Estimation of the muscle fibre semi-length under varying joint positions on the basis of non-invasively extracted motor unit action potentials

Advances in EMG measurement techniques, analysis procedures, and the impact of muscle mechanics on future requirements for the methodology

Muscular coordination enables locomotion and interaction with the environment. For more than 50 years electromyography (EMG) has provided insights into the central nervous system control of individual muscles or muscle groups, enabling both fine and gross motor functions. This information is available either at individual motor units (Mus) level or on a more global level from the coordination of different muscles or muscle groups. In particular, non-invasive EMG methods such as surface EMG (sEMG) or, more recently, spatial mapping methods (High-Density EMG - HDsEMG) have found their place in research into biomechanics, sport and exercise, ergonomics, rehabilitation, diagnostics, and increasingly for the control of technical devices. With further technical advances and a growing understanding of the relationship between EMG and movement task execution, it is expected that with time, especially non-invasive EMG methods will become increasingly important in movement sciences. However, while the total number of publications per year on non-invasive EMG methods is growing exponentially, the number of publications on this topic in journals with a scope in movement sciences has stagnated in the last decade. This review paper contextualizes non-invasive EMG development over the last 50 years, highlighting methodological progress. Changes in research topics related to non-invasive EMG were identified. Today non-invasive EMG procedures are increasingly used to control technical devices, where muscle mechanics have a minor influence. In movement science, however, the effect of muscle mechanics on the EMG signal cannot be neglected. This explains why non-invasive EMG's relevance in movement sciences has not developed as expected.

Reliability of resting intramuscular fiber conduction velocity evaluation

Characterization of the least number of muscle fibers analyzed for a quick and reliable, evaluation of intramuscular fiber conduction velocity (MFCV) is of importance for sport scientists. The aim of this study was to evaluate the reliability of vastus lateralis' intramuscular MFCV measuring either 25 or 50 different muscle fibers per participant, as well as to compare intramuscular MFCV measured in 25 (C₂₅ ), 50 (C₅₀ ), or 140 (C₁₄₀ ) muscle fibers. Resting vastus lateralis' MFCV was measured in 21 young healthy males (age 22.1±2.4 years) using intramuscular microelectrodes in different days. Test-retest reliability of MFCV's parameters was calculated for C₂₅ and C₅₀ , while MFCV was compared among C₂₅ , C₅₀ , and C₁₄₀ . Significant differences of MFCV parameters were observed between C₂₅ condition and those of C₅₀ and C₁₄₀ . The differences in MFCV values between conditions C₅₀ and C₁₄₀ were non-significant. A close correlation was found for MFCV between C₅₀ and C₁₄₀ (r=0.884-0.988, P=.000). All reliability measures of MFCV measured with 50 fibers were high (eg, ICC=0.813-0.980, P=.000), in contrast to C₂₅ (eg, ICC=0.023-0.580 P>.05). In conclusion, an average of 50 different fibers per subject is sufficient to provide a quick and reliable intramuscular evaluation of vastus lateralis MFCV.

Territory and fiber orientation of vastus medialis motor units: A Surface electromyography investigation

INTRODUCTION

The aim of this study was to determine whether muscle fibers innervated by single motor neurons are confined in small subvolumes of the vastus medialis (VM) and if motor unit fiber orientation depends on their position within the muscle.

METHODS

Single motor units were identified from a grid of surface electrodes. The size of their surface representation and fiber orientation were extracted using an algorithm validated on simulated signals.

RESULTS

The action potentials of 77 motor units were represented locally on the skin (10th-90th percentiles: 14-25 mm). According to simulations, this indicates territories smaller than 11.8-64.8 mm. Motor units in distal regions of VM had fibers at a greater angle than those in proximal regions (R = -0.54, P < 0.001).

CONCLUSION

Motor units with small territories and varying fiber orientations may be an anatomical predisposition to regulate how regions within VM apply forces to the patella. This could help to redistribute loads within the joint in painful conditions.

