Suprascapular neuropathy restricted to the infraspinatus muscle in volleyball players

Six volleyball players developed weakness and atrophy of the infraspinatus muscle on the dominant side. Electromyography (EMG) disclosed denervation and motor unit loss restricted to the infraspinatus muscle, the supraspinatus and other shoulder muscles remaining normal. Infraspinatus neuropathy may occur as a professional hazard in volleyball players. A lesion of the suprascapular nerve at the spinoglenoid notch is implied.     

Recording and physical characteristics of disposable concentric needle EMG electrodes

There is currently considerable interest in using disposable concentric needle (CN) electrodes for clinical electromyography (EMG). To determine how these electrodes compare with reusable CN electrodes, we have compared signals recorded by these two electrode types from the same muscle in normal subjects. We also made similar recordings with two groups of reusable electrodes. There was no difference in the features of motor unit action potentials (MUAPs) recorded by the two groups of reusable electrodes. Disposable electrodes performed satisfactorily in conventional EMG examination. However, compared to reusable electrodes, the disposable electrodes recorded MUAPs with smaller amplitude and area but with the same area:amplitude ratio and MUAP duration. The physical and electrical properties of the CNE groups were also investigated. Disposable electrodes had lower electrical resistance and greater capacitance than reusable electrodes when measurements were made in saline. Photomicrographs showed that the disposable electrodes had smaller recording surfaces and that the central wire was frequently eccentric in the cannula. The differences in electrical recording characteristics could be due to differences in the size of the recording surface, eccentric placement of the central wire in the cannula or differences in the metal used for the central wire. We conclude that electrical and physical testing may not predict the recording characteristics of needle electrodes. Electrophysiologic testing is necessary to determine how EMG signals recorded by new types of electrodes compare with those recorded by currently used electrodes.     

Surface EMG crosstalk between knee extensor muscles: experimental and model results

Surface electromyographic (EMG) crosstalk between vastus lateralis, vastus medialis, and rectus femoris muscles was evaluated by selective electrical stimulation of one muscle and recording from the stimulated and another muscle with linear surface arrays of eight electrodes. The ratio between the amplitude of the signals recorded over nonstimulated and stimulated muscles and their correlation coefficient were used as indices to quantify crosstalk. Single-differential and double-differential detection systems were used with interelectrode distances in the range 10-40 mm. The multichannel EMG signals clearly showed that crosstalk is largely due to nonpropagating potentials that correspond in time to the end of the propagation of the action potentials generated by the stimulated muscle. The crosstalk signal increased with increasing interelectrode distance and was statistically higher for single- than for double-differential recordings. The correlation-based indices of crosstalk were poorly correlated with the amplitude-based indices. Moreover, the characteristic spectral frequencies of the signals detected over the nonstimulated muscles were statistically higher than those from the stimulated muscles. A mathematical model of signal generation was used to explain the experimental findings. This study clarifies many controversial findings of past investigations and creates the basis for crosstalk interpretation, simulation, and reduction.     

Atypical shoulder muscle activation in multidirectional instability.

OBJECTIVE: Surface and intramuscular electromyography was used to investigate shoulder muscle activity in subjects with multidirectional instability (MDI). METHODS: Subjects (seven MDI, 11 control) performed repetitive shoulder abduction/adduction, flexion/extension and internal/external rotation movements on an isokinetic dynamometer. The activity of the deltoid, infraspinatus, supraspinatus, latissimus dorsi, and pectoralis major muscles were recorded using double-differential surface and intramuscular fine-wire electrodes. A repeated measures analysis of variance evaluated group differences in the amplitude, onset, termination and duration of the muscle activity. RESULTS: Significant activation parameter differences for the supraspinatus, infraspinatus, posterior deltoid and pectoralis major muscles were found in the subjects with MDI. The rotator cuff and posterior deltoid muscles demonstrated abbreviated periods of activity when performing internal/external rotation, despite activation amplitudes that were similar to the controls. In contrast, the activation of the pectoralis major differed from the control group in both the amplitude and time domains when performing shoulder extension. CONCLUSIONS: MDI is associated with atypical patterns of muscle activity that occur even when highly constrained movements are used to elicit the activity. SIGNIFICANCE: In addition to glenohumeral hyperlaxity, the results suggest that dysfunctional neuromuscular control of the rotator cuff is also a contributing factor to the pathoetiology of MDI.     

