key: cord-0289116-vdz6bvrs
authors: Fernandes, G. L.; Orssatto, L. B. R.; Sakugawa, R. L.; Trajano, G. S.
title: Reduced motor unit discharge rates in gastrocnemius lateralis, but not in gastrocnemius medialis or soleus, in runners with Achilles tendinopathy
date: 2022-05-07
journal: nan
DOI: 10.1101/2022.05.05.22274750
sha: 465effa9d92c91e413a9860c701a9daa450c6f5b
doc_id: 289116
cord_uid: vdz6bvrs

Objectives: Deficits in muscle performance could be a consequence of a reduced ability of a motor neuron to increase the rate in which it discharges. This study aimed to investigate motor unit (MU) discharge properties of each Triceps surae muscle (TS), and TS torque steadiness during submaximal intensities in runners with Achilles tendinopathy (AT). Methods: We recruited runners with (n=12) and without (n=13) mid-portion AT. MU discharge rate was analysed for each of the TS muscles, using High-Density surface electromyography during 10 and 20% isometric plantar flexor contractions. Results: MU mean discharge rate was lower in the Gastrocnemius lateralis (GL) in AT compared to controls. In AT, GL MU mean discharge rate did not increase as torque increased from 10% peak torque, 8.24pps (95%CI: 7.08 to 9.41) to 20%, 8.52pps (7.41 to 9.63, p=0.540); however, in controls, MU discharge rate increased as torque increased from 10%, 8.39pps (7.25 to 9.53) to 20%, 10.07pps (8.89 to 11.25, p<0.001). There were no between-group difference in Gastrocnemius medialis (GM) or Soleus (SOL) MU discharge rates. We found no between-groups differences in coefficient of variation of MU discharge rate in any of the TS muscles nor in TS torque steadiness. Conclusion: Our data demonstrates that runners with AT may have a reduced neural drive to GL, failing to increase MU discharge rate to adjust for the increase in torque demand. Further research is needed to understand how interventions focusing on increasing neural drive to GL would affect muscle function in runners with AT.

compared to controls. Force-sharing contribution of individual muscles of the triceps surae was 83 estimated for each muscle based on the root mean squared (RMS) of surface EMG (electromy-84 ography) signal amplitude. Even though data from surface EMG signal is somewhat limited in 85 estimating changes in neural drive to a specific muscle 11 , this result suggests that individual 86 muscles recruitment strategies might be altered in AT. 87

From a neurophysiological perspective, the force exerted by a muscle depends, partly, 88 on the recruitment and discharge rates of the motor units 10 . Thus, deficits in motor performance 89 could be a result of a reduced ability to recruit motor units and/or to increase the rate at which 90 motor neurones discharge 10 . The analysis of individual motor unit discharge rates from each 91 muscle of the triceps surae 12 is a more accurate and robust way of investigating the central 92 nervous system strategy of recruitment of the triceps surae muscle (i.e. neural drive), compared 93 to the typical and limited interference EMG 13 . This method has been also used in other studies 94 to estimate changes in neural drive to specific muscles in individuals with ACL injury 14 and 95 patellofemoral pain 15 . Thus, if individuals with AT have reduced neural drive to one or more 96 muscles of the triceps surae, motor unit mean discharge rate of the affected muscle would be 97 reduced compared to the other muscles and to controls 8, 16, 17 . 98

Furthermore, reduced control of the plantar flexors could create tendon overload, pro-99 gressing to early stages of tendinopathy 9 . Increased fluctuation in torque (torque steadiness) is 100 associated with painful musculoskeletal conditions such as knee osteoarthritis or patellofemo-101 ral pain 15, 18 , and could occur as consequence of greater variation in motor unit discharge rate 102 19 . Coefficient of variation of motor unit discharge represents, at an individual muscle level, 103 the ability to effectively control muscle torque and it is an important measure that can help 104 explain motor performance 10 . Chronic musculoskeletal disorders display reduced torque 105 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. and if they were unsure, they were asked which leg they would use to kick a ball. Participants 155 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) was observed between contractions, and the highest value was used. Thereafter, participants 160 performed three trapezoidal submaximal isometric contractions (3×10% and 3×20% peak 161 torque) in a randomised order. For each intensity participants had 4 attempts to get familiarised 162 with task before recordings. Rate of torque rise and decline was standardised at 10% peak 163 torque/s between contractions with different intensities, with a 10-s sustained plateau at the following the estimated muscle fibres orientation using a bi-adhesive layer with a conductive 171 paste to ensure good skin-electrode contact and conductibility. One 32-channels electrode 172 matrice (ELSCH032NM6, OTBioelettronica, Torino, Italy) was placed on GM, one 32-173 channels electrode matrice on GL and two 32-channels electrodes matrices on SOL, one 174 laterally and one medially to the Achilles tendon, to ensure sufficient data were gathered for 175 motor unit analysis. The ground strap electrode (WS2, OTBioelettronica, Torino, Italy) was 176 dampened and positioned around the ankle joint of the tested leg. The EMG signals were 177 recorded in monopolar mode, amplified (256x), band-passed filtered (10-500Hz) and converted 178

