Discharge properties of motor units of the abductor hallucis muscle during cramp contractions

Current developments in surface electromyography

Background/aim

Surface electromyography (surface EMG) is a primary technique to detect the electrical activities of muscles through surface electrodes. In recent years, surface EMG applications have grown from conventional fields into new fields. However, there is a gap between the progress in the research of surface EMG and its clinical acceptance, characterized by the translational knowledge and skills in the widespread use of surface EMG among the clinician community. To reduce this gap, it is necessary to translate the updated surface EMG applications and technological advances into clinical research. Therefore, we aimed to present a perspective on recent developments in the application of surface EMG and signal processing methods.

Materials and methods

We conducted this scoping review following the Joanna Briggs Institute (JBI) method. We conducted a general search of PubMed and Web of Science to identify key search terms. Following the search, we uploaded selected articles into Rayyan and removed duplicates. After prescreening 133 titles and abstracts, we assessed 91 full texts according to the inclusion criteria.

Results

We concluded that surface EMG has made innovative technological progress and has research potential for routine clinical applications and a wide range of applications, such as neurophysiology, sports and art performances, biofeedback, physical therapy and rehabilitation, assessment of physical exercises, muscle strength, fatigue, posture and postural control, movement analysis, muscle coordination, motor synergies, modelling, and more. Novel methods have been applied for surface EMG signals in terms of time domain, frequency domain, time-frequency domain, statistical methods, and nonlinear methods.

Conclusion

Translating innovations in surface EMG and signal analysis methods into routine clinical applications can be a helpful tool with a growing and valuable role in muscle activation measurement in clinical practices. Thus, researchers must build many more interfaces that give opportunities for continuing education and research with more contemporary techniques and devices.

EMG-Phänomene peripherer motorisch axonaler Übererregbarkeit

Bei der Nadel-Elektromyographie (EMG) besitzen Phänomene der vermehrten Erregbarkeit von Muskelfasern und von motorischen Axonen Bedeutung für die Diagnostik neuromuskulärer Erkrankungen. Zur motorisch axonalen Übererregbarkeit gehören spontane Phänomene wie Faszikulationen, spontane kontinuierliche Einzelentladungen der motorischen Einheit (SKEME), Myokymien, neuromyotone Entladungsserien und Krampi. Ferner gehören dazu reizinduzierte Phänomene wie manche A-Wellen, reizinduzierte komplex repetitive Entladungen oder tetanischen Spasmen bei Elektrolytstörungen. In der vorliegenden Übersicht wird der Kenntnisstand zu den verschiedenen Phänomenen motorisch axonaler Übererregbarkeit referiert. Ein Schwerpunkt liegt dabei auf den SKEME als neuem Mitglied der Gruppe spontaner Potenziale aus dem motorischen Axon.

The Effects of Spinal Manipulation on Motor Unit Behavior

Over recent years, a growing body of research has highlighted the neural plastic effects of spinal manipulation on the central nervous system. Recently, it has been shown that spinal manipulation improved outcomes, such as maximum voluntary force and limb joint position sense, reflecting improved sensorimotor integration and processing. This study aimed to further evaluate how spinal manipulation can alter neuromuscular activity. High density electromyography (HD sEMG) signals from the tibialis anterior were recorded and decomposed in order to study motor unit changes in 14 subjects following spinal manipulation or a passive movement control session in a crossover study design. Participants were asked to produce ankle dorsiflexion at two force levels, 5% and 10% of maximum voluntary contraction (MVC), following two different patterns of force production (“ramp” and “ramp and maintain”). A significant decrease in the conduction velocity (p = 0.01) was observed during the “ramp and maintain” condition at 5% MVC after spinal manipulation. A decrease in conduction velocity suggests that spinal manipulation alters motor unit recruitment patterns with an increased recruitment of lower threshold, lower twitch torque motor units.

Case Studies in Management of Muscle Cramps

Muscle cramps, defined as a painful contraction of a muscle or muscle group, are a common symptom most people have experienced throughout their lifespan. In some cases cramps can be frequent, severe, and disabling, thus requiring medical assessment and intervention. Physiologic states such as pregnancy and exercise are associated with excessive muscle cramps, as are numerous medical and neurologic conditions, medications such as diuretics and statins, and peripheral nerve hyperexcitability syndromes. Treatment options for muscle cramps are limited, although recent studies have shown that mexiletine could be a safe and efficient alternative for patients with amyotrophic lateral sclerosis.

