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acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No 5, 2018. DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

  

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation and 

acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No 5, 2018. DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

RESEARCH ARTICLE  

 

A FEASIBILITY STUDY TO INVESTIGATE IF THERE IS A CORRELATION 

BETWEEN SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 
 
Buis A1*, Guarato F2, Law J1, Ralston Z1,  & Courtney A3 

 
1 Department of Biomedical Engineering, University of Strathclyde, Glasgow, Scotland, UK. 
2 Department of Electronic and Electric Engineering, University of Strathclyde, Glasgow, Scotland. UK. 
3 The London Orthotic Consultancy Ltd, Kingston-Upon-Thames, Greater London, England, UK.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
*CORRESPONDING AUTHOR  

Dr. Arjan Buis, Department of Biomedical Engineering, University of Strathclyde, Wolfson Centre,  

106 Rottenrow, Glasgow, G4 0NW, Scotland, UK. 

ORCID number: https://orcid.org/0000-0003-3947-293X 

Email: arjan.buis@strath.ac.uk  

DOI: https://doi.org/10.33137/cpoj.v1i1.30354                                                                       

CPOJ EDITORS-IN-CHIEF 

Dr. Silvia U Raschke, British Columbia Institute of Technology (BCIT), Canada 

Dr. Thorvaldur Ingvarsson, Orthopaedic Surgeon, University of Iceland, Iceland 

ABSTRACT 
 

BACKGROUND: Background:   Acoustic emission from structures subject to 

external loads can be monitored to detect internal damage before destruction occurs. 

It is hypothesised that deformation of soft tissue will emit acoustic signals which 

may aid early detection of deep tissue injury, particularly in the lower limb amputee 

population. No previous studies have applied this method to biological soft tissue. 

OBJECTIVE: To determine if skeletal muscle tissue produced measurable acoustic 

emission during dynamic tensile loading with the aim to establish a reliable 

biomarker for lower limb prosthetic socket fit quantification and prosthetic health. 

STUDY DESIGN: Experimental study design. 

METHODOLOGY: In this research article, Sus scrofa domesticus (pork) muscle and 

Gallus gallus domesticus (chicken) muscle specimens (10mm width x 45mm height 

x 4mm depth) were submerged into saline baths while an Instron testing machine 

applied displacement controlled tensile loads. Time stamped, load, displacement 

and acoustic signal (hydrophone) data was collected. 

FINDINGS: The pork muscle was tested to failure being subject to tensile load. Prior 

to failure, no peaks were found in the amplitude or frequency of the acoustic signal 

to indicate that either tissue deformation or failure was occurring.  Data gathered 

during chicken muscle testing was inconclusive. 

CONCLUSIONS: Results displayed that tensile testing of pork intercostal muscle 

produced tissue deformation and failure with no detectable change in the amplitude 

or frequency of the background sound during tensile loading. The other specimens 

failed before reaching the same levels of tensile load. Further studies are required in 

order to address the numerous limitations of this study. 

1 

ARTICLE INFO 

Received: May 08, 2018 

Accepted: August 24, 2018 

Published: August 28, 2018 

CITATION 

Buis A, Guarato F, Law J, 

Ralston Z, Courtney A. A 

feasibility study to investigate if 

there is a correlation between 

soft tissue deformation and 

acoustic emission. Canadian 

Prosthetics & Orthotics Journal, 

Volume 1,  Issue 1,  No 5,  2018. 

KEYWORDS 

Prosthetics, Acoustic Emission, 

Deep Tissue Injury, Pressure 

Ulcer, Tissue Shear, Prosthetic 

Socket Design, Diabetic Foot, 

Soft Tissue, Lower Limb 

Prosthetic, Tensile Load. 