Electromyography‐related pain: Muscle selection is the key modifiable study characteristic

Introduction: The aim of this study was to estimate the effects of patient, provider, and study characteristics on electromyography (EMG)‐related pain. Methods: Patients undergoing EMG rated their EMG‐related pain after each muscle was studied on a 100‐point visual analog scale (VAS). Investigators recorded the order in which the muscles were sampled, the total time spent with the needle in each muscle, and whether electrical endplate noise was noted. Results: A total of 1781 muscles were studied in 304 patients. Eleven muscles were associated with significantly more or less pain than the others. Endplate noise was associated with more pain (5.4 mm, 95% CI 2.8–7.0). There was a small, but significant effect from needling time (0.02 mm, 95% CI 0.00–0.04). Conclusions: Among factors that electromyographers can control, muscle selection has the greatest impact on pain. Our data include an extensive list of muscle‐specific EMG‐related pain scores. Provider and other study characteristics have little or no impact on EMG‐related pain. Muscle Nerve 49:570–574, 2014     

A convenient method to reduce crosstalk in surface EMG. Cobb Award-winning article, 2001.

OBJECTIVES: Crosstalk in surface EMG can be reduced by the use of spatial filters. We compared a variety of spatial filters to establish the most effective and the least complex method to reduce crosstalk. METHODS: Six different spatial filters described in the literature were tested in 8 healthy volunteers. Electrode arrays were placed over the anterior tibial and triceps surae muscles. Selective muscle activation was achieved both by supramaximal nerve stimulation and by maximal voluntary contraction. Selectivity of activation was guaranteed by using intramuscular wire electrodes. Crosstalk was quantified by dividing the amount of EMG activity recorded during pure agonist activation (i.e. the muscle directly under the electrode array) by the EMG activity recorded during pure antagonist activation. This was done for both compound muscle action potentials and voluntary muscle activation. The amount of crosstalk recorded with the different spatial filters was compared with that recorded with a standard bipolar lead. RESULTS: Crosstalk was most reduced by the "double-differential" (DD) filter, yielding an up to 6-fold improvement of EMG selectivity. We then compared signals recorded with this DD filter with those recorded with the less complex "branched electrode". As expected on theoretical grounds, signals from both filter types were identical. CONCLUSIONS: Crosstalk is best reduced using a "double-differentiating" recording technique, which can be achieved easily using a branched electrode instead of a standard bipolar lead. This technique can be used with all conventional EMG equipment.     

Recording characteristics of the surface EMG electrodes

Routine motor nerve conduction studies are conducted using surface EMG electrodes. Most techniques of estimating the number of motor units (MUs) are based on surface EMG recordings. Therefore, it is important to assess the uptake area of these electrodes. We recorded surface EMG motor unit action potentials (SMUAPs) from the biceps muscle of normal subjects. The SMUAP amplitude fell from 42 μV for the superficially located MUs (i.e., within 10 mm of skin surface) to 11 μV for the deep MUs (i.e., more than 20 mm from the skin surface). We infer that the pickup radius of the surface electrode is less than 20 mm. The implications of the limited uptake area of the surface electrodes to the analysis of compound muscle action potentials, estimation of the number of MUs, and the surface EMG recordings are discussed. © 1994 John Wiley & Sons, Inc.     

Effect of shoulder angle on the activation pattern of the elbow extensors during a submaximal isometric fatiguing contraction

The aim of this study was to examine the effect of shoulder angle on the electromyographic (EMG) activation pattern of the elbow extensors during a fatiguing contraction. Ten young men (23.5 ± 1.7) were tested on two occasions with the elbow angle at 90° and the shoulder at either 0° or 90° of flexion. EMG was recorded by fine wire electrodes inserted into the lateral, medial, and long heads of the triceps brachii and the anconeus. An EMG‐torque relationship was determined prior to a sustained isometric contraction at 20% of maximum voluntary contraction (MVC) until target failure. Endurance time was shorter, and postfatigue MVC torque was lower at 90° (40.4 ± 12.7 Nm) versus 0° (47.9 ± 14.7 Nm) of flexion. EMG activity of the long head during the final 10% of the fatiguing contraction was significantly greater at 90° versus 0° with no effect of shoulder angle on any other muscle portions. The findings suggest that measures from one muscle portion of the elbow extensors are not representative of the whole group, and the relative activation of the two‐joint long head was changed depending on shoulder angle during a fatigue task. Muscle Nerve, 2010     

