Recruitment Order Of Motor Units Research Articles

M-type potassium currents differentially affect activation of motoneuron subtypes and tune recruitment gain.

The size principle is a key mechanism governing the orderly recruitment of motor units and is believed to be dependent on passive properties of the constituent motoneurons. However, motoneurons are endowed with voltage-sensitive ion channels that create non-linearities in their input-output functions. Here we describe a role for the M-type potassium current, conducted by KCNQ channels, in the control of motoneuron recruitment in mice. Motoneurons were studied with whole-cell patch clamp electrophysiology in transverse spinal slices and identified based on delayed (fast) and immediate (slow) onsets of repetitive firing. M-currents were larger in delayed compared to immediate firing motoneurons, which was not reflected by variations in the presence of Kv7.2 or Kv7.3 subunits. Instead, a more depolarized spike threshold in delayed-firing motoneurons afforded a greater proportion of the total M-current to become activated within the subthreshold voltage range, which translated to a greater influence on their recruitment with little influence on their firing rates. Pharmacological activation of M-currents also influenced motoneuron recruitment at the population level, producing a rightward shift in the recruitment curve of monosynaptic reflexes within isolated mouse spinal cords. These results demonstrate a prominent role for M-type potassium currents in regulating the function of motor units, which occurs primarily through the differential control of motoneuron subtype recruitment. More generally, these findings highlight the importance of active properties mediated by voltage-sensitive ion channels in the differential control of motoneuron recruitment, which is a key mechanism for the gradation of muscle force. KEY POINTS: M-currents exert an inhibitory influence on spinal motor output. This inhibitory influence is exerted by controlling the recruitment, but not the firing rate, of high-threshold fast-like motoneurons, with limited influence on low-threshold slow-like motoneurons. Preferential control of fast motoneurons may be linked to a larger M-current that is activated within the subthreshold voltage range compared to slow motoneurons. Larger M-currents in fast compared to slow motoneurons are not accounted for by differences in Kv7.2 or Kv7.3 channel composition. The orderly recruitment of motoneuron subtypes is shaped by differences in the contribution of voltage-gated ion channels, including KCNQ channels. KCNQ channels may provide a target to dynamically modulate the recruitment gain across the motor pool and readily adjust movement vigour.

The orderly recruitment of motor units may be modified when a muscle is acting as an antagonist.

Despite the perceived importance of antagonist muscle activity, it is unknown if motor unit (MU) behavior at recruitment differs when a muscle acts as an antagonist versus agonist. Fourteen healthy participants performed ramped, isometric elbow flexor or extensor contractions to 50% or 100% maximal voluntary contraction (MVC) torque. Surface and fine-wire intramuscular electromyographic (EMG) recordings were sampled from biceps and triceps brachii. During agonist contractions, low-threshold MUs (recruited at <10% MVC torque) were sampled in all participants, with a total of 107 and 90 for biceps and triceps brachii, respectively. For ramped MVCs, antagonist surface EMG coactivation (% amplitude during agonist MVC) was 8.3 ± 6.6% for biceps and 15.2 ± 7.3% for triceps brachii. However, antagonist single MU activity was recorded from only four participants, with only one of these individuals having antagonist MUs recorded from both muscles. All antagonist MUs were successfully detected during agonist contractions, but many (∼40%) had a recruitment threshold >10% MVC torque. For MUs recorded during both agonist and antagonist contractions, discharge rate at recruitment was seemingly lower for antagonist than agonist contractions. Coexistence of typical levels of surface EMG-derived coactivation with scant antagonist MU recordings suggests that coactivation in these muscles is primarily the result of cross talk. Based on the limited antagonist MU data detected, MUs recruited early during an agonist contraction are not necessarily among those first recruited during an antagonist contraction. These findings highlight the possibility of a modification of orderly recruitment when a motoneuron pool is acting as an antagonist.NEW & NOTEWORTHY Modest levels of coactivation are widely considered essential for appropriate motor control; however, minimal attention has been given to recruitment patterns of motor units (MUs) from antagonist muscles. Despite the successful recording of many low-threshold MUs during agonist contractions, we recorded no antagonist MUs in most participants. Of the units recorded, only ∼60% matched those recruited at <10% of maximal torque when the muscle acted as an agonist, which suggests a modified recruitment order for antagonist MUs.