OPEN  ACCESS 

SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 

ABBREVIATIONS 

AE  : Acoustic Emission 

DTI : Deep Tissue Injury 

DPU: Deep Pressure Ulcers 

https://doi.org/10.33137/cpoj.v1i1.30354
https://orcid.org/0000-0003-3947-293X
mailto:arjan.buis@strath.ac.uk
https://doi.org/10.33137/cpoj.v1i1.30354


 

Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

OPEN  ACCESS 

SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
 

INTRODUCTION 

In the UK, 72% of lower limb amputations result from 

dysvascularity and/or neuropathy, with the majority of 

amputations occurring at trans-tibial level (1).  The two 

commonly used trans-tibial socket designs are based on 

conflicting principles, both of which have a lack of 

substantiation regarding the internal conditions of the 

residual limb within the socket (2). The first design, the 

PTB socket, applies specific loads to pressure-tolerant 

areas, whereas the hydrocast socket aims to dispense 

pressure uniformly through the “stiffest path principle” 

(3). It is advocated that hydrocast sockets have a greater 

ability to generate “ideal” pressure distribution together 

with a reduction in internal shear (2).There is, at present, 

no consensus on what defines an “optimal” socket 

design or fit (3, 4).  

There is a growing belief among researchers that the 

‘coupling’ stiffness between the hard prosthetic socket 

and the weight bearing structure, the skeleton, is a major 

factor in optimising the quality of a socket design (3, 5), 

as it can reduce the amount of tissue deformation during 

load bearing and consequently reduce the shear stresses 

in the soft tissues (3-5).  Due to the aetiology of these 

amputations, the residual limb is at increased risk of 

developing a deep tissue injury (DTI) due to peripheral 

neuropathy, impaired blood supply and/or poor skin 

condition (6).  If a DTI is misdiagnosed or diagnosed 

late in this population, there can be a risk of revision 

amputation being necessary due to increasing tissue 

necrosis. This severely impacts the patient’s quality of 

life and places excess strain on the healthcare system 

(7). This highlights the avid prerequisite for 

comprehensive assessment of prosthetic socket fit and 

function.   

To date, research focusing on residuum-socket interface 

pressures has been unable to establish a direct 

relationship between interface pressures and the 

magnitude of deep tissue stresses. These studies have 

inadequately quantified the mechanical stresses within 

deep internal tissues (8, 9).  As a result, interfacial 

pressures alone are unlikely to change socket design 

principles and additional performance/bio-  markers are 

therefore needed. 

Deep Tissue Injury (DTI) 

Ulceration as a direct result of pressure can be 

characterised into six differing classifications according 

to the National Pressure Ulcer Advisory Panel. 

However, they are broadly divided into two groups: 

superficial or deep (10). Superficial ulcers can be 

triggered by a variety of mechanisms, including a 

combination of moisture and heat, in conjunction with 

frictional and shear forces, infection, poor nutrition 

and/or peripheral neuropathy (8, 10-12).  These ulcers 

are confined to the epidermis, can be identified by visual 

inspection of the skin (10) and have been confirmed 

reversible (8, 13). 

On the contrary, deep pressure ulcers (DPUs) are 

secondary to DTIs and have a different mechanism of 

origin (8). The term DTI designates an ulcer that 

experiences high pressure at the bone-muscle interface, 

below intact skin (11, 13). A DTI can have the visual 

appearance of a deep bruise and can often be mistaken 

for a superficial pressure ulcer, resulting in appropriate 

diagnosis often occurring at a late stage (6, 8, 10). As 

deep skeletal muscle undergoes necrosis secondary to 

pressure, it begins to stiffen which transfers the stresses 

to more superficial layers of tissue (8, 9, 12).  As a 

result, necrotic tissue will develop in these superficial 

layers continuing the positive-feedback cycle of muscle 

stiffening and tissue necrosis until the ulcer appears on 

the epidermal layer (8, 12).  Exemplar situations in 

which DTIs can often occur include immobile patients 

in bed, wheelchair bound patients or users of 

orthoses/prostheses - the primary focus of this study (14, 

15). 

DTI’s have historically been alleged to be instigated by 

factors which can include pressure-related ischaemia 

and/or ischaemia-reperfusion injuries due to the quick 

flow of blood rushing back to ischaemic tissues upon 

withdrawal of the applied load (6, 8, 11, 13, 16, 17). 

However, more contemporary studies suggest that 

tissue deformation and ischaemia related deformation 

injuries can also contribute to a DTI (13) . Interestingly, 

it has been shown that tissue deformation results in 

permanent muscle damage following two hours of 

compressive loading whereas muscle damage due to 

ischaemia can be reversible (14, 17). This information 

highlights the necessity of imminent investigation 

regarding the compression of soft tissues and the 

applied deformation forces on soft tissues, in order to 

reduce the risk of a DTI. 