Single-wire recording of regional activity in the human masseter muscle

A new approach is presented for recording regional muscle responses by means of a single intramuscular wire electrode. The technique exploits differences in impedance due to the locations of the recording and indifferent electrodes and also relies upon bandpass filtering. The method is demonstrated by recording the activity of the right masseter muscle at three different sites during voluntary tooth clenching. The results indicate that satisfactory separation of regional muscle activity can be obtained and that the recorded signals can be sampled digitally for quantitative purposes. As this method for recording jaw muscle activity avoids the problem of variable interelectrode distances commonly associated with conventional bipolar wire electrodes, it is advantageous when recording from human muscles in the facial and intraoral regions.     

Muscle‐selective block using intrafascicular high‐frequency alternating current

High‐frequency alternating current (HFAC) applied to a peripheral nerve can reversibly block skeletal muscle contractions. We evaluated the ability of HFAC delivered via intrafascicular electrodes to selectively block activation of targeted muscles without affecting activation of other muscles. Utah slanted electrode arrays (USEAs) were implanted into the sciatic nerves of five cats, and HFAC was delivered to individual USEA electrodes. The effects of HFAC block were monitored by recording evoked electromyograms (EMGs) and three‐dimensional endpoint forces. In each animal, activity evoked in targeted muscles by nerve cuff stimulation could be selectively abolished by HFAC delivered via individual USEA electrodes. Two mechanisms of blockade were evoked: selective neuromuscular blocks were achieved with 500–8000‐HZ HFAC, and selective nerve conduction block was achieved in one animal using 16‐kHZ HFAC. These results show that intrafascicular HFAC can be used to block selected muscles independent of activation of other muscles. Muscle Nerve 42: 339–347, 2010     

Suprascapular nerve lesions at the spinoglenoid notch: report of three cases and review of the literature.

Lesions of the suprascapular nerve can occur at the supraspinatus notch (SSN) or at the spinoglenoid notch (SGN). Electromyographic (EMG), evaluation of the infraspinatus, and especially the supraspinatus muscles distinguishes SGN from SSN lesions. Three cases of SGN lesions, which are more common than SSN lesions, are presented.     

Decoding the neural drive to muscles from the surface electromyogram

This brief review discusses the methods used to estimate the neural drive to muscles from the surface electromyogram (EMG). Surface EMG has been classically used to infer the neural activation of muscle by associating its amplitude with the number of action potentials discharged by a population of motor neurons. Although this approach is valuable in some applications, the amplitude of the surface EMG is only a crude indicator of the neural drive to muscle. More advanced methods are now available to estimate the neural drive to muscle from the surface EMG. These approaches identify the discharge times of a few motor units by decomposing the EMG signal to determine the relative changes in neural activation. This approach is reliable in several conditions and muscles for isometric contractions of moderate force, but is limited to the few superficial units that can be identified in the recordings.     

The standard concentric needle cannula cannot replace the Macro EMG electrode

ObjectiveTo establish the usefulness of the single use and affordable standard concentric EMG electrode as a substitute for the expensive standard macro electrode.MethodsMacro EMG performed with macro electrode is compared with recordings from the uninsulated cannula of a standard EMG electrode at two different recording depths in the tibialis anterior muscle. This was performed both in muscles with signs of collateral reinnervation and without.ResultsThe amplitude of the motor units recorded with the uninsulated concentric needle cannula were lower for the deeply recorded motor units compared to motor unit potential (MUP) amplitudes recorded with the standard macro electrode. The deeply recorded concentric needle (CN) cannula recorded MUPs amplitudes were also lower than superficially recorded CN cannula MUPs. The standard Macro EMG signals show no difference between deeply and superficially recorded motor units.ConclusionThe uninsulated cannula of the concentric needle electrode cannot replace the standard Macro EMG electrode due to technical reasons, probably from different effects of shunting of the bare cannula in deep vs. superficially recorded motor units.SignificanceThe standard CN electrode could not be used as substitute for the standard Macro EMG needle.     

