Massimiliano Gobbo

The surface mechanomyogram as a tool to describe the influence of fatigue on biceps brachii motor unit activation strategy. Historical basis and novel evidence.

The surface mechanomyogram (MMG) (detectable at the muscle surface as MMG by accelerometers, piezoelectric contact sensors or other transducers) is the summation of the activity of single motor units (MUs). Each MU contribution is related to the pressure waves generated by the active muscle fibres. The first part of this article will review briefly the results obtained by our group studying the possible role of motor unit recruitment and firing rate in determining the characteristics of the MMG during stimulated and voluntary contractions. The second part of this article will study the MMG and EMG during a short isometric force ramp from 0 to 90% of the maximal voluntary contraction (MVC) in fresh and fatigued biceps brachii. The aim is to verify whether changes in motor unit activation strategy in voluntarily fatigued muscle could be specifically reflected in the time and frequency domain parameters of the MMG. MMG-RMS vs. %MVC: at fatigue the MMG-RMS did not present the well known increment, when effort level increases, followed by a clear decrement at near-maximal contraction levels. MMG-MF vs. %MVC: compared to fresh muscle the fatigued biceps brachii showed an MF trend significantly shifted towards lower values and the steeper MF increment, from 65 to 85% MVC, was not present. The alteration in the MMG and EMG parameters vs. %MVC relationships at fatigue seems to be related to the impossibility of recruiting fast, but more fatigable MUs, and to the lowering of the global MUs firing during the short isometric force ramp investigated.

Muscle motor point identification is essential for optimizing neuromuscular electrical stimulation use

Transcutaneous neuromuscular electrical stimulation applied in clinical settings is currently characterized by a wide heterogeneity of stimulation protocols and modalities. Practitioners usually refer to anatomic charts (often provided with the user manuals of commercially available stimulators) for electrode positioning, which may lead to inconsistent outcomes, poor tolerance by the patients, and adverse reactions. Recent evidence has highlighted the crucial importance of stimulating over the muscle motor points to improve the effectiveness of neuromuscular electrical stimulation. Nevertheless, the correct electrophysiological definition of muscle motor point and its practical significance are not always fully comprehended by therapists and researchers in the field. The commentary describes a straightforward and quick electrophysiological procedure for muscle motor point identification. It consists in muscle surface mapping by using a stimulation pen-electrode and it is aimed at identifying the skin area above the muscle where the motor threshold is the lowest for a given electrical input, that is the skin area most responsive to electrical stimulation. After the motor point mapping procedure, a proper placement of the stimulation electrode(s) allows neuromuscular electrical stimulation to maximize the evoked tension, while minimizing the dose of the injected current and the level of discomfort. If routinely applied, we expect this procedure to improve both stimulation effectiveness and patient adherence to the treatment.The aims of this clinical commentary are to present an optimized procedure for the application of neuromuscular electrical stimulation and to highlight the clinical implications related to its use.

Effect of accelerometer location on mechanomyogram variables during voluntary, constant-force contractions in three human muscles.

To understand better the features of the mechanomyogram (MMG) with different force levels and muscle architectures, the MMG signals detected at many points along three muscles were analysed by the application of a linear array of MMG sensors (up to eight) over the skin. MMG signals were recorded from the biceps brachii, tibialis anterior and upper trapezius muscles of the dominant side of ten healthy male subjects. The accelerometers were aligned along the direction of the muscle fibres. One accelerometer was located over the distal muscle innervation zone, and the other six or seven accelerometers were placed over the muscle, forming an array of sensors with fixed distances between them. The array covered almost the entire muscle length in all cases. MMG signals detected from adjacent accelerometers had similar shapes, with correlation coefficients ranging from about 0.5 to about 0.9. MMG amplitude and characteristic spectral frequencies significantly depended on accelerometer location. The MMG amplitude was maximum at the muscle belly for the biceps brachii and the tibialis anterior. Higher MMG characteristic spectral frequencies were associated with higher amplitudes in the case of the biceps brachii, whereas the opposite was observed for the tibialis anterior muscle. In the upper trapezius, the relationship between characteristic spectral frequencies, MMG amplitude and contraction force depended on the accelerometer location. This suggested that MMG spectral features do not only reflect the mechanical properties of the recruited muscle fibres but depend on muscle architecture and motor unit territorial distribution. It was concluded that the location of the accelerometer can have an influence on both amplitude and spectral MMG features, and this dependence should be considered when MMG signals are used for muscle assessment.