Portnoy et al assessed the specific forces acting on a 

transtibial residuum and analysed the internal 

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

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SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
mechanical condition of the soft tissues during static 

weight bearing. They established that in a transtibial 

residuum, soft tissues are subject to tensile, compressive 

and both internal and external shear forces during static 

and dynamic loading. It was concluded that the 

magnitude of tensile and shear strain within the tissues 

was much greater than the compressive strain (2) . 

Differing academics seconded this when they 

demonstrated that DTIs tend to develop in areas where 

highest shear strain values have been recorded during 

tissue loading (14) and that tissue damage increases 

with increasing shear strain (13). Thus, it has been 

concluded that the force in a transtibial socket most 

likely to cause a DTI is shear force. 

Shear forces can be separated into internal and external 

categories. Shear can develop when external frictional 

forces stretch the top layer of the skin as it slides against 

a supporting surface (e.g. against a bone or against a 

prosthetic socket) (18, 19). This theory can then be 

applied to internal shear forces where subcutaneous 

tissue and muscle glide over one another. Internal and 

external shear forces interact. For example, the presence 

of internal shear increases around the ischial tuberosities 

where the skin has been stretched. This type of shear is 

commonly seen in pressure ulcer formation, especially 

around the sacrum, however, this can occur on any part 

of the body. Both external frictional forces and internal 

tissue shear contribute to the six certified classifications 

of pressure ulcers, however, these two forces are more 

significant in DTIs than in superficial ulcers (18).  

Contemporary publications exploring socket fit have 

utilised sensors in order to research socket-interface 

pressures in transtibial users (20) . However, it is now 

recognised that the condition of the deep muscle tissue 

is not accounted for in these studies and that the close 

proximity of the deep vascular structures to bony 

prominences is likely to result in much higher pressures 

than present on the surface of the residuum (8).  

Therefore, the sole use of interface sensors at the skin’s 

surface is inadequate in detecting high pressures at the 

bone-muscle interface. Consequently, this method does 

not highlight DTI risk and thus, does not truly quantify 

conclusive results with regard to an ‘optimal socket fit’. 

Detection of DTI’s could therefore be used as an aid to 

assess the biomechanical fit of a prosthetic socket, along 

with other methods, using tissue deformation as a 

parameter of measurement. 

Measurement of DTI in Clinical Practice                                                                                                                                                                                                                     

At present, there are several current clinical techniques 

which have a body of supporting research substantiating 

their usage in the detection of DTIs. However, these 

current methods prove to be invasive (serum chemistry), 

provide latent information i.e. not real time (urine and 

serum chemistry along with MRI), expensive (MRI) or 

require specialist knowledge to interpret (sonography 

and MRI). Currently, sonography seems most 

promising in DTI detection due to portability and 

economic factors, however, further research is required 

to define the specific characteristics. 

Another possible method of detecting the presence of a 

DTI, which holds pre-empted potential, may be to 

analyse acoustic emission (AE) that may arise from 

shear stress. 

When an external load is applied to a material, 

deformation occurs as a result of the relative movement 

between atomic chains, with this movement emitting 

acoustic waves (21, 22).  The phenomenon is known as 

AE and currently has well established applications in 

structural and civil engineering.  AE detection is 

classified as a non-destructive and a non-invasive real-

time method to assess the integrity of the internal 

structure of a material (21-23).  It does not require an 

input signal as the acoustic waves are generated by the 

material and transducers can be placed anywhere on the 

materials surface to monitor acoustic output (22).  

However, the detection of AE may be affected by 

background noise and there is potential for the signal to 

be distorted as it propagates through a material. 

Therefore, mechanically advanced microphones may be 

obligatory in order to accurately capture the signals (21, 

23).    

Sound waves are released as muscle contracts/is loaded 

and these waves may then be detected using acoustic 

myography (AMG), a specialised AE technique (24-

28). It is believed that the noise arises from the lateral 

oscillations of fibres (24, 28, 29). Interestingly, the 

contractile force is proportional to the volume of the 

acoustic noise (24, 25).  This signal has been utilised, by 

adapting AMG, to analyse muscle activity and fatigue 

(24, 26, 28, 29) and to control an externally powered 

upper limb prosthesis (27). The movement which occurs 

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

OPEN  ACCESS 

SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
within muscle tissue during deformation could 

potentially release AE waves as atomic dislocations 

produce pressure waves that travel through the tissue 

and are detectable on the surface (23, 24) . Tendons and 

skeletal muscles that are composed of uni-directional 

fibres have the potential to develop shear stresses when 

these tissues are loaded multi-directionally (30), which 

can be identified by AE. 