to digital signal at 2048Hz by a 16-bit wireless amplifier (Sessantaquattro, OTBioelettronica, 179 Torino, Italy), before being stored for offline analysis. Since the matrice adapter device 180 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint (AD2x32SE, OTBioelettronica, Torino, Italy) has only 2 channels for electrode connection, 181 each intensity of the protocol had to be performed twice, once with electrodes connected to the 182 gastrocnemii and a second time with the electrodes connected SOL, this order was randomised 183 for each intensity. Torque signal was recorded and analysed with OT Biolab+ software. HD-184 EMG signal was recorded and analysed offline, decomposed into motor unit spike trains, then 185 converted into instantaneous discharge rates with specialised software using blind source 186 separation decomposition technique using DEMUSE tool software (v.4.1; The University of 187

Maribor, Slovenia) 25 . For each muscle and for each intensity, the 2 best contractions, with the 188 lowest deviation from trapezoidal torque trajectory, were combined in one file and motor unit 189 tracked across the 2 contractions at the same intensity for analysis. All motor units were 190 visually inspected, erroneous discharge times were excluded, and missed discharges included. 191

Manual inspection is required to reduce automatic decomposition discharge errors and improve 192 data reliability 26 . Only motor units with a pulse-to-noise ratio >30dB, sensitivity > 90%, were 193 used for data analysis 24,25 . For participants that yield no good quality motor units after motor 194 unit tracking across the 2 contractions at the same intensity, the best single contraction was 195 used for analysis with the motor unit discharge characteristics inspected as mentioned above. 196

Due to the reduced number of the same motor units found across intensities, motor unit tracking 197 across intensities was not feasible and therefore not used for analysis. Motor units were 198 collected from the 10 seconds isometric plateau, the first and last two seconds were excluded 199 and analysis of mean motor unit mean discharge rate, coefficient of variation of motor unit 200 discharge rate and torque steadiness were performed from the central 6 seconds of the isometric 201 plateau. Motor unit mean discharge rate, coefficient of variation of motor unit discharge rate 202 and were calculated for each muscle during 10% and 20% peak torque, trapezoidal 203 contractions. 204 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint Torque steadiness was analysed as the coefficient of variation in torque for each torque 205 intensity tested. Torque was filtered (10Hz, 4th order, low pass). To reduce intrasubject 206 variability, data from the coefficient of variability of torque was averaged across the 4 207 contractions for each of the two torque intensities tested, the 2 contractions recorded during 208 gastrocnemii testing and from the 2 contractions during SOL testing 27 . 209 210

All analyses were performed using R studio (version 1.3.1093). Models were fitted 212 using the lme4 package 30. Separate linear mixed-effect models were used to compare motor 213 unit mean discharge rates and coefficient of variation of motor unit discharge rate of identified 214 motor units for each muscle (SOL, GM, GL); between intensities (10% and 20%) and groups 215 (AT and control). We tested the model using a random intercept (participant ID) and slope 216 (recruitment threshold by intensity) for each participant in the study to account for the influence 217 of motor unit populations and the correlation between repeated observations in each participant. 218

The estimated marginal mean difference and 95% confidence intervals (CI) for all variables 219 (motor unit mean discharge rate, coefficient of variation of motor unit discharge rate, between 220 groups and torque steadiness) were determined using the emmeans package31. Normality 221 assumptions were confirmed by analysis of the histogram of residuals, Q-Q Plot and the 222 residual-predicted scatterplot. Independent t-test was used to compare torque steadiness 223 between groups for each torque intensity. An alpha level of 5% was set for statistical 224 significance for all tests, and when appropriate, Bonferroni post-hoc analysis was performed. 225

