Spinal Mechanisms Contribute to Differences in the Time to Failure of Submaximal Fatiguing Contractions Performed With Different Loads

2008 ◽  
Vol 99 (3) ◽  
pp. 1096-1104 ◽  
Author(s):  
Malgorzata Klass ◽  
Morgan Lévénez ◽  
Roger M. Enoka ◽  
Jacques Duchateau
Keyword(s):  
Biceps Brachii ◽  
Motor Evoked Potential ◽  
H Reflex ◽  
Time To Failure ◽  
Elbow Flexors ◽  
Task Failure ◽  
Constant Torque ◽  
Rate Of Increase ◽  
Position Task ◽  
Fatiguing Contractions

This study compared the mechanisms that limit the time to failure of a sustained submaximal contraction at 20% of maximum when the elbow flexors either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). The surface electromyogram (EMG), the motor-evoked potential (MEP) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus were recorded in biceps brachii during the two tasks. Although the time to failure for the position task was only 44% of that for the force task, the rate of increase of the average EMG (aEMG; % initial MVC) and MEP area (% Mmax) did not differ significantly during the two tasks. At task failure, however, the increases in normalized aEMG and MEP area were significantly ( P < 0.05) greater for the force task (36.4 and 219.9%) than for the position task (22.4 and 141.7%). Furthermore, the superimposed mechanical twitch (% initial MVC), evoked by TMS during a brief MVC of the elbow flexors immediately after task failure, was increased similarly in both tasks. Although the normalized H-reflex area (% Mmax) decreased during the two fatiguing contractions, the reduction was more rapid and greater during the position task (59.8%) compared with the force task (34.7%). Taken together, the results suggest that spinal mechanisms were a major determinant of the briefer time to failure for the position task.

scholarly journals Electromyographic measures of muscle activation and changes in muscle architecture of human elbow flexors during fatiguing contractions

2008 ◽  
Vol 104 (6) ◽  
pp. 1720-1726 ◽  
Author(s):  
Thorsten Rudroff ◽  
Didier Staudenmann ◽  
Roger M. Enoka
Keyword(s):  
Muscle Activation ◽  
Biceps Brachii ◽  
Surface Emg ◽  
Muscle Architecture ◽  
Surface Measurement ◽  
Emg Activity ◽  
Time To Failure ◽  
Elbow Flexors ◽  
Emg Amplitude ◽  
Rate Of Increase

The study compared changes in intramuscular and surface recordings of EMG amplitude with ultrasound measures of muscle architecture of the elbow flexors during a submaximal isometric contraction. Ten subjects performed a fatiguing contraction to task failure at 20% of maximal voluntary contraction force. EMG activity was recorded in biceps brachii, brachialis, and brachioradialis muscles using intramuscular and surface electrodes. The rates of increase in the amplitude of the surface EMG for the long and short heads of biceps brachii and brachioradialis were greater than those for the intramuscular recordings measured at different depths. The amplitude of the intramuscular recordings from three muscles increased at a similar rate ( P = 0.13), as did the amplitude of the three surface recordings from two muscles ( P = 0.83). The increases in brachialis thickness (27.7 ± 5.7 to 30.9 ± 3.5 mm; P < 0.05) and pennation angle (10.9 ± 3.5 to 16.5 ± 4.8°; P = 0.003) were not associated with the increase in intramuscular EMG amplitude ( P > 0.58). The increase in brachioradialis thickness (22.8 ± 4.8 to 25.5 ± 3.4 mm; P = 0.0075) was associated with the increase in the amplitude for one of two intramuscular EMG signals ( P = 0.007, r = 0.79). The time to failure was more strongly associated with the rate of increase in the amplitude of the surface EMG than that for the intramuscular EMG, which suggests that the surface measurement provides a more appropriate measure of the change in muscle activation during a fatiguing contraction.

