The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Roger M. Enoka - One of the best experts on this subject based on the ideXlab platform.
-
reflex responsiveness of a human Hand Muscle when controlling isometric force and joint position
Clinical Neurophysiology, 2007Co-Authors: Katrina S. Maluf, Benjamin K Barry, Zachary A Riley, Roger M. EnokaAbstract:Objective: This study compared reflex responsiveness of the first dorsal interosseus Muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net Muscle torque. Methods: Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered. Results: The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load. Conclusions: Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects. Significance: Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus Muscle. � 2007 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
-
rate coding is compressed but variability is unaltered for motor units in a Hand Muscle of old adults
Journal of Neurophysiology, 2007Co-Authors: Benjamin K Barry, Michael A Pascoe, Mark Jesunathadas, Roger M. EnokaAbstract:The discharge of single motor units (n = 34) in the first dorsal interosseus Muscle and the fluctuations in force during steady contractions were measured across a range of index finger abduction f...
-
Muscle activation and time to task failure differ with load type and contraction intensity for a human Hand Muscle
Experimental Brain Research, 2005Co-Authors: Katrina S. Maluf, Minoru Shinohara, Jennifer L. Stephenson, Roger M. EnokaAbstract:Time to failure for sustained isometric contractions of the elbow flexors is briefer when maintaining a constant elbow angle while supporting an inertial load (position task) compared with exerting an equivalent torque against a rigid restraint (force task). Our primary purpose was to determine whether the effects of load type on time to task failure exist when motor unit recruitment cannot be enhanced during a sustained submaximal contraction of an intrinsic Hand Muscle. A second purpose was to determine whether a greater reserve remains in the Muscle after early failure of the position task. Two groups of 10 strength-matched men performed the force and position tasks at either 20% or 60% of maximal force (MVC) with the first dorsal interosseus, followed by a second force task at the same relative intensity. The rate of increase in surface EMG was greater ( P = 0.002) and time to failure was briefer ( P = 0.005) for the position task (593±212 s) compared with the force task (983±328 s) at 20% MVC, whereas there were no task differences in these variables at 60% MVC ( P ≥ 0.200). Time to failure for the second force tasks did not differ at either contraction intensity ( P ≥0.743). These results demonstrate that previously observed effects of load type generalize to a Hand Muscle, although only for low-intensity contractions. For the position task at low forces, Muscle activity increased more rapidly and no additional reserve remained in the Muscle at failure compared with the force task. We propose that the briefer time to failure for the position task during sustained, low-intensity contractions is due to earlier recruitment of the motor unit pool.
-
practice reduces motor unit discharge variability in a Hand Muscle and improves manual dexterity in old adults
Journal of Applied Physiology, 2005Co-Authors: Kurt W Kornatz, Evangelos A Christou, Roger M. EnokaAbstract:A steadiness-improving intervention was used to determine the contribution of variability in motor unit discharge rate to the fluctuations in index finger acceleration and manual dexterity in older...
-
discharge rate variability influences the variation in force fluctuations across the working range of a Hand Muscle
Journal of Neurophysiology, 2005Co-Authors: Chet T Moritz, Michael A Pascoe, Benjamin K Barry, Roger M. EnokaAbstract:The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus Muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units (P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC (P < 0.01) and remained constant at higher forces (1.2-1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability (P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a Muscle.
Michelle D Harran - One of the best experts on this subject based on the ideXlab platform.
-
differential poststroke motor recovery in an arm versus Hand Muscle in the absence of motor evoked potentials
Neurorehabilitation and Neural Repair, 2019Co-Authors: Heidi M Schambra, Meret Branscheidt, Martin A Lindquist, Jasim Uddin, Levke Steiner, Benjamin Hertler, Nathan Kim, Jessica Berard, Michelle D HarranAbstract:Background. After stroke, recovery of movement in proximal and distal upper extremity (UE) Muscles appears to follow different time courses, suggesting differences in their neural substrates. Objective. We sought to determine if presence or absence of motor evoked potentials (MEPs) differentially influences recovery of volitional contraction and strength in an arm Muscle versus an intrinsic Hand Muscle. We also related MEP status to recovery of proximal and distal interjoint coordination and movement fractionation, as measured by the Fugl-Meyer Assessment (FMA). Methods. In 45 subjects in the year following ischemic stroke, we tracked the relationship between corticospinal tract (CST) integrity and behavioral recovery in the biceps (BIC) and first dorsal interosseous (FDI) Muscle. We used transcranial magnetic stimulation to probe CST integrity, indicated by MEPs, in BIC and FDI. We used electromyography, dynamometry, and UE FMA subscores to assess Muscle-specific contraction, strength, and inter-joint coordination, respectively. Results. Presence of MEPs resulted in higher likelihood of Muscle contraction, greater strength, and higher FMA scores. Without MEPs, BICs could more often volitionally contract, were less weak, and had steeper strength recovery curves than FDIs; in contrast, FMA recovery curves plateaued below normal levels for both the arm and Hand. Conclusions. There are shared and separate substrates for paretic UE recovery. CST integrity is necessary for interjoint coordination in both segments and for overall recovery. In its absence, alternative pathways may assist recovery of volitional contraction and strength, particularly in BIC. These findings suggest that more targeted approaches might be needed to optimize UE recovery.