Data is presented as mean (± 95% CI). 226 227

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint participant) and 539 motor units in the control group (average of 41 per participant) across all 231 muscles and intensities. A total of 120 motor units were found in SOL at 10% peak torque in 232 the AT group and 120 motor units at 20%. In the control group, we found a total of 127 motor 233 units at 10% and 114 motor units at 20% peak torque. GM analysis yield 101 motor units in 234 the AT at 10% peak torque and 128 motor units at 20%. In the control group, we found a total 235 of 113 motor units at 10% and 116 motor unit at 20%. For GL analysis, we identified a total of 236 20 motor units at 10% peak torque in the AT and 29 motor units at 20%. In the control group, 237

we found a total of 43 motor units at 10% and 26 motor units at 20%. GL was the muscle with 238 the least amount motor unit found in single contractions, and some were lost during motor unit 239 tracking between the two contractions of the same intensity. 240 241

Analysis of SOL motor unit mean discharge rate showed difference between torque 243 intensities (F=20.118, p<0.001, η2p=0.04) but no differences between groups (F=0.324, 244 p=0.574) or intensity × group interaction (F=0.512, p=0.474). There was an increase in motor 245 unit mean discharge rate in both groups as torque increased. In the AT, motor unit mean 246 discharge rate increased from 6.98pps (6.72 to 7.24) at 10% peak torque to 7.29pps (7.02 to 247 7.55) at 20%; the motor unit mean discharge rate in the control group also increased, from 248 7.40pps (7.16 to 7.63) at 10% peak torque to 7.86 pps (7.55 to 8.16), (Figure 1) . 249 250 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Motor unit mean discharge rate of Soleus during 10% and 20% peak isometric contraction. 253

Each dot represents a single motor unit data point, coloured by participants. Mean and 95% 254 confidence interval are offset to the left to facilitate visualisation. pps = pulse per second. 255 256 Similar to SOL, in GM analysis, we observed differences in motor unit mean discharge 257 rate between different torque intensities (F=75.554, p<0.001, η2p=0.15) but no differences 258 between groups (F=0.488, p=0.492) or intensity × group interaction (F=1.063, p=0.303). In 259 both groups, motor unit mean discharge rate increases with the increase in torque intensity. In 260 the AT group, motor unit mean discharge rate increased as torque increased from 10%, 8.38pps 261 (7.44 to 9.33) to 20% peak torque, 9.54pps (8.61 to 10.48). The same was observed in the 262 control group, motor unit mean discharge rate increased as torque increased from 10%, 8.63pps 263 (7.75 to 9.51) to 20%, 10.10pps (9.21 to 11.00) (Figure 2 ). On the other hand, in GL, we found 264 an intensity × group interaction (F=27.955, p=0.001, η2p=0.11). While in the AT, motor unit 265 mean discharge rate did not change as torque increased from 10% peak torque, 8.24pps (7.08 266 to 9.41) to 20%, 8.52pps (7.41 to 9.63, p=0.540); however, in the control group, motor unit 267 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint mean discharge rate increased as torque increased from 10%, 8.39pps (7.25 to 9.53) to 20% 268 peak torque, 10.07pps (8.89 to 11.25, p<0.001), (Figure 3 , ** denotes statistical difference). 269

The control group had a higher motor unit mean discharge rate at 20% torque compared to the 270 AT group. Motor unit mean discharge rate of Gastrocnemius lateralis during 10% and 20% peak isomet-283 ric contraction. Each dot represents a single motor unit data point, coloured by participants. 284

Mean and 95% confidence interval are offset to the left, to facilitate visualisation. pps = pulse 285 per second. ** denotes statistical difference, p<0.001. 286 287