Task failure during fatiguing contractions performed by humans

2005 ◽  
Vol 99 (2) ◽  
pp. 389-396 ◽  
Author(s):  
Katrina S. Maluf ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Pressor Response ◽  
Emg Activity ◽  
Time To Failure ◽  
Postural Activity ◽  
Rate Of Increase ◽  
Position Task ◽  
Rate Coding ◽  
The Difference ◽  
Fatiguing Contractions

By comparing the physiological adjustments that occur when two similar fatiguing contractions are performed to failure, it is possible to identify mechanisms that limit the duration of the more difficult task. This approach has been used to study two fatiguing contractions, referred to as the force and position tasks, which differed in the type of feedback given to the subject and the amount of support provided by the surroundings. Even though the two tasks required a similar net muscle torque during submaximal isometric contractions, the duration that the position task could be sustained was consistently much briefer than that for the force task. The position task involved a greater rate of increase in EMG activity and more marked changes in motor unit recruitment and rate coding compared with the force task. These observations are consistent with the hypothesis that the motor unit pool was recruited more rapidly during the position task. The difference in motor unit behavior appeared to be caused by variation in synaptic input, likely involving heightened sensitivity of the stretch reflex during the position task. Upon repeat performances of the two fatiguing contractions, some subjects were able to increase the time to failure for the force task but not the position task. Furthermore, the time to failure for the position task could be influenced by the postural demands associated with maintaining the position of the limb, and the difference in the two durations was enhanced when the postural activity evoked a pressor response. These observations indicate that the difference in the duration of the two fatiguing contractions was attributable to differences in the control strategy used to sustain the tasks and the magnitude of the associated postural activity.

scholarly journals Load Type Influences Motor Unit Recruitment in Biceps Brachii During a Sustained Contraction

2009 ◽  
Vol 102 (3) ◽  
pp. 1725-1735 ◽  
Author(s):  
Stéphane Baudry ◽  
Thorsten Rudroff ◽  
Lauren A. Pierpoint ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Time To Failure ◽  
Single Motor Unit ◽  
Recruitment Threshold ◽  
Task Failure ◽  
Position Task

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task ( P = 0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% ( P = 0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (−16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

scholarly journals Cortical and Spinal Modulation of Antagonist Coactivation During a Submaximal Fatiguing Contraction in Humans

2008 ◽  
Vol 99 (2) ◽  
pp. 554-563 ◽  
Author(s):  
Morgan Lévénez ◽  
S. Jayne Garland ◽  
Malgorzata Klass ◽  
Jacques Duchateau
Keyword(s):  
Evoked Potentials ◽  
Biceps Brachii ◽  
Motor Evoked Potentials ◽  
H Reflex ◽  
Elbow Flexors ◽  
Triceps Brachii ◽  
Task Failure ◽  
Antagonist Muscles ◽  
Antagonist Coactivation ◽  
Stimulation Of

This study investigates the control mechanisms at the cortical and spinal levels of antagonist coactivation during a submaximal fatiguing contraction of the elbow flexors at 50% of maximal voluntary contraction (MVC). We recorded motor-evoked potentials in the biceps brachii and triceps brachii muscles in response to magnetic stimulation of the motor cortex (MEP) and corticospinal tract (cervicomedullary motor-evoked potentials—CMEPs), as well as the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus, before, during, and after the fatigue task. The results showed that although the coactivation ratio did not change at task failure, the MVC torque produced by the elbow flexors declined by 48% ( P < 0.01) with no change in MVC torque for the elbow extensors. While the MEP and CMEP areas (normalized to Mmax) of the biceps brachii increased (∼50%) over the first 40% of the time to task failure and then plateaued, both responses in the triceps brachii increased (∼150–180%) gradually throughout the fatigue task. In contrast to the monotonic increase in the MEP and CMEP of the antagonist muscles, the H-reflex of the triceps brachii exhibited a biphasic modulation, increasing during the first part of the contraction before declining subsequently to 65% of its initial value. Collectively, these results suggest that the level of coactivation during a fatiguing contraction is mediated by supraspinal rather than spinal mechanisms and involves differential control of agonist and antagonist muscles.

scholarly journals Task- and time-dependent modulation of Ia presynaptic inhibition during fatiguing contractions performed by humans

2011 ◽  
Vol 106 (1) ◽  
pp. 265-273 ◽  
Author(s):  
Stéphane Baudry ◽  
Adam H. Maerz ◽  
Jeffrey R. Gould ◽  
Roger M. Enoka
Keyword(s):  
Presynaptic Inhibition ◽  
Angular Position ◽  
H Reflex ◽  
Time Dependent ◽  
Initial Value ◽  
Ia Afferents ◽  
Position Task ◽  
Motor Neuron Pool ◽  
Extensor Carpi Radialis ◽  
Fatiguing Contractions