Muscle-joint unit transfer function derived from torque and surface mechanomyogram in humans using different stimulation protocols

UNLABELLED Torque and laser detected surface mechanomyogram (MMG) analysis after electrical stimulation of human tibialis anterior (TA) of 14 male subjects was aimed to: (a) obtain the dynamic responses of TA muscle-joint unit from a long (LP, about 1h) and short (SP, 12.5s) stimulation protocol; (b) compare the resulting transfer function parameters from the two signals. The sinusoidal amplitude modulation of a 30 Hz stimulation train (SST) changed the number of the recruited motor units, and hence the isometric torque and the TA surface position in the same fashion. Subject instrumentation and SST amplitude range definition took about 25 min. SP: seven consecutive modulation frequencies (0.4, 6.0, 1.0, 4.5, 1.8, 3.0, and 2.5 Hz). LP: fourteen 5s long isolated frequencies (0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 4.0, 5.0, and 6.0 Hz), 5 min rest in between. Poles position (Hz) and added delay (ms) for phase correction with respect to the input sine (parameters of a critically damped II order system) were: torque 2.44+/-0.27 Hz (SP) or 2.32+/-0.33 Hz (LP) and 18.3+/-2.2 ms (SP) or 17.2+/-4.5 ms (LP); MMG 2.28+/-0.30 Hz (SP) or 2.30+/-0.44 Hz (LP) and 17.4+/-5.6 ms (SP) or 17.4+/-6.4 ms (LP). Differences were never statistically significant. CONCLUSION it is possible to characterise the in vivo mechanics of muscle-joint unit with a short (few seconds) stimulation protocol affordable in clinical environment using both torque and MMG signals.

Non-invasive assessment of single motor unit mechanomyographic response and twitch force by spike-triggered averaging

A method for non-invasive assessment of single motor unit (MU) properties from electromyographic (EMG), mechanomyographic (MMG) and force signals is proposed. The method is based on the detection and classification of single MU action potentials from interference multichannel surface EMG signals and on the spike-triggered average of the MMG (detected by an accelerometer) and force signals. The first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles were investigated at contraction levels of 2% and 5% of the maximum voluntary contraction (MVC) force. A third contraction was performed by selective activation of a single MU with surface MU action potential visual feedback provided to the subject. At 5% MVC, the mean (±standard error) single MU MMG peak-to-peak value was 11.0±1.8 mm s−2 (N=17) and 32.3±6.5 mm s−2 (N=20) for the FDI and AMD muscles, respectively. The peak of the twitch force was, at the same contraction livel, 7.41±1.34 mN and 14.42±2.92 mN, for the FDI and ADM muscles, respectively. The peak-to-peak value of the MMG was significantly different for the same MU at different contraction levels, indicating a non-linear summation of the single MU contributions. For the FDI muscle, the MMG peak-to-peak value of individual MUs was 21.5±7.8 mm s−2, when such MUs were activated with visual feedback provided to the subject, whereas, for the same MUs, it was 11.8±3.8 mm s−2, when the subject maintained a constant force level of 2% MVC. The method proposed allows the non-invasive assessment of single MU membrane and contractile properties during voluntary contractions.

Electromyogram and force fluctuation during different linearly varying isometric motor tasks

The purpose of this work was to verify if deviation from the mirror-like behaviour of the motor units activation strategy (MUAS) and de-activation strategy (MUDS) and the degree of the error of the motor control system, during consecutive linearly increasing-decreasing isometric tension tasks, depend on the maximum reached tension and/or on the rate of tension changes. In 12 male subjects the surface EMG and force produced by the first dorsal interosseus activity were recorded during two (a and b) trapezoid isometric contractions with different plateau (a: 50% maximal voluntary contraction (MVC) and b: 100% MVC) and rate of tension changes (a: 6.7% MVC/s and b: 13.3% MVC/s) during up-going (UGR) and down-going (DGR) ramps. Ten steps (ST) 6s long at 5, 10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC were also recorded. The root mean square (RMS) and mean frequency (MF) from EMG and the relative error of actual force output with respect to the target (% ERR) were computed. The EMG-RMS/% MVC and EMG-MF/% MVC relationships were not overlapped when the ST and DGR as well as the UGR and DGR data were compared. The % ERR/% MVC relationships during a and b contractions differed from ST data only below 20% MVC. It can be concluded that MUAS and MUDS are not mirroring one each other because MU recruitment or de-recruitment threshold may be influenced by the maximum effort and by the % MVC/s of UGR and DGR. The role of MUs mechanical and/or central nervous system hysteresis on force decrement control is discussed.

