Reliability of the mechanomyogram detected with an accelerometer during voluntary contractions

Implementation of surface mechanomyography as a novel approach for objective evaluation of phasic muscle stretch reflexes in people with type 2 diabetes

INTRODUCTION

Diabetic peripheral neuropathy is the most common complication of diabetes producing metabolic disruptions in the peripheral nervous system. Alteration in the predictable nature of tendon reflexes is the most common indicator suggesting the possibility of diabetic neuropathy. Evaluation of tendon reflexes is a part of various clinical scoring systems that assess neuropathy. The conventional reflex grading scales are subjective, lack temporal data, and have high inter-rater variability. Hence, an indigenous quantification tool was developed to evaluate the tendon reflexes in order to assess diabetic peripheral neuropathy.

MATERIALS AND METHODS

A cross-sectional study was carried out in 140 healthy volunteers and 140 patients with type 2 diabetes. The mean age of controls and diabetics (49.1 ± 8.9, 50.7 ± 7.5) years, weight (66.9 ± 9.4, 69.8 ± 11.5) kilograms and BMI (24.5 ± 3.8, 26.1 ± 4.7), respectively. All of them are subjected to evaluation of tendon reflexes using the reflex quantification tool comprised of surface mechanomyography and electrogoniometry that can provide various static and dynamic variables of tendon reflex.

RESULTS

The dynamic variables such as reflex amplitude, muscle velocity and angular velocity were significantly low in diabetic patients (p: <0.001) whereas latency and duration (p: <0.001) were prolonged. Furthermore, no significant difference was observed in the application of tendon striking force (p: 0.934) among the participants.

CONCLUSION

The current study demonstrates that the proposed reflex quantification tool provides several dynamic variables of patellar tendon reflex, which are significantly affected and altered in diabetic patients suggesting the involvement of peripheral neurons.

3D muscle networks based on vibrational mechanomyography

Objective. Muscle network modeling maps synergistic control during complex motor tasks. Intermuscular coherence (IMC) is key to isolate synchronization underlying coupling in such neuromuscular control. Model inputs, however, rely on electromyography, which can limit the depth of muscle and spatial information acquisition across muscle fibers.Approach. We introduce three-dimensional (3D) muscle networks based on vibrational mechanomyography (vMMG) and IMC analysis to evaluate the functional co-modulation of muscles across frequency bands in concert with the longitudinal, lateral, and transverse directions of muscle fibers. vMMG is collected from twenty subjects using a bespoke armband of accelerometers while participants perform four hand gestures. IMC from four superficial muscles (flexor carpi radialis, brachioradialis, extensor digitorum communis, and flexor carpi ulnaris) is decomposed using matrix factorization into three frequency bands. We further evaluate the practical utility of the proposed technique by analyzing the network responses to various sensor-skin contact force levels, studying changes in quality, and discriminative power of vMMG.Main results. Results show distinct topological differences, with coherent coupling as high as 57% between specific muscle pairs, depending on the frequency band, gesture, and direction. No statistical decrease in signal strength was observed with higher contact force.Significance. Results support the usability vMMG as a tool for muscle connectivity analyses and demonstrate the use of IMC as a new feature space for hand gesture classification. Comparison of spectrotemporal and muscle network properties between levels of force support the robustness of vMMG-based network models to variations in tissue compression. We argue 3D models of vMMG-based muscle networks provide a new foundation for studying synergistic muscle activation, particularly in out-of-clinic scenarios where electrical recording is impractical.

A Novel Mechanomyography (MMG) Sensor Based on Piezo-Resistance Principle and with a Pyramidic Microarray

Flexible piezoresistive sensors built by printing nanoparticles onto soft substrates are crucial for continuous health monitoring and wearable devices. In this study, a mechanomyography (MMG) sensor was developed using a flexible piezoresistive MMG signal sensor based on a pyramidal polydimethylsiloxane (PDMS) microarray sprayed with carbon nanotubes (CNTs). The experiment was conducted, and the results show that the sensitivity of the sensor can reach 0.4 kPa-1 in the measurement range of 0~1.5 kPa, and the correlation reached 96%. This has further implications for the possibility that muscle activation can be converted into mechanical movement. The integrity of the sensor in terms of its MMG signal acquisition was tested based on five subjects who were performing arm bending and arm extending movements. The results of this test were promising.

Toward a robust swallowing detection for an implantable active artificial larynx: a survey

Total laryngectomy consists in the removal of the larynx and is intended as a curative treatment for laryngeal cancer, but it leaves the patient with no possibility to breathe, talk, and swallow normally anymore. A tracheostomy is created to restore breathing through the throat, but the aero-digestive tracts are permanently separated and the air no longer passes through the nasal tracts, which allowed filtration, warming, humidification, olfaction, and acceleration of the air for better tissue oxygenation. As for phonation restoration, various techniques allow the patient to talk again. The main one consists of a tracheo-esophageal valve prosthesis that makes the air passes from the esophagus to the pharynx, and makes the air vibrate to allow speech through articulation. Finally, swallowing is possible through the original tract as it is now isolated from the trachea. Yet, many methods exist to detect and assess a swallowing, but none is intended as a definitive restoration technique of the natural airway, which would permanently close the tracheostomy and avoid its adverse effects. In addition, these methods are non-invasive and lack detection accuracy. The feasibility of an effective early detection of swallowing would allow to further develop an implantable active artificial larynx and therefore restore the aero-digestive tracts. A previous attempt has been made on an artificial larynx implanted in 2012, but no active detection was included and the system was completely mechanic. This led to residues in the airway because of the imperfect sealing of the mechanism. An active swallowing detection coupled with indwelling measurements would thus likely add a significant reliability on such a system as it would allow to actively close an artificial larynx. So, after a brief explanation of the swallowing mechanism, this survey intends to first provide a detailed consideration of the anatomical region involved in swallowing, with a detection perspective. Second, the swallowing mechanism following total laryngectomy surgery is detailed. Third, the current non-invasive swallowing detection technique and their limitations are discussed. Finally, the previous points are explored with regard to the inherent requirements for the feasibility of an effective swallowing detection for an artificial larynx. Graphical Abstract.

