures (Dz), in 192

which the difference between mean values were standardized by the deviation of the difference.6 193

As a general guideline, Dz in the range of 0.20–0.50 was considered to be a small effect, 0.50–194

0.80 a medium effect, and a value of more than 0.80 as a large effect.6 An alpha level of .05 was 195

used for all tests of significance. 196

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197

RESULTS 198

Average active ankle plantarflexion and dorsiflexion range of motion of participants were 34° ± 199

6° and 15° ± 9°, respectively. Compared with unshod stance, footwear with a 10–mm heel offset 200

increased mean ankle plantarflexion by 4° ± 2° during quiet bipedal stance, which was further 201

increased to 8° ± 3° with addition of a 12–mm in–shoe heel raise (t11 = 7.0, P = .01). 202

203

< Insert Table 1 around here > 204

205

As demonstrated in TABLE 1, there were no statistically significant differences in 206

temporospatial gait parameters between conditions. Similarly there were no statistically 207

significant differences in the majority of kinetic parameters (TABLE 2), with the exceptions of a 208

statistically significant increase in the magnitude of the first vertical ground reaction force 209

maximum (t11 = 3.4, P = .02) and minimum (t11 = -4.1, P = .01). Although the orthotic heel raise 210

was associated with only a 2% increase in F1 and a 2% decrease in F2, the effect size exceeded 211

Cohen’s convention for a large effect (Dz = 0.80), suggesting the increase in F1 and decrease in 212

F2 was relatively consistent across participants (TABLE 2). 213

214

< Insert Table 2 around here > 215

216

Ultrasound transmission speed in the Achilles tendon was highly reproducible during steady–217

state treadmill walking; with a mean within–subject coefficient of variation of ultrasonic maxima 218

and minima ranging between 0.2% ± 0.1% and 1.0 ± 0.6%. Inclusion of an orthotic heel raise 219

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within the shoe lowered peak ultrasound transmission speeds within the Achilles tendon, with a 220

12m/s reduction noted in the magnitude of P1 (t11 = -2.3, P = .04), a 14m/s reduction in P2 (t11 = -221

3.5, P = .01) and a 50m/s reduction noted in M2 (t11 = -3.3, P = .01). Although the heel raise also 222

tended to lower the magnitude of M1 (t10 = -2.0, P = .07), the effect was not significant at the .05 223

level (TABLE 3). Effect sizes for the difference in ultrasonic transmission velocity with heel lifts 224

exceeded Cohen’s convention for moderate (Dz= 0.50–0.79) to large (Dz> 0.80) effects. 225

226

< Insert Table 3 around here > 227

228

DISCUSSION 229

This study used transmission-mode ultrasonography to evaluate the pattern of loading of the 230

Achilles tendon in standard running shoes and with the addition of a commercially available in–231

shoe orthotic heel lift. We have previously shown that a traditional running shoe design which 232

incorporated an intrinsic 10–mm heel offset induced global changes in vertical ground reaction 233

force and temporospatial gait parameters in healthy adults that acted to increase tensile load in 234

the Achilles tendon, as defined by an increase in ultrasound transmission speed (≈ 25 m/s) in the 235

tendon during walking.39 Here we advanced this work to show that the addition of a 12–mm 236

orthotic heel lift added to this standard shoe had negligible effect on basic gait parameters but 237

resulted in a significant reduction in ultrasound transmission speed in the Achilles tendon (12–50 238

m/s), suggesting that the heel lift resulted in reduced loading of the Achilles tendon during 239

walking; thereby counteracting, in part, the shoe–induced increase in tendon loading observed in 240

our earlier work.39 241

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It is noteworthy that the in–shoe heel raise used in this study had no significant effect on any of 243

the recorded temporospatial gait parameters compared with the shoe only condition, although it 244

did induce small but statistically significant alterations in vertical ground reaction force. While, 245

on average, F1 was increased by 2% and F2 decreased by 2% with the addition of the heel raise, 246

the effect was large according to Cohen’s convention (Dz > 0.80), suggesting the change in F1 247

and F2 although clinically small was relatively consistent across participants. While similar 248

alterations in F1 and F2 have been reported previously, albeit only in shoes with a heel height 249

greater than 5.1cm and accompanied by changes in temporospatial gait parameters,11 the reason 250

for a selective increase in F1 and decrease in F2 with use of the orthotic heel raise in the current 251

study is unknown. With an effective mass of just 10 g, it is unlikely that the 20N change in F1 252

and F2 reflect inertial effects associated with the orthotic device itself. 253

254

In the current study, the axial transmission speed of ultrasound in the Achilles tendon was 255

comparable to that reported previously for human tendon (≈ 1900–2050 m.s-1),29, 30, 41 and was of 256

a similar pattern to direct measures of force in the Achilles tendon during walking when recorded 257

by implanted force transducers.13, 18 As described by Komi et al.,18 force in the Achilles tendon is 258

typically reduced after heel contact prior to increasing again during the first part of stance. In the 259

current study both the first and second peaks in ultrasound velocity (P1, P2) were highly 260

reproducible during steady–state walking (mean within–subject coefficient of <1%), which is 261

consistent with earlier observations that the peak–to–peak amplitude of Achilles tendon force is 262

invariant across a range of walking speeds (1.1–1.8 m.s-1).13 Interestingly, use of the orthotic heel 263

lift in the current study, was associated with a lower peak magnitude of the speed of ultrasound 264

transmission in the Achilles tendon but increased the change (range) in ultrasonic velocity 265

