Evidence for a fatigue-induced reflex inhibition of motoneuron firing rates

1987 ◽  
Vol 58 (1) ◽  
pp. 125-137 ◽  
Author(s):  
J. J. Woods ◽  
F. Furbush ◽  
B. Bigland-Ritchie
Keyword(s):  
Blood Supply ◽  
Muscle Activation ◽  
Biceps Brachii ◽  
Voluntary Contraction ◽  
Adductor Pollicis ◽  
Full Recovery ◽  
Firing Rates ◽  
Peripheral Muscle ◽  
Pressure Cuff ◽  
Before And After

1. In previous studies on the adductor pollicis and biceps brachii muscles we suggested that motoneuron firing rates are inhibited by a reflex from the muscle during fatigue, since: the firing rates decline during a sustained maximal voluntary contraction (MVC); recovery of MVC firing rates is prevented if the fatigued state of the muscle is preserved for 3 min by local occlusion of its blood supply; and full recovery occurs during this time once the blood supply to the peripheral muscle is restored. These findings were confirmed in the present study for quadriceps contractions. 2. These results do not necessarily imply an inhibitory reflex. The lower firing rates recorded from the muscle fibers during an MVC following 3 min of postfatigue ischemia may have been caused by either reduced subject effort (decreased muscle activation by the CNS) or impaired peripheral impulse transmission under these conditions. The present experiments, carried out on the quadriceps and adductor pollicis muscles, were designed to test this alternative explanation. 3. For both muscles, MVC contractions were sustained for 40 s with a blood pressure cuff inflated to 200 mmHg. This was followed by 3 min ischemic rest and a second 20-s MVC before cuff release. Three minutes after the blood supply to the muscle was restored a third 20-s MVC was made. Single shocks were delivered to the muscle throughout to record twitches from the relaxed muscle (Tr) before and after each MVC, and any twitches super-imposed on the voluntary contractions (Ts). The degree to which the muscle could be activated by voluntary effort was assessed from the ratio [1 - Ts/Tr]. For adductor pollicis, changes in the amplitude of the evoked M-waves were also measured. 4. Spike frequencies were only recorded during quadriceps experiments. These declined by 30% during the initial 40-s MVC. No recovery was seen in the second MVC following 3 min ischemic rest, but full recovery occurred within 3 min of cuff release. 5. Failure to retain full muscle activation was frequently seen in all three MVCs. However, for many well-motivated subjects twitch occlusion showed no reduction in the degree to which either the adductor pollicis or quadriceps muscles could be activated voluntarily during the MVC executed after 3 min of ischemic rest compared with that performed 3 min after the blood supply had been restored.(ABSTRACT TRUNCATED AT 400 WORDS)

Motor unit firing rates during isometric voluntary contractions performed at different muscle lengths

2004 ◽  
Vol 82 (8-9) ◽  
pp. 769-776 ◽  
Author(s):  
Alejandro Del Valle ◽  
Christine K Thomas
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Muscle Length ◽  
Surface Emg ◽  
Voluntary Contraction ◽  
Force Frequency ◽  
Triceps Brachii ◽  
Firing Rates ◽  
Motor Unit Firing ◽  
Voluntary Contractions

Firing rates of motor units and surface EMG were measured from the triceps brachii muscles of able-bodied subjects during brief submaximal and maximal isometric voluntary contractions made at 5 elbow joint angles that covered the entire physiological range of muscle lengths. Muscle activation at the longest, midlength, and shortest muscle lengths, measured by twitch occlusion, averaged 98%, 97%, and 93% respectively, with each subject able to achieve complete activation during some contractions. As expected, the strongest contractions were recorded at 90° of elbow flexion. Mean motor unit firing rates and surface EMG increased with contraction intensity at each muscle length. For any given absolute contraction intensity, motor unit firing rates varied when muscle length was changed. However, mean motor unit firing rates were independent of muscle length when contractions were compared with the intensity of the maximal voluntary contraction (MVC) achieved at each joint angle.Key words: muscle activation, length–tension relationships, force–frequency relationships.

scholarly journals Acute Effects Using Light-Emitting Diode Therapy (LEDT) for Muscle Function during Isometric Exercise in Asthma Patients: A Pilot Study

2019 ◽  
Vol 2019 ◽  
pp. 1-10
Author(s):  
Ivan Peres Costa ◽  
Fabiano Politti ◽  
Lawrence P. Cahalin ◽  
Etiene Farah Teixeira de Carvalho ◽  
Dirceu Costa ◽  
...  
Keyword(s):  
Light Emitting Diode ◽  
Isometric Exercise ◽  
Simultaneous Recording ◽  
Voluntary Contraction ◽  
Muscle Performance ◽  
Asthmatic Patients ◽  
Peripheral Muscle ◽  
Before And After ◽  
Asthma Patients ◽  
Long Time

