Isometric Index Finger Abduction Research Articles

Sex differences in laterality of motor unit firing behavior of the first dorsal interosseous muscle in strength-matched healthy young males and females

PurposeThe purpose of this study was to compare laterality in motor unit firing behavior between females and males.MethodsTwenty-seven subjects (14 females) were recruited for this study. The participants performed ramp up and hold isometric index finger abduction at 10, 30, and 60% of their maximum voluntary contraction (MVC). High-density surface electromyography (HD-sEMG) signals were recorded in the first dorsal interosseous (FDI) muscle and decomposed into individual motor unit (MU) firing behavior using a convolution blind source separation method.ResultsIn total, 769 MUs were detected (females, n = 318 and males, n = 451). Females had a significantly higher discharge rate than males at each relative torque level (10%: male dominant hand, 13.4 ± 2.7 pps vs. female dominant hand, 16.3 ± 3.4 pps; 30%: male dominant hand, 16.1 ± 3.9 pps vs. female dominant hand, 20.0 ± 5.0 pps; and 60%: male dominant hand, 19.3 ± 3.8 vs. female dominant hand, 25.3 ± 4.8 pps; p < 0.0001). The recruitment threshold was also significantly higher in females than in males at 30 and 60% MVC. Furthermore, males exhibited asymmetrical discharge rates at 30 and 60% MVC and recruitment thresholds at 30 and 60% MVC, whereas no asymmetry was observed in females.ConclusionIn the FDI muscle, compared to males, females exhibited different neuromuscular strategies with higher discharge rates and recruitment thresholds and no asymmetrical MU firing behavior. Notably, the findings that sex differences in neuromuscular activity also occur in healthy individuals provide important information for understanding the pathogenesis of various diseases.

Measures of motor segmentation from rapid isometric force pulses are reliable and differentiate Parkinson's disease from age-related slowing.

Some people with Parkinson's disease (PD) have disruptions in motor output during rapid isometric muscle contractions. Measures of such disruptions (motor segmentation) may help clarify disease subtype, progression, or effects of therapeutic interventions. We investigated the potential utility of segmentation measures by testing two hypotheses that are fundamental to measurement and evaluation. First, measures of motor segmentation are reliable from day to day (intraclass correlation coefficient > 0.8). Second, that measures of motor segmentation have the sensitivity to differentiate between people with PD and older adults. 10 subjects with PD had a mean age of 70.1years, Hoehn-Yahr stage < 3, and median levodopa equivalent daily dose of 350mg. Older adult (mean age 81.9years) reference data are from a previously published study. Each subject provided approximately 87 rapid isometric index finger abduction force pulses up to 65% of their maximal isometric force for calculation of force pulse measures. Measures were computed for the excitation, transition, and relaxation phases of each force pulse. Measures of motor segmentation had high reliability and presented large (Cohen's D > 0.8) and significant (p < 0.05) group differences. In bivariate plots of selected measures, motor segmentation marked a departure of PD from age-related slowing. Across all subjects, greater segmentation was associated with greater impairments in rate control and a longer time to reach peak force (all Spearman's ρ > 0.8). These results support the potential utility of the motor segmentation measures by satisfying requirements for reliability and the sensitivity to indicate deviations from age-related slowing in motor output.

Hemisphere Asymmetry during Different Levels of Co-activation of Antagonist Muscles: a Transcranial Magnetic Stimulation Study

Contralateral and ipsilateral silent periods were recorded from first dorsal interosseous muscles of the dominant and the non-dominant hand in response to contralateral and ipsilateral transcranial magnetic stimulation (TMS) in twelve right-handed healthy volunteers during isometric index finger abduction and co-activation of antagonist muscles.&#x0D; In the dominant hand, contralateral silent period recorded during coactivation of antagonist muscles was significantly shorter than contralateral silent period recorded during isometric index finger abduction. In the nondominant hand, we did not find significant differences.&#x0D; In the dominant hand, ipsilateral silent period recorded during co-activation of antagonist muscles was significantly shorter than ipsilateral silent period recorded during isometric index finger abduction. In the non-dominant hand, we did not find significant differences.&#x0D; In conclusion, our main finding was the hemisphere asymmetry during co-activation of antagonist muscles. Both, contralateral and ipsilateral silent periods recorded from the dominant right hand were shorter in comparison to those recorded from the non-dominant hand. These findings illustrate the role of co-activation for the better and precise usage of the dominant hand.