Basic characteristics between mechanomyogram and muscle force during twitch and tetanic contractions in rat skeletal muscles

The mechanomyogram (MMG) is a signal measured by various vibration sensors for slight vibrations induced by muscle contraction, and it reflects the muscle force during electrically induced-contraction or until 60%-70% maximum voluntary contraction, so the MMG is considered an alternative and novel measurement tool for muscle strength. We simultaneously measured the MMG and muscle force in the gastrocnemius (GC), vastus intermedius (VI), and soleus (SOL) muscles of rats. The muscle force was measured by attaching a hook to the tendon using a load cell, and the MMG was measured using a charged-coupled device-type displacement sensor at the middle of the target muscle. The MMG-twitch waveform was very similar to that of the muscle force; however, the half relaxation time and relaxation time (10%), which are relaxation parameters, were prolonged compared to those of the muscle force. The MMG amplitude correlated with the muscle force. Since stimulation frequencies that are necessary to evoke tetanic progression have a significant correlation with the twitch parameter, there is a close relationship between twitch and tetanus in the MMG signal. Therefore, we suggest that the MMG, which is electrically induced and detected by a laser displacement sensor, may be an alternative tool for measuring muscle strength.

Indices reflecting muscle contraction performance during exercise based on a combined electromyography and mechanomyography approach

Electromyography (EMG) and mechanomyography (MMG) have been used to directly evaluate muscle function through the electromechanical aspect of muscle contraction. The purpose of this study was to establish new absolute indices to describe muscle contraction performance during dynamic exercise by combining EMG and displacement MMG (dMMG) measured simultaneously using our previously developed MMG/EMG hybrid transducer system. Study participants were eight healthy male non-athletes (controls) and eight male athletes. EMG and dMMG of the vastus medialis were measured for 30 s during four cycles of recumbent bicycle pedaling (30, 60, 90, and 120 W) and on passive joint movement. Total powers were calculated based on the time domain waveforms of both signals. Muscle contraction performance was verified with the slope of regression line (SRL) and the residual sum of squares (RSS) obtained from EMG and dMMG correlation. EMG and dMMG has increased with the work rate. Force and EMG were similar between groups, but dMMG showed a significant difference with load increase. Athletes had significantly higher SRL and significantly lower RSS than controls. The average value divided by SRL and RSS was higher in athletes than in controls. The indices presented by the combined approach of EMG and dMMG showed a clear contrast between the investigated groups and may be parameters that reflect muscle contraction performance during dynamic exercise.

Wearable MMG-Plus-One Armband: Evaluation of Normal Force on Mechanomyography (MMG) to Enhance Human-Machine Interfacing

In this paper, we introduce a new mode of mechanomyography (MMG) signal capture for enhancing the performance of human-machine interfaces (HMIs) through modulation of normal pressure at the sensor location. Utilizing this novel approach, increased MMG signal resolution is enabled by a tunable degree of freedom normal to the sensor-skin contact area. We detail the mechatronic design, experimental validation, and user study of an armband with embedded acoustic sensors demonstrating this capacity. The design is motivated by the nonlinear viscoelasticity of the tissue, which increases with the normal surface pressure. This, in theory, results in higher conductivity of mechanical waves and hypothetically allows to interface with deeper muscle; thus, enhancing the discriminative information context of the signal space. Ten subjects (seven able-bodied and three trans-radial amputees) participated in a study consisting of the classification of hand gestures through MMG while increasing levels of contact force were administered. Four MMG channels were positioned around the forearm and placed over the flexor carpi radialis, brachioradialis, extensor digitorum communis, and flexor carpi ulnaris muscles. A total of 852 spectrotemporal features were extracted (213 features per each channel) and passed through a Neighborhood Component Analysis (NCA) technique to select the most informative neurophysiological subspace of the features for classification. A linear support vector machine (SVM) then classified the intended motion of the user. The results indicate that increasing the normal force level between the MMG sensor and the skin can improve the discriminative power of the classifier, and the corresponding pattern can be user-specific. These results have significant implications enabling embedding MMG sensors in sockets for prosthetic limb control and HMI.

Neck sensor-supported hyoid bone movement tracking during swallowing

Hyoid bone movement is an important physiological event during swallowing that contributes to normal swallowing function. In order to determine the adequate hyoid bone movement, clinicians conduct an X-ray videofluoroscopic swallowing study, which even though it is the gold-standard technique, has limitations such as radiation exposure and cost. Here, we demonstrated the ability to track the hyoid bone movement using a non-invasive accelerometry sensor attached to the surface of the human neck. Specifically, deep neural networks were used to mathematically describe the relationship between hyoid bone movement and sensor signals. Training and validation of the system were conducted on a dataset of 400 swallows from 114 patients. Our experiments indicated the computer-aided hyoid bone movement prediction has a promising performance when compared with human experts' judgements, revealing that the universal pattern of the hyoid bone movement is acquirable by the highly nonlinear algorithm. Such a sensor-supported strategy offers an alternative and widely available method for online hyoid bone movement tracking without any radiation side-effects and provides a pronounced and flexible approach for identifying dysphagia and other swallowing disorders.

Mechanomyography for Intraoperative Assessment of Cortical Breach During Instrumented Spine Surgery

OBJECTIVE

We sought to determine the utility of mechanomyography (MMG) in detecting and preventing pedicle breach in instrumented lumbar spine surgery.

METHODS

In a prospective nonrandomized trial without controls, we selected consecutive patients to undergo intraoperative MMG during instrumented lumbar spine surgery. MMG testing was performed at the original pilot hole, after tapping, and after screw placement, with the minimum current to elicit a recorded MMG response. All patients underwent a postoperative computed tomography scan, and a single radiologist interpreted each pedicle to identify breach. Chi-square test was used to compare patients with and without breaches. Two sample Student's t-tests were used to compare changes in functional outcomes. Sensitivity and specificity of MMG were computed using receiver operating characteristic curve analysis.