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without influencing the duration of the gait cycle. Hence the in–shoe heel lift was associated with 266

a reduced magnitude but greater change in loading of the Achilles tendon throughout a gait cycle. 267

Interestingly, the peak change in ultrasound velocity with heel lifts (≈50 m/s) is similar to the 268

absolute difference in range noted between walking and running (≈54 m/s) over the entire gait 269

cycle.41 Thus, assuming that in–shoe orthotic heel lifts represent an effective treatment option 270

for Achilles tendon disorders based on self-reported outcomes,15, 25 this observation raises the 271

intriguing possibility that a reduction in the magnitude of load within the Achilles tendon, rather 272

than the rate of loading within the tendon, may be more important in achieving symptomatic 273

benefit for Achilles tendinopathy. Moreover, it may be argued that the load–reducing effect of an 274

in–shoe orthotic heel lift may be clinically counter–intuitive in management of tendinopathy, as 275

Achilles tendinopathy is often associated with reduced muscle activity in the Triceps Surae,2 and 276

heavy eccentric and/or concentric exercise is currently advocated for the management of 277

tendinopathy.17 Nonetheless, use of an in–shoe orthotic heel lift (12 mm) combined with 278

footwear that already incorporates a positive heel offset (10 mm) may be clinically useful, 279

particularly early in an progressive–loading intervention program, in which management is often 280

initially aimed at lowering tension within the Achilles tendon.25, 34 281

282

Study Limitations 283

The use of acoustic transmission techniques to investigate Achilles tendon loading is not without 284

limitations. The propagation of acoustic waves in soft tissue media is influenced by several 285

factors, including the structure, temperature, and density of the tissue through which it passes.26 286

Although tendon density and temperature were reasonably assumed to be stable during gait 287

conditions in the current study, there is evidence, albeit in animal models, that the composition of 288

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the Achilles tendon may vary along its length.8 Thus it is unknown whether the effects observed 289

in the current study were uniform across the entire tendon structure. Similarly, the mechanical 290

properties and response of the Achilles tendon to load may differ between sexes, with 291

tendinopathy and over the life–span.1, 19, 23 While gender–differences in tendon properties are not 292

universally reported,27 it is important to note that this study was limited to a small sample of 293

healthy adult males walking at one (preferred) gait speed, in a single shoe design and on the 294

same day. Thus, while it is pertinent to hypothesize about the effects of an in–shoe orthotic heel 295

lift on Achilles tendon disorders, it is unclear whether the results are directly applicable to 296

populations with Achilles tendon injury. Further studies exploring Achilles tendon loading 297

patterns in those with mid-portion and insertional tendinopathy and in repaired tendons following 298

rupture are warranted. Similarly, the effect of the in–shoe orthotic heel lift observed in the 299

current study may have represented only a transient response and may not be directly 300

transferable to Achilles tendon loading with habitual use, in children, females, older aged 301

cohorts, different shoe designs, or at markedly faster or slower gait speeds. Moreover, treadmill 302

systems are known to induce both spatial and temporal constraints on gait. While some studies 303

have observed that treadmills have no significant effect on fascicle behavior of the Triceps Surae 304

compared to over–ground walking, others have noted alter neuromuscular co–ordination and 305

subsequent lower extremity joint moments and powers during walking, which may mitigate the 306

effects of orthotic heel lifts.21, 22 Hence, the findings of this study may not be representative of 307

unconstrained walking in settings outside of the laboratory. 308

309

Nonetheless, the findings of the current study suggest that the addition of a 12–mm orthotic heel 310

lift to standard running shoes, which already have an intrinsic 10–mm heel offset, effectively 311

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counteracts the influence of footwear, to lower tensile load in the Achilles tendon. Although 312

further research into potential mechanisms underpinning the observed effect is required, the 313

findings of this study tend to support the addition of orthotic heel lifts to footwear for the 314

rehabilitation of Achilles tendon disorders where management is aimed at lowering tension 315

within the tendon. 316

317

CONCLUSIONS 318

The addition of an effective 12mm in–shoe orthotic heel lift to standard running shoes lowered 319

the speed of axial transmission of ultrasound in the Achilles tendon. Hence orthotic heel lifts 320

appear to counteract the increase in Achilles tendon loading noted with use of a standard 321

running shoe alone. These findings support the continued clinical use of in–shoe orthotic heel 322

lifts in the rehabilitation of Achilles tendon disorders in which the management plan aims to 323

lower tension within the tendon. 324

325

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KEY POINTS 326

327

Findings: 328

Addition of a 12mm in–shoe orthotic heel lift to standard running shoes lowered the speed 329

of axial transmission of ultrasound and thus tensile load, in the Achilles tendon. 330

Implications: 331

Addition of in–shoe orthotic heel lifts to standard running shoes reduced loading of the 332