-
differential poststroke motor recovery in an arm versus Hand Muscle in the absence of motor evoked potentials
Neurorehabilitation and Neural Repair, 2019Co-Authors: Heidi M Schambra, Meret Branscheidt, Martin A Lindquist, Jasim Uddin, Levke Steiner, Benjamin Hertler, Nathan Kim, Jessica Berard, Michelle D HarranAbstract:Background. After stroke, recovery of movement in proximal and distal upper extremity (UE) Muscles appears to follow different time courses, suggesting differences in their neural substrates. Objec...
Katrina S. Maluf - One of the best experts on this subject based on the ideXlab platform.
-
reflex responsiveness of a human Hand Muscle when controlling isometric force and joint position
Clinical Neurophysiology, 2007Co-Authors: Katrina S. Maluf, Benjamin K Barry, Zachary A Riley, Roger M. EnokaAbstract:Objective: This study compared reflex responsiveness of the first dorsal interosseus Muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net Muscle torque. Methods: Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered. Results: The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load. Conclusions: Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects. Significance: Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus Muscle. � 2007 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
-
Muscle activation and time to task failure differ with load type and contraction intensity for a human Hand Muscle
Experimental Brain Research, 2005Co-Authors: Katrina S. Maluf, Minoru Shinohara, Jennifer L. Stephenson, Roger M. EnokaAbstract:Time to failure for sustained isometric contractions of the elbow flexors is briefer when maintaining a constant elbow angle while supporting an inertial load (position task) compared with exerting an equivalent torque against a rigid restraint (force task). Our primary purpose was to determine whether the effects of load type on time to task failure exist when motor unit recruitment cannot be enhanced during a sustained submaximal contraction of an intrinsic Hand Muscle. A second purpose was to determine whether a greater reserve remains in the Muscle after early failure of the position task. Two groups of 10 strength-matched men performed the force and position tasks at either 20% or 60% of maximal force (MVC) with the first dorsal interosseus, followed by a second force task at the same relative intensity. The rate of increase in surface EMG was greater ( P = 0.002) and time to failure was briefer ( P = 0.005) for the position task (593±212 s) compared with the force task (983±328 s) at 20% MVC, whereas there were no task differences in these variables at 60% MVC ( P ≥ 0.200). Time to failure for the second force tasks did not differ at either contraction intensity ( P ≥0.743). These results demonstrate that previously observed effects of load type generalize to a Hand Muscle, although only for low-intensity contractions. For the position task at low forces, Muscle activity increased more rapidly and no additional reserve remained in the Muscle at failure compared with the force task. We propose that the briefer time to failure for the position task during sustained, low-intensity contractions is due to earlier recruitment of the motor unit pool.
Janet L Taylor - One of the best experts on this subject based on the ideXlab platform.
-
long interval intracortical inhibition in a human Hand Muscle
Experimental Brain Research, 2011Co-Authors: Chris J Mcneil, Peter G Martin, Simon C Gandevia, Janet L TaylorAbstract:When two motor cortical stimuli are delivered with an interstimulus interval of 50–200 ms, the response (motor evoked potential; MEP) to the second stimulus is typically suppressed. This phenomenon is termed long-interval intracortical inhibition (LICI), although data from one subject suggest that facilitation is possible. Moreover, we recently showed that suppression can be mediated at a spinal level. We characterized LICI more fully by exploring a broad range of contraction strengths and test stimulus intensities. MEPs were evoked in first dorsal interosseous by transcranial magnetic stimulation over the motor cortex. Single test and paired (conditioning-test interval of 100 ms) stimuli at intensities of 100–160% resting motor threshold were delivered at rest or during brief contractions of 10, 25, or 100% maximal voluntary force. Inhibition or facilitation was quantified with the standard ratio in which conditioned MEPs were expressed as a percentage of unconditioned MEPs. Inhibition was greatest at weak–moderate contraction strengths and least at rest and during maximal efforts. Both at rest and during maximal efforts, MEPs evoked by strong stimuli were facilitated. In a subset of subjects, cervicomedullary stimulation was used to activate the corticospinal tract to identify possible spinal influences on changes to MEPs. Contraction strength and test stimulus intensity each had different effects on unconditioned and conditioned MEP size, and hence, LICI is highly dependent on both factors. Further, because motoneurons are facilitated during contraction but disfacilitated after a strong conditioning stimulus, the standard ratio of LICI is of questionable validity during voluntary contractions.