The aim of this study was to evaluate the effectiveness of acute application of LEDT in improving peripheral muscle performance during isometric exercise in patients with asthma. Eleven patients, with a mean age 38 ± 10, underwent a single LEDT and sham application in the femoral quadriceps’ dominant member (cluster with 50 LED λ = 850 nm, 50 mW, 15 s; 37.5 J), 48 h apart in a randomized crossover design. Before and after LEDT and sham application, the patients were submitted an isometric endurance test (60% of the maximum isometric voluntary contraction), up to the limit of tolerance simultaneous recording of surface electromyography. There were no statistically significant differences between groups at the time of contraction (before 41±14 versus 44±16; after 46±12 versus 45±20 s) during the isometric contraction test and inflammatory markers before and after a single LEDT application. A single application of LEDT in the parameters and dose according to the equipment used in the study were not able to promote differences in the time of contraction and the fatigue response in asthmatic patients. However, the chronic effects of LEDT application for improving muscle performance in these patients are unknown and may present different responses during applications for a long time.

Older adults can maximally activate the biceps brachii muscle by voluntary command

1998 ◽  
Vol 84 (1) ◽  
pp. 284-291 ◽  
Author(s):  
Sophie J. De Serres ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Muscle Activation ◽  
Biceps Brachii ◽  
Maximum Voluntary Contraction ◽  
The Elderly ◽  
Voluntary Contraction ◽  
Elderly Subjects ◽  
Biceps Brachii Muscle ◽  
Activation Level ◽  
Voluntary Command

De Serres, Sophie J., and Roger M. Enoka. Older adults can maximally activate the biceps brachii muscle by voluntary command. J. Appl. Physiol. 84(1): 284–291, 1998.—Because some of the decline in strength with age may be explained by an impairment of muscle activation, the purpose of this study was to determine the activation level achieved in biceps brachii by older adults during a maximum voluntary contraction (MVC). This capability was assessed with two superimposition techniques: one calculated the activation level that was achieved during an MVC, and the other provided an estimate of the expected MVC force based on extrapolation with submaximal forces. The activation level in biceps brachii was incomplete (<100%) for the young ( n = 16) and elderly ( n = 16) subjects, with the elderly subjects exhibiting the greater deficit. In contrast, there was no difference between the measured and expected MVC forces for either group of subjects, whether the extrapolation involved a third-order polynomial or linearization of the data. Because of the lower signal-to-noise ratio associated with the measurement of activation level and the greater number of measurements that contributed to the estimate of the expected MVC force, we conclude that the older adults were able to achieve complete activation of the biceps brachii muscle during an MVC.

scholarly journals Effect of Acute Static Stretching on the Activation Patterns Using High-Density Surface Electromyography of the Gastrocnemius Muscle during Ramp-Up Task

Sensors ◽  
2021 ◽  
Vol 21 (14) ◽  
pp. 4841
Author(s):  
Noriaki Maeda ◽  
Makoto Komiya ◽  
Yuichi Nishikawa ◽  
Masanori Morikawa ◽  
Shogo Tsutsumi ◽  
...  
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Gastrocnemius Muscle ◽  
Peak Torque ◽  
Voluntary Contraction ◽  
Motor Unit Recruitment ◽  
Static Stretching ◽  
Activation Patterns ◽  
Muscle Activation Patterns ◽  
Before And After

This study aimed to evaluate motor unit recruitment during submaximal voluntary ramp contraction in the medial head of the gastrocnemius muscle (MG) by high-density spatial electromyography (SEMG) before and after static stretching (SS) in healthy young adults. SS for gastrocnemius was performed in 15 healthy participants for 2 min. Normalized peak torque by bodyweight of the plantar flexor, muscle activity at peak torque, and muscle activation patterns during ramp-up task were evaluated before and after SS. Motor unit recruitment during the submaximal voluntary contraction of the MG was measured using SEMG when performing submaximal ramp contractions during isometric ankle plantar flexion from 30 to 80% of the maximum voluntary contraction (MVC). To evaluate the changes in the potential distribution of SEMG, the root mean square (RMS), modified entropy, and coefficient of variation (CV) were calculated from the dense surface EMG data when 10% of the MVC force was applied. Muscle activation patterns during the 30 to 80% of MVC submaximal voluntary contraction tasks were significantly changed from 50 to 70% of MVC after SS when compared to before. The variations in motor unit recruitment after SS indicate diverse motor unit recruitments and inhomogeneous muscle activities, which may adversely affect the performance of sports activities.

scholarly journals Changes in muscle activation can prolong the endurance time of a submaximal isometric contraction in humans