RESULTS

There were 122 consecutive instrumented lumbar surgery patients enrolled, with a total of 890 lumbar pedicle screws tested with MMG. The medial or inferior breach rate was 2.25%, with no statistically significant difference in Oswestry Disability Index or visual analog scale between patients who breached and who did not. For the MMG measurement from the original pilot hole, the area under the receiver operating characteristic was 0.835; the maximum combination of sensitivity (80.42%) and specificity (80.6%) was found using MMG current ≤12 mA. We found that an MMG cutoff of >12 mA resulted in a 99.5% likelihood of no medial or inferior breach.

CONCLUSIONS

MMG can be safely used during instrumented lumbar spine surgery. A cutoff value of >12 mA for MMG can accurately predict and prevent medial and inferior pedicle screw breach.

Real-time continuous recognition of knee motion using multi-channel mechanomyography signals detected on clothes

Mechanomyography (MMG) signal has been recently investigated for pattern recognition of human motion. In theory, it is no need of direct skin contact to be detected and unaffected by changes in skin impedance. So, it is hopeful for developing wearable sensing device with clothes. However, there have been no studies so far to detect MMG signal on clothes and verify the feasibility of pattern recognition. For this study, 4-channel MMG signals were detected on clothes from the thigh muscles of 8 able-bodied participants. The support vector machines (SVM) classifier with 4 common features was used to recognize 6 knee motions and the average accuracy of nearly 88% was achieved. The accuracy can be further improved up to 91% by introducing a new proposed feature of the difference of mean absolute value (DMAV), but not by root mean square (RMS) or mean absolute value (MAV). Furthermore, the first-order Markov chain model was combined with the SVM classifier and it can avoid the misclassifications in some cases. For application to wearable power-assisted devices, this study would promote the developments of more flexible, more comfortable, and minimally obtrusive wearable sensing devices with clothes and recognition techniques of human motion intention.

Verification of nerve decompression using mechanomyography

BACKGROUND CONTEXT

Assessment of nerve root decompression in surgery is largely based on visualization and tactile feedback. Often times, visualization can be limited, such as in minimally invasive surgery, and tactile feedback is a subjective assessment that makes the evaluation of successful nerve decompression difficult. Electromyography (EMG) has been proposed as an assessment tool, but EMG responses are often difficult to quantify. Alternatively, mechanomyography (MMG) provides a quantifiable response with high signal-to-noise ratio compared with EMG. MMG provides a sensitive tool to accurately quantify mechanical responses to motor action potentials generated by electrical stimulus, allowing more reliable assessment of nerve decompression.

PURPOSE

The aim of this study was to assess the ability of MMG to quantitatively demonstrate successful nerve root decompression.

STUDY DESIGN

Prospective cohort, Therapeutic Level III, Urban Level I Trauma Center.

PATIENT SAMPLE

A total of 46 patients (72 affected nerve roots) undergoing decompression procedures for lower extremity radiculopathy caused by nerve root compression were enrolled in the study. The study population included 15 patients with herniated nucleus pulposus (HNP) and 31 with lateral recess stenosis (LRS).

OUTCOME MEASURE

Visual analog scale (VAS) score.

METHODS

A total of 72 nerves roots in 46 patients undergoing lumbar decompression procedures, for lower extremity radicular symptoms, were tested using MMG. Nerves were stimulated upstream from the compression site, and the lowest threshold current needed to generate a muscle response was determined. Signal response sizes were recorded before and after decompression. VAS scores were collected pre- and postoperatively.

RESULTS

Of the patients, 90% (65/72) had elevated stimulation thresholds (>1 milliamp [mA]) before decompression. After decompression, 98% of patients (64/65) with elevated current thresholds exhibited a drop in threshold of ≥1 mA (p<.001). A postdecompression increase in response amplitude was recorded in all patients. VAS scores improved postdecompression (6.8 vs. 1.1, p<.001) with a positive correlation between decreased stimulation thresholds and degree of improvement in VAS scores (p<.001).

CONCLUSION

MMG is an effective tool that can be used to differentiate normal and compressed nerves by quantifying the mechanomyographic response to a stimulating current. MMG allows one to measure the effect of decompression, judge its effectiveness in real time, and eliminate the subjectivity seen in tactile feedback methods. When the adequacy of decompression is uncertain, MMG can guide the surgeon toward additional or alternative procedures to ensure complete nerve root decompression.

Muscle stiffness estimation using a system identification technique applied to evoked mechanomyogram during cycling exercise

The aims of this study were to develop a method to extract the evoked mechanomyogram (MMG) during cycling exercise and to clarify muscle stiffness at various cadences, workloads, and power. Ten young healthy male participants were instructed to pedal a cycle ergometer at cadences of 40 and 60 rpm. The loads were 4.9, 9.8, 14.7, and 19.6 N, respectively. One electrical stimulus per two pedal rotations was applied to the vastus lateralis muscle at a knee angle of 80° in the down phase. MMGs were measured using a capacitor microphone, and the MMGs were divided into stimulated and non-stimulated sequences. Each sequence was synchronously averaged. The synchronously averaged non-stimulated MMG was subtracted from the synchronously averaged stimulated MMG to extract an evoked MMG. The evoked MMG system was identified and the poles of the transfer function were calculated. The poles and mass of the vastus lateralis muscle were used to estimate muscle stiffness. Results showed that muscle stiffness was 186-626 N /m and proportional to the workloads and power. In conclusion, our method can be used to assess muscle stiffness proportional to the workload and power.

