Behaviour of motor unit action potential rate, estimated from surface EMG, as a measure of muscle activation level

BACKGROUND: Surface electromyography (EMG) parameters such as root-mean-square value (RMS) are commonly used to assess the muscle activation level that is imposed by the central nervous system (CNS). However, RMS is influenced not only by motor control aspects, but also by peripheral properties of the muscle and recording setup. To assess motor control separately, the number of motor unit action potentials (MUAPs) per second, or MUAP Rate (MR) is a potentially useful measure. MR is the sum of the firing rates of the contributing MUs and as such reflects the two parameters that the CNS uses for motor control: number of MUs and firing rate. MR can be estimated from multi-channel surface EMG recordings. The objective of this study was to explore the behaviour of estimated MR (eMR) in relation to number of active MUs and firing rate. Furthermore, the influence of parameters related to peripheral muscle properties and recording setup (number of fibers per MU, fiber diameter, thickness of the subcutaneous layer, signal-to-noise-ratio) on eMR was compared with their influence on RMS. METHODS: Physiological parameters were varied in a simulation model that generated multi-channel EMG signals. The behaviour of eMR in simulated conditions was compared with its behaviour in experimental conditions. Experimental data was obtained from the upper trapezius muscle during a shoulder elevation task (20–100 N). RESULTS: The simulations showed strong, monotonously increasing relations between eMR and number of active MUs and firing rate (r(2 )> 0.95). Because of unrecognized superimpositions of MUAPs, eMR was substantially lower than the actual MUAP Rate (aMR). The percentage of detected MUAPs decreased with aMR, but the relation between eMR and aMR was rather stable in all simulated conditions. In contrast to RMS, eMR was not affected by number of fibers per MU, fiber diameter and thickness of the subcutaneous layer. Experimental data showed a strong relation between eMR and force (individual second order polynomial regression: 0.96 < r(2 )< 0.99). CONCLUSION: Although the actual number of MUAPs in the signal cannot be accurately extracted with the present method, the stability of the relation between eMR and aMR and its independence of muscle properties make eMR a suitable parameter to assess the input from the CNS to the muscle at low contraction levels non-invasively

Behavioral modeling of human choices reveals dissociable effects of physical effort and temporal delay on reward devaluation

There has been considerable interest from the fields of biology, economics, psychology, and ecology about how decision costs decrease the value of rewarding outcomes. For example, formal descriptions of how reward value changes with increasing temporal delays allow for quantifying individual decision preferences, as in animal species populating different habitats, or normal and clinical human populations. Strikingly, it remains largely unclear how humans evaluate rewards when these are tied to energetic costs, despite the surge of interest in the neural basis of effort-guided decision-making and the prevalence of disorders showing a diminished willingness to exert effort (e.g., depression). One common assumption is that effort discounts reward in a similar way to delay. Here we challenge this assumption by formally comparing competing hypotheses about effort and delay discounting. We used a design specifically optimized to compare discounting behavior for both effort and delay over a wide range of decision costs (Experiment 1). We then additionally characterized the profile of effort discounting free of model assumptions (Experiment 2). Contrary to previous reports, in both experiments effort costs devalued reward in a manner opposite to delay, with small devaluations for lower efforts, and progressively larger devaluations for higher effort-levels (concave shape). Bayesian model comparison confirmed that delay-choices were best predicted by a hyperbolic model, with the largest reward devaluations occurring at shorter delays. In contrast, an altogether different relationship was observed for effort-choices, which were best described by a model of inverse sigmoidal shape that is initially concave. Our results provide a novel characterization of human effort discounting behavior and its first dissociation from delay discounting. This enables accurate modelling of cost-benefit decisions, a prerequisite for the investigation of the neural underpinnings of effort-guided choice and for understanding the deficits in clinical disorders characterized by behavioral inactivity

Mechanomyographic amplitude and frequency responses during dynamic muscle actions: a comprehensive review