Research Hypothesis and Objectives 

The objectives of this study are to firstly select a 

microphone suitable for this experimental work to 

enable simultaneous recording of acoustic frequencies 

and deformation data from tissue specimens under 

tensile loading, to visually examine data for acoustic 

peaks at critical points in the deformation data collected 

and then to present the results with future 

recommendations. This study hypothesizes that acoustic 

signals are emitted from skeletal muscle tissue when it 

is deformed during loading. 

METHODS  

Specimens and Equipment 

Two baseline tests were carried out using a sample of 

fabric webbing. This style of webbing was selected as it 

is manufactured from a weaved structure composing of 

uni-directional fibers with low elasticity – crudely 

comparable with soft tissue. The tensile testing was 

performed with an Instron testing machine 

(Electropuls™ E10000) and allowed to exercise a 

displacement control test. This means that the tissue 

under investigation is stretched with a selected loading 

rate samples were loaded under tension (1mm/s to 

20mm) and simultaneous length increase is recorded. 

Biological test specimens firstly included intercostal 

muscle and bone from sus scrofa domesticus (pork). 

Additionally, domensticus (chicken) was selected, 

ensuring the muscle belly of the gastronomicus and its 

attachment to bone was intact. Bone was utilised in 

order to maintain tissue structure throughout testing 

(Figure1).  These biological test specimens were 

selected due to the organized uni-directional nature of 

the fibers and the ease of availability. The specimens 

were dissected when the muscle was frozen, however, 

due, to the initial freezing process, the muscles were 

contorted and had to be thawed and refrozen into a more 

easily dissectible state. The specimens were thawed at 

room temperature prior to testing. Samples were 

prepared by isolating the muscle and cutting the bone at 

the attachment points into a size suitable for the clamps 

to receive (Figure 2), with an approximate width of 1cm 

per unit. Bone was used as a clamping point to minimise 

slippage during testing. 

Figure1. Pork specimen clamped in bath via rib bones.  

Enlarged view of specimen illustrating preparation and size 

of sample. 

 

Figure 2. View looking downwards into bath with chicken 

specimen clamped in place (Test 1).  As part of specimen was 

clamped out with the bath, a rod was used to conduct the 

“tapping” signal to synchronise data collection. 

 

An Instron testing machine (Electropuls™ E10000) 

with a 1kN load cell was used to apply a tensile force to 

each specimen.  Specimens were clamped onto serrated 

grips and submerged in a physiological saline solution 

(Composition: 0.9g sodium chloride – Purified water 

QS 100ml) bath (30mm x 30mm x 30mm) to simulate 

an in vitro environment (Figure 3).  As sound 

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

OPEN  ACCESS 

SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
propagates through liquid at a faster rate than through 

air, the test environment was anticipated to aid acoustic 

detection, providing an improved medium for the 

hydrophone.  Unfortunately, the temperature of the 

saline could not be controlled in this study due to the 

customisation of the test equipment. 

An omni-directional hydrophone with an inbuilt 

preamplifier was placed within the bath to detect 

acoustic signals. The selected hydrophone had a 

frequency range of 0.1Hz – 180kHz (Bruel and Kjaer 

Type 8103) and a voltage sensitivity of 25.4µV/Pa. The 

sensitivity range of the Hydrophone is within the 

acoustic range between 20Hz and 20KHz as illustrated 

in Figure 4 and capable to operate in the target range.  

The hydrophone was consistently positioned 10mm 

from the tissue under analysis. A digital oscilloscope 

(Gratten GA1000CAL) recorded hydrophone data in 

real-time and had a sampling rate of 40MHz.  

The Instron and hydrophone data were recorded 

simultaneously with the load cell “tapped” at the 

beginning of each test to act as a data synchronisation 

point. 