Changes in joint angle,muscle‐tendon complex length,muscle contractile tissue displacement,and modulation of EMG activity during acute whole‐body vibration

It has been suggested that vibration causes small changes in muscle length, but to the best of our knowledge, these have yet to be demonstrated during whole‐body vibration (WBV). This was an observational study to determine whether acute WBV would result in muscle lengthening. We hypothesized that acute WBV would increase electromyography (EMG) activity concurrently with measurable changes in muscle contractile length. Nine healthy males performed two conditions on a Galileo vibration machine for 15 s at 0 HZ (resting) and 6 HZ at a set knee angle of 18°. Muscle tendon complex length, contractile tissue displacement of the medial gastrocnemius muscle, and EMG of soleus, tibialis anterior, and vastus lateralis muscles were measured. At 6 HZ the medial gastrocnemius (MG) muscle tendon complex (MTC) amplitude (375 μm) was significantly greater (P < 0.05) compared to 0 HZ (35 μm). The MG contractile length (CD) amplitude at 6 HZ (176 μm) was significantly greater (P < 0.01) compared to 0 HZ (4 μm). Significant increases (P < 0.05) in EMG modulation were found for all muscles during the 6 HZ compared to the 0 HZ condition. The major finding was that ≈50% of the elongation occurred within the muscle itself and was associated with preceding changes in EMG. This indicates muscle lengthening may be a prerequisite for eliciting stretch reflexes. In conclusion, there is a temporal association between EMG activity and muscle contractile tissue displacement where low‐frequency WBV results in small muscle length changes and increases muscle activation. Muscle Nerve, 2009     

Effects of high‐pass filtering on MUAP metrics

Previous observations suggest that elevated high‐pass filter settings (1,600–3,200 HZ ) can reveal greater motor unit action potential (MUAP) complexity (turns). We assessed the effect of high‐pass filter settings (500, 1,000, 2,000 HZ ) on MUAP metrics. MUAPs were recorded with a concentric needle and initially extracted by a decomposition software algorithm at 10 HZ –10 kHZ and further filtered offline at 500, 1,000, and 2,000 HZ . When reanalyzed by the decomposition software there were marked reductions in peak–peak amplitude, area, area‐to‐amplitude ratio, and duration at the 500 HZ filter with lesser subsequent reductions at higher filter settings. In contrast, turn and phase counts did not change significantly. Individual MUAPs tracked across filter settings showed rare increases in turn count at the 500 HZ setting but a subsequent decrease in counts with higher filter settings. We conclude that the routine use of elevated high‐pass filters, as in quantitative EMG analysis, does not enhance turn count. Muscle Nerve, 2009     

Normalization reduces the spatial dependency of the jaw‐stretch reflex activity in the human masseter muscle

The jaw‐stretch reflex is the short‐latency response in the jaw‐closing muscles after a sudden stretch. The hypothesis whether normalization of the jaw‐stretch reflex amplitude with respect to prestimulus electromyographic (EMG) activity will make the amplitude more independent of the location of the electrodes over the masseter muscle was tested. A 5 × 6 electrode grid was used to record the jaw‐stretch reflex from 25 sites over the right masseter muscle of 15 healthy men. The results showed that there was a significant site dependency of the prestimulus EMG activity and the reflex amplitude. High cross‐correlation coefficients were found between the spatial distribution of mean prestimulus EMG activities and reflex amplitude. When the reflex amplitude was normalized with respect to the prestimulus EMG activity, no site dependency was found. In conclusion, normalization of the jaw‐stretch reflex amplitude by the prestimulus EMG activity strongly reduces its spatial dependency. Muscle Nerve, 2010     

Effect of antagonistic voluntary contraction on motor responses in the forearm.

OBJECTIVE: We investigated the effects of voluntary contraction of agonist and antagonist muscles on motor evoked potentials (MEP) and on myoelectric activities in the target (agonist) muscle following transcranial magnetic stimulation (TMS). METHODS: The left extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles were studied in 16 healthy subjects. H reflexes, MEP induced by TMS, and background electromyographic (EMG) activity were recorded using surface electrodes at rest and during voluntary contraction of either agonist or antagonist muscles. RESULTS: Voluntary contraction of antagonist muscles (at 10% of maximum contraction) enhanced the amplitudes of MEP for both muscles. The H reflex of the FCR muscle was inhibited by contraction (10% of maximum) of the ECR muscle. Background EMG activity did not differ between H-reflex trials and TMS trials. Enhancement of MEP amplitudes and background EMG activity during voluntary antagonist contraction was comparable in the two muscles. Appearance rate of MEP recorded by needle electrodes in response to subthreshold TMS was increased by antagonistic voluntary contraction. CONCLUSION: Facilitation occurs during voluntary contraction of antagonist muscles. Differences between the effects of voluntary contraction of the ECR muscle for the MEP and the H reflex of the FCR suggest that cortical facilitatory spread occurs between agonist and antagonist muscles.     

