scholarly journals Discharge properties of motor units during steady isometric contractions performed with the dorsiflexor muscles

2012 ◽  
Vol 112 (11) ◽  
pp. 1897-1905 ◽  
Author(s):  
Mark Jesunathadas ◽  
Malgorzata Klass ◽  
Jacques Duchateau ◽  
Roger M. Enoka
Keyword(s):  
Tibialis Anterior ◽  
Interspike Interval ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Isometric Contractions ◽  
Discharge Characteristics ◽  
Recruitment Threshold ◽  
Discharge Rates ◽  
Contraction Times ◽  
Experimental Values

The purpose of this study was to record the discharge characteristics of tibialis anterior motor units over a range of target forces and to import these data, along with previously reported observations, into a computational model to compare experimental and simulated measures of torque variability during isometric contractions with the dorsiflexor muscles. The discharge characteristics of 44 motor units were quantified during brief isometric contractions at torques that ranged from recruitment threshold to an average of 22 ± 14.4% maximal voluntary contraction (MVC) torque above recruitment threshold. The minimal [range: 5.8–19.8 pulses per second (pps)] and peak (range: 8.6–37.5 pps) discharge rates of motor units were positively related to the recruitment threshold torque ( R2 ≥ 0.266; P < 0.001). The coefficient of variation for interspike interval at recruitment was positively associated with recruitment threshold torque ( R2 = 0.443; P < 0.001) and either decreased exponentially or remained constant as target torque increased above recruitment threshold torque. The variability in the simulated torque did not differ from the experimental values once the recruitment range was set to ∼85% MVC torque, and the association between motor twitch contraction times and peak twitch torque was defined as a weak linear association ( R2 = 0.096; P < 0.001). These results indicate that the steadiness of isometric contractions performed with the dorsiflexor muscle depended more on the distributions of mechanical properties than discharge properties across the population of motor units in the tibialis anterior.

Pain-induced changes in motor unit discharge depend on recruitment threshold and contraction speed

2021 ◽  
Author(s):  
Eduardo Martinez-Valdes ◽  
Francesco Negro ◽  
Michail Arvanitidis ◽  
Dario Farina ◽  
Deborah Falla
Keyword(s):  
Discharge Rate ◽  
Tibialis Anterior Muscle ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Inhibitory Effect ◽  
Voluntary Contraction ◽  
Recruitment Threshold ◽  
Contraction Speed ◽  
Discharge Rates ◽  
Hypertonic Saline Injection

At high forces, the discharge rates of lower and higher threshold motor units (MU) are influenced in a different way by muscle pain. These differential effects may be particularly important for performing contractions at different speeds since the proportion of lower and higher threshold MUs recruited varies with contraction velocity. We investigated whether MU discharge and recruitment strategies are differentially affected by pain depending on their recruitment threshold (RT), across a range of contraction speeds. Participants performed ankle dorsiflexion sinusoidal-isometric contractions at two frequencies (0.25Hz and 1Hz) and two modulation amplitudes [5% and 10% of the maximum voluntary contraction (MVC)] with a mean target torque of 20%MVC. High-density surface electromyography recordings from the tibialis anterior muscle were decomposed and the same MUs were tracked across painful (hypertonic saline injection) and non-painful conditions. Torque variability, mean discharge rate (MDR), DR variability (DRvar), RT and the delay between the cumulative spike train and the resultant torque output (neuromechanical delay, NMD) were assessed. The average RT was greater at faster contraction velocities (p=0.01) but was not affected by pain. At the fastest contraction speed, torque variability and DRvar were reduced (p<0.05) and MDR was maintained. Conversely, MDR decreased and DRvar and NMD increased significantly during pain at slow contraction speeds (p<0.05). These results show that reductions in contraction amplitude and increased recruitment of higher threshold MUs at fast contraction speeds appears to compensate for the inhibitory effect of nociceptive inputs on lower threshold MUs, allowing the exertion of fast submaximal contractions during pain.