Mechanomyographic parameter extraction methods: an appraisal for clinical applications

The research conducted in the last three decades has collectively demonstrated that the skeletal muscle performance can be alternatively assessed by mechanomyographic signal (MMG) parameters. Indices of muscle performance, not limited to force, power, work, endurance and the related physiological processes underlying muscle activities during contraction have been evaluated in the light of the signal features. As a non-stationary signal that reflects several distinctive patterns of muscle actions, the illustrations obtained from the literature support the reliability of MMG in the analysis of muscles under voluntary and stimulus evoked contractions. An appraisal of the standard practice including the measurement theories of the methods used to extract parameters of the signal is vital to the application of the signal during experimental and clinical practices, especially in areas where electromyograms are contraindicated or have limited application. As we highlight the underpinning technical guidelines and domains where each method is well-suited, the limitations of the methods are also presented to position the state of the art in MMG parameters extraction, thus providing the theoretical framework for improvement on the current practices to widen the opportunity for new insights and discoveries. Since the signal modality has not been widely deployed due partly to the limited information extractable from the signals when compared with other classical techniques used to assess muscle performance, this survey is particularly relevant to the projected future of MMG applications in the realm of musculoskeletal assessments and in the real time detection of muscle activity.

Analysis of crosstalk in the mechanomyographic signals generated by forearm muscles during different wrist postures

INTRODUCTION

In this study, we analyzed the crosstalk in mechanomyographic (MMG) signals generated by the extensor digitorum (ED), extensor carpi ulnaris (ECU), and flexor carpi ulnaris (FCU) muscles of the forearm during wrist flexion (WF) and extension (WE) and radial (RD) and ulnar (UD) deviations.

METHODS

Twenty right-handed men (mean ± SD age=26.7 ± 3.83 years) performed the wrist postures. During each wrist posture, MMG signals were detected using 3 accelerometers. Peak cross-correlations were used to quantify crosstalk.

RESULTS

The level of crosstalk ranged from 1.69 to 64.05%. The wrist postures except the RD did not influence the crosstalk significantly between muscle pairs. However, muscles of the forearm compartments influenced the level of crosstalk for each wrist posture significantly.

CONCLUSIONS

The results may be used to improve our understanding of the mechanics of the forearm muscles during wrist postures.

Mechanomyography and muscle function assessment: a review of current state and prospects

Previous studies have explored to saturation the efficacy of the conventional signal (such as electromyogram) for muscle function assessment and found its clinical impact limited. Increasing demand for reliable muscle function assessment modalities continues to prompt further investigation into other complementary alternatives. Application of mechanomyographic signal to quantify muscle performance has been proposed due to its inherent mechanical nature and ability to assess muscle function non-invasively while preserving muscular neurophysiologic information. Mechanomyogram is gaining accelerated applications in evaluating the properties of muscle under voluntary and evoked muscle contraction with prospects in clinical practices. As a complementary modality and the mechanical counterpart to electromyogram; mechanomyogram has gained significant acceptance in analysis of isometric and dynamic muscle actions. Substantial studies have also documented the effectiveness of mechanomyographic signal to assess muscle performance but none involved comprehensive appraisal of the state of the art applications with highlights on the future prospect and potential integration into the clinical practices. Motivated by the dearth of such critical review, we assessed the literature to investigate its principle of acquisition, current applications, challenges and future directions. Based on our findings, the importance of rigorous scientific and clinical validation of the signal is highlighted. It is also evident that as a robust complement to electromyogram, mechanomyographic signal may possess unprecedented potentials and further investigation will be enlightening.

MEASUREMENT AND ESTIMATION OF MUSCLE CONTRACTION STRENGTH USING MECHANOMYOGRAPHY BASED ON ARTIFICIAL NEURAL NETWORK ALGORITHM

Muscle contraction strength estimation using mechanomyographic (MMG) signal is typically calculated by the root mean square (RMS) amplitude. Raw MMG signal is processed by rectification, low-pass filtering, and mapping. In this work, beside RMS amplitude, another significant parameter of MMG signal, i.e. frequency variance (VAR), is introduced and used for constructing an algorithm for estimating the muscle contraction strength. Seven participants produced isometric contractions about the elbow while MMG signal and generated torque (resultant of muscle contraction strength) of biceps brachii were recorded. We found that MMG RMS increased monotonously and VAR decreased under incremental voluntary contractions. Based on these results, a two-layer neural network was utilized for the model of estimating the muscle contraction strength from MMG RMS and VAR. Experimental evaluation was performed under constant posture and sinusoidal contractions at 0.5 Hz, 0.25 Hz, 0.125 Hz, and random frequency. The results of the proposed algorithm and MMG RMS linear mapping were also compared. The proposed algorithm has better accuracy than linear mapping for all contraction frequencies. The mean absolute error decreased 6% for the 0.5Hz contraction, 43% for 0.25 Hz contraction, 52% for 0.125 Hz contraction, and 30% for random frequency contraction.

MUSCLES WITHIN MUSCLES: A MECHANOMYOGRAPHIC ANALYSIS OF MUSCLE SEGMENT CONTRACTILE PROPERTIES WITHIN HUMAN GLUTEUS MAXIMUS

The aim of this investigation was to determine the contractile properties of motor units within 3 segments of the gluteus maximus utilizing a laser-based mechanomyographic (MMG) technique. The intention was to determine whether there were segmental differences in motor unit contractile properties and whether these differences may be related to the muscle segment's function and its fibre type composition.

Ten subjects were recruited from the student population at the University of Wollongong. Maximal percutaneous neuromuscular stimulation (PNS) was delivered to the medial and lateral portions of three (cranial, middle, caudal) muscle segments of the gluteus maximus by an MMG stimulator. An MMG laser sensor measured the lateral displacement of the muscle segment belly resulting from the development of maximal isometric tension. Parameters characterizing the MMG waveforms were statistically compared to determine variations in contractile properties both within (medial to lateral) and between segments.

Our results indicated that the contractile properties of motor units varied significantly (p < 0.05) between, but not within (medial to lateral), the three segments of the gluteus maximus. Most the gluteus maximus. Most notably, segment contraction time (tc) decreased significantly (p < 0.05) in a cranio to caudal direction suggesting a variation in muscle fibre type composition within the three segments of the muscle. Even when corrected for differences in muscle belly displacement between subjects, the cranial segment was found to have a significantly (p < 0.05) longer contraction time than the two more caudal segments. The results suggest that the gluteus maximus was composed of muscle segments that were physiologically, as well as anatomically, designed to fulfil particular roles during everyday motor tasks. Based upon these results, the MMG technique appears to have considerable utility for the non-invasive assessment of muscle segment contractile properties for both laboratory and clinical applications.