Validity and everyday clinical applicability of lumbar muscle fatigue assessment methods in patients with chronic non-specific low back pain: a systematic review

Abstract Purpose: This systematic literature review aimed at examining the validity and applicability in everyday clinical rehabilitation practise of methods for the assessment of back muscle fatiguability in patients with chronic non-specific low back pain (CNSLBP). Methods: Extensive research was performed in MEDLINE, Cumulative Index of Nursing and Allied Health Literature (CINAHL), Embase, Physiotherapy Evidence Database (PEDro) and Cochrane Central Register of Controlled Trials (CENTRAL) databases from their inception to September 2014. Potentially relevant articles were also manually looked for in the reference lists of the identified publications. Studies examining lumbar muscle fatigue in people with CNSLBP were selected. Two reviewers independently selected the articles, carried out the study quality assessment and extracted the results. A modified Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) scale was used to evaluate the scientific rigour of the selected works. Results: Twenty-four studies fulfilled the selection criteria and were included in the systematic review. We found conflicting data regarding the validity of methods used to examine back muscle fatigue. The Biering-Sorensen test, performed in conjunction with surface electromyography spectral analysis, turned out to be the most widely used and comparatively, the most optimal modality currently available to assess objective back muscle fatigue in daily clinical practise, even though critical limitations are discussed. Conclusions: Future research should address the identification of an advanced method for lower back fatigue assessment in patients with CNSLBP which, eventually, might provide physical therapists with an objective and reliable test usable in everyday clinical practise. Implications for Rehabilitation Despite its limitations, the Biering-Sorensen test is currently the most used, convenient and easily available fatiguing test for lumbar muscles. To increase validity and reliability of the Biering-Sorensen test, concomitant activation of synergistic muscles should be taken into account. Pooled mean frequency and half-width of the spectrum are currently the most valid electromyographic parameters to assess fatigue in chronic non-specific low back pain. Body mass index, grading of pain and level of disability of the study population should be reported to enhance research quality.

Association Between Sagittal Balance and Scoliosis in Patients with Parkinson Disease: A Cross-sectional Study.

ObjectiveThe aim of this study was to describe the association between scoliosis and sagittal balance parameters in Parkinson disease patients. DesignThis is a cross-sectional study. ResultsFifty percent of the cohort presented a scoliosis larger than 11 degrees; 84% of the patients with scoliosis presented a thoracolumbar curve, 10% presented a thoracic one, and 6% presented a lumbar one. The group with scoliosis curves presented a lower spinosacral angle (111.6 [21.9] degrees vs. 121.7 [9.8] degrees, P < 0.05), whereas thoracic kyphosis, lumbar lordosis, and spinopelvic angle were similar. Pelvic incidence, pelvic tilt, and sacral slope were not statistically different. In the scoliosis group, the authors found negative correlations for lumbar lordosis/spinopelvic angle, sacral slope/spinosacral angle, and lumbar lordosis/pelvic tilt. Moreover, the sacral slope/pelvic tilt correlation was positive in patients without scoliosis and negative in others. The two groups did not present differences regarding age, years of disease, Hoehn-Yahr score, and Unified Parkinson Disease Rating Scale-motor section. ConclusionsPelvic parameters were similar in the two groups, whereas spinosacral angle was lower in patients with scoliosis. The prevalence of scoliosis in Parkinson disease was higher than what was previously described and the thoracolumbar spine was the mostly affected.

Principal Component Analysis Applied to Surface Electromyography: A Comprehensive Review

Surface electromyography (sEMG) records muscle activities from the surface of muscles, which offers a wealth of information concerning muscle activation patterns in both research and clinical settings. A key principle underlying sEMG analyses is the decomposition of the signal into a number of motor unit action potentials (MUAPs) that capture most of the relevant features embedded in a low-dimensional space. Toward this, the principal component analysis (PCA) has extensively been sought after, whereby the original sEMG data are translated into low-dimensional MUAP components with a reduced level of redundancy. The objective of this paper is to disseminate the role of PCA in conjunction with the quantitative sEMG analyses. Following the preliminaries on the sEMG methodology and a statement of PCA algorithm, an exhaustive collection of PCA applications related to sEMG data is in order. Alongside the technical challenges associated with the PCA-based sEMG processing, the envisaged research trend is also discussed.

L6E9 Myoblasts Are Deficient of Myostatin and Additional TGF- Members Are Candidates to Developmentally Control Their Fiber Formation

This work provides evidence that the robust myoblast differentiation observed in L6E9 cells is causally linked to deficiency of myostatin, which, conversely, has been found to be expressed in C2C12 cells. However, despite the absence of endogenous myostatin, L6E9 myoblasts expressed functional Activin receptors type II (ActRIIs) and follistatin as well as the highly related TGF-β members Activins and GDF11, suggesting that in this cell line the regulation of fiber size might be under the control of multiple regulators regardless of myostatin. In line with this hypothesis, delivery of a dominant-negative ActRIIb form or the increase of follistatin, as obtained via Trichostatin treatment or stable transfection of a short human follistatin form, enhanced the L6E9 cell differentiation and further increased the size of myotubes, suggesting that L6E9 myoblasts provide a spontaneous myostatin knock-out in vitro model to study TGF-β ligands involved in developmental regulation of fiber size.