-
stimulus waveform influences the efficacy of repetitive transcranial magnetic stimulation
Journal of Affective Disorders, 2007Co-Authors: Janet L Taylor, Colleen LooAbstract:Abstract Background Optimising stimulus parameters is important in maximising the efficacy of repetitive transcranial magnetic stimulation (rTMS) in treatment applications. RTMS over motor cortex has been reported as more effective in producing corticospinal inhibition when a monophasic rather than a biphasic stimulus waveform is used. However, non-optimal coil orientation and high intensities of monophasic rTMS may have influenced previous results. Methods In eight healthy subjects, we measured motor evoked potentials (MEPs) in a Hand Muscle after monophasic and biphasic rTMS (1 Hz for 15 min) over the motor cortex with the coil always in the optimal orientation. MEPs were evoked by both monophasic and biphasic stimuli. Results MEPs were initially significantly reduced after monophasic but not biphasic rTMS. However, a late reduction was seen after biphasic rTMS. Limitations These motor cortical findings may not be directly applicable to prefrontal rTMS. Conclusions This study confirms that low frequency rTMS with monophasic pulses produces more corticospinal inhibition than with biphasic pulses, even when the direction of current and intensity are as well-matched as possible.
Mark Hallett - One of the best experts on this subject based on the ideXlab platform.
-
cortical silent period duration and its implications for surround inhibition of a Hand Muscle
European Journal of Neuroscience, 2012Co-Authors: Brach Poston, Sahana N Kukke, Rainer Paine, Sophia Francis, Mark HallettAbstract:Surround inhibition is a neural mechanism that assists in the focusing of excitatory drive to Muscles responsible for a given movement (agonist Muscles) by suppressing unwanted activity in Muscles not relevant to the movement (surround Muscles). The purpose of the study was to determine the contribution of γ-aminobutyric acidB receptor-mediated intracortical inhibition, as assessed by the cortical silent period (CSP), to the generation of surround inhibition in the motor system. Eight healthy adults (five women and three men, 29.8 ± 9 years) performed isometric contractions with the abductor digiti minimi (ADM) Muscle in separate conditions with and without an index finger flexion movement. The ADM motor evoked potential amplitude and CSP duration elicited by transcranial magnetic stimulation were compared between a control condition in which the ADM was activated independently and during conditions involving three phases (pre-motor, phasic, and tonic) of the index finger flexion movement. The motor evoked potential amplitude of the ADM was greater during the control condition compared with the phasic condition. Thus, the presence of surround inhibition was confirmed in the present study. Most critically, the CSP duration of the ADM decreased during the phasic stage of finger flexion compared with the control condition, which indicated a reduction of this type of intracortical inhibition during the phasic condition. These findings indicate that γ-aminobutyric acidB receptor-mediated intracortical inhibition, as measured by the duration of the CSP, does not contribute to the generation of surround inhibition in Hand Muscles.
-
changes in motor cortex excitability during ipsilateral Hand Muscle activation in humans
Clinical Neurophysiology, 2000Co-Authors: Wolf Muellbacher, Stefano Facchini, Babak Boroojerdi, Mark HallettAbstract:Abstract Objectives : To test whether unilateral Hand Muscle activation involves changes in ipsilateral primary motor cortex (M1) excitability. Methods : Single- and paired-pulse transcranial magnetic stimulation (TMS) of the right hemisphere was used to evoke motor evoked potentials (MEPs) from the resting left abductor pollicis brevis (APB) in 9 normal volunteers. We monitored changes in motor threshold (MT), MEP recruitment, intracortical inhibition (ICI) and intracortical facilitation (ICF) while the ipsilateral right APB was either at rest or voluntarily activated. Spinal motoneuron excitability was assessed using F-wave recording procedures. Results : Voluntary Muscle activation of the ipsilateral APB significantly facilitated the MEPs and F-waves recorded from the contralateral APB. Facilitation was observed with Muscle activation >50% of the maximum voluntary force and with stimulus intensities >20% above the individual resting motor threshold. Intracortical inhibition significantly decreased in the ipsilateral M1, while there was no significant change in intracortical facilitation during this maneuver. Conclusions : Unilateral Hand Muscle activation changes the excitability of homotopic Hand Muscle representations in both the ipsilateral M1 and the contralateral spinal cord. While the large proportion of MEP facilitation most likely occurred at a spinal level, involvement of the ipsilateral hemisphere may have contributed to the enlargement of magnetic responses.
-
plasticity of cortical Hand Muscle representation in patients with hemifacial spasm
Neuroscience Letters, 1999Co-Authors: Joachim Liepert, Leonardo G. Cohen, Mark Hallett, Celia Orejaguevara, Martin Tegenthoff, J P MalinAbstract:To investigate interactions between face and Hand representations in the human motor cortex, we studied patients with a hemifacial spasm before and after treatment with Botulinum toxin. Focal transcranial magnetic stimulation was used to assess the cortical motor output map of the abductor pollicis brevis Muscle (APB) on both sides. Prior to therapy the representation of the APB ipsilateral to the facial Muscle contractions (iAPB) was significantly smaller than on the contralateral side. Two weeks after successful therapy, the iAPB output area was significantly enlarged and expanded into the direction of the face representation. The results indicate activity dependent interactions between Hand and face representations in the adult human motor cortex.