Maturation of persistent and hyperpolarization-activated inward currents shapes the differential activation of motoneuron subtypes during postnatal development.

The size principle underlies the orderly recruitment of motor units; however, motoneuron size is a poor predictor of recruitment amongst functionally defined motoneuron subtypes. Whilst intrinsic properties are key regulators of motoneuron recruitment, the underlying currents involved are not well defined. Whole-cell patch-clamp electrophysiology was deployed to study intrinsic properties, and the underlying currents, that contribute to the differential activation of delayed and immediate firing motoneuron subtypes. Motoneurons were studied during the first three postnatal weeks in mice to identify key properties that contribute to rheobase and may be important to establish orderly recruitment. We find that delayed and immediate firing motoneurons are functionally homogeneous during the first postnatal week and are activated based on size, irrespective of subtype. The rheobase of motoneuron subtypes becomes staggered during the second postnatal week, which coincides with the differential maturation of passive and active properties, particularly persistent inward currents. Rheobase of delayed firing motoneurons increases further in the third postnatal week due to the development of a prominent resting hyperpolarization-activated inward current. Our results suggest that motoneuron recruitment is multifactorial, with recruitment order established during postnatal development through the differential maturation of passive properties and sequential integration of persistent and hyperpolarization-activated inward currents.

Distribution of M-Wave and H-Reflex in Hand Muscles Evoked via Transcutaneous Nerve Stimulation: A Preliminary Report.

Neuromuscular electrical stimulation (NMES) targeting the muscle belly is commonly used to restore muscle strength in individuals with neurological disorders. However, early onset of muscle fatigue is a major limiting factor. Transcutaneous nerve stimulation (TNS) can delay muscle fatigue compared with traditional NMES techniques. However, the recruitment of Ia afferent fibers has not be specifically targeted to maximize muscle activation through the reflex pathway, which can lead to more orderly recruitment of motor units, further delaying fatigue. This preliminary study assessed the distribution of M-wave and H-reflex of intrinsic and extrinsic finger muscles. TNS was delivered using an electrode array placed along the medial side of the upper arm. Selective electrode pairs targeted the median and ulnar nerves innervating the finger flexors. High-density electromyography (HD EMG) was utilized to quantify the spatial distribution of the elicited activation of finger intrinsic and extrinsic muscles along the hand and forearm. The spatial patterns were characterized through isolation of the M-wave and H-reflex across various stimulation levels and EMG channels. Our preliminary results showed that, by altering the stimulation amplitude, distinct M-wave and H-reflex responses were evoked across EMG channels. In addition, distinct stimulation locations appeared to result in varied levels of reflex recruitment. Our findings indicate that it is possible to adjust stimulation parameters to maximize reflex activation, which can potentially facilitate physiological recruitment order of motoneurons.

Electrical muscle stimulation: Application and potential role in aging society

Neurodegenerative diseases and sarcopenia become more prevalent as individuals age and, therefore, represent a serious issue for the healthcare system. Several studies have reported the relationship between physical activity and reduced incidence of dementia or cognitive deterioration. Thus, exercise and strength training are most recommended treatments, but it is proving difficult to engage individuals to initiate exercise and strength training. Electrical muscle stimulation (EMS) may provide an alternative and more efficient solution. Although EMS has undergone a decline in use, mainly because of stimulation discomfort, new technologies allow painless application of strong contractions. Such activation can be applied in higher exercise dosages and more efficiently than people are likely to achieve with exercise. Unlike orderly recruitment of motor units (MUs) during low intensity voluntary exercise, EMS activates large fast-twitch MUs with glycolytic fibers preferentially and this could have benefit for prevention and treatment of diabetes and chronic diseases associated with muscle atrophy that ultimately lead to bed-ridden conditions. Recent evidence highlights the potential for EMS to make a major impact on these and other lifestyle related diseases and its role as a useful modality for orthopedic and cardiac rehabilitation. This paper will discuss the potential for EMS to break new ground in effective interventions in these frontiers of medical science.