Presynaptic modulation of Ia afferents converging onto the motor neuron pool of the extensor carpi radialis (ECR) was compared during contractions (20% of maximal force) sustained to failure as subjects controlled either the angular position of the wrist while supporting an inertial load (position task) or exerted an equivalent force against a rigid restraint (force task). Test Hoffmann (H) reflexes were evoked in the ECR by stimulating the radial nerve above the elbow. Conditioned H reflexes were obtained by stimulating either the median nerve above the elbow or at the wrist (palmar branch) to assess presynaptic inhibition of homonymous (D1 inhibition) and heteronymous Ia afferents (heteronymous Ia facilitation), respectively. The position task was briefer than the force task ( P = 0.001), although the maximal voluntary force and electromyograph for ECR declined similarly at failure for both tasks. Changes in the amplitude of the conditioned H reflex were positively correlated between the two conditioning methods ( P = 0.02) and differed between the two tasks ( P < 0.05). The amplitude of the conditioned H reflex during the position task first increased (129 ± 20.5% of the initial value, P < 0.001) before returning to its initial value ( P = 0.22), whereas it increased progressively during the force task to reach 122 ± 17.4% of the initial value at failure ( P < 0.001). Moreover, changes in conditioned H reflexes were associated with the time to task failure and force fluctuations. The results suggest a task- and time-dependent modulation of presynaptic inhibition of Ia afferents during fatiguing contractions.

Accessory muscle activity contributes to the variation in time to task failure for different arm postures and loads

2007 ◽  
Vol 102 (3) ◽  
pp. 1000-1006 ◽  
Author(s):  
Thorsten Rudroff ◽  
Benjamin K. Barry ◽  
Amy L. Stone ◽  
Carolyn J. Barry ◽  
Roger M. Enoka
Keyword(s):  
Mean Arterial Pressure ◽  
Arterial Pressure ◽  
Muscle Activity ◽  
Maximal Voluntary Contraction ◽  
Force Transducer ◽  
Voluntary Contraction ◽  
Time To Failure ◽  
Elbow Flexors ◽  
Accessory Muscle ◽  
Position Task

Time to failure and electromyogram activity were measured during two types of sustained submaximal contractions with the elbow flexors that required each subject to exert the same net muscle torque with the forearm in two different postures. Twenty men performed the tasks, either by maintaining a constant force while pushing against a force transducer (force task), or by supporting an equivalent load while maintaining a constant elbow angle (position task). The time to failure for the position task with the elbow flexed at 1.57 rad and the forearm horizontal was less than that for the force task (5.2 ± 2.6 and 8.8 ± 3.6 min, P = 0.003), whereas it was similar when the forearm was vertical (7.9 ± 4.1 and 7.8 ± 4.5 min, P = 0.995). The activity of the rotator cuff muscles was greater during the position tasks (25.1 ± 10.1% maximal voluntary contraction) compared with the force tasks (15.2 ± 5.4% maximal voluntary contraction, P < 0.001) in both forearm postures. However, the rates of increase in electromyogram of the accessory muscles and mean arterial pressure were greater for the position task only when the forearm was horizontal ( P < 0.05), whereas it was similar for the elbow flexors. These findings indicate that forearm posture influences the difference in the time to failure for the two fatiguing contractions. When there was a difference between the two tasks, the task with the briefer time to failure involved greater rates of increase in accessory muscle activity and mean arterial pressure.

scholarly journals The effects of forearm position and contraction intensity on cortical and spinal excitability during a submaximal force steadiness task of the elbow flexors

2020 ◽  
Vol 123 (2) ◽  
pp. 522-528
Author(s):  
Alexandra F. Yacyshyn ◽  
Samantha Kuzyk ◽  
Jennifer M. Jakobi ◽  
Chris J. McNeil
Keyword(s):  
Evoked Potential ◽  
Biceps Brachii ◽  
Cortical Excitability ◽  
Motor Evoked Potential ◽  
Elbow Flexor ◽  
Elbow Flexors ◽  
Neural Excitability ◽  
Force Steadiness ◽  
Maximal Force ◽  
Spinal Excitability