Investigation of innervation zone shift with continuous dynamic muscle contraction

Innervation zone (IZ) has been identified as the origin of action potential propagation in isometric contraction. However, IZ shifts with changes in muscle length during muscle activity. The IZ shift has been estimated using raw EMG signals. This study aimed to investigate the movement of IZ location during continuous dynamic muscle contraction, using a computer program. Subjects flexed their elbow joint as repetitive dynamic muscle contractions. EMG signals were recorded from the biceps brachii muscle using an eight-channel surface electrode array. Approximately 100 peaks from EMG signals were detected for each channel and summed to estimate the IZ location. For each subject, the estimated IZ locations were subtracted from the IZ location during isometric contractions with the elbow flexed at 90°. The results showed that the IZ moved significantly with elbow joint movement from 45° to 135°. However, IZ movement was biased with only a 3.9 mm IZ shift on average when the elbow angle was acute but a 16 mm IZ shift on average when it was obtuse. The movement of IZ location during continuous dynamic muscle contraction can be investigated using this signal processing procedure without subjective judgment.

Spatial distribution of surface action potentials generated by individual motor units in the human biceps brachii muscle

This study analyses the spatial distribution of individual motor unit potentials (MUPs) over the skin surface and the influence of motor unit depth and recording configuration on this distribution. Multichannel surface (13×5 electrode grid) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25mm) electromyographic (EMG) signals were concurrently recorded with monopolar derivations from the biceps brachii muscle of 10 healthy subjects during 60-s isometric contractions at 20% of the maximum torque. Multichannel monopolar MUPs of the target motor unit were obtained by spike-triggered averaging of the surface EMG. Amplitude and frequency characteristics of monopolar and bipolar MUPs were calculated for locations along the fibers' direction (longitudinal), and along the direction perpendicular (transverse) to the fibers. In the longitudinal direction, monopolar and bipolar MUPs exhibited marked amplitude changes that extended for 16-32mm and 16-24mm over the innervation and tendon zones, respectively. The variation of monopolar and bipolar MUP characteristics was not symmetrical about the innervation zone. Motor unit depth had a considerable influence on the relative longitudinal variation of amplitude for monopolar MUPs, but not for bipolar MUPs. The transverse extension of bipolar MUPs ranged between 24 and 32mm, whereas that of monopolar MUPs ranged between 72 and 96mm. The mean power spectral frequency of surface MUPs was highly dependent on the transverse electrode location but not on depth. This study provides a basis for the interpretation of the contribution of individual motor units to the interference surface EMG signal.

Effect of position of electrodes relative to the innervation zone onsurface EMG

We investigated the effect of the position of electrodes relative to the innervation zone (IZ) of the biceps brachii muscle during isometric elbow flexion using eight-channel surface array electrodes. We estimated the location of the IZ near the centre of the muscle in 20 male subjects. The pulse peaks from electromyogram (EMG) waveforms were detected for each channel and averaged, the triphasic pulse was determined, and the peak values of the first and third phases were compared. The results showed significantly greater pulse values for the first phase when the electrode placement was proximal to the estimated IZ, and for the third phase when the electrode placement was distal to the estimated IZ. Using this method, the positional relationship between electrodes and IZ can be determined using a surface EMG waveform recorded with a pair of bipolar electrodes. This method may be clinically useful in confirming the reliability of a recorded surface EMG.

Monopolar surface electromyography: a better tool to assess motoneuron excitability upon passive muscle stretching

Bipolar and monopolar surface electromyography (sEMG) are known procedures to measure the H-reflex. However, signal cancellation is a potential experimental problem of bipolar sEMG. The results of our study show that monopolar sEMG was the more sensitive procedure to differentiate motoneuron excitability at different passive muscle stretching speeds as it overcame signal cancellation.

Analysis of motor units with high-density surface electromyography

Although the behaviour of individual motor units is classically studied with intramuscular EMG, recently developed techniques allow its analysis also from EMG recorded in multiple locations over the skin surface (high-density surface EMG). The analysis of motor units from the surface EMG is useful when the insertion of needles is not desirable or not possible. Moreover, surface EMG allows the measure of motor unit properties which are difficult to assess with invasive technology (e.g., muscle fiber conduction velocity or location of innervation zones) and may increase the number of detectable motor units with respect to selective intramuscular recordings. Although some limitations remain, both the discharge pattern and muscle fiber properties of individual motor units can currently be analyzed non-invasively. This review presents the conditions and methodologies which allow the investigation of motor units with surface EMG.