Recruitment Curves during Different Types of Muscle Activity in Non-dominant Hand: A Transcranial Magnetic Stimulation Study

Motor evoked potentials (MEPs) were recorded from first dorsal interosseous muscle of non-dominant hand in response to contralateral transcranial magnetic stimulation (TMS) in seven right-handed healthy volunteers during relaxed muscles (without electromyorgaphic activity and zero force production), isometric index finger abduction (20% of individual measured maximum voluntary contraction in direction of abduction) and co-activation of antagonist muscles (simultaneously activated antagonist muscles, matching level equal to 20% of individual measured maximum voluntary contraction in direction of abduction by increasing the angle stiffness without producing of external force). The excitability of motor cortex was assessed by the amplitudes of MEPs recorded in response to increasing stimulation intensity: 100%, 110%, 120%, 130%, 140% of individually measured motor threshold at relax. The aim of the present study was using the method of transcranial magnetic stimulation to investigate the effect of different types of muscle activity in non-dominant hand. The secondary purpose was to compare new collected data with our previous data about dominant hand. At non-dominant hand we found significant changes between relax condition and each of the two active motor tasks almost at all five investigated TMS intensities. Also, we found that MEP amplitudes during abduction were significantly bigger than MEP amplitudes during co-activation of antagonist muscles, both in non-dominant hand and in dominant hand. We observed changes between MEP amplitudes of non-dominant and dominant hand during the performance of the same motor task.

Influence of preceding muscle activity on perceptually guided force production during superimposed ballistic contraction

Current study attempted to clarify whether preceding muscle activity influences the perceptually guided force production during superimposed ballistic contractions using isometric abduction of index finger. Subjects were verbally asked to produce ballistic force at requested percentages of their MVC with and without preceding muscle activity at 10% and 20%MVC. Our results showed that the ballistic force and EMG activity were increased significantly with preceding muscle activity at 20%MVC, but no change for 10%MVC. These results suggest that preceding muscle activity at sufficient intensity facilitates spinal motoneuron excitability, which could alter the relationship between the produced force and sense of effort.

Unravelling the Modulation of Intracortical Inhibition During Motor Imagery: An Adaptive Threshold-Hunting Study

Motor imagery (MI) is the mental simulation of an action without any apparent muscular contraction. By means of transcranial magnetic stimulation (TMS), few studies revealed a decrease of short-interval intracortical inhibition (SICI) within the primary motor cortex. However, this decrease is ambiguous, as one would expect greater inhibition during MI to prevent overt motor output. The current study investigated the extent of SICI modulation during MI through a methodological and a conceptual reconsideration of (i) the importance of parameters to assess SICI (Exp.1) and (ii) the inhibitory process within the primary motor cortex as an inherent feature of MI (Exp.2). Participants performed two tasks: (1) rest and (2) imagery of isometric abduction of the right index finger. Using TMS, motor evoked potentials were elicited in the right first dorsal interosseous (FDI) muscle. An adaptive threshold-hunting paradigm was used, where the stimulus intensity required to maintain a fixed motor evoked potential amplitude was quantified. To test SICI, we conditioned the test stimulus with a conditioning stimulus (CS) of different intensities. Results revealed an Intensity by Task interaction showing that SICI decreased during MI as compared to rest only for the higher CS intensity (Exp.1). At the lowest CS intensities, a Task main effect revealed that SICI increased during MI (Exp.2). SICI modulation during MI depends critically on the CS intensity. By optimising CS intensity, we have shown that SICI circuits may increase during MI, revealing a potential mechanism to prevent the production of a movement while the motor system is activated.