Task-dependent recruitment across ankle extensor muscles and between mechanical demands is driven by the metabolic cost of muscle contraction.

The nervous system is faced with numerous strategies for recruiting a large number of motor units within and among muscle synergists to produce and control body movement. This is challenging, considering multiple combinations of motor unit recruitment may result in the same movement. Yet vertebrates are capable of performing a wide range of movement tasks with different mechanical demands. In this study, we used an experimental human cycling paradigm and musculoskeletal simulations to test the theory that a strategy of prioritizing the minimization of the metabolic cost of muscle contraction, which improves mechanical efficiency, governs the recruitment of motor units within a muscle and the coordination among synergist muscles within the limb. Our results support our hypothesis, for which measured muscle activity and model-predicted muscle forces in soleus-the slower but stronger ankle plantarflexor-is favoured over the weaker but faster medial gastrocnemius (MG) to produce plantarflexor force to meet increased load demands. However, for faster-contracting speeds induced by faster-pedalling cadence, the faster MG is favoured. Similar recruitment patterns were observed for the slow and fast fibres within each muscle. By contrast, a commonly used modelling strategy that minimizes muscle excitations failed to predict force sharing and known physiological recruitment strategies, such as orderly motor unit recruitment. Our findings illustrate that this common strategy for recruiting motor units within muscles and coordination between muscles can explain the control of the plantarflexor muscles across a range of mechanical demands.

Size-dependent dendritic maladaptations of hypoglossal motor neurons in SOD1G93A mice.

The total motor neuron (MN) somato-dendritic surface area is correlated with motor unit type. MNs with smaller surface areas innervate slow (S) and fast fatigue-resistant (FR) motor units, while MNs with larger surface areas innervate fast fatigue-intermediate (FInt) and fast fatigable (FF) motor units. Differences in MN surface area (equivalent to membrane capacitance) underpin the intrinsic excitability of MNs and are consistent with the orderly recruitment of motor units (S > FR > FInt > FF) via the Size Principle. In amyotrophic lateral sclerosis (ALS), large MNs controlling FInt and FF motor units exhibit earlier denervation and death, compared to smaller and more resilient MNs of type S and FR motor units that are spared until late in ALS. Abnormal dendritic morphologies in MNs precede neuronal death in human ALS and in rodent models. We employed Golgi-Cox methods to investigate somal size-dependent changes in the dendritic morphology of hypoglossal MNs in wildtype and SOD1G93A mice (a model of ALS), at postnatal (P) day ~30 (pre-symptomatic), ~P60 (onset), and ~P120 (mid-disease) stages. In wildtype hypoglossal MNs, increased MN somal size correlated with increased dendritic length and spines in a linear fashion. By contrast, in SOD1G93A mice, significant deviations from this linear correlation were restricted to the larger vulnerable MNs at pre-symptomatic (maladaptive) and mid-disease (degenerative) stages. These findings are consistent with excitability changes observed in ALS patients and in rodent models. Our results suggest that intrinsic or synaptic increases in MN excitability are likely to contribute to ALS pathogenesis, not compensate for it.