Design and evaluation of a novel microphone-based mechanomyography sensor with cylindrical and conical acoustic chambers

Mechanomyography has recently been proposed as a control modality for alternative access technologies for individuals with disabilities. However, MMG recordings are highly susceptible to contamination from limb movements. Pressure-based transducers are touted to be the most robust to external movement although there is some debate about their optimal chamber geometry, in terms of low frequency gain and spectral flatness. To investigate the question of preferred geometry, transducers with cylindrical and conical chambers of varying dimensions were designed, manufactured and tested. Using a computer-controlled electrodynamic shaker, the frequency response of each chamber geometry was empirically derived. Of the cylindrical chambers, the highest gain and the flattest frequency response was exhibited by a chamber 10 mm in diameter and 5-7 mm in height. However, conical chambers offered an average rise in gain of 6.79 ± 1.06 dB/Hz over that achievable with cylindrical geometries. The highest gain and flattest response was achieved with a transducer consisting of a low-frequency MEMS microphone, a 4 μm aluminized mylar membrane and a rigid conical chamber 7 mm in diameter and 5mm in height. This design is recommended for MMG applications where limb movement is prevalent.

Propagation direction of natural mechanical oscillations in the biceps brachii muscle during voluntary contraction

The aim of the study was to determine the directionality of the coupling of mechanical vibrations across the biceps brachii muscle at different frequencies of interest during voluntary contraction. The vibrations that are naturally generated by skeletal muscles were recorded by a two-dimensional array of skin mounted accelerometers over the biceps brachii muscle (surface mechanomyogram, S-MMG) during voluntary isometric contractions in ten healthy young men. As a measure of the similarity of vibration between a given pair of accelerometers, the spatial coherence of S-MMG at low (f<25Hz) and high (f>25Hz) frequency bands were investigated to determine if the coupling of the natural mechanical vibrations were due to the different physiological muscle activity at low and high frequencies. In both frequency bands, spatial coherence values for sensor pairs aligned longitudinally along the proximal to distal ends of the biceps were significantly higher compared with those for the sensor pairs oriented perpendicular to the muscle fibers. This difference was more evident at the higher frequency band. The findings indicated that coherent mechanical oscillations mainly propagated along the longitudinal direction of the biceps brachii muscle fibers at high frequencies (f>25Hz).

A durability study of a paracorporeal pulsatile electro-mechanical pneumatic biventricular assist device

In 2002, the paracorporeal pulsatile electro-mechanical pneumatic ventricular assist device (VAD) began to be developed by the Korea Artificial Organ Center at Korea University under a Health & Medical Technology Research and Development program which finished in 2008. In vitro durability testing was conducted on the paracorporeal pulsatile pneumatic VAD to determine device durability and to evaluate device failures. The 1- and 2-year reliability of the paracorporeal pulsatile pneumatic VAD was shown to be 91.2% and 54.9%, respectively, with an 80% confidence level. Failure modes were analyzed using fault tree analysis, with customized software continuously acquiring data during the test period. After this period, 21 in vivo animal tests were done, with 14 cases of left atrium to left ventricle (LV) inflow cannulation (36Fr)/outflow grafting to descending aorta, and seven cases of apex cannulation of LV to descending aorta (12 mm). The longest postoperative day (182 days) in Korea was recently recorded in in vivo animal testing (bovine, 90 kg, male, 3.5-4.0 L/min flow rate, and 55 bpm).

Estimation of elbow flexion force during isometric muscle contraction from mechanomyography and electromyography

Mechanomyography (MMG) is the muscle surface oscillations that are generated by the dimensional change of the contracting muscle fibers. Because MMG reflects the number of recruited motor units and their firing rates, just as electromyography (EMG) is influenced by these two factors, it can be used to estimate the force exerted by skeletal muscles. The aim of this study was to demonstrate the feasibility of MMG for estimating the elbow flexion force at the wrist under an isometric contraction by using an artificial neural network in comparison with EMG. We performed experiments with five subjects, and the force at the wrist and the MMG from the contributing muscles were recorded. It was found that MMG could be utilized to accurately estimate the isometric elbow flexion force based on the values of the normalized root mean square error (NRMSE = 0.131 ± 0.018) and the cross-correlation coefficient (CORR = 0.892 ± 0.033). Although MMG can be influenced by the physical milieu/morphology of the muscle and EMG performed better than MMG, these experimental results suggest that MMG has the potential to estimate muscle forces. These experimental results also demonstrated that MMG in combination with EMG resulted in better performance estimation in comparison with EMG or MMG alone, indicating that a combination of MMG and EMG signals could be used to provide complimentary information on muscle contraction.

On functional motor adaptations: from the quantification of motor strategies to the prevention of musculoskeletal disorders in the neck-shoulder region

BACKGROUND

Occupations characterized by a static low load and by repetitive actions show a high prevalence of work-related musculoskeletal disorders (WMSD) in the neck-shoulder region. Moreover, muscle fatigue and discomfort are reported to play a relevant initiating role in WMSD.

AIMS

To investigate relationships between altered sensory information, i.e. localized muscle fatigue, discomfort and pain and their associations to changes in motor control patterns.

MATERIALS & METHODS

In total 101 subjects participated. Questionnaires, subjective assessments of perceived exertion and pain intensity as well as surface electromyography (SEMG), mechanomyography (MMG), force and kinematics recordings were performed.

