Index Finger Abduction Research Articles

History Dependency of Muscle Strength Recovery from a Fatiguing Intermittent Task

Muscle fatigue and recovery are complex processes influencing muscle force generation capacity. While fatigue reduces this capacity, muscle recovery acts to restore the unfatigued muscle state. Many factors can potentially affect muscle recovery, among these may be a task dependency of recovery following an exercise. However, little has been reported regarding the history dependency of recovery after fatiguing contractions. Recently, we investigated the dependency of the fatigue process on cycle time during low to moderate exertion levels of intermittent muscle contraction (Rashedi & Nussbaum, 2016). A dependency of localized muscle fatigue on cycle time was shown, even though there was a consistent level of overall physical demand. It was concluded that the difference in fatigue development might be related to recovery processes occurring during execution of the intermittent task. In the present study, we focused on the potential effect of contraction history on post-fatigue recovery. Based on the expected dependency of recovery to task demands during exercise, it was hypothesized that post-fatigue recovery will also be affected by the history of exercise-induced muscle fatigue. We examined the dependency of muscle recovery subsequent to four different histories of fatiguing muscle contractions, imposed using two cycle times (30 and 60 sec) during low to moderate levels (15% and 25% of maximum voluntary contraction (MVC)) of intermittent static exertions involving index finger abduction. All participants completed the intermittent contractions, in all four conditions, for 1 hour, and MVCs were obtained at fixed intervals during 1 hour of post-exercise recovery (i.e., at 0.2, 5, 10, 30, and 60 minutes). There was a clear and statistically-significant dependency of muscle recovery rate on the muscle capacity state existing immediately after fatiguing exercise. This dependency did not appear to be modified by either the cycle time or exertion level leading to that state. Similar results were found in the study of Iguchi et al. (2008), wherein the authors compared recovery between two different exertion histories while fatiguing muscles to the same level, though recovery was only monitored for 5 minutes. These results imply that the post-exercise rate of recovery is primarily influenced by the post-exercise muscle state. Such evidence may help improve existing models of muscle recovery (Rashedi & Nussbaum, 2015b), facilitating more accurate predictions of localized muscle fatigue development (Rashedi & Nussbaum, 2015a), and thereby helping to enhance muscle performance and reduce the risk of injury

Altered motor unit discharge patterns in paretic muscles of stroke survivors assessed using surface electromyography

Objective. Hemispheric stroke survivors often show impairments in voluntary muscle activation. One potential source of these impairments could come from altered control of muscle, via disrupted motor unit (MU) firing patterns. In this study, we sought to determine whether MU firing patterns are modified on the affected side of stroke survivors, as compared with the analogous contralateral muscle. Approach. Using a novel surface electromyogram (EMG) sensor array, coupled with advanced template recognition software (dEMG) we recorded surface EMG signals over the first dorsal interosseous (FDI) muscle on both paretic and contralateral sides. Recordings were made as stroke survivors produced isometric index finger abductions over a large force range (20%–60% of maximum). Utilizing the dEMG algorithm, MU firing rates, recruitment thresholds, and action potential amplitudes were estimated for concurrently active MUs in each trial. Main results. Our results reveal significant changes in the firing rate patterns in paretic FDI muscle, in that the discharge rates, characterized in relation to recruitment force threshold and to MU size, were less clearly correlated with recruitment force than in contralateral FDI muscles. Firing rates in the affected muscle also did not modulate systematically with the level of voluntary muscle contraction, as would be expected in intact muscles. These disturbances in firing properties also correlated closely with the impairment of muscle force generation. Significance. Our results provide strong evidence of disruptions in MU firing behavior in paretic muscles after a hemispheric stroke, suggesting that modified control of the spinal motoneuron pool could be a contributing factor to muscular weakness in stroke survivors.