Test Protocols 

WaveMatrix™ software gathered time, load and 

displacement data at a sampling rate of 1000Hz. The 

amplitude of the acoustic signal was recorded on the 

oscilloscope with a sampling rate of 40MHz.  Microsoft 

Excel™ was used to generate graphical representations 

of the WaveMatrix™ data.  The oscilloscope captured 

amplitude information which was imported to MatLab 

allowing for signal processing in the frequency and time 

domain by applying the Fourier transform to the full 

data set:  

 

𝑋(𝑘) =∑𝑥(𝑗)𝜔𝑁
−(𝑗−1)(𝑘−1)

𝑁

𝑗=1

 

                       where,  𝜔𝑁 = 𝑒(−2𝜋𝑖)/𝑁 

 

There is a lack of current available evidence to support 

the optimal tensile displacement rate and threshold 

during testing, as studies commonly select 1, 5 and 

10mm/s for tissue compression rates with no apparent 

justification (31, 32).  In this study, displacement rate 

and threshold was restricted by the collection of real-

time acoustic data from the oscilloscope display. 

 

 
Figure 3. Instron test machine setup including saline bath 

and hydrophone. 

 

 

 
Figure 4. Sound frequency spectrum. 

 

Two baseline tests with fabric webbing were conducted 

at a ramp rate of 1mm/s to a total displacement of 20mm 

by a tensile load.  One sus scrofa domesticus sample and 

three gallus gallus domesticus samples were loaded 

under tension at 1mm/s to 20mm/s.  WaveMatrix™ 

software was instructed to collect data for 2 seconds 

prior to the commencement of displacement to allow the 

“tapping” of the load cell to be recorded. 

 

RESULTS 

Tests with three different tissues have been conducted 

but not all have been successful. The narrative will 

describe the tests that were disregarded and the main 

body of the result section will describe the tests used for 

analysis.   

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

OPEN  ACCESS 

SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
Narrative 

A baseline test with a fabric with uni-directional fiber 

orientation, followed by experiments using animal 

specimen, Sus scrofa domesticus (pork) and Gallus 

gallus domensticus (chicken) respectively. During the 

first tests utilising the fabric specimen, the specimen 

slipped in the clamp which disturbed the recording and 

were disregarded. During tensile testing of the pork 

intercostal muscle specimen, the tissue failed by 

detaching from the bone. The chicken specimen utilised 

for test 2 slipped in the clamp during testing, being 

subject to a maximum load of 18N and was omitted 

from the data set.  

Baseline 

In test 2, the fabric failed after approximately 7s at a 

tensile load of 112N and at a displacement of 8mm 

(Figure 5). 

 

 

 

 

 

 

 

 

 

 

 

Figure5. Plot of load and displacement against time of 

baseline test 2 using fabric webbing.  Initial data spike from 

“tapping” of load cell for data synchronisation point clearly 

captured. 

 

Figure 6. Load/displacement against time of pork intercostal 

muscle under tensile load resulting in specimen failure at 34 

N. 

 

Sus scrofa domesticus Testing 

The inter-costal muscle failed after 3 seconds at a load 

of 34N and a displacement of 3mm (Figure 6). 

 

Gallus gallus domensticus Testing 

In test 1 with chicken muscle, the muscle bulk was 

entirely submerged in the bath, however, one end of the 

bone lay out with the bath due to the specimen size 

(Figure 2). As part of the specimen lay out with the bath, 

a rod was used to conduct the “tapping” signal in order 

to synchronise data collection.  This specimen 

successfully completed testing without fail and 

experienced a maximum load of 13N. 

The specimen utilised for test 2 comprised of a larger 

muscle belly which also lay out with the dimensions of 

the bath. In test 3, the specimen failed after 18.3s at a 

displacement of 16mm (Figure 7) as the tendon became 

detached from bone.  The peak load applied to the 

specimen was 20N, occurring at 12s.   

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 7. Load /displacement against time during test 3 on 

chicken tissue in which specimen failed.  

 

Acoustic Emission  data 

In both the fabric and the chicken specimens, the 

acoustic data collected during testing did not include 

enough data points. This resulted in inadequate findings 

on these samples.  Thus, acoustic data could only be 

analysed on the sole pork specimen.  The amplitude 

recording (Figure 8 and 9) detected the sound created by 

the “tapping” signal, with only the trace of background 

noise before and during the tensile testing phase.  Figure 

10 shows a plot of amplitude and frequency (Figure 10).  