Potentiating paired corticospinal-motoneuronal plasticity after spinal cord injury

BackgroundPaired corticospinal-motoneuronal stimulation (PCMS) increases corticospinal transmission in humans with chronic incomplete spinal cord injury (SCI).Objective/Hypothesis: Here, we examine whether increases in the excitability of spinal motoneurons, by performing voluntary activity, could potentiate PCMS effects on corticospinal transmission. MethodsDuring PCMS, we used 100 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the hand representation of the primary motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the first dorsal interosseous (FDI) muscle ∼1–2 ms before antidromic potentials were elicited in motoneurons by electrical stimulation of the ulnar nerve. PCMS was applied at rest (PCMSrest) and during a small level of isometric index finger abduction (PCMSactive) on separate days. Motor evoked potentials (MEPs) elicited by TMS and electrical stimulation were measured in the FDI muscle before and after each protocol in humans with and without (controls) chronic cervical SCI. ResultsWe found in control participants that MEPs elicited by TMS and electrical stimulation increased to a similar extent after both PCMS protocols for ∼30 min. Whereas, in humans with SCI, MEPs elicited by TMS and electrical stimulation increased to a larger extent after PCMSactive compared with PCMSrest. Importantly, SCI participants who did not respond to PCMSrest responded after PCMSactive and those who responded to both protocols showed larger increments in corticospinal transmission after PCMSactive. ConclusionsOur findings suggest that muscle contraction during PCMS potentiates corticospinal transmission. PCMS applied during voluntary activity may represent a strategy to boost spinal plasticity after SCI.

Force estimation in fatigue condition using a muscle-twitch model during isometric finger contraction

We propose a force estimation method in fatigue condition using a muscle-twitch model and surface electromyography (sEMG). The twitch model, which is an estimate of force by a single spike, was obtained from sEMG features and measured forces. Nine healthy subjects performed isometric index finger abduction until exhaustion for a series of dynamic contractions (0–20% MVC) to characterize the twitch model and static contractions (50% MVC) to induce muscle fatigue. Muscle fatigue was identified based on the changes of twitch model; the twitch peak decreased and the contraction time increased as muscle fatigue developed. Force estimation performance in non-fatigue and fatigue conditions was evaluated and its results were compared with that of a conventional method using the mean absolute value (MAV). In non-fatigue conditions, the performance of the proposed method (0.90 ± 0.05) and the MAV method (0.88 ± 0.06) were comparable. In fatigue conditions, the performance was significantly improved for the proposed method (0.87 ± 0.05) compared with the MAV (0.78 ± 0.09).

Increased Force Variability Is Associated with Altered Modulation of the Motorneuron Pool Activity in Autism Spectrum Disorder (ASD).

Force control deficits have been repeatedly documented in autism spectrum disorder (ASD). They are associated with worse social and daily living skill impairments in patients suggesting that developing a more mechanistic understanding of the central and peripheral processes that cause them may help guide the development of treatments that improve multiple outcomes in ASD. The neuromuscular mechanisms underlying force control deficits are not yet understood. Seventeen individuals with ASD and 14 matched healthy controls completed an isometric index finger abduction test at 60% of their maximum voluntary contraction (MVC) during recording of the first dorsal interosseous (FDI) muscle to determine the neuromuscular processes associated with sustained force variability. Central modulation of the motorneuron pool activation of the FDI muscle was evaluated at delta (0–4 Hz), alpha (4–10 Hz), beta (10–35 Hz) and gamma (35–60 Hz) frequency bands. ASD patients showed greater force variability than controls when attempting to maintain a constant force. Relative to controls, patients also showed increased central modulation of the motorneuron pool at beta and gamma bands. For controls, reduced force variability was associated with reduced delta frequency modulation of the motorneuron pool activity of the FDI muscle and increased modulation at beta and gamma bands. In contrast, delta, beta, and gamma frequency oscillations were not associated with force variability in ASD. These findings suggest that alterations of central mechanisms that control motorneuron pool firing may underlie the common and often impairing symptoms of ASD.