Surface-electromyographic analysis of the upper extremity muscles can aid in the rehabilitation of hemiparetic stroke survivors

Objective To explore any changes in the surface electromyography (sEMG) signals measured from the biceps brachii, flexor pollicis brevis and the first dorsal interosseous muscles of stroke survivors with hemiparesis during maximum isometric voluntary contractions. To correlate them with motor function recovery so as to provide a reference in making up individualized rehabilitation programs. Methods Twenty stroke survivors with hemiparesis were selected as the experimental group, and 10 healthy counterparts were recruited as the control group. For all of them, sEMG signals were recorded bilaterally from the biceps brachii, flexor pollicis brevis and the first dorsal interosseous muscles during maximum isometric voluntary contractions involving elbow flexion, thumb flexion and index finger abduction. For the stroke patients, such recording was performed repeatedly on the paretic side during the course of convalescence. Both root-mean-square (RMS) amplitude and median frequency (MDF) were calculated for the recorded surface EMG signals. The upper-extremity component of the Fugl-Meyer assessment scale (FMA-UE) and manual testing (MMT) of each muscle examined were performed with each stroke patient to assess motor function and upper extremity muscle strength. Results There were no significant differences in either RMS amplitude or MDF between the left and right sides of the healthy control subjects. Significant differences in both RMS amplitude and MDF were observed for the stroke survivors and between their values and those of the controls. The patients' unaffected side had significantly larger RMS amplitude and MDF than either side of the healthy control group. For the affected side of the stroke patients, both RMS amplitude and MDF tended to increase during the course of rehabilitation, with significant differences between the values obtained at the first and the last sessions. The FMA-UE scores and MMT results on the affected side were positively correlated with the RMS amplitudes and MDF results for the upper extremity muscles. Conclusions Surface electromyography is able to reflect the rehabilitation process and the recovery of muscle strength and motor function. It can be considered as a quantitative index for evaluating rehabilitation and a useful reference for designing targeted functional training programs. Key words: Stroke; Upper extremities; Surface electromyography; Rehabilitation

Force steadiness as a predictor of time to complete a pegboard test of dexterity in young men and women.

The purpose of the study was to evaluate the capacity of an expanded set of force steadiness tasks to explain the variance in the time it takes young men and women to complete the grooved pegboard test. In a single experimental session, 30 participants (mean ± SD) (24.2 ± 4.0 yr; 15 women) performed the grooved pegboard test, two tests of hand speed, measurements of muscle strength, and a set of submaximal, steady contractions. The steadiness tasks involved single and double actions requiring isometric contractions in the directions of wrist extension, a pinch between the index finger and thumb, and index finger abduction. Time to complete the grooved pegboard test ranged from 41.5 to 67.5 s. The pegboard times (53.9 ± 6.2 s) were not correlated with any of the strength measurements or the reaction time test of hand speed. A stepwise, multiple-regression analysis indicated that much of the variance (R(2) = 0.70) in pegboard times could be explained by a model that comprised two predictor variables derived from the steadiness tasks: time to match the target during a rapid force-matching task and force steadiness (coefficient of variation for force) during a single-action task. Moreover, the pegboard times were significantly faster for women (51.7 ± 6.8 s) than men (56.1 ± 4.9 s). Participants with slower pegboard times seemed to place a greater emphasis on accuracy than speed as they had longer times to match the target during the rapid force-matching task and exhibited superior force steadiness during the single-action task.

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.

Visuomotor Correction is a Robust Contributor to Force Variability During Index Finger Abduction by Older Adults

We examined aging-related differences in the contribution of visuomotor correction to force fluctuations during index finger abduction via the analysis of two datasets from similar subjects. Study (1) Young (N = 27, 23 ± 8 years) and older adults (N = 14, 72 ± 9 years) underwent assessment of maximum voluntary contraction force (MVC) and force steadiness during constant-force (CF) index finger abduction (2.5, 30, 65% MVC). For each trial, visual feedback of the force (VIS) was provided for 8–10 s and removed for 8–10 s (NOVIS). Visual gain of the force feedback at 2.5% MVC was high; 12- and 26-fold greater than the 30 and 65% MVC targets. Mean force, standard deviation (SD) of force, and coefficient of variation (CV) of force was calculated for detrended (<0.5 Hz drift removed) VIS and NOVIS data segments. Study (2) A similar group of 14 older adults performed discrete, randomly-ordered VIS or NOVIS trials at low target forces (1–3% MVC) and high visual gain. Study (1) For young adults the CV of force was similar between VIS and NOVIS for the 2.5% (4.8 vs. 4.3%), 30% (3.2 vs. 3.2%) and 65% (3.5 vs. 4.2%) target forces. In contrast, for older adults the CV of force was greater for VIS than NOVIS for 2.5% MVC (6.6 vs. 4.2%, p < 0.001), but not for the 30% (2.4 vs. 2.4%) and 65% (3.1 vs. 3.3%) target forces. At 2.5% MVC, the increase in CV of force for VIS compared with NOVIS was significantly greater (age × visual condition p = 0.008) for older than young adults. Study (2) Similarly, for older adults performing discrete, randomly ordered trials the CV of force was greater for VIS than NOVIS (6.04 vs. 3.81%, p = 0.01). When visual force feedback was a dominant source of information at low forces, normalized force variability was ~58% greater for older adults, but only 11% greater for young adults. The significant effect of visual feedback for older adults was not dependent on the order of presentation of visual conditions. The results indicate that impaired processing of visuomotor information underlies the greater motor variability observed in older adults during lab-based isometric contractions of a hand muscle.