RESULTS

Multi-channel SEMG and MMG revealed that the degree of heterogeneity of the trapezius muscle activation increased with fatigue. Further, the spatial organization of trapezius muscle activity changed in a dynamic manner during sustained contraction with acute experimental pain. A graduation of the motor changes in relation to the pain stage (acute, subchronic and chronic) and work experience were also found. The duration of the work task was shorter in presence of acute and chronic pain. Acute pain resulted in decreased activity of the painful muscle while in subchronic and chronic pain, a more static muscle activation was found. Posture and movement changed in the presence of neck-shoulder pain. Larger and smaller sizes of arm and trunk movement variability were respectively found in acute pain and subchronic/chronic pain. The size and structure of kinematics variability decreased also in the region of discomfort. Motor variability was higher in workers with high experience. Moreover, the pattern of activation of the upper trapezius muscle changed when receiving SEMG/MMG biofeedback during computer work.

DISCUSSION

SEMG and MMG changes underlie functional mechanisms for the maintenance of force during fatiguing contraction and acute pain that may lead to the widespread pain seen in WMSD. A lack of harmonious muscle recruitment/derecruitment may play a role in pain transition. Motor behavior changed in shoulder pain conditions underlining that motor variability may play a role in the WMSD development as corroborated by the changes in kinematics variability seen with discomfort. This prognostic hypothesis was further, supported by the increased motor variability among workers with high experience.

CONCLUSION

Quantitative assessments of the functional motor adaptations can be a way to benchmark the pain status and help to indentify signs indicating WMSD development. Motor variability is an important characteristic in ergonomic situations. Future studies will investigate the potential benefit of inducing motor variability in occupational settings.

Cross-correlation analysis of mechanomyographic signals detected in two axes

The purpose of this study was to use laser displacement sensors to examine the cross-correlation of surface mechanomyographic (MMG) signals detected from the rectus femoris muscle in perpendicular and transverse axes during isometric muscle actions of the leg extensors. Ten healthy men (mean +/- SD age = 22.1 +/- 1.6 years) and ten healthy women (age = 24.4 +/- 2.8 years) volunteered to perform submaximal to maximal isometric muscle actions of the dominant leg extensors. During each muscle action, two separate MMG signals were detected from the rectus femoris with laser displacement sensors. One MMG sensor was oriented in an axis that was perpendicular (PERP) to the muscle surface, and the second sensor was oriented in an axis that was transverse (TRAN) to the muscle surface. For each subject and force level, the MMG signals from the PERP and TRAN sensors were cross-correlated. The results showed maximum cross-correlation coefficients that ranged from R(x)(,y) = 0.273 to 0.989, but all subjects demonstrated at least one coefficient greater than 0.89. These findings showed a high level of association between the MMG signals detected in the perpendicular and transverse axes. Thus, it may not be necessary to detect MMG signals in multiple axes.

Laser-detected lateral muscle displacement is correlated with force fluctuations during voluntary contractions in humans

Fluctuations in muscle force during steady voluntary contractions result from the summation of twitch forces produced by asynchronous activation of multiple motor units. We hypothesized that oscillatory lateral muscle displacement, measured with a non-contact high-resolution laser displacement sensor, is correlated with force fluctuations during steady, voluntary contractions with a human muscle. Eight healthy young adults (20-33 yrs) performed steady isometric contractions with the first dorsal interosseus muscle. Contraction intensity ranged from 2.5% to 60% of the maximal voluntary contraction force. Oscillatory lateral displacement of the muscle surface was measured with a high-resolution laser displacement sensor (0.5 microm resolution), concurrently with abduction force of the index finger. In the time-domain analysis, there was a significant positive peak in the cross-correlation function between lateral muscle displacement and force fluctuations. In addition, the amplitude increased linearly with contraction intensity in both signals. In the frequency-domain analysis, frequency content was similar in both signals, and there was significant coherence between signals for the major frequency range of the signals (<5 Hz). In conclusion, laser-detected lateral displacement of a hand muscle is correlated with force fluctuations across a wide range of contraction intensity during steady voluntary contractions in humans.

Within-day and between-days reliability of quadriceps isometric muscle fatigue using mechanomyography on healthy subjects

This study aimed to examine within-day and between-days intratester reliability of mechanomyography (MMG) in assessing muscle fatigue. An accelerometer was used to detect the MMG signal from rectus femoris. Thirty one healthy subjects (15 males) with no prior knee problems initially performed three maximum voluntary contractions (MVCs) using an ISOCOM dynamometer. After 10 min rest, subjects performed a fatiguing protocol in which they performed three isometric knee extensions at 75% MVC for 40 s. The fatiguing protocol was repeated on two other days, two to four days apart for between-days reliability. MMG activity was determined by overall root mean squared amplitude (RMS), mean power frequency (MPF) and median frequency (MF) during a 40s contraction. RMS, MPF and MF linear regression slopes were also analysed. Intraclass Correlation Coefficients (ICC); ICC1,1 and ICC1,2 were used to assess within-day reliability and between-days reliability respectively. Standard error of measurement (SEM) and smallest detectable difference (SDD) described the within-subjects variability. MMG fatigue measures using linear regression slopes showed low reliability and large between-days error (ICC1,2=0.43-0.46; SDD=306.0-324.8% for MPF and MF slopes respectively). Overall MPF and MF, on the other hand, were reliable with high ICCs and lower SDDs compared to linear slopes (ICC1,2=0.79-0.83; SDD=21.9-22.8% for MPF and MF respectively). ICC1,2 for overall MMG RMS and linear RMS slopes were 0.81 and 0.66 respectively; however, the SDDs were high (56.4% and 268.8% respectively). The poor between-days reliability found in this study suggests caution in using MMG RMS, MPF and MF and their corresponding slopes in assessing muscle fatigue.