Mechanical Properties and Behaviour of Motor Units in the Tibialis Anterior During Voluntary Contractions

1997 ◽  
Vol 22 (6) ◽  
pp. 585-597 ◽  
Author(s):  
Michaël Van Cutsem ◽  
Patrick Feiereisen ◽  
Jacques Duchateau ◽  
Karl Hainaut
Keyword(s):  
Mechanical Properties ◽  
Tibialis Anterior ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Recruitment Threshold ◽  
Time To Peak ◽  
Spike Triggered Averaging ◽  
The Mean ◽  
Voluntary Contractions ◽  
Mean Time

The present work was carried out to analyse the properties and behaviour of Tibialis anterior motor units (MUs) during voluntary contractions in humans. A total of 528 single MU mechanical properties was recorded in 10 subjects by means of the spike-triggered averaging (STA) technique. MU recruitment thresholds and discharge frequencies were recorded during linearly increasing maximal voluntary contraction (MVC). The results indicate a mean (±SD) MU torque of 25.5 ± 21.5 mN•m. and a mean time-to-peak of 45.6 ± 13.6 ms. A comparison of the average MU twitch torque with that of the muscle allowed an estimate of about 300 MUs in the Tibialis anterior. A positive linear relationship was recorded between the MU twitch torque and the recruitment threshold. The mean minimal and maximal discharge frequencies of MUs were 8.4 ± 3.0 Hz and 33.2 ± 14.7 Hz, respectively. The results of the present work indicate that MU behaviour during voluntary contractions is different in the tibialis anterior and in the adductor pollicis. Key words: discharge frequency, recruitment threshold, motor unit count

Impulse propagation and muscle activation in long maximal voluntary contractions

1989 ◽  
Vol 67 (5) ◽  
pp. 1835-1842 ◽  
Author(s):  
C. K. Thomas ◽  
J. J. Woods ◽  
B. Bigland-Ritchie
Keyword(s):  
Tibialis Anterior ◽  
Muscle Activation ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Mass Action ◽  
M Wave ◽  
Force Reduction ◽  
Small Decline ◽  
Voluntary Contractions ◽  
Do So

With fatigue, force generation may be limited by several factors, including impaired impulse transmission and/or reduced motor drive. In 5-min isometric maximal voluntary contraction, no decline was seen in the peak amplitude of the tibialis anterior compound muscle mass action potential (M wave) either during or immediately after the voluntary effort, provided maximal nerve stimulation was retained. For first dorsal interosseous (FDI) muscle, M wave amplitudes declined by 19.4 +/- 1.6% during the first 2 min but did not change significantly thereafter, despite the continued force reduction (up to 94% in 5 min for both muscles). The duration of the FDI M waves increased (greater than 30%), suggesting that the small decline in amplitude was the result of increased dispersion between the responses of different motor units. Some subjects kept FDI maximally activated throughout, but when they used tibialis anterior, twitch occlusion and tetanic muscle stimulation showed that most subjects were usually only able to do so for the first 60 s and thereafter only during brief “extra efforts.” Thus force loss during isometric voluntary contractions sustained at the highest intensities results mainly from failure of processes within the muscle fibers.

Motor-Unit Synchronization Alters Spike-Triggered Average Force in Simulated Contractions

2002 ◽  
Vol 88 (1) ◽  
pp. 265-276 ◽  
Author(s):  
Anna M. Taylor ◽  
Julie W. Steege ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Contraction Time ◽  
Average Force ◽  
Twitch Force ◽  
Discharge Rates ◽  
Spike Triggered Averaging ◽  
Motor Unit Synchronization ◽  
Contraction Times ◽  
Twitch Characteristics

The purpose of the study was to quantify the effect of motor-unit synchronization on the spike-triggered average forces of a population of motor units. Muscle force was simulated by defining mechanical and activation characteristics of the motor units, specifying motor neuron discharge times, and imposing various levels of motor-unit synchronization. The model comprised 120 motor units. Simulations were performed for motor units 5–120 to compare the spike-triggered average responses in the presence and absence of motor-unit synchronization with the motor-unit twitch characteristics defined in the model. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; this kept the number of action potentials constant across the three levels of synchrony for each motor unit. Because there was some overlap of motor-unit twitches even at minimal discharge rates, the simulations indicated that spike-triggered averaging underestimates the twitch force of all motor units and the contraction time of motor units with contraction times longer than 49 ms. Although motor-unit synchronization increased the estimated twitch force and decreased the estimated contraction time of all motor units, spike-triggered average force changed systematically with the level of synchrony in motor units 59–120 (upper 90% of the range of twitch forces). However, the reduction in contraction time was similar for moderate and high synchrony. In conclusion, spike-triggered averaging appears to provide a biased estimate of the distribution of twitch properties for a population of motor units because twitch fusion causes an underestimation of twitch force for slow units and motor-unit synchronization causes an overestimation of force for fast motor units.

Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

2008 ◽  
Vol 33 (6) ◽  
pp. 1086-1095 ◽  
Author(s):  
Teatske M. Altenburg ◽  
Cornelis J. de Ruiter ◽  
Peter W.L. Verdijk ◽  
Willem van Mechelen ◽  
Arnold de Haan
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Discharge Rate ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Knee Extensors ◽  
Single Motor Unit ◽  
Discharge Rates ◽  
Active Motor

A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20° at 10°/s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%–47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 ± 0.19) and discharge rate (1.11 ± 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 ± 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 ± 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

scholarly journals Motor unit rate coding is severely impaired during forceful and fast muscular contractions in individuals post stroke

2013 ◽  
Vol 109 (12) ◽  
pp. 2947-2954 ◽  
Author(s):  
Li-Wei Chou ◽  
Jacqueline A. Palmer ◽  
Stuart Binder-Macleod ◽  
Christopher A. Knight
Keyword(s):  
Motor Unit ◽  
Force Production ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Paretic Limb ◽  
Discharge Rates ◽  
Rate Coding ◽  
Motor Unit Action ◽  
Unit Action ◽  
Rehabilitation Strategies

Information regarding how motor units are controlled to produce forces in individuals with stroke and the mechanisms behind muscle weakness and movement slowness can potentially inform rehabilitation strategies. The purpose of this study was to describe the rate coding mechanism in individuals poststroke during both constant ( n = 8) and rapid ( n = 4) force production tasks. Isometric ankle dorsiflexion force, motor unit action potentials, and surface electromyography were recorded from the paretic and nonparetic tibialis anterior. In the paretic limb, strength was 38% less and the rate of force development was 63% slower. Linear regression was used to describe and compare the relationships between motor unit and electromyogram (EMG) measures and force. During constant force contractions up to 80% maximal voluntary contraction (MVC), rate coding was compressed and discharge rates were lower in the paretic limb. During rapid muscle contractions up to 90% MVC, the first interspike interval was prolonged and the rate of EMG rise was less in the paretic limb. Future rehabilitation strategies for individuals with stroke could focus on regaining these specific aspects of motor unit rate coding and neuromuscular activation.

Human motor-unit recruitment during isometric contractions and repeated dynamic movements

1987 ◽  
Vol 57 (1) ◽  
pp. 311-324 ◽  
Author(s):  
C. K. Thomas ◽  
B. H. Ross ◽  
B. Calancie
Keyword(s):  
Motor Unit ◽  
Isometric Contraction ◽  
Rank Order ◽  
Human Subjects ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Isometric Contractions ◽  
Abductor Pollicis Brevis ◽  
Contraction Times ◽  
Abductor Pollicis Brevis Muscle

Spike-triggered averaging was used to determine the twitch tensions and contraction times of motor units in the abductor pollicis brevis muscle of two human subjects for two directions of isometric contraction: abduction and opposition of the thumb. During isometric contractions in each direction, the threshold force for motor-unit recruitment and the twitch amplitude were correlated linearly. These data suggested that an orderly pattern of recruitment, according to increasing twitch size, describes the function of the human abductor pollicis brevis muscle for each contraction direction. Rank order of motor-unit recruitment in each isometric contraction direction was correlated, but not identical. All units contributed tension in each direction of contraction, so no clear evidence was found for task-dependent motor units. In two subjects, motor-unit recruitment order during isometric contraction of the first dorsal interosseous and abductor pollicis brevis muscles was then compared with that of motor-unit pairs in both muscles during repetitive dynamic movements. Recruitment according to increasing twitch size was largely preserved during the repetitive opening and closing of scissors. The recruitment reversals that were observed were usually between pairs of units with similar thresholds.

scholarly journals Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

2011 ◽  
Vol 105 (2) ◽  
pp. 571-581 ◽  
Author(s):  
Michael A. Pascoe ◽  
Matthew R. Holmes ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Young Adults ◽  
Isometric Contraction ◽  
Biceps Brachii ◽  
Motor Units ◽  
Discharge Characteristics ◽  
Recruitment Threshold ◽  
Old Adults ◽  
Threshold Force ◽  
Target Force

The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference ( P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences ( P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.