Interhemispheric connectivity during bimanual isometric force generation.

Interhemispheric interactions through the corpus callosum play an important role in the control of bimanual forces. However, the extent to which physiological connections between primary motor cortices are modulated during increasing levels of bimanual force generation in intact humans remains poorly understood. Here we studied coherence between electroencephalographic (EEG) signals and the ipsilateral cortical silent period (iSP), two well-known measures of interhemispheric connectivity between motor cortices, during unilateral and bilateral 10%, 40%, and 70% of maximal isometric voluntary contraction (MVC) into index finger abduction. We found that EEG-EEG coherence in the alpha frequency band decreased while the iSP area increased during bilateral compared with unilateral 40% and 70% but not 10% of MVC. Decreases in coherence in the alpha frequency band correlated with increases in the iSP area, and subjects who showed this inverse relation were able to maintain more steady bilateral muscle contractions. To further examine the relationship between the iSP and coherence we electrically stimulated the ulnar nerve at the wrist at the alpha frequency. Electrical stimulation increased coherence in the alpha frequency band and decreased the iSP area during bilateral 70% of MVC. Altogether, our findings demonstrate an inverse relation between alpha oscillations and the iSP during strong levels of bimanual force generation. We suggest that interactions between neural pathways mediating alpha oscillatory activity and transcallosal inhibition between motor cortices might contribute to the steadiness of strong bilateral isometric muscle contractions in intact humans.

Correlates Between Force and Postural Tremor in Older Individuals with Essential Tremor.

Essential tremor (ET) is commonly associated with kinetic tremor. However, other forms of tremor, such as force and postural tremor, may occur in ET with less severity. This study objectively assessed force and postural tremor characteristics in ET with the purpose of identifying the relationships between these tremors. Ten individuals with ET (age 71 ± 5years) and ten healthy controls (age 70 ± 5years) participated in the study. Force tremor was quantified as fluctuations in index finger abduction force during isometric contractions at 10% maximum voluntary contraction (MVC) and 60% MVC. Postural tremor was quantified as index finger acceleration when the subjects held their entire arm unsupported, and when their arm was supported so that only the index finger could move. Time- and frequency-domain parameters were extracted from tremor data, and then correlations within, and between, tremor subtypes were examined. ET force tremor was dependent on contraction intensity whereas postural tremor was unaffected by the level of limb support. Significant correlations existed between frequency components of postural tremor and force tremor amplitude. Force tremor amplitude normalised to the level of contraction intensity correlated to the proportion of power for postural tremor. These correlations were observed for both contraction intensities and both levels of postural support. The proportion of power represents the output of central oscillators in ET patients and therefore correlated well to force tremor. Given that significant relationships existed between spectral features of postural tremor and the overall force tremor amplitude, it is clear that these tremor modalities are not completely independent in older adults with ET.

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.

Reduced Voluntary Activation During Brief and Sustained Contractions of a Hand Muscle in Secondary-Progressive Multiple Sclerosis Patients