Efficient parameterization of MUAP signal for identification of MU and volume conductor characteristics using neural networks

The motor unit action potential (MUAPs) shapes depend on the anatomy and the physiology of the contracted muscle. The aim of this work is the identification of some characteristics of the motor unit (MU) and the volume conductor, namely the MU depth, the innervation zone width and the thickness of fat and skin layers based on MUAP signal parameters. The relationship between these characteristics and MUAP parameters are non-linear and complex. Thus, the use of the neural networks approach becomes an efficient tool to put in evidence this relationship. We have used the similarity and the homogeneity of the parameter criterions to choose which parameters are appropriate for the extraction. Two identification systems are presented and compared, a global system and a separate one. In order to evaluate the performance of each system, we have tested them using several simulated MUAP signals corrupted with additive Gaussian noise at different signal to noise ratios (SNR). A new test is introduced in which the electrode radius, the bar electrode dimensions and inclination angles for the detection system, fixed during the training process, are changed.

Triceps surae stretch and voluntary contraction alters maximal M-wave magnitude

Reliability of the motor response (M-wave) is fundamental in many reflex studies; however it has recently been shown to change during some investigations. The aim of this investigation was to determine if triceps surae stretch and voluntary contraction, or recording and analysis techniques, affect the maximal M-wave magnitude. The maximal M-wave was investigated in human gastrocnemius and soleus during different foot positions and during triceps surae contraction. Both bipolar and monopolar-recoding methods, and area and peak-to-peak (PTP) amplitude analysis methods were used.

RESULTS

Maximal M-wave magnitude changed significantly between test muscle conditions, and is largest during dorsiflexion, probably due to changes in muscle bulk and recording electrode relationship. The maximal M-wave was up to 88% smaller when recorded by bipolar electrodes compared to monopolar electrodes, which is discussed in relation to signal cancellation. Area analysis provided more significant differences in M-wave magnitude between test muscle conditions than did PTP amplitude analysis, and the maximal M-wave shape changed significantly between test muscle conditions. This study suggests that maximal M-wave magnitude can vary depending on muscle condition, it highlights the importance of using correct recording and analysis techniques, and questions the reliability of using M-wave magnitude to monitor the relationship between the nerves and stimulating electrodes.

Clinical applications of high-density surface EMG: A systematic review

High density-surface EMG (HD-sEMG) is a non-invasive technique to measure electrical muscle activity with multiple (more than two) closely spaced electrodes overlying a restricted area of the skin. Besides temporal activity HD-sEMG also allows spatial EMG activity to be recorded, thus expanding the possibilities to detect new muscle characteristics. Especially muscle fiber conduction velocity (MFCV) measurements and the evaluation of single motor unit (MU) characteristics come into view. This systematic review of the literature evaluates the clinical applications of HD-sEMG. Although beyond the scope of the present review, the search yielded a large number of "non-clinical" papers demonstrating that a considerable amount of work has been done and that significant technical progress has been made concerning the feasibility and optimization of HD-sEMG techniques. Twenty-nine clinical studies and four reviews of clinical applications of HD-sEMG were considered. The clinical studies concerned muscle fatigue, motor neuron diseases (MND), neuropathies, myopathies (mainly in patients with channelopathies), spontaneous muscle activity and MU firing rates. In principle, HD-sEMG allows pathological changes at the MU level to be detected, especially changes in neurogenic disorders and channelopathies. We additionally discuss several bioengineering aspects and future clinical applications of the technique and provide recommendations for further development and implementation of HD-sEMG as a clinical diagnostic tool.

A new method to estimate signal cancellation in the human maximal M-wave

A new method is introduced that estimates EMG signal cancellation in surface recorded investigations. Its usefulness is demonstrated when determining changes in the maximal motor response (M-wave) magnitude during rest and voluntary contraction. The accuracy of recording and analysis methods and the reliability of the maximal M-wave were assessed in the human gastrocnemius and soleus. The maximal M-wave was recorded by bipolar surface electrodes placed 2 cm, 3 cm and 4 cm apart, and by monopolar (one active and one indifferent reference) surface electrodes. Up to 85% of the maximal M-wave was lost due to signal cancellation during bipolar recording. The maximal M-wave magnitude decreased consistently and significantly during triceps surae contraction compared to rest when recorded by monopolar electrodes, but not when recorded by bipolar electrodes. Area and peak-to-peak (PTP) amplitude analysis methods provided similar results when determining the magnitude of the maximal M-wave. This provides evidence that monopolar recording is superior to bipolar recording as it removes the signal cancellation error and allows the genuine changes in maximal M-wave magnitude to be observed.