The purpose of this review is to examine the literature that has investigated mechanomyographic (MMG) amplitude and frequency responses during dynamic muscle actions. To date, the majority of MMG research has focused on isometric muscle actions. Recent studies, however, have examined the MMG time and/or frequency domain responses during various types of dynamic activities, including dynamic constant external resistance (DCER) and isokinetic muscle actions, as well as cycle ergometry. Despite the potential influences of factors such as changes in muscle length and the thickness of the tissue between the muscle and the MMG sensor, there is convincing evidence that during dynamic muscle actions, the MMG signal provides valid information regarding muscle function. This argument is supported by consistencies in the MMG literature, such as the close relationship between MMG amplitude and power output and a linear increase in MMG amplitude with concentric torque production. There are still many issues, however, that have yet to be resolved, and the literature base for MMG during both dynamic and isometric muscle actions is far from complete. Thus, it is important to investigate the unique applications of MMG amplitude and frequency responses with different experimental designs/methodologies to continually reassess the uses/limitations of MMG

Soleus stretch reflex in subjects with cerebrovascular accident

Aims: The conclusion that circumferential pressure decreases reflex activity in subjects with neuromuscular disorders including cerebrovascular accidents has recently come under scrutiny due to the methods used to analyze its effects. Thus far, studies used to determine circumferential pressure’s efficacy on motor neuron reflex excitability have mainly used the H-reflex technique on resting muscle. The purpose of this study was to investigate the effect that circumferential pressure has on the soleus stretch reflex (SSR) when superimposed onto a voluntary ramp movement in patients with cerebrovascular accident (CVA)

Influência da posição do braço na relação EMG-força em músculos do braço Influence of arm position on the EMG-force relationship in arm muscles

A relação entre a amplitude do sinal eletromiográfico e a força muscular (EMG-força) tem sido tomada como medida indireta da força muscular. Este estudo, em 18 voluntárias saudáveis e destras, visou avaliar a influência da posição do braço na relação EMG-força em músculos do braço em três tarefas - flexão do braço (FB), abdução do braço (AB) e neutra do braço (NB) - enquanto se tomavam ambas as medidas: uma célula de carga foi acoplada ao conversor do eletromiógrafo para registrar simultaneamente força e sinal eletromiográfico. Foram analisados os sinais dos músculos bíceps braquial, braquiorradial e tríceps braquial, e estimada a força de flexão e de extensão do braço nas diferentes tarefas. A relação entre esses conjuntos de valores foi analisada estatisticamente, verificando se havia correlação entre força e sinal eletromiográfico. Os resultados mostraram não haver tal correlação nas tarefas avaliadas. A posição do braço não influenciou a relação EMG-força dos músculos avaliados, com exceção do tríceps braquial, cuja atividade eletromiográfica foi maior durante a tarefa NB. Conclui-se que, em isometria, as tarefas podem ser empregadas para ativar o bíceps braquial e o braquiorradial; a tarefa NB é a mais indicada para ativar o tríceps braquial.<br>The relationship between myoelectric signal amplitude and muscle strength (EMG-force) has been used as an indirect measure of muscle strength. The aim of this study, in 18 healthy, female, right-handed volunteers, was to assess the influence of arm position on the relationship EMG-force of arm muscles in three different tasks: arm flexion, arm abduction, and neutral arm position. Both myoelectric signals and strength measures were acquired simultaneously, by coupling a load-cell to the electromyograph transducer. Signals from the biceps brachii, braquioradialis, and triceps brachii muscles were analysed, and arm extension and flexion force was estimated. Relationships between these values were statistically analysed, searching for a correlation between myoelectric signal amplitude and muscle strength. Results showed no such correlation could be found during any of the tasks. Arm position did not influence EMG-force of the assessed muscles, to the exception of the triceps brachii muscle, which showed greater activity in the neutral arm task as compared to the other tasks. In isometric contractions, the tasks may be used to activate biceps brachii and braquioradialis; neutral arm position is indicated to activate the triceps brachii muscle