Isokinetic Plantar Flexion Endurance. Reliability and Validity of output/excitation Measurements

The reliability and validity of isokinetic measurement of plantar flexion endurance has been studied by a method previously described by us which utilizes simultaneous measurements of mechanical contractional work (CW) and integrated electromyogram (iEMG). The reliability was gauged by test/re-test with a two year's interval; while validity was assessed by myoelectric power spectrum analyses. The output/input balance (CW/iEMG) remained unchanged for the group as well as inter-individually. Changes in myoelectric power spectrum as function of number of contractions were clearly indicative of fatigue. Under the present conditions fatigue of fast twitch motor units may explain the rapid decreases in output and excitation followed by nearly steady state levels of all registered parameters. As is the case for non-fatigued isokinetic plantar flexions, the motor unit recruitment order appears quite stereotyped during plantar flexion fatigue. The findings of significantly lower mean power spectrum during the first part of each contraction than during the second part may support the size-principle described by Henneman.

EMG median frequency shifts without change in muscle oxygenation following novel locomotor training in individuals with incomplete spinal cord injury

Objectives To examine the effect of muscle fiber recruitment patterns on muscle oxygen utilization during treadmill walking in a group of individuals who have incomplete spinal cord injury. Methods 5 participants with motor incomplete spinal cord injury (Age; 42.2 ± 18.8 years, Male; n = 4) completed an over ground locomotor training program. Muscle utilization/oxygenation and activation of the medial gastrocnemius were measured by near infrared spectroscopy and surface electromyography pre- and post-over ground locomotor training during two separate treadmill walking bouts at self-selected speeds. Outcomes were changes in deoxygenation hemoglobin/myoglobin concentrations, and the change in median power of the power spectrum of the electromyography after training. Results A significant increase in median power of the power spectrum of the electromyography signal was observed during both bouts of treadmill walking, 6-minute walking bout and longer fatiguing bout (49% p = 0.047 and 48% p = 0.035, respectively) post-over ground locomotor training. There was no significant change in muscle utilization/oxygenation post-over ground locomotor training. There was no significant effect of median power of the power spectrum on deoxygenation hemoglobin/myoglobin during either of the walking bouts. Conclusions The main finding of the current study was that median power of the power spectrum significantly increased following 12 weeks of over ground locomotor training, with no significant change in deoxygenation hemoglobin/myoglobin. The recruitment of more and/or larger motor units was seen in conjunction with no changes in muscle oxygen utilization for the same walking task. Implications for Rehabilitation The reduction of skeletal muscle innervation in Spinal Cord Injury may adversely affect the orderly recruitment of motor units, which could in turn blunt the oxidative metabolic response during physical activity. Over-ground locomotor could be a useful tool in the rehabilitative process following an incomplete spinal cord injury.

Glutamatergic Neurotransmission at Rat Phrenic Motor Neurons

Glutamatergic (Glu) synaptic inputs at phrenic motor neurons (PhMNs) predominantly comprise descending axons from respiratory centers in the ventrolateral medulla. Descending synaptic inputs provide the drive for inspiratory as well as various other behaviors that result in higher force generation. We recently examined pre‐synaptic Glu terminals (VGLUT1 and VGLUT2‐positive) around the somal membrane of PhMNs grouped according to size using a robust confocal imaging‐based technique and semi‐automated image processing, and found an ~10% higher density of Glu terminals at PhMNs in the lower tertile of somal surface area. In parallel studies, we determined the density of AMPA and NMDA mRNA expression in PhMNs grouped according to size, and found that PhMNs in the lower tertile of somal surface area display ~25% higher density of AMPA (Gria2) and NMDA (Grin1) mRNA expression compared to PhMNs in the upper tertile of somal surface area. Assuming an orderly recruitment of motor units based on their intrinsic electrophysiological properties, PhMNs in the lower tertile of somal surface area are likely recruited first in order to accomplish lower force behaviors necessary for inspiration in normoxic as well as in hypoxic (10% O2) and hypercapnic (5% CO2) conditions. Thus, differences in glutamatergic neurotransmission across PhMNs of increasing size support the orderly recruitment of motor units, promoting recruitment of fatigue resistant units for inspiratory behaviors. In adult male rats, unilateral C2 hemisection (C2SH) abolishes inspiratory‐related diaphragm muscle (DIAm) activity ipsilateral to injury, but DIAm activity during higher force behaviors persists, reflecting the contribution of spared inputs, likely contralateral. Accordingly, we hypothesized that C2SH primarily disrupts Glu synaptic inputs to smaller PhMNs, whereas Glu synaptic inputs to larger PhMNs are preserved. By 7 days following unilateral C2SH, there was a ~45% reduction in ipsilateral Glu synaptic input to PhMNs compared to a ~20% reduction contralaterally. Ipsilateral to C2SH, there was an ~60% reduction in presynaptic Glu terminals at PhMNs in lower tertile of somal surface area compared to ~30% reduction at PhMNs in the upper tertile of size. Contralateral to C2SH, the reduction in presynaptic Glu terminals was similar across PhMNs grouped according to somal surface area. These results are consistent with a more pronounced effect of C2SH on inspiratory behaviors of the DIAm, and the preservation of higher force behaviors after C2SH. These results also indicate that Glu synaptic input to PhMNs varies depending on motor neuron size and likely reflects different motor control, including possibly distinct central pattern generator and premotor circuits, onto PhMNs recruited for motor behaviors with varying levels of force generation.Support or Funding InformationNIH Grants R01 HL96750 and HL146114

