Motor Unit Derecruitment Research Articles

Neuromotor control of spontaneous quiet breathing in awake rats evaluated by assessments of diaphragm EMG stationarity.

The neuromotor control of the diaphragm muscle (DIAm) is dynamic. The activity of the DIAm can be recorded via electromyography (EMG), which represents the temporal summation of motor unit action potentials. Our goal in the present study was to investigate DIAm neuromotor control during quiet spontaneous breathing (eupnea) in awake rats by evaluating DIAm EMG at specific temporal locations defined by motor unit recruitment and derecruitment. We evaluated the nonstationarity of DIAm EMG activity to identify DIAm motor unit recruitment and derecruitment durations. Combined with assessments of root mean square (RMS) and sum of squares (SS) EMG, the durations of these phases provide physiological information about the temporal aspects of motor control. During eupnea in awake rats (n = 10), the duration of motor unit recruitment comprised 61 ± 19 ms of the onset-to-peak duration (214 ± 62 ms) of the DIAm RMS EMG. The peak-to-offset duration of DIAm EMG activity was 453 ± 96 ms, with a terminating period of derecruitment of 161 ± 44 ms. The burst duration was 673 ± 128 ms. Both the RMS EMG amplitude and the SS EMG were higher at the completion of motor unit recruitment than at the start of motor unit derecruitment, suggesting that offset discharge rates were lower than onset discharge rates. Our analyses provide novel insights into the time domain aspects of DIAm neuromotor control and allow indirect estimates of the contribution of recruitment and frequency to RMS EMG amplitude during eupnea in awake rats.NEW & NOTEWORTHY We characterized three phases of neuromotor control-motor unit recruitment, sustained activity, and derecruitment-based on statistical assessments of stationarity of the diaphragm muscle (DIAm) EMG activity in awake rats. Our findings may allow indirect estimates of the contribution of motor unit recruitment and frequency coding toward generating force and provide novel insights about the temporal aspects of DIAm neuromotor control and descending respiratory drive in unanesthetized animals.

Muscle activation during leg-press exercise with or without eccentric overload.

This study sought to investigate the electromyographic activity of the vastus lateralis (VL) muscle during concentric-eccentric exercise using a new concept leg press machine enabling a preset overloading in the eccentric phase. Ten young males familiar with resistive exercise were recruited for this study. Tests were performed on a Leg-press Biostrength® (Technogym S.p.A., Italy). The load was set to 70% and 80% of one-repetition maximum (1-RM). The participants performed 2 sets of 6 repetitions at each relative load with (ECC +) and without (ISOW) an eccentric overload equivalent to 150% of the concentric load. A metronome was employed to maintain the selected cadence. Sets were separated by a 5-min rest. Surface electromyography (EMG) of VL was recorded and integrated (iEMG). Results showed a higher iEMG in ECC + with respect to ISOW at both intensities (+ 29% for 70% 1-RM, p < 0.01 and + 31% for 80% 1-RM, p < 0.001). No statistically significant differences were detected between concentric and eccentric phase in both ECC + conditions. Training with a 150% eccentric overload provides a ~ 30% greater motor unit recruitment of the VL muscle in leg press exercise. Moreover, the results show that the eccentric overloading provided by the Biostrength® machine enables training at the same level of neural activation of the concentric phase. Hence, the derecruitment of motor units, normally observed during the eccentric phase when using conventional training machines, was overcome using the Biostrength® machine; this observation seems particularly important for maximizing neuromuscular responses to strength training.

Changes in motor unit discharge patterns following strength training.

Changes in motor unit discharge patterns following strength training.

Reorganization of motor unit activity at different sites within the human masseter muscle during experimental masseter pain.

The aims were to test the hypotheses that experimental masseter muscle pain leads to recruitment and/or derecruitment of motor units at different sites within the masseter and that the patterns of change in motor unit activity differ between sites. Single motor unit (SMU) activity was recorded at two sites within the right masseter [superior/anterior, inferior/posterior (IP)] during isometric biting tasks (ramp, step level) on an intraoral force transducer in 17 participants during three experimental blocks comprising no infusion (baseline), 5% hypertonic saline infusion (pain), or isotonic saline infusion (control). A visual analog scale (VAS) was used to score pain intensity. The VAS scores were statistically significantly greater during infusion of hypertonic saline than during infusion of isotonic saline. No significant differences in force levels and rates of force change were found between experimental blocks. In comparison with isotonic saline infusion, SMUs could be recruited and derecruited at both sites during hypertonic saline infusion. The frequency of recruitment or derecruitment, in comparison with no change, was statistically significantly greater at the IP site than at the superior/anterior site. Experimental noxious masseter stimulation results in a reorganization of motor unit activity throughout the muscle, and the pattern of reorganization may be different in different regions of the muscle.