The electromyographic signal as a presymptomatic indicator of organophosphates in the body

Organophosphate (OP) compounds are present in household and agricultural pesticides as well as in nerve agents. The toxic effects of these chemicals result from their anticholinesterase activity, which disrupts nerve junctions and parasympathetic effector sites, leading to a variety of symptoms and possible death. When the anticholinesterase agents in OP compounds reach the neuromuscular junction, they cause a disruption in the firing of muscle fiber action potentials. This effect has the potential of altering the time course of the electromyographic (EMG) signal detected by surface electrodes. We investigated the association between OP compound dose, surface EMG changes, and overt signs of OP toxicity. Daily doses of 10-15 microg/kg of diisopropylfluorophosphate (DFP) were injected into the calf muscle of four rhesus monkeys while surface EMG signals were recorded from two thigh muscles bilaterally. With increasing number of doses, the EMG signal presented an increasing number of time gaps. The presence of the gaps was evident prior to any overt symptoms of cholinesterase toxicity. These findings can lead to the development of noninvasive technology for indicating the presence of OP compounds in muscle tissue prior to clinical abnormalities.     

Non-invasive monitoring of functionally distinct muscle activations during swallowing.

OBJECTIVES: Dysphagia is an important consequence of many diseases. As some of the muscles of deglutition tend to be deep to the surface, needle electrodes are typically used, but this limits the number of muscles that can be simultaneously recorded. Since control of swallowing involves central pattern generators (CPGs) which distribute commands to several muscles, monitoring several muscles simultaneously is desirable. Here we describe a novel method, based on computing the independent components (ICs) of the simultaneous sEMG recordings (Muscle Nerve Suppl 9 (2000) 9) to detect the underlying functional muscle activations during swallowing using only surface EMG (sEMG) electrodes. METHODS: Seven normal subjects repeatedly swallowed liquids of varying consistency while sEMG was recorded from 15 electrodes from the face and throat. Active areas of EMG were excised from the recordings and the ICs of the sEMG were calculated. RESULTS: The ICs demonstrated less swallow-to-swallow variability than the raw sEMG. The ICs, each consisting of a unique temporal waveform and a spatial distribution, provided a means to segregate the complex sequence of muscle activation into rigorously defined separate functional units. The temporal profiles of the ICs and their spatial distribution were consistent with prior needle EMG studies of the cricopharyngeal, superior pharyngeal constrictor, submental and possibly arytenoid muscles. Other components appeared to correspond to EKG artifact contaminating the EMG recordings, laryngeal excursion, tongue movement and activation of the buccal and/or masseter musculature At least two of the components were affected by the consistency of the liquids swallowed. Re-testing one subject a week later demonstrated good intertrial reliability. CONCLUSIONS: We propose that the ICs of the sEMG provide a non-invasive means to assess the complex muscle sequence activation of deglutition.     

An implantable system for In Vivo chronic electromyographic study in the larynx

Introduction: Electromyography (EMG) plays an important role in exploring the mechanisms of selective reinnervation. An implantable system can help provide chronological information regarding reinnervation of laryngeal muscles. This study was designed to develop an implantable system for repeated recordings of spontaneous and evoked EMG from laryngeal muscles. Methods: This implantable system has 4 bipolar stimulus cuffs for bilateral recurrent laryngeal nerves (RLNs) and superior laryngeal nerves (SLNs), and 4 EMG recording electrodes for bilateral vocal fold adductors (thyroarytenoid–lateral cricoarytenoid, TA‐LCA) and abductor (posterior cricoarytenoid, PCA) muscles. The system was implanted in 8 canines for up to 41 weeks. Results: The system showed good compatibility. Consistent EMG signals were recorded from both PCA and TA‐LCA muscles. Conclusion: We developed a long‐term implantable EMG system that is simple and capable of obtaining stable EMG recordings from canine laryngeal muscles with minimal risk of device breakage, trauma, or infection. Muscle Nerve 55: 706–714, 2017