Two-dimensional spatial distribution of surface mechanomyographical response to single motor unit activity

In order to better understand the mechanisms of generation of mechanomyography (MMG) signals, the two-dimensional distribution of surface MMG produced by the activity of single motor units was analyzed by a novel two-dimensional recording method. Motor unit action potentials were identified from intramuscular electromyographic (EMG) signals and used to trigger the averaging of MMG signals detected over the tibialis anterior muscle of 11 volunteers with a grid of 5x3 accelerometers (20-mm inter-accelerometer distance). The intramuscular wires were inserted between the first and second accelerometer in the middle column of the grid, proximal to the innervation zone. The subjects performed three contractions with visual feedback of the intramuscular EMG signals. In each contraction, a new motor unit was recruited at the minimum stable discharge rate (mean+/-S.D., N = 11 subjects, 7.3+/-2.3 pulse/s), resulting in torque of 2.4+/-2.8% of the maximal voluntary contraction (MVC), 4.6+/-2.7% MVC, and 6.3+/-3.1% MVC (all different, P < 0.01). For 23 out of 33 detected motor units, it was possible to extract the motor unit surface acceleration map (MUAM). A negative MUAM peak (-2.7+/-2.2 mm/s2) was detected laterally and a positive MUAM peak (4.1+/-2.4 mm/s2) medially (P < 0.001). The time-to-peak was shorter in the medial part of the muscle (2.9+/-0.4 ms) than in the other locations (3.4+/-0.5 ms, P < 0.001). The double integrated signals (muscle displacement) indicated negative deflection in the lateral part and inflation close to the tibia bone. The maps of acceleration showed spatial dependency in single motor unit MMG activities. The technique provides a new insight into motor unit contractile properties.

Spatial and force dependency of mechanomyographic signal features

The aim was to investigate with a novel technique the spatial inhomogeneity in surface mechanomyographic (MMG) response to muscle contraction at varying force levels. MMG signals were detected over the dominant tibialis anterior muscle of 10 volunteers using a 5 x 3 grid of accelerometers. The subjects performed 3 s long isometric contractions at forces ranging from 0% to 100% of the maximal force (10% increment) in a randomised order. From the two-dimensional MMG recordings, maps of absolute and normalized temporal and spectral MMG descriptors were obtained. The centroid and entropy of these maps were computed to describe the spatial centre of activity and degree of homogeneity, respectively. MMG absolute amplitude did not depend on location over the muscle while normalized amplitude did and the centroid shifted with increasing force. Amplitude increased with force and its entropy decreased. Absolute and normalized spectral variables depended on location over the muscle and their centroid shifted with increasing force. In addition, the dependency of absolute and normalized spectral variables on force was affected by location. These results highlight limitations when using single-channel MMG features for the assessment of motor unit control strategies, due to a substantial effect of position on the relation between force and MMG characteristics.

Effects of electromyographic and mechanomyographic biofeedback on upper trapezius muscle activity during standardized computer work

The purpose of this laboratory study was to investigate the effects of surface electromyography (EMG)- and mechanomyography (MMG)-based audio and visual biofeedback during computer work. Standardized computer work was performed for 3 min with/without time constraint and biofeedback in a randomized order. Biofeedback was given on the basis of an individual preset threshold value for the right trapezius EMG and MMG signal and a time factor (repetition of events above the threshold). The duration of muscle activity above the preset threshold, the right trapezius EMG and MMG root mean square (RMS) values as well as the work performance in terms of number of completed graph/mouse clicks/errors, the rating of perceived exertion (RPE) and the usefulness of the biofeedback were assessed. The duration of muscle activity above the threshold was significantly lower with MMG compared with EMG as source of biofeedback (p < 0.05). Biofeedback led to a significant decrease in the right trapezius EMG RMS, lower RPE and decreased number of errors and mouse clicks, but also decreased number of completed graphs (p < 0.05). Audio and visual biofeedbacks were as effective. MMG-based biofeedback is a potential reliable alternative to EMG in ergonomics. A lowering of the trapezius muscle activity may contribute to diminish the risk of work related musculoskeletal disorders development.

Comparison of a piezoelectric contact sensor and an accelerometer for examining mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii

The purpose of this study was to compare a piezoelectric contact sensor with an accelerometer for measuring the mechanomyographic (MMG) signal from the biceps brachii during submaximal to maximal isokinetic and isometric forearm flexion muscle actions. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), 10 adults (mean+/-SD age=22.8+/-2.7yrs) performed randomly ordered, submaximal step muscle actions of the dominant forearm flexors in 20% increments from 20% to 80% PT and MVC. Surface MMG signals were recorded simultaneously from a contact sensor and an accelerometer placed over the belly of the biceps brachii muscle. During the isokinetic and isometric muscle actions, the contact sensor and accelerometer resulted in linear increases in normalized MMG amplitude with torque (r(2) range=0.84-0.97) but the linear slope of the normalized MMG amplitude versus isokinetic torque relationship for the accelerometer was less (p<0.10) than that of the contact sensor. There was no significant (p>0.05) relationship for normalized MMG mean power frequency (MPF, %max) versus isokinetic and isometric torque for the contact sensor, but the accelerometer demonstrated a quadratic (R(2)=0.94) or linear (r(2)=0.83) relationship for the isokinetic and isometric muscle actions, respectively. There were also a number of significant (p<0.05) mean differences between the contact sensor and accelerometer for normalized MMG amplitude or MPF values. These findings indicated that in some cases involving dynamic and isometric muscle actions, the contact sensor and accelerometer resulted in different torque-related responses that may affect the interpretation of the motor control strategies involved.

Spectral moments of mechanomyographic signals recorded with accelerometer and microphone during sustained fatiguing contractions

The aim of this study was to analyse the trends of the first three power spectral moments of the mechanomyogram (MMG) signal recorded by a microphone (MMG(MIC)) and an accelerometer (MMG(ACC)) during sustained contractions. MMG signals were recorded from the biceps brachii muscle in 14 healthy male subjects during a 3 min isometric elbow flexion at 30% of the maximal voluntary contraction. MMG absolute and normalised root mean square (RMS), mean power frequency (MNF), power spectral variance (Mc2), and skewness (mu3) were computed. For both MMG(MIC) and MMG(ACC), absolute and normalised RMS and Mc2 increased while MNF and mu3 decreased with contraction time (P<0.001). The rates of change of RMS over time were significantly correlated (P<0.001) for MMG(MIC) and MMG(ACC) but not correlated for spectral moments. The coefficient of variation of RMS was higher for MMG(MIC) than for MMG(ACC), while the opposite was observed for mu3 (P<0.05). It was concluded that higher order spectral moments of the MMG signal change during sustained contraction, indicating a complex modification of the shape of the power spectrum and not just scaling of the bandwidth. This is most likely due to the additional motor unit recruitment with fatigue and to the non-linear summation of motor unit contributions to the signal. Moreover, the characteristics of MMG signals recorded with microphones and accelerometers have important differences, which should be taken into account when comparing results from different studies.