Cycle time influences the development of muscle fatigue at low to moderate levels of intermittent muscle contraction

Localized muscle fatigue (LMF) during a repetitive task can be influenced by several aspects such as the level and duration of exertions. Among these aspects, though, the influence of cycle time remains unclear. Here, the effect of cycle time on LMF and performance was examined for a simple biomechanical system during repetitive static efforts. Participants performed 1-h trials of intermittent isometric index finger abduction with a duty cycle of 50% in all combinations of two cycle times (30 and 60s) and two exertion levels (15% and 25% of maximum voluntary capacity). Measures of discomfort, performance (force fluctuations), and muscle capacity (voluntary strength and low-frequency twitch responses) were obtained, all of which demonstrated a beneficial effect of the 30s cycle time. Specifically, the shorter cycle time led to lower rates of increase in perceived discomfort, lower rates of increase in force fluctuations, lower rates of decrease in voluntary capacity, and smaller changes in twitch responses. These benefits, reflecting less LMF development in the shorter cycle time, were quite consistent between genders and the two levels of effort. Results of this study can be used to modify current models predicting work–rest allowance and/or LMF, helping to enhance performance and reduce the risk of adverse musculoskeletal outcomes.

Visually guided targeting enhances bilateral force variability in healthy older adults

This study observed the effect of visual feedback on between-limb force variability relationships in young and older adults. Abduction force was examined in healthy young (n = 15, 25 ± 4 years) and older adults (n = 18, 71 ± 6 years) during simultaneous isometric contractions of both index fingers. Target forces ranged from 5% to 30% maximum voluntary contraction (MVC), where force variability and first dorsal interosseus activity were measured while (1) subjects viewed visual targets for both index fingers, (2) a visual target was provided for the dominant index finger only, and (3) visual targets were removed for both index fingers during bilateral isometric contractions. When subjects were provided with bilateral visual feedback during simultaneous contractions at low forces (5% and 10% MVC), older adults produced greater force variability than younger subjects (p = 0.002). However, when bilateral visual feedback was removed, age-related differences in variability were no longer present. Between-limb force variability differences existed at higher force outputs (20% and 30% MVC) when visual feedback was removed for the nondominant limb during bilateral isometric index finger abduction (p = 0.002). The control of bilateral force variability is compromised in older adults when visuomotor processes are engaged. However, age-related differences in force variability are abolished when no task-related visual feedback is available, and isometric contractions are based on internally guided feedback.

A Study on Estimation of Joint Force Through Isometric Index Finger Abduction With the Help of SEMG Peaks for Biomedical Applications.

We propose a new method to estimate joint force using a biomechanical muscle model and peaks of surface electromyography (SEMG). The SEMG measurement was carried out from the first dorsal interosseous muscle during isometric index finger abduction. The SEMG peaks were used as the input of the biomechanical muscle model which is a transfer function to generate the force. The force estimation performance ( R(2) ) was evaluated using the proposed method with nine healthy subjects, and a former method using a mean absolute value (MAV), which is the full-wave rectified and averaged (or low-pass filtered) signal of SEMG in a time window, was compared with the proposed method; the performance of the proposed method (0.94 ± 0.03) was better than that of MAV (0.90 ± 0.02). The proposed method could be widely applied to quantitative analysis of muscle activities based on SEMG.

Excitability changes in primary motor cortex just prior to voluntary muscle relaxation.

We postulated that primary motor cortex (M1) activity does not just decrease immediately prior to voluntary muscle relaxation; rather, it is dynamic and acts as an active cortical process. Thus we investigated the detailed time course of M1 excitability changes during muscle relaxation. Ten healthy participants performed a simple reaction time task. After the go signal, they rapidly terminated isometric abduction of the right index finger from a constant muscle force output of 20% of their maximal voluntary contraction force and performed voluntary muscle relaxation. Transcranial magnetic stimulation pulses were randomly delivered before and after the go signal, and motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous muscle. We selected the time course relative to an appropriate reference point, the onset of voluntary relaxation, to detect excitability changes in M1. MEP amplitude from 80 to 60 ms before the estimated electromyographic offset was significantly greater than that in other intervals. Dynamic excitability changes in M1 just prior to quick voluntary muscle relaxation indicate that cortical control of muscle relaxation is established through active processing and not by simple cessation of activity. The cortical mechanisms underlying muscle relaxation need to be reconsidered in light of such dynamics.

Removing visual feedback for a single limb alters between-limb force tremor relationships during isometric bilateral contractions.