 

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

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SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
DISCUSSION 

This study investigated the hypothesis that animal soft 

tissue emits acoustic noise when subject to deformation 

from an externally applied load. Due to the limited 

number of trials performed in this study, statistical 

analysis could not be performed.   

 

 
Figure 8. Amplitude against time (s) of tensile test on pork 

specimen. Data spike from “tapping” of load cell clearly 

displayed. 

 

 
Figure 9. Zoomed in display of amplitude against time (s) for 

tensile test on pork specimen.  This illustrates the absence of 

any sound which may be associated with deformation of the 

specimen. 

 

 
Figure 10. Plot of amplitude against frequency for pork 

specimen showing no discernible change in frequency of the 

recorded signal. 

 

During testing of the pork intercostal muscle specimen, 

failure occurred as the soft tissue detached from the 

bone, thus indicating tissue deformation was likely 

taking place at the bony attachment and not within the 

muscle bulk.  As a result, further testing on these 

specimens was not conducted. 

In the limited data collected for these samples, no event 

occurred in the acoustic data to indicate tissue was being 

deformed or failed.  The only event clearly captured in 

either the amplitude or frequency reading was the 

“tapping” of the load cell which was required in order 

to synchronise the data.  

The above results also highlight that during 

experimentation of the chicken tissue, inconclusive 

results were collected and thus no correlation could be 

explored. However, as the maximum load measured 

during testing of pork (35N) was higher than that 

recorded on any of the tests on chicken (20N), it may be 

postulated that no correlation would be seen between 

the acoustic output and tissue deformation in these tests 

either.   

The fabric webbing failed at a load of 112N. 112N may 

have been a high enough load to produce an AE data 

recording reading, however, unfortunately there was no 

recoding in this study. These results are therefore unable 

to indicate a correlation between tissue deformation and 

emission of an acoustic signal. 

Though the Instron testing machine applied a tensile 

loading force in this feasibility study, this is not the 

same as shear, which, as previously mentioned, has an 

important role in DTI development (13, 14, 18). 

However, it may be assumed that internal shear stresses 

were present during tensile loading, as the pressure 

gradient induced by the tensile loading may give rise to 

internal shear stresses (3, 5, 24). 

There were several limitations to this feasibility study. 

A degree of slippage was noted between each specimen 

and the clamps during loading. Consequently, the 

specimens may not fully subjected to the applied load. 

The interaction between skeletal muscle and other 

biological tissues was not taken into account in this 

study, as the aim was to investigate whether or not 

acoustic signals are emitted from isolated skeletal 

muscle undergoing deformation. Future work would be 

required to investigate this phenomenon in a cadaveric 

limb segment. 

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

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SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
It may not be appropriate to extrapolate in vitro results 

to the in vivo environment due to the large number of 

assumptions encountered and the complexity of muscle 

structure. Most biological tissues (including skeletal 

muscle and connective tissue) are described as 

viscoelastic in nature (3, 5, 33). This means they exhibit 

both viscous (i.e. fluid-like behavior resisting shear) and 

elastic (i.e. rapidly returning to original state) 

characteristics when strained. Specifically, with muscle 

it displays elasticity when it returns to its original shape 

when stretched then released (34). A muscle’s viscosity 

is the internal resistance to motion (35). As a result of 

this, viscoelasticity time is also an important factor as 

viscosity dissipates the shear force (32, 36). This is 

further corroborated by porcine muscle displaying 

different levels of stress to a variety of strain rates (34). 

Considering the loading of a trans-tibial residual limb 

during ambulation with a prosthesis, the residuum will 

be subject to tensile, compressive and shear forces 

simultaneously and in a cyclic manner.  The residual 

limb is loaded for 62% of the gait cycle (37) but it will 

also be subject to external forces during swing phase 

due to the inertia of the swinging limb.  The vertical 

displacement of the tibia due to socket pistoning during 

the gait cycle has been investigated in many studies (38-

40) with the mean value of the collective movement in 

the proximodistal direction throughout a full gait cycle 

being 57mm while using a patellar-tendon bearing 

socket (40).  The relative movement between the tibia 

and the surrounding soft tissue will lead to tissue 

deformation.  However, tibial movement not only 

occurs in the coronal plane but also in the sagittal plane, 

so deforming forces are being applied to various 

locations on the residuum and in different directions. 