Comparison of Different Time-Frequency Analyses Techniques Based on sEMG-Signals in Table Tennis: A Case Study

Abstract The surface EMG signal in the action of dynamic contraction has more movement interference compared to sustained static contractions. In addition, the recruitment and de-recruitment of motor units causes a faster change in the surface EMG signal’s proprieties. Therefore, more complex techniques are required to extract information from the surface EMG signal. The standardized protocol for surface myoelectric signal measurement in table tennis was a case study in this research area. The Autoregressive method based on the Akaike Information Criterion, the Wavelet method based on intensity analysis, and the Hilbert-Huang transform method were used to estimate the muscle fatigue and non-fatigue condition. The result was that the Hilbert-Huang transform method was shown to be more reliable and accurate for studying the biceps brachii muscle in both conditions. However, the Wavelet method based on intensity analysis is more reliable and accurate for the pectoralis major muscle, deltoideus anterior muscle and deltoideus medialis muscle. The results suggest that different time-frequency analysis techniques influence different muscle analyses based on surface EMG signals in fatigue and non-fatigue conditions

Sex differences in variances of multi-channel surface electromyography distribution of the vastus lateralis muscle during isometric knee extension in young adults.

The aim of the present study was to compare spatial electromyographic potential distribution during force production between healthy young female and male using multi-channel surface electromyography (multi-SEMG). Thirty healthy subjects (15 females) performed sustained isometric knee extension at 10% maximal voluntary contraction (MVC) task for 120s. Multi-SEMG signals from the vastus lateralis muscle were detected and the modified entropy, coefficient of variation (CV), and correlation coefficient determined. The modified entropy and CV showed significant interaction and difference between females and males at all time points during the 10% MVC task. The correlation coefficient in females was significantly lower at 90 and 120s than that of males. The multi-SEMG potential distribution pattern in females showed more varied motor unit recruitment during sustained low-intensity isometric contraction than that of males. Variations in motor unit recruitment may result from recruitment and/or de-recruitment of motor units.

A self-organizing neural network for neuromuscular control

Adaptive technology holds great promise for sensorimotor rehabilitation in people suffering from spinal cord injury, neuromuscular disease and stroke. With a long-term goal of developing adaptive technology for diagnosis and rehabilitation of neuromuscular dysfunction, we begin the development of a self-organizing neural network (SNN) that compensates for reduced neural drive. We suggest that the self-organizing architecture that adds or deletes nodes online to generate suitable compensatory muscle excitation (Figure ​(Figure1A)1A) is an apt mechanism to emulate the motor pool behavior of recruitment and de-recruitment of motor units during muscle force generation. Using a virtual muscle [1] resembling the human biceps brachii, we demonstrate the augmentation of neural excitation by the SNN to compensate for abnormal muscle force due to change in the number of motor units. Figure 1 A. Schematic showing the virtual muscle-SNN system; Φ1, Φ2, .. Φn are radial basis functions and w1, w2, ..wn are weights for summation. B. Simulation of normal (Slow-Fast motor unit ratio - 2:4), abnormal (Slow-Fast motor unit ...

Electromyographic, mechanomyographic, and metabolic responses during cycle ergometry at a constant rating of perceived exertion.

Ten subjects performed four 8-min rides (65%-80% peak oxygen consumption) to determine the physical working capacity at the OMNI rating of perceived exertion (RPE) threshold (PWCOMNI). Polynomial regression analyses were used to examine the patterns of responses for surface electromyographic (EMG) amplitude (EMG AMP), EMG mean power frequency (EMG MPF), mechanomyographic (MMG) AMP, and MMG MPF of the vastus lateralis as well as oxygen consumption rate, respiratory exchange ratio (RER), and power output (PO) were examined during a 1-h ride on a cycle ergometer at a constant RPE that corresponded to the PWCOMNI. EMG AMP and MMG MPF tracked the decreases in oxygen consumption rate, RER, and PO, while EMG MPF and MMG AMP tracked RPE. The decreases in EMG AMP and MMG MPF were likely attributable to decreases in motor unit (MU) recruitment and firing rate, while the lack of change in MMG AMP may have resulted from a balance between MU de-recruitment as PO decreased, and an increase in the ability of activated fibers to oscillate. The current findings suggested that during submaximal cycle ergometry at a constant RPE, MU de-recruitment and mechanical changes within the muscle may influence the perception of effort via feedback from group III and IV afferents.

Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects.

Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (R2 > 0.69). Unit recruitment occurred over a wide force range (0.2–103% of 50 Hz force). Significant unit rate modulation occurred during spasms (frequency at 25% maximal force: 8.8 Hz, 3.3 SD; at maximal force: 16.1 Hz, 4.1 SD). Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD). Coactive unit pairs that fired for more than 4 s showed high (R2 > 0.7, n = 4) or low (R2:0.3–0.7, n = 12) rate-rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

Electromyogram features during linear torque decrement and their changes with fatigue.

Surface electromyogram (EMG) spike shape analysis (SSA) has recently been proposed as an adjunct tool to EMG time and frequency domain analysis to increase our knowledge of motor unit (MU) control strategies. The study was aimed to understand more in MU deactivation strategy during torque decrement, and its possible changes in fatigued muscle, using a combination of traditional time and frequency domain analysis and SSA techniques. EMG was detected from the biceps brachii of 11 untrained male subjects during static down-going ramp contractions (90-0% of the maximal voluntary contraction, MVC) under non-fatigued (DGR) and fatigued (FDGR) conditions. The root mean square (RMS) and mean frequency (MF), as well as SSA parameters, were calculated on 1-s EMG windows centred on each 10 % MVC step for both conditions. In both the DGR and FDGR EMG-RMS, mean spike amplitude and mean spike slope decreased by 50 % in the 90-60 % MVC. The mean spike frequency also decreased by 50 % in the 30-10 % MVC. Except the "mean number of spikes per second" all the other estimated EMG parameters were significantly different during FDGR compared to DGR. The dynamics of EMG parameters during torque decrement would support a MU deactivation strategy which relies more on MU de-recruitment in the high % MVC range and more on firing rate reduction in the low % MVC range. The adopted integrated approach to EMG signal processing could indicate that SSA is an important tool to disclose alterations in motor control due to fatigue.

Analysis of Muscle Fatigue Progression in Biceps Brachii Using Surface Electromyography Signals and Wavelet Packet Entropies

Event Abstract Back to Event Analysis of Muscle Fatigue Progression in Biceps Brachii Using Surface Electromyography Signals and Wavelet Packet Entropies Karthick P A1* and Ramakrishnan S1 1 Research Scholar, Biomedical Engineering Group, Department of Applied Mechanics, India Muscle fatigue is a neuromuscular condition where muscles fail to generate the expected or required force to perform the task continuously. It is associated with both central and peripheral nervous systems. This condition can also be caused due to neuromuscular disorders such as Parkinson’s disease, Carcinoma, Facioscapulohumeral dystrophy, Myotonic dystrophy and Hereditary motor and sensory neuropathy type I. Surface electromyography (sEMG) is a non-invasive technique, which records the electrical activity of neuromuscular system. It is a complex, nonstationary and multicomponent signal. Most of the real life activities such as cyclic exercise and walking are based on dynamic contraction of muscles. Degree of nonstationarity is increased in dynamic contractions due to recruitment and de-recruitment of motor units, movement of innervations zones with respect to electrode and changes in muscle fiber length, firing rate and conduction velocity. The sEMG signals are often analyzed in the time domain, frequency domain and time-frequency domain. Although several methods of signal analyses are reported in the literature, the analysis of fatigue progression still remains a challenging task. In this work, an attempt has been made to analyze progression of fatigue in biceps brachii muscle using surface EMG signals and wavelet packet entropies. The sEMG signals are recorded from biceps brachii of fifty healthy volunteers under well defined protocol and are preprocessed. The preprocessed signals are divided into six equal epochs. Further, these signals are subjected to wavelet packet transform. The entropies such as Shannon, threshold, norm, sure and log energy are extracted from the wavelet packet coefficients. The results show that Shannon, threshold and norm entropies are found to be distinct in all zones. Also the seperability of these entropies are appreciable between zone 1 and zone 6 which corresponds to non-fatigue and fatigue zone respectively. The t-test performed gives a p-value less than 0.0001 implying that the features are extremely significant. The demographics of subjects, representative sEMG signals and threshold entropy for all zones are shown below. Figure 1 Figure 2 Keywords: biceps brachii, muscle fatigue progression, surface EMG, Wavelet packet entropy, threshold entropy, shannan entropy, norm entropy Conference: Neuroinformatics 2014, Leiden, Netherlands, 25 Aug - 27 Aug, 2014. Presentation Type: Poster, not to be considered for oral presentation Topic: Electrophysiology Citation: P A K and S R (2014). Analysis of Muscle Fatigue Progression in Biceps Brachii Using Surface Electromyography Signals and Wavelet Packet Entropies. Front. Neuroinform. Conference Abstract: Neuroinformatics 2014. doi: 10.3389/conf.fninf.2014.18.00048 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Apr 2014; Published Online: 04 Jun 2014. * Correspondence: Mr. Karthick P A, Research Scholar, Biomedical Engineering Group, Department of Applied Mechanics, Chennai, Tamil nadu, India, Tamilnadu, 600 036, India, pakarthick1@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract Supplemental Data The Authors in Frontiers Karthick P A Ramakrishnan S Google Karthick P A Ramakrishnan S Google Scholar Karthick P A Ramakrishnan S PubMed Karthick P A Ramakrishnan S Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Mechanomyographic and Electromyographic Responses During Fatiguing Eccentric Muscle Actions of the Leg Extensors