This study examined how force tremor and muscle activity are altered between limbs when a visual target is removed for one limb during bilateral index finger abduction. Isometric index finger abduction force was examined in healthy adults (23 ± 4 years) when both index fingers abducted simultaneously. Abduction forces ranged from 5 to 20% maximum voluntary contraction, and these target forces were displayed on a PC monitor in front of the subject. Force tremor and first dorsal interosseous (FDI) activity were first collected while subjects viewed visual targets for both index fingers and then when the visual target was removed for the non-dominant index finger. Subjects successfully matched the force amplitudes generated for both limbs regardless of visual condition. When the visual target was removed for one limb, force tremor increased in this limb (p < 0.01). Different power spectral profiles were evident for each FDI EMG when targets were available for both limbs (p < 0.05); however, when one target was removed, the pattern of FDI EMG for the limb without a visual target closely reflected FDI EMG for the limb which had the visual target. The CNS actively modulates muscle activity in each limb to perform visually guided isometric contractions. Given that the goal was to match force output with both limbs, the requirements of the task must be established from the limb that had a visual target, and a copy of those motor commands appears to have been sent to the FDI of the limb without a visual target.

Alterations in the Peak Amplitude Distribution of the Surface Electromyogram Poststroke

We introduce a new method to examine the spinal motoneuron involvement after stroke using a surface electromyography (EMG) recording system. Fourteen chronic stroke survivors with mild to severe muscle weakness participated in the study. Surface EMG signals were collected from the first dorsal interosseous muscle while subjects performed isometric index finger abduction with paretic or contralateral hand at different matched force levels. Compared with the contralateral muscles, different patterns of peak amplitude distribution were observed at the paretic muscles, which could be induced by motor unit pathological alterations following a stroke. Compared with the conventional electrophysiological methods, the peak amplitude distribution analysis proposed in this study provides a convenient approach to help identify specific mechanisms of muscle weakness and other symptoms after stroke.

Transcranial magnetic stimulation and peristimulus frequencygram

ObjectiveThe aim of our study was to use peristimulus frequencygram (PSF) constructed from single motor unit recordings to further characterise the responses evoked by low intensity TMS. MethodsTwelve healthy subjects (age 32±11 years) received single-pulse TMS over the first dorsal interosseus (FDI) motor area during weak isometric index finger abduction. Several hundred stimuli were delivered at a frequency of ∼0.3Hz and at an intensity of 79–110% of active motor threshold. FDI electromyogram (EMG) was recorded with surface and intramuscular fine wire electrodes. For single motor units, data analysis involved construction of a peristimulus time histogram (PSTH) and PSF. Surface EMG analysis involved signal averaging. Cumulative sums (CUSUMs) were calculated for SEMG, PSTH, and PSF data. ResultsForty-five single motor units were identified. The average number of stimuli per unit was 201±112. Characterisation of the response evoked by TMS differed with the use of SEMG, PSTH, and PSF CUSUMs. ConclusionsThe duration of the EMG silence that follows the MEP during voluntary contraction was longer in the PSF than SEMG and PSTH. SignificanceThese findings highlight the importance of using both probability and frequency-based analysis when determining the duration of inhibitory events in peripheral recordings.

Physiological changes underlying bilateral isometric arm voluntary contractions in healthy humans

Many bilateral motor tasks engage simultaneous activation of distal and proximal arm muscles, but little is known about their physiological interactions. Here, we used transcranial magnetic stimulation to examine motor-evoked potentials (MEPs), interhemispheric inhibition at a conditioning-test interval of 10 (IHI(10)) and 40 ms (IHI(40)), and short-interval intracortical inhibition (SICI) in the left first dorsal interosseous (FDI) muscle during isometric index finger abduction. The right side remained at rest or performed isometric voluntary contraction with the FDI, biceps or triceps brachii, or the tibialis anterior. Left FDI MEPs were suppressed to a similar extent during contraction of the right FDI and biceps and triceps brachii but remained unchanged during contraction of the right tibialis anterior. IHI(10) and IHI(40) were decreased during contraction of the right biceps and triceps brachii compared with contraction of the right FDI. SICI was increased during activation of the right biceps and triceps brachii and decreased during activation of the right FDI. The present results indicate that an isometric voluntary contraction with either a distal or a proximal arm muscle, but not a foot dorsiflexor, decreases corticospinal output in a contralateral active finger muscle. Transcallosal inhibitory effects were strong during bilateral activation of distal hand muscles and weak during simultaneous activation of a distal and a proximal arm muscle, whereas GABAergic intracortical activity was modulated in the opposite manner. These findings suggest that in intact humans crossed interactions at the level of the motor cortex involved different physiological mechanisms when bilateral distal hand muscles are active and when a distal and a proximal arm muscle are simultaneously active.