Thus, in amputees, DTI is more likely to occur over long 

periods of cyclical gait motion in a multi-directional 

orientation. If it were possible to record acoustic data 

over a longer period of time, a slower more realistic 

strain rate could be employed along with compression 

testing to recreate forces experienced by the residual 

limb. 

The values chosen for this study were not based on 

quantitative evidence (due to a significant lack of 

literature investigating the loading rates of internal soft 

tissue in a residual limb during gait) but on values 

selected in preceding studies in which tissue damage 

was examined through displacement (31, 32). 

Furthermore, the results presented in this feasibility 

study are not statistically significant as a very limited 

number of tests were conducted. It has also been noted 

that this study did not investigate a range of tissue 

loading rates and thus future work would preferably 

investigate these as well as cyclic loading conditions.  

Moreover, the sample numbers collected from the 

acoustic data in this study did not allow for sufficient 

examination of the correlation between tissue 

deformation and acoustic output. 

Another point of note is that, ideally, biological 

specimens should be prepared and tested within a few 

hours of death to preserve the mechanical properties. It 

has been documented that freezing muscle alters and 

reduces its strength (41). In contrast, Huang et al has 

documented that thawing then refreezing tendon in less 

than three cycles does not significantly alter the 

mechanical properties (42). Despite these conflicting 

findings, biological tissue should be tested fresh if 

possible or it should be preserved in a manner that 

reflects the in vivo environment with minimum 

freezing. 

An alternative transducer may be a contact microphone 

which is placed on the surface of the material being 

tested.  The inclusion of a pre-amplifier may also 

enhance the detection of any sound signal being emitted.  

Further investigation is required to carry out a robust 

correlation analysis between tissue deformation and 

acoustic emission.  This would ideally incorporate a 

variety of transducers to detect sound output, testing of 

various tissues and the application of a range of tissue 

displacement rates. 

The acoustic emission of tissue under shear is most 

likely be over shadowed by muscle noise and sliding 

tissues during human movement (mechanomyogram 

(MMG)). In addition, the noise generated by the 

prosthetic intervention itself and contact with clothing 

will in all probability render a clinical application 

useless. 

CONCLUSIONS 

This feasibility study provides a foundation of results 

regarding tissue deformation and AE. Results displayed 

that tensile testing of pork intercostal muscle produced 

tissue deformation and failure with no detectable change 

in the amplitude or frequency of the background sound 

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Buis A, Guarato F, Law J, Ralston Z, Courtney A. A feasibility study to investigate if there is a correlation between soft tissue deformation 

and acoustic emission. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 1, No5, 2018.DOI: https://doi.org/10.33137/cpoj.v1i1.30354 

 

 

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SOFT TISSUE DEFORMATION AND ACOUSTIC EMISSION 

 
Volume 1, Issue 1, Article No. 5, August 2018 

 

 
during tensile loading. There is a more than strong 

argument that acoustic shear emission during soft tissue 

deformation is over shadowed by muscle activation 

noise (MMG) and noise generated in the prosthetic 

system during gait. 

The other specimens failed before reaching the same 

levels of tensile load, and, as testing of other tissue did 

not reach the same levels of tensile load, it is unlikely 

that any acoustic events could have been detected from 

these specimens either. Further studies are required in 

order to address the numerous limitations of this study.  

DECLARATION OF CONFLICTING 

INTERESTS 

No conflict of interest declared. This research did not 

receive any specific grant from funding agencies in the 

public, commercial, or not-for-profit sectors.  

COMPETING INTERESTS 

None declared 

SOURCES OF SUPPORT 

University of Strathclyde – experimental work took 

place on campus. 

ETHICAL APPROVAL 

Not required 

AUTHOR CONTRIBUTION  

Arjan Buis: Conceptualization, formal analysis, 

investigation, methodology, visualization, writing 

original, review & editing. 

Francesco Guarato: Conceptualization, formal 

analysis, investigation, methodology, visualization. 

Jason Law: formal analysis, investigation, 

methodology, visualization, writing original.   

Zoe Ralston: formal analysis, investigation, 

methodology, visualization, writing original.   

Anna Courtney: methodology, visualization, Writing 

original. review & editing. 

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