SOL had no difference in coefficient of variation of motor unit discharge rate between 289 intensities (F=2.963, p=0.086) or between groups (F=0.151, p=0.700). In the AT, the 290 coefficient of variation of motor unit discharge rate was 10.3% (8.8 to 11.8) at 10% peak torque 291 and 9.3% (7.9 to 10.7) at 20%; in the control group, the coefficient of variation of motor unit 292 discharge rate was 10.2% (8.8 to 11.7) at 10% peak torque and 10.0% (8.6 to 11.5). GM 293 presented difference in coefficient of variation of motor unit discharge rate between intensities 294 (F=51.203, p<0.001, η 2 p=0.10) but not between groups (F=3.673, p=0.07) and had no intensity 295 × group interaction (F=0.872, p=0.350). In the AT, the coefficient of variation of motor unit 296 discharge rate was 12.15% (11.1 to 13.1) at 10% peak torque and 9.16% (8.1 to 10.1) at 20%; 297 in the control group, the coefficient of variation of motor unit discharge rate was 10.75% (9.8 298 to 11.7) at 10% peak torque and 8.45% (7.4 to 9.5). In GL, as well as in GM, we observed 299 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint difference between intensities (F=5.222, p=0.024, η 2 p=0.05) but not between groups (F=0.661, 300 p=0.428) or intensity × group interaction (F=2.779, p=0.098). In the AT, the coefficient of 301 variation of motor unit discharge rate was 10.9% (8.5 to 13.2) at 10% peak torque and 11.9% 302 (9.6 to 14.0) at 20%; in the control group, the coefficient of variation of motor unit discharge 303 rate was 12.7% (10.5 to 14.9) at 10% peak torque and 12.9% (10.5 to 15.2). 304 305

There were no differences in torque steadiness analysis between groups in either of the 307 two intensities analysed. Mean coefficient of variation of torque at 10% peak torque, in the AT 308 group was 1.06 (0.79 to 1.32) and 1.13 (0.90 to 1.37, p=0.656) in the control group; and at 309 20%, mean coefficient of variation in torque in the AT group was 0.80 (0.57 to 1.03) and 0.92 310 (0.73 to 1.11, p=0.375) in the control group. 311 312 4. Discussion 313

The present study aimed to determine if runners with chronic mid-portion AT had re-315 duced neural drive to the triceps surae and if there were muscle-specific differences in motor 316 unit discharge characteristics within the triceps surae. For that, we analysed motor unit mean 317 discharge rate and coefficient of variation of motor unit discharge rate of each individual mus-318 cle of the triceps surae during isometric contractions of increasing intensities. We also aimed 319 to determine if the AT group had reduced torque steadiness. 320

Our data indicate that runners with AT have reduced neural drive to GL during the 321 increase in plantar flexor isometric torque output. We confirmed our primary hypothesis, 322 demonstrating a muscle-specific difference in neural drive in the AT group and a reduced 323 neural drive to GL during the increase in plantar flexor torque, not observed in the control 324 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint group. However, we had also hypothesised that the neural drive to the triceps surae of the AT 325 group would be reduced, but GM and SOL were no different from controls. Furthermore, we 326 did not confirm our second hypothesis, as we found no differences in the coefficient of variation 327 of motor unit discharge rate in any of the muscles, nor did we find differences in triceps surae 328 torque steadiness between groups. 329 330

It has been previously identified that the three muscles of the triceps surae, although 332 synergists as ankle plantar flexors, may have an independent neural drive from one another, 333 allowing independent recruitment strategies for better joint control 12 . Our study also observed 334 independent neural drive within the triceps surae. Further, we found different neural strategies 335 between groups in only one of the three muscles of the triceps surae. Our data show that the 336 AT group does not use the GL as effectively as healthy controls to match the increase in plantar 337 flexor torque intensity. Although there was no difference in motor unit mean discharge rate 338 between groups in GL when comparing the same level of torque, the AT group had a reduced 339 motor unit mean discharge rate with the increase in torque, outlining a change in recruitment 340 strategy in GL that was not observed in the control group. Similar findings have been reported 341 in another study with runners with AT 8 . They used the physiological cross-sectional area and 342 normalised RMS EMG to calculate the index of force of each muscle and estimate individual 343 muscles' contribution to triceps surae force production. GL had a significantly lower contribu-344 tion to overall triceps surae force output; thus, reduced neural drive compared to healthy coun-345 terparts. Muscle force depends on motor unit discharge rate, which is proportional to the neural 346 drive to the muscle. In healthy individuals, motor unit discharge rate increases to adjust for an 347 increase in torque intensity 10 . Contrary to what was previously suggested in the literature 4 , we 348 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint found no differences in SOL motor unit mean discharge during the increase in torque, which 349 suggests that SOL contribution to plantar flexor force is not impaired in AT. 350

The reduced neural drive to GL observed in our study seems relevant in the persistent 351 muscle deficits observed in AT 6 . Perhaps current treatment strategies for AT fail in effectively 352 rehabilitating GL function; therefore, maintaining this reduced neural drive and contribution to 353 force production during ankle plantar flexion. Individual muscles of the triceps surae have 354 independent neural drive 12 . Therefore, finding strategies to increase GL recruitment and 355 contribution during exercise is important, as altered muscle coordination (i.e. individual muscle 356 contribution for force production within a muscle group) may lead to unequal loading to the 357 Achilles tendon 28 . Performing heel raises with the foot positioned with toes pointed inwards, 358 significantly increased GL motor unit discharge rate compared to toes neutral in healthy 