Muscular cramp: causes and management

Muscular cramp is a common symptom in healthy people, especially among the elderly and in young people after vigorous or peak exercise. It is prominent in a number of benign neurological syndromes. It is a particular feature of chronic neurogenic disorders, especially amyotrophic lateral sclerosis. A literature review was undertaken to understand the diverse clinical associations of cramp and its neurophysiological basis, taking into account recent developments in membrane physiology and modulation of motor neuronal excitability. Many aspects of cramping remain incompletely understood and require further study. Current treatment options are correspondingly limited.

Muscle cramps: A comparison of the two-leading hypothesis

Exercise-Associated Muscle Cramps (EAMC) are a common painful condition of muscle spasms. Despite scientists tried to understand the physiological mechanism that underlies these common phenomena, the etiology is still unclear. From 1900 to nowadays, the scientific world retracted several times the original hypothesis of heat cramps. However, recent literature seems to focus on two potential mechanisms: the dehydration or electrolyte depletion mechanism, and the neuromuscular mechanism. The aim of this review is to examine the recent literature, in terms of physiological mechanisms of EAMC. A comprehensive search was conducted on PubMed and Google Scholar. The following terminology was applied: muscle cramps, neuromuscular hypothesis (or thesis), dehydration hypothesis, Exercise-Associated muscle cramps, nocturnal cramps, muscle spasm, muscle fatigue. From the initial literature of 424 manuscripts, sixty-nine manuscripts were included, analyzed, compared and summarized. Literature analysis indicates that neuromuscular hypothesis may prevails over the initial hypothesis of the dehydration as the trigger event of muscle cramps. New evidence suggests that the action potentials during a muscle cramp are generated in the motoneuron soma, likely accompanied by an imbalance between the rising excitatory drive from the muscle spindles (Ia) and the decreasing inhibitory drive from the Golgi tendon organs. In conclusion, from the latest investigations there seem to be a spinal involvement rather than a peripheral excitation of the motoneurons.

Age-related changes in motor unit firing pattern of vastus lateralis muscle during low-moderate contraction

Age-related changes in motor unit activation properties remain unclear for locomotor muscles such as quadriceps muscles, although these muscles are preferentially atrophied with aging and play important roles in daily living movements. The present study investigated and compared detailed motor unit firing characteristics for the vastus lateralis muscle during isometric contraction at low to moderate force levels in the elderly and young. Fourteen healthy elderly men and 15 healthy young men performed isometric ramp-up contraction to 70 % of the maximal voluntary contractions (MVC) during knee extension. Multichannel surface electromyograms were recorded from the vastus lateralis muscle using a two-dimensional grid of 64 electrodes and decomposed with the convolution kernel compensation technique to extract individual motor units. Motor unit firing rates in the young were significantly higher (~+29.7 %) than in the elderly (p < 0.05). There were significant differences in firing rates among motor units with different recruitment thresholds at each force level in the young (p < 0.05) but not in the elderly (p > 0.05). Firing rates at 60 % of the MVC force level for the motor units recruited at <20 % of MVC were significantly correlated with MVC force in the elderly (r = 0.885, p < 0.0001) but not in the young (r = 0.127, p > 0.05). These results suggest that the motor unit firing rate in the vastus lateralis muscle is affected by aging and muscle strength in the elderly and/or age-related strength loss is related to motor unit firing/recruitment properties.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Preferred sensor sites for surface EMG signal decomposition

Technologies for decomposing the electromyographic (EMG) signal into its constituent motor unit action potential trains have become more practical by the advent of a non-invasive methodology using surface EMG (sEMG) sensors placed on the skin above the muscle of interest (De Luca et al 2006 J. Neurophysiol. 96 1646-57 and Nawab et al 2010 Clin. Neurophysiol. 121 1602-15). This advancement has widespread appeal among researchers and clinicians because of the ease of use, reduced risk of infection, and the greater number of motor unit action potential trains obtained compared to needle sensor techniques. In this study we investigated the influence of the sensor site on the number of identified motor unit action potential trains in six lower limb muscles and one upper limb muscle with the intent of locating preferred sensor sites that provided the greatest number of decomposed motor unit action potential trains, or motor unit yield. Sensor sites rendered varying motor unit yields throughout the surface of a muscle. The preferred sites were located between the center and the tendinous areas of the muscle. The motor unit yield was positively correlated with the signal-to-noise ratio of the detected sEMG. The signal-to-noise ratio was inversely related to the thickness of the tissue between the sensor and the muscle fibers. A signal-to-noise ratio of 3 was found to be the minimum required to obtain a reliable motor unit yield.