Discharge properties of abductor hallucis before, during, and after an isometric fatigue task

2013 ◽  
Vol 110 (4) ◽  
pp. 891-898 ◽  
Author(s):  
Luke A. Kelly ◽  
Sebastien Racinais ◽  
Andrew G. Cresswell
Keyword(s):  
Discharge Rate ◽  
Weight Bearing ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Abductor Hallucis ◽  
Recruitment Threshold ◽  
Longitudinal Arch ◽  
Discharge Rates ◽  
Significant Difference ◽  
Ramp Contractions

Abductor hallucis is the largest muscle in the arch of the human foot and comprises few motor units relative to its physiological cross-sectional area. It has been described as a postural muscle, aiding in the stabilization of the longitudinal arch during stance and gait. The purpose of this study was to describe the discharge properties of abductor hallucis motor units during ramp and hold isometric contractions, as well as its discharge characteristics during fatigue. Intramuscular electromyographic recordings from abductor hallucis were made in 5 subjects; from those recordings, 42 single motor units were decomposed. Data were recorded during isometric ramp contractions at 60% maximum voluntary contraction (MVC), performed before and after a submaximal isometric contraction to failure (mean force 41.3 ± 15.3% MVC, mean duration 233 ± 116 s). Motor unit recruitment thresholds ranged from 10.3 to 54.2% MVC. No significant difference was observed between recruitment and derecruitment thresholds or their respective discharge rates for both the initial and postfatigue ramp contractions (all P > 0.25). Recruitment threshold was positively correlated with recruitment discharge rate ( r = 0.47, P < 0.03). All motor units attained similar peak discharge rates (14.0 ± 0.25 pulses/s) and were not correlated with recruitment threshold. Thirteen motor units could be followed during the isometric fatigue task, with a decline in discharge rate and increase in discharge rate variability occurring in the final 25% of the task (both P < 0.05). We have shown that abductor hallucis motor units discharge relatively slowly and are considerably resistant to fatigue. These characteristics may be effective for generating and sustaining the substantial level of force that is required to stabilize the longitudinal arch during weight bearing.

Positive Proprioceptive Feedback Elicited By Isometric Contractions of Ankle Flexors on Pretibial Motoneurons in Cats

2002 ◽  
Vol 88 (5) ◽  
pp. 2207-2214 ◽  
Author(s):  
L. Brizzi ◽  
L. H. Ting ◽  
D. Zytnicki
Keyword(s):  
Electrical Stimulation ◽  
Positive Feedback ◽  
Tibialis Anterior ◽  
Extensor Digitorum Longus ◽  
Motor Units ◽  
Extensor Digitorum ◽  
Isometric Contractions ◽  
Postsynaptic Potentials ◽  
Group I ◽  
Stimulation Of

Pretibial flexor motoneurons were recorded intracellularly in anesthetized cats during unfused isometric contractions of a subpopulation of motor units from either tibialis anterior (TA) or extensor digitorum longus (EDL) muscles. The contractions elicited excitatory postsynaptic potentials in 23 of 28 pretibial flexor motoneurons. No effect was observed in the remaining motoneurons. In control experiments, the effects of electrical stimulation of afferents within the TA nerve were investigated to help identify afferents responsible for the contraction-induced positive feedback. This feedback was ascribed to actions of Ia fibers because the pattern of the contraction-induced excitatory potentials was consistent with the known pattern of Ia discharge; in control experiments, electrical stimulation of group I fibers elicited only monosynaptic excitatory potentials; and the distribution of both the contraction-induced positive feedback among motor nuclei as well as the electrically evoked Ia excitatory monosynaptic potentials were restricted to homonymous and synergic motoneurons. Observation of the Ia contraction-induced positive feedback was facilitated by the absence of Ib autogenic inhibition. This contraction-induced Ia excitatory feedback in ankle flexors might either reinforce Ia-induced reflexes when these muscles are lengthened or help to lift the leg over an obstacle.

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