Heterogeneous glutamatergic receptor mRNA expression across phrenic motor neurons in rats.

The diaphragm muscle comprises various types of motor units that are recruited in an orderly fashion governed by the intrinsic electrophysiological properties (membrane capacitance as a function of somal surface area) of phrenic motor neurons (PhMNs). Glutamate is the main excitatory neurotransmitter at PhMNs and acts primarily via fast acting AMPA and N-methyl-D-aspartic acid (NMDA) receptors. Differences in receptor expression may also contribute to motor unit recruitment order. We used single cell, multiplex fluorescence in situ hybridization to determine glutamatergic receptor mRNA expression across PhMNs based on their somal surface area. In adult male and female rats (n=9) PhMNs were retrogradely labeled for analyses (n=453 neurons). Differences in the total number and density of mRNA transcripts were evident across PhMNs grouped into tertiles according to somal surface area. A~25% higher density of AMPA (Gria2) and NMDA (Grin1) mRNA expression was evident in PhMNs in the lower tertile compared to the upper tertile. These smaller PhMNs likely comprise type S motor units that are recruited first to accomplish lower force, ventilatory behaviors. In contrast, larger PhMNs with lower volume densities of AMPA and NMDA mRNA expression presumably comprise type FInt and FF motor units that are recruited during higher force, expulsive behaviors. Furthermore, there was a significantly higher cytosolic NMDA mRNA expression in small PhMNs suggesting a more important role for NMDA-mediated glutamatergic neurotransmission at smaller PhMNs. These results are consistent with the observed order of motor unit recruitment and suggest a role for glutamatergic receptors in support of this orderly recruitment. Cover Image for this issue: doi: 10.1111/jnc.14747.

Closed-loop functional optogenetic stimulation

Optogenetics has been used to orchestrate temporal- and tissue-specific control of neural tissues and offers a wealth of unique advantages for neuromuscular control. Here, we establish a closed-loop functional optogenetic stimulation (CL-FOS) system to control ankle joint position in murine models. Using the measurement of either joint angle or fascicle length as a feedback signal, we compare the controllability of CL-FOS to closed-loop functional electrical stimulation (CL-FES) and demonstrate significantly greater accuracy, lower rise times and lower overshoot percentages. We demonstrate orderly recruitment of motor units and reduced fatigue when performing cyclical movements with CL-FOS compared with CL-FES. We develop and investigate a 3-phase, photo-kinetic model to elucidate the underlying mechanisms for temporal variations in optogenetically activated neuromusculature during closed-loop control experiments. Methods and insights from this study lay the groundwork for the development of closed-loop optogenetic neuromuscular stimulation therapies and devices for peripheral limb control.