*Poster 180: Effect of Bilateral Isometric Forces in Different Directions on Motor Cortical Function

Objective: The activity in the M1 reflects the direction of movements. The purpose of this study was to investigate physiological changes in the M1 during generation of bilateral isometric forces in different directions. Design: Randomized. Setting: Research laboratory. Participants: 12 right-handed healthy volunteers participated in this study. Interventions: None. Main Outcome Measures: We used transcranial magnetic stimulation to examine motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI), and interhemispheric inhibition (IHI) in the left first dorsal interosseous (FDI) muscle during 10% of isometric index finger abduction while the contralateral side performed 30% of isometric index finger abduction (bilateral force, opposite direction) or adduction (bilateral force, same direction), or remained at rest (unilateral force).

The Effect of Bilateral Isometric Forces in Different Directions on Motor Cortical Function in Humans

The activity in the primary motor cortex (M1) reflects the direction of movements, but little is known about physiological changes in the M1 during generation of bilateral isometric forces in different directions. Here, we used transcranial magnetic stimulation to examine motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI), and interhemispheric inhibition (IHI) in the left first dorsal interosseous (FDI) during isometric index finger abduction while the right index finger remained at rest or performed isometric forces in different directions (abduction or adduction) and in different postures (prone and supine). Left FDI MEPs were suppressed during bilateral compared with unilateral forces, with a stronger suppression when the right index finger force was exerted in the adduction direction regardless of hand posture. IHI targeting the left FDI increased during bilateral compared with unilateral forces and this increase was stronger during right index finger adduction despite the posture of the right hand. SICI decreased to a similar extent during both bilateral forces in both hand postures. Thus generation of index finger isometric forces away from the body midline (adduction direction), regardless of the muscle engaged in the task, down-regulates corticospinal output in the contralateral active hand to a greater extent than forces exerted toward the body midline (abduction direction). Transcallosal inhibition, but not GABAergic intracortical circuits, was modulated by the direction of the force. These findings suggest that during generation of bimanual isometric forces the M1 is driven by "extrinsic" parameters related to the hand action.

The effect of test TMS intensity on short-interval intracortical inhibition in different excitability states

The paired-pulse transcranial magnetic stimulation (TMS) paradigm is increasingly employed to examine intracortical inhibitory processes in different motor tasks. Short-interval intracortical inhibition (SICI) has been shown to vary with the size of the MEP elicited by the test TMS pulse. This suggests that factors that alter MEP size, such as changes in cortical excitability, may confound the interpretation of SICI. However, the effect of excitability on SICI has not been systematically investigated. The present study examined SICI in 11 volunteers. The effect of test TMS intensities ranging from 90 to 150% resting motor threshold (RMT) on SICI was examined in three excitability states in the right first dorsal interosseous muscle: rest, isometric abduction of the left index finger (Contra) and isometric abduction of the right index finger (Active). For all excitability states SICI was not observed when test TMS intensity was less than 110% resting motor threshold. This was true even for the Active condition in which 90 and 100% test TMS intensities elicited large and consistent MEPs. For all conditions moderately suprathreshold test TMS intensities (110-120% RMT) yielded the greatest measure of SICI; increasing test TMS intensities resulted in a progressive reduction in the estimate of SICI. These results suggest that estimates of SICI are systematically affected by the intensity of the test TMS pulse, regardless of excitability state. The results suggest that SICI should be examined using a constant test TMS intensity regardless of changes in cortical excitability and test MEP size.