Achilles tendon during walking and are appropriate in the rehabilitation of Achilles tendon 333

disorders where management aims to lower tension within the tendon. 334

Caution: 335

This study evaluated the immediate response of healthy adults to a 12mm in–shoe orthotic 336

heel lift during treadmill walking in a single shoe design and may not be directly transferable 337

to Achilles tendon loading with habitual use, in older or pathological cohorts, different shoe 338

designs or at markedly faster or slower gait speeds. 339

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437

438

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439

FIGURE LEGENDS 440

FIGURE 1. Ultrasound transmission speed was measured in the right Achilles tendon using a 441

custom built ultrasonic device (U), which was maintained in close contact with the skin by 442

means of an acoustic coupling medium and elasticized straps (omitted for illustrative purposes). 443

The endblocks of a strain–gauge electrogoniometer (G) were fixed to the skin over the lateral 444

aspect of the fibula and calcaneus using double sided adhesive tape and further fixed with 445

surgical tape. Inset: Close up image of five element ultrasound probe. 446

447 FIGURE 2. Commercially available in–shoe orthotic heel lifts (inset) were added to the 448

rearfoot of a standard running shoe design, which incorporated a single density EVA midsole 449

and an intrinsic heel offset (elevation) of 10 mm. 450

451 FIGURE 3. Vertical ground reaction force parameters derived from the instrumented treadmill 452

system. The force–time integral (FTI), or total foot impulse, is represented by the area under the 453

curve (shaded), while the peak force loading rate (PFLR) was the maximum rate of force change 454

within the first 20ms of the gait cycle. The time to force maxima and local minimum was 455

expressed as a percentage of the stance phase duration. 456

457 FIGURE 4. A typical curve for ultrasound transmission speed (solid lines) and vertical ground 458

reaction force (dashed lines) recorded for two gait cycles from the right Achilles tendon during 459

shod walking with (gray line) and without (black line) an orthotic heel lift at matched speed. 460

Note that the acoustic velocity in the Achilles tendon was characterized by two maxima (P1, P2) 461

and two minima (M1, M2) during each step. 462

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U

G

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0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100

TF3

TF2

TF1

F2F1 F3

Time (% stance phase duration)

For

ce (

N)

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TABLE 1. Mean (SD) temporal and spatial gait parameters during shod walking with and without an additional 12mm heel raise.

Footwear Heel Raise Dz

n 12 12

Speed (km/h) 3.4 3.4 -0.37 (0.7) (0.7)

Cadence (steps/min) 96.7 96.2 0.22 (12.9) (13.2)

Step width (cm) 10.6 10.8 -0.19 (2.1) (1.9)

Step length (cm) 57.2 57.3 0.01 (4.4) (5.8)

Stance phase duration (% GC) 68 68 -0.40 (3) (3)

Load response (%SPD) 18 18 -0.42 (3) (3)

Single support (%SPD) 32 32 0.55 (3) (2)

Pre-swing (% SPD) 18 18 -0.51 (3) (2)

Swing phase duration (%GC) 32 32 0.40 (3) (3)

Note there are no statistically significant differences between conditions Dz, Cohen’s effect size statistic for repeated measures n, Sample size GC, Gait cycle SPD, Stance phase duration  

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TABLE 2. Mean (SD) kinetic gait parameters during shod walking with and without an additional 12mm heel raise.

Footwear Heel Raise Dz

n 12 12

F1 (BW) 1.14 1.16 * 0.98 (0.09) (0.10)

TF1 (% SPD) 30 30 -0.29 (4) (3)

F2 (BW) 0.92 0.90 * -1.18 (0.07) (0.09)

TF2 (% SPD) 49 49 -0.26 (3) (3)

F3 (BW) 1.14 1.13 -0.39 (0.10) (0.10)

TF3 (% SPD) 72 72 -0.36 (5) (5)

FTI (BWs) 0.65 0.65 -0.17 (0.07) (0.08)

PFLR (BW/S) 108 98 -0.36 (46) (29)

* statistically significant difference between conditions (P <.05) n, Sample size Dz, Cohen’s effect size statistic for repeated measures F1–F3, Vertical ground reaction force maxima (F1 & F3) and topic minimum (F2) TF1–TF3, Timing of vertical ground reaction force maxima and topic minimum FTI, Total–foot impulse (force–time integral) PFLR, Peak force loading rate BW, Body weights SPD, Stance phase duration

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TABLE 3. Mean (SD) ultrasonic velocity in the Achilles tendon during shod walking with and without an additional 12mm heel raise. 

Footwear Heel Raise Dz

n 12 12

M1 1867 1834 -0.61 (60) (56)

P1 2047 2035 * 0.65 (71) (62)

M2 1844 1794 * -1.04 (51) (70)

P2 1943 1929 * 1.00 (55) (48)

Range 202 244 * 0.76 (34) (65)

* statistically significant difference between conditions (P <.05) n, Sample size Dz, Cohen’s effect size statistic for repeated measures M1–M2, Minimum axial transmission speed of ultrasound in the Achilles tendon P1–P2, Peak axial transmission speed of ultrasound in the Achilles tendon

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