Mechanotendography: description and evaluation of a new method for investigating the physiological mechanical oscillations of tendons using a piezo-based measurement system

The mechanotendography (MTG) analyzes mechanical oscillations of tendons during muscular actions. It can be assessed as equivalent to mechanomyography just applied for tendons. Since this method is unknown, the aim of this investigation was to evaluate the technical reliability of a piezo-based measurement system used for MTG. The reliability measurements were performed using audio files played by a subwoofer. The thereby generated mechanical pressure waves were recorded by a piezoelectric sensor based measurement system. The piezo sensor was fixed onto the subwoofer’s coverage. An audio of 40 Hz-sine oscillations and, to stay close to human applications, four different formerly in vivo recorded MTG-signals from Achilles and triceps brachii tendon were converted into audio files and were used as test signals. Five trials with each audio were performed. One audio was used for repetition trials on another day. The correlation of the recorded signals were estimated by the Spearman correlation coefficient (MCC), the intraclass-correlation-coefficient (ICC(3,1)), Cronbach’s alpha (CA) and by mean distances (MD) between the signals. They were compared between repetition and random matched signals. The repetition trials show high correlations (MCC: 0.86 ± 0.13, ICC: 0.89 ± 0.12, CA: 0.98 ± 0.03), low MD (0.03 ± 0.03V) and differ significantly from the random matched signals (MCC: 0.15 ± 0.10, ICC: 0.17 ± 0.09, CA: 0.37 ± 0.16, MD: 0.19 ± 0.01V) (p = 0.001 – 0.043). This speaks for an excellent reliability of the piezo-based measurement system in a technical setting. Since research showed that the skin above superficial tendons oscillates adequately, we estimate this tool as valid for the application in musculoskeletal systems. It might provide further insight into the functional behavior of tendons during muscular activity.

117 oscillations of tendons during isometric muscle action can be detected by piezoelectric 118 sensor based measurement systems, which will be presented here. We estimate this 119 technique as unique and innovative. It is suggested to name this method 120 mechanotendography (MTG). It can be considered as analogy to MMG but applied for 121 tendons [3,28,29].

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Because of the novelty of this method using a piezo-based measurement system, 123 there is the need of evaluation. The measurement system is adopted from music. The 124 piezoelectric sensors and amplifiers are usually used to pickup and amplify auditory signal 125 from instruments. Thereby, they have been proofed to be suitable to take off harmonic 126 oscillations. Anyway, the justifiable question remains, whether or not piezoelectric 127 sensors are suitable to record stochastically distributed mechanical oscillations in 128 frequency areas around 20 Hz like those produced by muscles or tendons.

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Accordingly, the first step is to investigate whether or not this measurement

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To examine trial-to-trial reliability, five repetitions of each audio were played and 187 recorded by the piezo-based measurement system at the same day. The 230 Results

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To illustrate the reproducibility of the oscillation characteristics of the original 232 MTG-signals and the recorded corresponding audio-signal from this investigation, Figure   233 2 shows exemplarily one original MTGAchilles-signal and the corresponding recorded 234 signal in the present setting using the subwoofer with fixed MTG-sensor. As shown, the 235 frequency is reproduced precisely, the amplitudes differ.

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The curve shapes of the identical recorded trials are shown in Figure 3. In each 238 diagram, five or, respectively, ten repetitions are displayed and reveal a good 239 reproducibility. The day-to-day trials of the audioMTGAchilles_1 are displayed in Figure   240 4. As can be seen, the 10 signals lie highly reproducible one above the other. For 241 quantification of this, the parameters (1) to (4) are regarded.

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For each comparison a high effect size of r > 0.90 is obtained, which underlines 268 the significant differences between the identical and random groups.

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However, not all piezoelectric sensors are suitable for the use of MMG or MTG.
344 We presented here the -in our experience -most appropriate ones. However, we already 345 had a new batch of the Shadow SH 4001 sensor, which have changed in quality. Therefore, 346 we switched to the Shadow SH-SV1, which proofed to be suitable. There are other 347 pickups, which turned out to be suitable with regard to the SNR. However, the one we 348 tested had a larger diameter and, therefore, turned out to be not as practicable for fixing 349 onto the skin above the muscle belly or tendon. Beside the choice of a suitable 350 piezoelectric sensor, an essential factor is the used amplifier. As mentioned in the method 351 setting, for MMG and MTG there is the need of an amplifier, which is capable to amplify 352 low frequency ranges, too. Since the Nobels preamp booster pre-1 turned out to be 353 suitable and reveals extremely clear signals, they are used in our investigations.   422 therefore, could reveal further insights into the quality of motor controlling processes 423 and, in general, into motor control.

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The Achilles tendon, e.g., is alternated tightened and released during walking. Due 425 to the impacts of floor during heel strike and the contraction of the triceps surae muscle 426 during push off, the sinew is tightened. The behavior of the tendinous string during and 427 after this impact is influenced by the mentioned active drives of the muscles but also by 428 its passive mechanical properties. The tension and length influences the resonance 429 frequency like it is the case for a chord of a guitar. Thereby, the tendon function as band 430 pass. Therefore, certain frequencies are suppressed and the surrounding soft tissues will 431 have a vibration damping effect. It is therefore assumed that the oscillations of tendons 432 not simply reflect muscular vibrations, but the behavior is highly influenced by the tension 433 and the vibrations of their driving muscles. It is supposed that if the motor control is 434 restricted, changes in the mechanical oscillating behavior of tendons might reflect them 435 and, therefore, investigating those mechanical tendinous oscillations might provide a 436 more functional insight into the properties of tendons. Hence, the non-invasive and easy 437 applicable method of mechanotendography could be a promising option to be applied in 438 further studies investigating the musculoskeletal-system to enlarge the knowledge of the 439 behavior of those relevant bodily structures in healthy and diseased persons and to 440 examine this promising tool for probable applications in diagnostics.

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The repetition trials showed that the used piezo-based measurement system is 443 suited to measure mechanical oscillations reproducibly. It is concluded that the MTG is a 444 reliable and valid tool to measure tendinous oscillations. It seems reasonably transferable 445 to muscular oscillations (mechanomyography).