Elbow flexor force steadiness is less with the forearm pronated (PRO) compared with neutral (NEU) or supinated (SUP) and may relate to neural excitability. Although not tested in a force steadiness paradigm, lower spinal and cortical excitability was observed separately for biceps brachii in PRO, possibly dependent on contractile status at the time of assessment. This study aimed to investigate position-dependent changes in force steadiness as well as spinal and cortical excitability at a variety of contraction intensities. Thirteen males (26 ± 7 yr; means ± SD) performed three blocks (PRO, NEU, and SUP) of 24 brief (~6 s) isometric elbow flexor contractions (5, 10, 25 or 50% of maximal force). During each contraction, transcranial magnetic stimulation or transmastoid stimulation was delivered to elicit a motor-evoked potential (MEP) or cervicomedullary motor-evoked potential (CMEP), respectively. Force steadiness was lower in PRO compared with NEU and SUP ( P ≤ 0.001), with no difference between NEU and SUP. Similarly, spinal excitability (CMEP/maximal M wave) was lower in PRO than NEU (25 and 50% maximal force; P ≤ 0.010) and SUP (all force levels; P ≤ 0.004), with no difference between NEU and SUP. Cortical excitability (MEP/CMEP) did not change with forearm position ( P = 0.055); however, a priori post hoc testing for position showed excitability was 39.8 ± 38.3% lower for PRO than NEU at 25% maximal force ( P = 0.006). The data suggest that contraction intensity influences the effect of forearm position on neural excitability and that reduced spinal and, to a lesser extent, cortical excitability could contribute to lower force steadiness in PRO compared with NEU and SUP. NEW & NOTEWORTHY To address conflicting reports about the effect of forearm position on spinal and cortical excitability of the elbow flexors, we examine the influence of contraction intensity. For the first time, excitability data are considered in a force steadiness context. Motoneuronal excitability is lowest in pronation and this disparity increases with contraction intensity. Cortical excitability exhibits a similar pattern from 5 to 25% of maximal force. Lower corticospinal excitability likely contributes to relatively poor force steadiness in pronation.

scholarly journals Muscle activity and time to task failure differ with load compliance and target force for elbow flexor muscles

2011 ◽  
Vol 110 (1) ◽  
pp. 125-136 ◽  
Author(s):  
Thorsten Rudroff ◽  
Jamie N. Justice ◽  
Matthew R. Holmes ◽  
Stephen D. Matthews ◽  
Roger M. Enoka
Keyword(s):  
Muscle Activity ◽  
Elbow Flexor ◽  
Rating Of Perceived Exertion ◽  
Isometric Contractions ◽  
Time To Failure ◽  
Task Failure ◽  
Position Task ◽  
Flexor Muscles ◽  
Elbow Flexor Muscles ◽  
Target Force

The primary purpose of this study was to determine the influence of load compliance on time to failure during sustained isometric contractions performed with the elbow flexor muscles at four submaximal target forces. Subjects pulled against a rigid restraint during the force task and maintained a constant elbow angle, while supporting an equivalent inertial load during the position task. Each task was sustained for as long as possible. Twenty-one healthy adults (23 ± 6 yr; 11 men) participated in the study. The maximal voluntary contraction (MVC) force was similar ( P = 0.95) before the subjects performed the force and position tasks at each of the four target forces: 20, 30, 45, and 60% of MVC force. The time to task failure was longer for the force tasks (576 ± 80 and 325 ± 70 s) than for the position tasks (299 ± 77 and 168 ± 35 s) at target forces of 20 and 30% ( P < 0.001), but was similar for the force tasks (178 ± 35 and 86 ± 14 s) and the position tasks (132 ± 29 and 87 ± 14 s) at target forces of 45 and 60% ( P > 0.19). The briefer times to failure for the position task at the lower forces were accompanied by greater rates of increase in elbow flexor muscle activity, mean arterial pressure, heart rate, and rating of perceived exertion. There was no difference in the estimates of external mechanical work at any target force. The dominant mechanisms limiting time to failure of sustained isometric contractions with the elbow flexor muscles appear to change at target forces between 30 and 45% MVC, with load compliance being a significant factor at lower forces only.