2003 ◽  
Vol 94 (1) ◽  
pp. 108-118 ◽  
Author(s):  
Sandra K. Hunter ◽  
Roger M. Enoka
Keyword(s):  
Isometric Contraction ◽  
Muscle Activation ◽  
Voluntary Contraction ◽  
Endurance Time ◽  
Force Fluctuations ◽  
Rate Of Increase ◽  
Before And After ◽  
Young Subjects ◽  
Reduced Rate ◽  
Fatiguing Contractions

Fourteen young subjects (7 men and 7 women) performed a fatiguing isometric contraction with the elbow flexor muscles at 20% of maximal voluntary contraction (MVC) force on three occasions. Endurance time for session 3 [1,718 ± 1,189 (SD) s] was longer than for session 1 (1,225 ± 683 s) and session 2 (1,410 ± 977 s). Five men and four women increased endurance time between session 1 and 3 by 60 ± 28% (responders), whereas two men and three women did not (−3 ± 11%; nonresponders). The MVC force was similar for the responders and nonresponders, both before and after the fatiguing contraction. Fatiguing contractions were characterized by an increase in the electromyogram (EMG) amplitude and number of bursts during the fatiguing contractions. The responders achieved a similar level of EMG at exhaustion but a reduced rate of increase in the EMG across sessions. The rate of increase in EMG across sessions declined for the nonresponders, but it remained greater than that of the responders. The increase in burst rate during the contractions declined across sessions with a negative relation between burst rate and endurance time ( r = −0.42). Normalized force fluctuations increased during the fatiguing contractions, and there was a positive relation ( r = 0.60) between the force fluctuations and burst rate. Changes in mean arterial pressure and heart rate during the fatiguing contraction were similar for the responders and nonresponders across the three sessions. The results indicate that those subjects who increased the endurance time of a submaximal contraction across three sessions did so by altering the level and pattern of muscle activation.

Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions

2003 ◽  
Vol 95 (4) ◽  
pp. 1648-1655 ◽  
Author(s):  
M. A. E. Oskouei ◽  
B. C. F. van Mazijk ◽  
M. H. C. Schuiling ◽  
W Herzog
Keyword(s):  
Muscle Activation ◽  
Knee Extensor ◽  
Voluntary Contraction ◽  
Twitch Force ◽  
Actual Force ◽  
Before And After ◽  
Target Force ◽  
The Mean ◽  
Voluntary Contractions ◽  
Maximal Effort

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol ( P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.

scholarly journals Distinguishing intrinsic from extrinsic factors underlying firing rate saturation in human motor units

2015 ◽  
Vol 113 (5) ◽  
pp. 1310-1322 ◽  
Author(s):  
Andrew J. Fuglevand ◽  
Rosemary A. Lester ◽  
Richard K. Johns
Keyword(s):  
Firing Rate ◽  
Biceps Brachii ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Tendon Vibration ◽  
Cellular Mechanisms ◽  
Extrinsic Factors ◽  
Additional Increase ◽  
Firing Rates ◽  
Synaptic Excitation

During voluntary contraction, firing rates of individual motor units (MUs) increase modestly over a narrow force range beyond which little additional increase in firing rate is seen. Such saturation of MU discharge may be a consequence of extrinsic factors that limit net synaptic excitation acting on motor neurons (MNs) or may be due to intrinsic properties of the MNs. Two sets of experiments involving recording of human biceps brachii MUs were carried out to evaluate saturation. In the first set, the extent of saturation was quantified for 136 low-threshold MUs during isometric ramp contractions. Firing rate-force data were best fit by a saturating function for 90% of MUs recorded with a maximum rate of 14.8 ± 2.0 impulses/s. In the second set of experiments, to distinguish extrinsic from intrinsic factors underlying saturation, we artificially augmented descending excitatory drive to biceps MNs by activation of muscle spindle afferents through tendon vibration. We examined the change in firing rate caused by tendon vibration in 96 MUs that were voluntarily activated at rates below and at saturation. Vibration had little effect on the discharge of MUs that were firing at saturation frequencies but strongly increased firing rates of the same units when active at lower frequencies. These results indicate that saturation is likely caused by intrinsic mechanisms that prevent further increases in firing rate in the presence of increasing synaptic excitation. Possible intrinsic cellular mechanisms that limit firing rates of motor units during voluntary effort are discussed.

Observations on force enhancement in submaximal voluntary contractions of human adductor pollicis muscle

2005 ◽  
Vol 98 (6) ◽  
pp. 2087-2095 ◽  
Author(s):  
Ali E. Oskouei ◽  
Walter Herzog
Keyword(s):  
Steady State ◽  
Muscle Activation ◽  
Isometric Force ◽  
Fiber Type ◽  
Voluntary Contraction ◽  
Adductor Pollicis ◽  
Postactivation Potentiation ◽  
Force Enhancement ◽  
Adductor Pollicis Muscle ◽  
Voluntary Contractions

It has been observed consistently and is well accepted that the steady-state isometric force after active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so-called force enhancement has not been studied for submaximal voluntary efforts; therefore, it is not known whether this property affects everyday movements. The purpose of this study was to determine whether there was force enhancement during submaximal voluntary contractions. Human adductor pollicis muscles ( n = 17) were studied using a custom-built dynamometer, and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction. The steady-state isometric force and activation after active stretch were compared with the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in 8 of the 17 subjects, whereas there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater postactivation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during submaximal voluntary contractions in a subset of the populations and suggest that it may affect everyday voluntary movements in this subset. On the basis of follow-up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance and therefore possibly to the fiber-type distribution in the adductor pollicis muscle.