Torque variability was measured during 10 and 20% relative peak isometric torque 369 plateau. We found no differences between groups in coefficient of variation of motor unit 370 discharge in any of the muscles of the triceps surae nor did we find differences in triceps surae 371 torque steadiness. All three muscles of the triceps surae were equally matched to controls in 372 the two submaximal intensities tested. coefficient of variation of motor unit discharge 373 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint represents, at an individual muscle level, the ability to effectively control muscle torque and it 374 is an important measures that can help explain motor performance 19, 30 . Fluctuations in torque, 375 coefficient of variation of motor unit discharge rate and torque steadiness are more variable in 376 lower intensities than in higher torque intensities, hence why 10 and 20% intensities were used 377 for analysis 19 . 378

Based on our findings, the ability of the triceps surae in controlling torque during 379 submaximal contractions is not affected in runners with AT, which aligns with another study 31 . 380

Torque steadiness is affected by pain 18 , and reduced torque steadiness has been reported in 381 other chronic 18 and painful musculoskeletal conditions 15,20 . In our study, we used submaximal 382 torque intensities and none of our participants reported pain during testing; however, we cannot 383 assert if such changes in torque steadiness and coefficient of variation of motor unit discharge 384 rate would not occur during activities that provoke pain in this group, such as running. 385 386

We were unable to effectively track the same motor unit from 10 to 20% peak torque. 388

We tried tracking the same motor units across the two intensities, but this has markedly reduced 389 the number of motor units left for analysis. Although both intensities used in this study are 390 considered as of low threshold, each motor unit is unique from another, and motor unit tracking 391 would have provided more robust information about each motor unit unique response to the 392 increase in torque. The EMG device used was limited to up to 2 × 32-channel adaptor, not 393 allowing sampling of all three muscles at the same time. Future studies might need to consider 394 using devices that allow recording of all three muscles during the same contractions and using 395 electrodes with more channels (i.e. 64 electrode matrices) to increase the number of motor unit 396 identified during decomposition when estimating neural drive to the triceps surae. Another 397 limitation that should be highlighted is the type of contraction used for analysis. HD-EMG 398 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint analysis provides reliable estimates of motor unit discharge rates 26 ; however, it requires 399 isometric contractions for motor unit analysis. Thus, the observations of neural drive from this 400 study cannot be extrapolated into dynamic tasks such as heel raises or running. Furthermore, 401 we used submaximal intensities of relative peak isometric torque, as this facilitates motor unit 402 identification, therefore, it is possible that during higher torque intensities, which demand more 403 torque of each individual muscles, the differences observed in this study would be greater. 404 405

Our data suggest that runners with mid-portion AT have a muscle-specific deficit in the 407 triceps surae, possibly creating heterogeneous loading to the Achilles tendon and contributing 408 for the high recurrencies 32 of AT. We observed reduced motor unit discharge rate, (i.e. reduced 409 neural drive) in GL during the increase in plantar flexor torque demands but not in GM or SOL. 410 This deficit in motor unit excitability in GL might be greater during activities that require 411 greater plantar flexor torque, which could contribute to overload the Achilles tendon. Different 412 strategies to try and increase GL activation during plantar flexion resistance training could be 413 beneficial for AT, such as adopting different feet position during heel raise. Such rehabilitation 414 strategy should be studied in patients with AT to further understand how the reduced 415 contribution of GL impacts Achilles tendinopathy and how implementing strategies to increase 416 the neural drive to GL would affect AT patient outcomes. 417 418 Authors' contribution 419 GLF and GST designed the study. GLF and LBRO conducted experiments. GLF analysed the 420 data and drafted the first version of the manuscript. RLS developed a MATLAB script for 421 motor unit discharge rate, recruitment threshold, coefficient of variation of motor unit 422 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint pathology: What is its merit in clinical practice and research? Br J Sports Med. 447 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted May 7, 2022. ; https://doi.org/10.1101/2022.05.05.22274750 doi: medRxiv preprint

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The authors declare having received no funding for this study. 433The authors thank all the volunteers who participated in this study for their contribution to the 434 development and achievement of this research. 435 436