Optical cuff for optogenetic control of the peripheral nervous system

Objective. Nerves in the peripheral nervous system (PNS) contain axons with specific motor, somatosensory and autonomic functions. Optogenetics offers an efficient approach to selectively activate axons within the nerve. However, the heterogeneous nature of nerves and their tortuous route through the body create a challenging environment to reliably implant a light delivery interface. Approach. Here, we propose an optical peripheral nerve interface—an optocuff—, so that optogenetic modulation of peripheral nerves become possible in freely behaving mice. Main results. Using this optocuff, we demonstrate orderly recruitment of motor units with epineural optical stimulation of genetically targeted sciatic nerve axons, both in anaesthetized and in awake, freely behaving animals. Behavioural experiments and histology show the optocuff does not damage the nerve thus is suitable for long-term experiments. Significance. These results suggest that the soft optocuff might be a straightforward and efficient tool to support more extensive study of the PNS using optogenetics.

Effects of microgravity on characteristics of the accuracy control of movements

Effects of microgravity on characteristics of the accuracy control of movements

Organization of the motor-unit pool for different directions of isometric contraction of the first dorsal interosseous muscle.

Muscle force generation involves recruitment and firing rate modulation of motor units (MUs). The control of MUs in producing multidirectional forces remains unclear. We studied MU recruitment and firing properties, recorded from the first dorsal interosseous muscle, for 3 different directions of contraction: abduction; abduction/flexion combination; and flexion. MUs were recruited systematically at higher threshold force during flexion. Larger MUs were recruited and firing rates of MUs were lower during abduction. There was an orderly recruitment of MUs according to MU size regardless of contraction direction, obeying the "size principle." Firing rates of earlier-recruited MUs were consistently higher than later-recruited MUs, affirming the "onion-skin" property. Our findings suggest that the size principle and onion-skin organization together provide a general description of MU recruitment patterns and firing properties. The directional alternations of MU control properties likely reflect changes in neural drive to the muscle. Muscle Nerve 57: E85-E93, 2018.

Neural control of lengthening contractions.

A number of studies over the last few decades have established that the control strategy employed by the nervous system during lengthening (eccentric) differs from those used during shortening (concentric) and isometric contractions. The purpose of this review is to summarize current knowledge on the neural control of lengthening contractions. After a brief discussion of methodological issues that can confound the comparison between lengthening and shortening actions, the review provides evidence that untrained individuals are usually unable to fully activate their muscles during a maximal lengthening contraction and that motor unit activity during submaximal lengthening actions differs from that during shortening actions. Contrary to common knowledge, however, more recent studies have found that the recruitment order of motor units is similar during submaximal shortening and lengthening contractions, but that discharge rate is systematically lower during lengthening actions. Subsequently, the review examines the mechanisms responsible for the specific control of maximal and submaximal lengthening contractions as reported by recent studies on the modulation of cortical and spinal excitability. As similar modulation has been observed regardless of contraction intensity, it appears that spinal and corticospinal excitability are reduced during lengthening compared with shortening and isometric contractions. Nonetheless, the modulation observed during lengthening contractions is mainly attributable to inhibition at the spinal level.

Effect of support deprivation on the order of motor unit recruitment

Motor unit (MU) activity in the leg extensors has been tested by maintenance of a small isometric effort under the conditions of support deprivation enabled by dry immersion with simultaneous mechanic stimulation of the foot support zones. The analysis of MU interspike interval (ISI) histograms in the heads of two leg extensors (m. soleus and m. gastrocnemius lat.) revealed that the order of MU recruitment is extremely dependent on the support input activity. In immersion, the order of MU recruitment was rear-ranged during the isometric effort maintenance test, which revealed weaker involvement of small tonic MUs. Large MUs with longer ISIs (up to 260 ms) and high variability replaced small tonic MUs with relatively short ISIs (100 ms) and low variability. Daily support stimulation under the conditions of immersion was favorable for maintaining the normal pattern of MU recruitment.