The purpose of the current study was to examine the patterns of responses for torque, mechanomyographic (MMG) amplitude, MMG frequency, electromyographic (EMG) amplitude, and EMG frequency across 30 repeated maximal eccentric muscle actions of the leg extensors. Eleven moderately trained females performed an eccentric fatigue protocol at 30°/s with MMG and EMG signals recorded from the vastus lateralis. The results indicated there were significant (P<.05) decreases in MMG frequency (linear, r2=.395), EMG frequency (linear, r2=.177), and torque (linear, r2=.570; % decline=9.8±13.3%); increases in MMG amplitude (linear, r2=.783); and no change in EMG amplitude (r2=.003). These findings suggested that the neural strategies used to modulate torque during fatiguing eccentric muscle actions involved de-recruitment of motor units, reduced firing rates, and synchronization. In addition, the decreases in eccentric torque were more closely associated with changes in MMG frequency than EMG frequency. Thus, these findings indicated that MMG frequency, compared with EMG frequency, more accurately tracks fatigue during repeated maximal eccentric muscle actions.

Therapeutic Implications of a Radiographic ‘Drop Sign’ Following an Elbow Dislocation

p 5 0.03) and delay in grip initiation (F5 6.147, p5 0.018). Conclusion: As expected, the uninjured hands were stronger than the injured hands. The slope of the forcegenerationphasewas less steep for the injured hand, indicating a decrease in the rate of force development. This injury-induced neuromuscular maladaptation is likely due to a reduction in the number and/or size of motor units and in their capacity to fire synchronously and at their highest firing rate. An unexpected finding was the steeper slope of the forcedecay phase for the uninjured hands, which indicated that the uninjured hands fatigued faster than the injured hands. The most plausible explanation for this finding is that participants did not exert true maximal effort with their injured hand. Thus, fatigue (motor unit de-recruitment) occurred at a slower rate than in the uninjured hand which contracted maximally.We suggest that protective mechanisms prevent maximal exertion in the injured hand, even when a person intends to exert full effort. Future studies should investigate the nature of those protective mechanisms, whether they are local (neuromuscular) or psychological (due to fear of pain and/or fear of reinjury)and further explore gender differences.

Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans

The alpha-motoneurone afterhyperpolarization (AHP) duration correlates with a number of its muscle unit properties in animal preparations. In humans, the interval death rate (IDR) analysis has been used to estimate the time course of human motoneurone AHP based on the pattern of motor unit firing. The purpose of this experiment was first, to examine the relationship between estimated AHP time course and the minimal firing rate of the motor unit and second, to examine the relationship between the AHP and motor unit contractile properties in the tibialis anterior (TA) muscle. Motor unit data were obtained from the TA muscle during low force isometric contractions lasting 600 s. Muscle unit twitch characteristics were determined using spike-triggered averaging (STA) and the motoneurone AHP time course was estimated using the IDR analysis. Minimal discharge rate and derecruitment threshold torque were determined for 2 s preceding motor unit derecruitment. The AHP time constant and minimal discharge rate were negatively correlated, whereas the derecruitment threshold torque was not associated with the AHP time constant. The estimated AHP duration, however, is considerably shorter than the mean ISI of the minimal discharge rate suggesting that synaptic noise and AHP duration are important factors in dictating the minimal discharge rate in low force voluntary contractions in humans. The AHP time constant did not vary significantly with motor unit twitch amplitude; however, significant positive relationships were found between the AHP time constant and the temporal properties of the motor unit twitch. The calculated AHP time course using the IDR analysis, therefore, is a reasonable estimate and coupled with motor unit properties attained with STA, it provides a powerful method to describe low-threshold motor units.

Unifying hypothesis for the motoneuronal code in neurological disorders

The elucidation of the motoneuronal code in disorders characterized by prominent motor deficits, such as stroke, Parkinson disease (PD), chronic cerebellar degeneration (CCD) and amyotrophic lateral sclerosis (ALS), would have major impacts for early diagnosis, understanding of disease progression, monitoring the effects of conventional or emerging therapies, and rehabilitation. We propose that the fundamental rules of motoneuronal coding are violated in these neurological diseases. Unraveling of the motoneuronal signatures will lead to a new classification of movement disorders. We suggest that: (1) paresia due to a stroke is associated with a reduction in the mean motor unit firing rates (MUFR), abnormal commonality between pairs of motor units (MU), and an abnormal MU/force relationship; (2) in PD, MU do not discharge at the appropriate MUFR at given recruitment thresholds due to underactivity of the motor cortex. Treatment restores the linear relationship between recruitment threshold and MUFR; (3) in CCD, the hierarchical organization of MU is impaired, the commonality between pairs of MU is deregulated, the derecruitment of MU is abnormal, and EMG discharges show periods of electrical silences in MU discharges (ESIMUD); (4) in the early stages of ALS, a premature recruitment of high-threshold MU at low force levels occurs. The hierarchical organization is lost. Fluctuations in timing intervals between successive discharges of given MU are increased. Impaired MU contractile properties resulting from upper motor neuron disease are associated with decreased maximal M wave following lower motor neuron disease. The decline in mechanical force output following fatigue is associated with frequency shifts in the myoelectric signal and reduced muscle fiber conduction velocity. We suggest that patterns of MU discharges are distinct in acute diseases and in chronic neurological conditions, as a result of plasticity, compensatory patterns and reorganization of motor cortex output involving changes in functional connectivity in the central nervous system (CNS).

Persistent inward currents in motoneuron dendrites: Implications for motor output

The dendrites of motoneurons are not, as once thought, passive conduits for synaptic inputs. Instead they have voltage-dependent channels that provide the capacity to generate a very strong persistent inward current (PIC). The amplitude of the PIC is proportional to the level of neuromodulatory input from the brainstem, which is mediated primarily by the monoamines serotonin and norepinephrine. During normal motor behavior, monoaminergic drive is likely to be moderately strong and the dendritic PIC generates many of the characteristic features of motor unit firing patterns. Most of the PIC activates at or below recruitment threshold and thus motor unit firing patterns exhibit a linear increase just above recruitment. The dendritic PIC allows motor unit derecruitment to occur at a lower input level than recruitment, thus providing sustained tonic firing with little or no synaptic input, especially in low-threshold units. However the dendritic PIC can be readily deactivated by synaptic inhibition. The overall amplification due to the dendritic PIC and other effects of monoamines on motoneurons greatly increases the input-output gain of the motor pool. Thus the brainstem neuromodulatory input provides a mechanism by which the excitability of motoneurons can be varied for different motor behaviors. This control system is lost in spinal cord injury but PICs nonetheless recover near-normal amplitudes in the months following the initial injury. The relationship of these findings to the cause of the spasticity syndrome developing after spinal cord injury is discussed.

Sustaining a constant effort by the tongue and hand: effects of acute fatigue.