Experimentally verified model of mechanomyograms recorded during single motor unit contractions

The aim of the paper is to create a model which enables to observe the mechanomyographic (MMG) wave generated during single motor unit contractions in a muscle, while the muscle is immersed in paraffin oil. The muscle model is described as a rheological membrane. Both the muscle and the medium models have been built by using Stiff-Finite-Element-Method (SFEM), which allows one to simulate the muscle surface displacement and the acoustic propagation of this effect in the oil. Such a modelling enables one to determine the impact of the rheological properties of the liquid environment on the shape of the MMG wave. In order to verify the model, the MMG signals and the contraction forces have been recorded in vivo from the medial gastrocnemius muscle of a rat. In these experiments single motor units were stimulated with various stimulation frequencies. A piezotransducer, immersed in paraffin oil, has been used to record the MMG signal recording. The signals recorded during individual twitches of the motor units have been used to estimate the parameters of the model. Subsequently, the model has been experimentally verified. The signals recorded in experiments during unfused and fused tetani have been compared with the simulated model responses in the analogous stimulation program. It has been observed that the MMG signals obtained with the proposed linear model have been consistent with the results of in vivo experiments.

Non-invasive characterization of single motor unit electromyographic and mechanomyographic activities in the biceps brachii muscle

The aim of the study was to investigate amplitude and frequency content of single motor unit (MU) electromyographic (EMG) and mechanomyographic (MMG) responses. Multi-channel surface EMG and MMG signals were detected from the dominant biceps brachii muscle of 10 volunteers during isometric voluntary contractions at 20%, 50%, and 80% of the maximal voluntary contraction (MVC) force. Each contraction was performed three times in the experimental session which was repeated in three non-consecutive days. Single MU action potentials were identified from the surface EMG signals and their times of occurrence used to trigger the averaging of the MMG signal. At each contraction level, the MUs with action potentials of highest amplitude were identified. Single MU EMG and MMG amplitude and mean frequency were estimated with normalized standard error of the mean within subjects (due to repetition of the measure in different trials and experimental sessions) smaller than 15% and 7%, respectively, in all conditions. The amplitude of the action potentials of the detected MUs increased with increasing force (mean +/- SD, 244 +/- 116 microV at 20% MVC, and 1426 +/- 638 microV at 80% MVC; P < 0.001) while MU MMG amplitude increased from 20% to 50% MVC (40.5 +/- 20.9 and 150 +/- 88.4 mm/s(2), respectively; P<0.001) and did not change significantly between 50% and 80% MVC (129 +/ -82.7 mm/s(2) at 80% MVC). MU EMG mean frequency decreased with contraction level (20% MVC: 97.2 +/- 13.9 Hz; 80% MVC: 86.2 +/- 11.4 Hz; P < 0.001) while MU MMG mean frequency increased (20% MVC: 33.2 +/- 6.8 Hz; 80% MVC: 40.1 +/- 6.1 Hz; P < 0.001). EMG peak-to-peak amplitude and mean frequency of individual MUs were not correlated with the corresponding variables of MMG at any contraction level.

Validation of an accelerometer for determination of muscle belly radial displacement

A commercial variable-capacitance micromachined accelerometer was validated for muscle belly radial displacement measurement. The displacement was calculated by the acceleration data being integrated twice and was compared with the results obtained simultaneously by an accurate mechanical displacement sensor based on an optical encoder. The aim of the investigation was to evaluate the accuracy and precision of an accelerometer for tensiomyography, which is a method for the detection of skeletal muscle contractile properties on the basis of muscle belly radial displacement. A hundred measurements at a bandwidth of 2300 Hz were performed. It was shown that the accuracy and precision in determination of the maximum displacement and the time of the maximum displacement from the calculated curve were satisfactory, in spite of the standard deviation of the twice-integrated acceleration growing approximately linearly with time. The results were accurate enough since the elapsed time from the beginning of the integration was small (less than 75 ms). The measured maximum displacement ranges were between 9.2 and 10.2 mm. The mean relative error was less than 1% (SD = 0.02mm) for the maximum displacement and about 1% (SD = 0.6 ms) for the time to maximum displacement. The accuracy of the half-relaxation time determination was more uncertain because of the relatively high relative error of -2.4% (SD = 3 ms). Results showed that a commercial micromachined accelerometer could be suitable for the measurement of muscle belly radial displacement and used for development of a future miniaturised and flexible system for the measurement of similar displacements.

Experimental muscle pain increases mechanomyographic signal activity during sub-maximal isometric contractions

This study was designed to investigate the local effect of experimental muscle pain on the MMG and the surface EMG during a range of sub-maximal isometric contractions. Muscle pain was induced by injections of hypertonic saline into the biceps brachii muscle in 12 subjects. Injections of isotonic saline served as a control. Pain intensity and location, MMG and surface EMG from the biceps brachii were assessed during static isometric (0%, 10%, 30%, 50% and, 70% of the maximal voluntary contraction) and ramp isometric (0-50% of the maximal voluntary contraction) elbow flexions. MMG and surface EMG signals were analyzed in the time and frequency domain. Experimentally induced muscle pain induced an increase in root mean square values of the MMG signal while no changes were observed in the surface EMG. Most likely this increase reflects changes in the mechanical contractile properties of the muscle and indicates compensatory mechanisms, i.e. decreased firing rate and increased twitch force to maintain a constant force output in presence of experimental muscle pain. Under well-controlled conditions, MMG recordings may be more sensitive than surface EMG recordings and clinically useful for detecting non-invasively increased muscle mechanical contributions during muscle pain conditions.