A OVER DAMPED PERSON IDENTIFICATION SYSTEM USING EMG SIGNAL

EMG signal distinct from one person to another person. Body mass, muscle fiber pattern, muscle size, motion of subject, neuromuscular activity, neurotransmitter activity in different areas within the muscle, different density of bone, changes in blood flow in the muscle, fatigue, skin conductivity, motor unit firing pattern, motor unit paths, distribution of heat in the muscle, skinfat layer, motor unit recruitment order and characteristics of muscle, strength and force generated by the muscle changing from person to person. Change in measurement of EMG signal depends on many factors. Irrespective of which factor changes the EMG signal but physiologic dependent singularity is different and hence Electromyogram (EMG) can be used for person identification. Useful information present in EMG signal need to be extracted to discriminate different persons. In this work, EMG signal acquired from a single channel instrumentation system. Cepstrum technique used to extract features from EMG signal. The Vector Quantization (VQ) is used to build person models. 50 healthy persons EMG data is recorded in three sessions at different days. Person identification system developed using cepstrum and VQ provide good result in performance.

Recruitment of rat diaphragm motor units across motor behaviors with different levels of diaphragm activation

Phrenic motor neurons are recruited across a range of motor behaviors to generate varying levels of diaphragm muscle (DIAm) force. We hypothesized that DIAm motor units are recruited in a fixed order across a range of motor behaviors of varying force levels, consistent with the Henneman Size Principle. Single motor unit action potentials and compound DIAm EMG activities were recorded in anesthetized, neurally intact rats across different motor behaviors, i.e., eupnea, hypoxia-hypercapnia (10% O2 and 5% CO2), deep breaths, sustained airway occlusion, and sneezing. Central drive [estimated by root-mean-squared (RMS) EMG value 75 ms after the onset of EMG activity (RMS75)], recruitment delay, and onset discharge frequencies were similar during eupnea and hypoxia-hypercapnia. Compared with eupnea, central drive increased (∼25%) during deep breaths, and motor units were recruited ∼12 ms earlier (P < 0.01). During airway occlusion, central drive was ∼3 times greater, motor units were recruited ∼30 ms earlier (P < 0.01), and motor unit onset discharge frequencies were significantly higher (P < 0.01). Recruitment order of motor unit pairs observed during eupnea was maintained for 98%, 87%, and 84% of the same pairs recorded during hypoxia-hypercapnia, deep breaths, and airway occlusion, respectively. Reversals in motor unit recruitment order were observed primarily if motor unit pairs were recruited <20 ms apart. These results are consistent with DIAm motor unit recruitment order being determined primarily by the intrinsic size-dependent electrophysiological properties of phrenic motor neurons.

A two-step model to optimise transcutaneous electrical stimulation of the human upper arm

Purpose – A novel model of the upper arm under transcutaneous electrical stimulation with multi-pad electrodes is presented and experimentally validated. The model aims at simulating and analysing the effects of surface electrical stimulation on biceps brachii. The paper aims to discuss these issues. Design/methodology/approach – Both the passive properties of tissues surrounding nerve bundles and the active characteristics of the nervous system are included. The output of the proposed model is nerve recruitment and muscle contraction. Findings – Simulations and experimental tests on six healthy young adults have been conducted and results show that the proposed model gives information on electrically elicited muscle contraction in accordance with in-vivo tests and literature on motor unit recruitment order. Tests with different electrodes configurations show that the spatial distribution of active electrodes is a critical factor in electrically elicited muscle contractions, and that multi-pad electrodes can optimise the stimulation effectiveness and patient comfort with sequences of biphasic pulses of 350 μs at 30 pulses/s and threshold values of 2 mA. Originality/value – Results encourage the use of the proposed model of the upper arm as a valid and viable solution for predicting the behaviour of the neuromuscular system when surface electrical stimulation is applied, thus optimising the design of neuroprosthetics.