Mechanisms of cramp contractions: peripheral or central generation?

We analysed the cramp threshold (i.e. the minimum frequency of electrical stimulation capable of inducing a cramp) and the behaviour of individual motor units during cramps electrically elicited in the absence (intact condition) and presence (blocked condition) of a peripheral nerve block in eight healthy subjects. The cramp threshold was significantly greater in the blocked than in the intact condition (18 ± 3 Hz vs. 13 ± 3 Hz; P = 0.01). Cramp duration and peak EMG amplitude in the intact condition (55.6 ± 19.2 s and 47.5 ± 24.8 μV, respectively) were significantly greater compared to the blocked condition (2.6 ± 1.3 s and 13.9 ± 8.8 μV; P < 0.01). All motor units identified in the blocked condition (n = 38) had a shorter interval of activity and a greater discharge rate compared to the intact condition (n = 37) (respectively, 1.1 ± 1.0 s vs. 29.5 ± 21.8 s, P < 0.0001; 25.7 ± 11.6 pulses s(-1) vs. 20.0 ± 5.9 pulses s(-1); P < 0.05). The motor unit activity detected during the blocked condition corresponded to spontaneous discharges of the motor nerves, while in the intact condition the motor unit discharge patterns presented the typical characteristics of motor neuron discharges. These results indicate a spinal involvement at the origin of cramps and during their development.

Myofascial trigger points: spontaneous electrical activity and its consequences for pain induction and propagation

Active myofascial trigger points are one of the major peripheral pain generators for regional and generalized musculoskeletal pain conditions. Myofascial trigger points are also the targets for acupuncture and/or dry needling therapies. Recent evidence in the understanding of the pathophysiology of myofascial trigger points supports The Integrated Hypothesis for the trigger point formation; however unanswered questions remain. Current evidence shows that spontaneous electrical activity at myofascial trigger point originates from the extrafusal motor endplate. The spontaneous electrical activity represents focal muscle fiber contraction and/or muscle cramp potentials depending on trigger point sensitivity. Local pain and tenderness at myofascial trigger points are largely due to nociceptor sensitization with a lesser contribution from non-nociceptor sensitization. Nociceptor and non-nociceptor sensitization at myofascial trigger points may be part of the process of muscle ischemia associated with sustained focal muscle contraction and/or muscle cramps. Referred pain is dependent on the sensitivity of myofascial trigger points. Active myofascial trigger points may play an important role in the transition from localized pain to generalized pain conditions via the enhanced central sensitization, decreased descending inhibition and dysfunctional motor control strategy.

Spinal involvement and muscle cramps in electrically elicited muscle contractions

Electrical stimulation of innervated muscles has been investigated for many decades with alternations of high and low clinical interest in the fields of rehabilitation medicine and sports sciences. Early work demonstrated that afferent fibers have lower thresholds and are usually activated first (therefore eliciting an H-reflex). In the case of nerve trunk stimulation, the order of recruitment is mostly conditioned by the axonal dimension and excitability threshold. In the case of muscle motor point stimulation, the spatial distribution of nerve branches plays a predominant role. Sustained stimulation produces a progressive increase of force that is often maintained in subsequent voluntary activation by stroke patients. This observation suggested a facilitation mechanism at the spinal and/or supraspinal level. Such facilitation has been observed in healthy subjects as well, and may explain the generation of cramps elicited during stimulation and sustained for dozens of seconds after the stimulation has been interrupted. The most recent interpretations of facilitation resulting from peripheral stimulation focused on presynaptic (potentiation of neurotransmitter release from afferent fibers) or postsynaptic (generation of "persistent inward currents" in spinal motor neurons or interneurons) mechanisms. The renewed attention to these phenomena is once more increasing the interest toward electrical stimulation of the neuromuscular system. This is an opportunity for a structured investigation of the field aimed to resolving elements of confusion and controversy that still plague this area of electrophysiology.