A constant-effort task was used previously as a potential assessment technique for fatigue. Participants sustained submaximal target effort levels with the tongue and hand against soft air-filled bulbs. For 80% of all trials, pressure decreased exponentially to a positive asymptote. In addition, pressure decreased faster when the muscles were fatigued than when they were rested. This study attempted to replicate the previous findings with new participants and to extend the findings to include surface electromyographic (EMG) data. Pressure and surface EMG signals were collected simultaneously while 10 neurologically normal young adults performed the constant-effort task at 50% of maximum pressure with the tongue and the hand. Eighty-one percent of the pressure data were modeled by a negative exponential equation with a nonzero asymptote. Seventy-three percent of the corresponding EMG data also fit this mathematical model. The pressure signals decayed more slowly than the corresponding EMG signals, particularly for the hand. After participants fatigued the tongue and hand with repeated brief maximal voluntary contractions, the time constants were reduced (rate of decay increased) for the tongue but not the hand. These results corroborate the previous finding that the time constant, determined from an exponential curve-fitting procedure, is a replicable measure. Furthermore, the reduction in the time constant after inducing acute fatigue in the tongue was replicated, although this same relationship was not replicated for the hand. The EMG data suggest that decreases in neuromuscular drive, including increased early adaptation, motor unit derecruitment, and motor unit desynchronization, contributed to the decrease in pressure during the constant-effort task, especially after acute fatigue was induced. These observations support the hypothesis that the task reflects, at least in part, central fatigue processes.

Role of the tendon in the dynamic performance of three different load-moving muscles.

The effect of the tendon's viscoelastic properties on the dynamic performance of three different load-moving muscles was determined. The frequency response models of the cat's medial gastrocnemius (MG), extensor digitorum longus (EDL), and tibialis anterior (TA) with and without their tendons were derived under sinusoidal shortening-lengthening, manipulated by orderly recruitment and derecruitment of motor units together with firing rate increase and decrease. The passive load sizes applied to the muscles were approximately 30%-40% of each muscle's maximal isometric force. It was shown that the tendon has a moderate effect on the dynamic response of muscles while moving loads of fixed mass. The MG and EDL without their tendons show a decrease in high frequency gain (2-5 dB) and increasing phase lag angles (7 degrees-9 degrees). In contrast, the TA without its tendon shows an increase in high frequency gain (2 dB) and decreasing phase lag angles (20 degrees) compared with the same muscle with the tendon. It was concluded that tendon's viscoelastic properties have a moderate effect during load-moving contractions, influencing the dynamic performance of different muscles in a different manner.

Upper airway pressure receptors alter expiratory muscle EMG and motor unit firing

To examine the effects of upper airway negative pressure (UAW NP) afferents on respiratory muscle activity during expiration (TE), diaphragm electromyograms (EMG) and triangularis sterni EMG and single motor unit activity were recorded from supine anesthetized tracheotomized cats while they breathed 100% O2. The period of TE during which the diaphragm was electrically active (TE-1) and the period of TE during which the diaphragm was quiescent (TE-2) were both increased with continuous UAW NP (P less than 0.001 and P less than 0.05, respectively), as was TE-1 as a percent of TE (P less than 0.001). Continuous UAW NP reduced peak triangularis sterni EMG (P less than 0.001) and delayed its expiratory onset (P less than 0.005) but did not alter its duration of firing. Changes in triangularis sterni EMG were due to a combination of complete cessation of motor unit activity (2 of 17 motor units), a reduction in mean motor unit firing frequency (P less than 0.02), and a delay in the expiratory onset of motor unit activity (P less than 0.001). Qualitatively similar results were obtained when UAW NP was applied during inspiration only. We conclude that 1) UAW NP has reciprocal stimulatory and inhibitory influences on diaphragm and triangularis sterni muscle electrical activity, respectively, during expiration, and 2) the reductions in triangularis sterni EMG are due to both motor unit derecruitment and a slowing of motor unit firing frequency.

Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recovered from eight pentobarbital anesthetized cats. Augemented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 ± 5 to 12 ± 2 arbitrary units ( P < 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths ( P < 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During testing breathing the motor units had a mean relative expiratory onset time of 46 ± 4% of expiration, and a mean firing frequency of 19 ± 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency ( P < 0.05) and number of impulses per breath ( P < 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.