Motor-unit discharge rates in maximal voluntary contractions of three human muscles

1983 ◽  
Vol 50 (6) ◽  
pp. 1380-1392 ◽  
Author(s):  
F. Bellemare ◽  
J. J. Woods ◽  
R. Johansson ◽  
B. Bigland-Ritchie
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Motor Units ◽  
Adductor Pollicis ◽  
Maximum Force ◽  
Mean Values ◽  
Firing Rates ◽  
Discharge Rates ◽  
The Mean ◽  
Voluntary Contractions

Single motor<unit firing rates have been recorded during maximal voluntary contractions using tungsten microelectrodes. Over 300 units from four subjects were sampled from each of three muscles. These were the biceps brachii, adductor pollicis, and soleus, chosen because of known differences in their fiber<type composition and contractile properties. In all cases the contraction maximality was assured by delivering single supramaximal shocks during the voluntary contractions. All motor units were deemed to have already been fully activated if no additional force resulted. Thus for each muscle, the firing rates elicited by a maximal voluntary effort are sufficient to generate a fully fused tetanus in each motor unit. For the biceps brachii and adductor pollicis muscles, the mean firing rates (+/- SD) were 31.1 +/- 10.1 and 29.9 +/- 8.6 Hz, respectively, while for soleus they were only 10.7 +/- 2.9 HZ. For each muscle the firing rates distribution covered approximately a four-fold range about the mean value. The mean firing rates for each muscle varied roughly in proportion to their respective twitch contraction and half relaxation times. These contractile time measurements for both biceps brachii and adductor pollicis agreed well with the mean values reported for human fast-twitch motor units, while those for soleus fell in the range observed for human slow-twitch units. An argument is presented that suggests that, in response to voluntary effort, the range of discharge rates of each motor-unit pool is limited to those only just sufficient to produce maximum force in each motor unit. This suggestion is based on the relationship between the range of motor-unit firing frequencies observed during maximum voluntary contractions, their range of contraction times, and the stimulation frequencies required for maximum force generation. The implications of this hypothesis for motor control are discussed.

Voluntary muscle activation is impaired by core temperature rather than local muscle temperature

2006 ◽  
Vol 100 (4) ◽  
pp. 1361-1369 ◽  
Author(s):  
Melissa M. Thomas ◽  
Stephen S. Cheung ◽  
Geoff C. Elder ◽  
Gordon G. Sleivert
Keyword(s):  
Skin Temperature ◽  
Core Temperature ◽  
Muscle Activation ◽  
Afferent Input ◽  
Voluntary Contraction ◽  
Voluntary Activation ◽  
Muscle Temperature ◽  
Local Skin ◽  
Peripheral Muscle ◽  
The Right

Fatigue during hyperthermia may be due in part to a failure of the central nervous system to fully activate the working muscles. We investigated the effects of passive hyperthermia on maximal plantar flexor isometric torque (maximal isometric voluntary contraction) and voluntary activation to determine the roles of local skin temperature, core temperature, and peripheral muscle temperature in fatigue. Nine healthy subjects were passively heated from 37.2 to 39.5°C (core temperature) and then cooled back down to 37.9°C using a liquid-conditioning garment, with the right leg kept at a thermoneutral temperature throughout the protocol, whereas the left leg was allowed to heat and cool. Passive heating resulted in significant decreases in torque from [mean (SD)] 172 N·m (SD 39) to 160 N·m (SD 44) and in voluntary activation from 96% (SD 2) to 91% (SD 5) in the heated leg, and maximal isometric voluntary contraction decreased similarly from 178 N·m (SD 37) to 165 N·m (SD 38) and voluntary activation from 97% (SD 2) to 94% (SD 5) in the thermoneutral leg. The initiation of cooling, which produced a rapid decrease in skin temperature and cardiovascular strain [heart rate reserve decreased from 58% (SD 12) to 31% (SD 12)], did not immediately restore either torque or voluntary activation. However, when core temperature was lowered back to normal, torque and voluntary activation were restored to baseline values. It was concluded that an increase in core temperature is a factor responsible for reducing voluntary activation during brief voluntary isometric contractions and that temperature-induced changes in the contractile properties of muscle and local thermal afferent input from the skin do not contribute significantly to the decrement in torque.

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