Carbohydrate Mouth Rinsing: Improved Neuromuscular Performance During Isokinetic Fatiguing Exercise

2017 ◽  
Vol 12 (8) ◽  
pp. 1031-1038 ◽  
Author(s):  
Ilenia Bazzucchi ◽  
Federica Patrizio ◽  
Francesco Felici ◽  
Andrea Nicolò ◽  
Massimo Sacchetti
Keyword(s):  
Biceps Brachii ◽  
Glucose Solution ◽  
Mouth Rinse ◽  
Elbow Flexors ◽  
Neuromuscular Performance ◽  
Fatiguing Exercise ◽  
Isokinetic Performance ◽  
Mouth Rinsing ◽  
Maltodextrin Solution ◽  
Fatiguing Contractions

Purpose:To determine whether repeated carbohydrate (CHO) mouth rinsing would improve neuromuscular performance during high-intensity fatiguing contractions.Methods:Eighteen young men (age 26.1 ± 5.0 y, BMI 22.9 ± 1.9) performed 3 maximal voluntary isometric contractions (MVICPRE). Immediately after, they completed 10-second mouth rinse with 6.4% maltodextrin solution (MAL), 7.1% glucose solution (GLU), water (W), artificially sweetened solution (PLA), or a control trial with no rinse (CON) in a crossover protocol. Subjects performed 5 sets of 30 isokinetic fatiguing contractions at 180°/s, and an MVICPOST with their elbow flexors was performed after each mouth rinse. Mechanical and electromyographic (EMG) signals were recorded from the biceps brachii and parameters of interest analyzed.Results:When rinsing the mouth with a solution containing CHO, independently of the sweetness, isokinetic performance was enhanced as shown by the greater total work achieved in comparison with CON. The decay of torque and mean fiber-conduction velocity (MFCV) recorded at the end of the fatiguing task was lower when rinsing the mouth with GLU than with CON. The torque recorded during the MVICPOST was greater with CHO with respect to CON, and this was associated to a lower decay of MFCV.Conclusions:CHO mouth rinse counteracts fatigue-induced decline in neuromuscular performance, supporting the notion that CHO rinse may activate positive afferent signals able to modify motor output. Repeated mouth rinsing with sweet and nonsweet CHO-containing solutions can improve neuromuscular performance during an isokinetic intermittent fatiguing task.

scholarly journals Age and load compliance alter time to task failure for a submaximal fatiguing contraction with the lower leg

2010 ◽  
Vol 108 (6) ◽  
pp. 1510-1519 ◽  
Author(s):  
Erin E. Griffith ◽  
Tejin Yoon ◽  
Sandra K. Hunter
Keyword(s):  
Young Adults ◽  
Force Control ◽  
Position Control ◽  
Lower Leg ◽  
Emg Activity ◽  
Time To Failure ◽  
Control Task ◽  
Task Failure ◽  
Old Adults ◽  
Rate Of Increase

The purpose of this study was to compare the time to failure and muscle activation of young and old adults for a sustained isometric submaximal contraction with the dorsiflexor muscles when the foot was restrained to a force transducer (force-control task) compared with supporting an equivalent inertial load unrestrained in the sagittal plane (position-control task). Seventeen young (23.6 ± 6.5 yr) and 12 old (70.0 ± 5.0 yr) adults performed the force-control and position-control tasks at 30% maximal voluntary contraction (MVC) until task failure on separate days. Despite the similar load torque for each task, time to failure was longer for the force-control than position-control task (10.4 ± 4.5 vs. 8.6 ± 3.4 min, P = 0.03) for the young and old adults. The old adults, however, had a longer time to task failure than the young adults for both tasks (11.4 ± 4.4 vs. 8.1 ± 2.1 min, P = 0.01), with no interaction of age and task ( P = 0.83). The rate of increase in agonist and antagonist root-mean-square EMG, agonist EMG bursting activity, mean arterial pressure, and heart rate during the fatiguing contraction was greater for the position-control than force-control task for the young and old adults. The old adults had a less rapid rate of increase in EMG activity, fluctuations in motor output, and cardiovascular measures than the young adults for both tasks. Development of fatigue can be manipulated in young and old adults by providing greater support to the foot and less ankle compliance during daily and ergonomic tasks that require prolonged activation of the lower leg. Minimizing load compliance to one degree of freedom during a position-control task maintained the greater fatigue resistance with age for an isometric contraction.

Sign in / Sign up

Export Citation Format

Share Document