Background. Secondary-progressive multiple sclerosis (SPMS) patients have structural cortical damage resulting in increased compensatory cortical activity during (submaximal) performance. However, functional effects of changed cortical output are difficult to measure. The interpolated-twitch technique allows for measurement of voluntary activation (VA) necessary for force production. This study aimed to determine VA, force, and muscle fatigue during brief and sustained contractions in SPMS patients. Because fatigue effects are not confined to the motor system, we additionally examined fatiguing effects on cognitive performance. Methods. Twenty-five SPMS and 25 sex-, age-, and education-matched participants performed brief (5 seconds) and sustained (2 minutes) maximal index finger abductions. To evaluate VA, double-pulse twitches were evoked before, during, and after contractions. Additionally, data were compared with data obtained in relapsing–remitting multiple sclerosis (RRMS) patients. Subjects also performed choice-reaction time tasks before and after the sustained contraction. Results. During brief contractions, VA (85% vs 94%, P = .004) and force (25 N vs 32 N, P = .011) were lower for SPMS patients than controls. During sustained contractions, VA (P = .001) was also lower, resulting in greater force decline (73% vs 63%, P < .001) and reduced peripheral fatigue (19% vs 50%, P < .001). Comparisons with RRMS resulted in lower VA, greater force decline, and greater estimated central fatigue in SPMS. SPMS patients were slower (P < .001) and made more errors (P < .001) than controls, but neither group reduced their performance after the sustained contraction. Conclusion. SPMS patients had lower VA than RRMS patients and controls. The importance of voluntary activation for muscle force and fatigability warrants targeted rehabilitation strategies.

Single Limb Fatigue Compromises The Ability To Generate Force In The Opposite Limb

Performing strong voluntary contractions with a single limb produces unintended muscle activity in the homologous muscle of the opposite resting limb. The mechanisms underlying these crossed-effects are likely to involve supraspinal centres, as neural activity in the active motor cortex is known to spread within and between motor cortices during repeated muscle contractions. Although strong contractions result in unintended activity in the resting opposite limb, it is unknown how brief contractions or fatigue inducing contractions affect the maximum force generating ability of the opposite limb. PURPOSE: To determine how intermittent maximal contractions of a single limb influences the ability to produce force in the opposite limb. METHODS: Twelve healthy participant (age: 23 ± 3.4 yr) were recruited into the study. Motor evoked potentials (MEP), EMG of the first dorsal interosseous (FDI), and index finger abduction forces were obtained before, during, and after a single limb intermittent maximal contraction protocol. The contraction sets consisted of four 4 s contractions separated by 2 s of rest for the dominant limb- followed a 10 s period where MEP, FDI EMG and force data were collected for the non-dominant limb. Contractions were continued until MVC reduced by 50%. RESULTS: In unfatigued muscles, only small increases in unintended activity occurred when the opposite limb was maximally contracting (∼5% increase in force with no changes in MEP amplitude). After one contraction set was performed by the dominant limb, the capacity to generate force in the non-dominant limb was significantly reduced by 15% which corresponded to a 20% reduction in MEP amplitude. Maximal force output for both limbs significantly reduced across the duration of contraction sets, where maximum force output declined by 52% for the exercising limb and 36% for the contralateral limb when the criteria for termination was achieved. CONCLUSIONS: Crossed-effects were evident across all conditions; however fatiguing contractions were most effective in eliciting responses in the contralateral limb. Given that repeated single limb contractions reduced the maximal force generating capacity in the opposite limb, it appears that inhibitory mechanisms were activated on the opposite side of the body, presumably at the level of the motor cortex.

Age-related Differences in Pre- and Post-synaptic Motor Cortex Inhibition are Task Dependent.

Previous research has shown age-related differences in short- (SICI) and long-interval intracortical inhibition (LICI) in both resting and active hand muscles, suggesting that healthy ageing influences post-synaptic motor cortex inhibition. However, it is not known how the ageing process effects the pre-synaptic interaction of SICI by LICI, and how these pre- and post-synaptic intracortical inhibitory circuits are modulated by the performance of different motor tasks in older adults. To examine age-related differences in pre- and post-synaptic motor cortex inhibition at rest, and during index finger abduction and precision grip. In 13 young (22.3±3.8 years) and 15 old (73.7±4.0 years) adults, paired-pulse transcranial magnetic stimulation (TMS) was used to measure SICI (2ms inter-stimulus interval; ISI) and LICI (100 and 150ms ISI), whereas triple-pulse TMS was used to investigate SICI when primed by LICI. We found no age-related difference in SICI at rest or during index finger abduction, but significantly greater SICI in older subjects during precision grip. Older adults showed reduced LICI in resting muscle (at an ISI of 150ms), with no age-related differences in LICI during either task. When SICI was primed by LICI, disinhibition of motor cortex was reduced in older adults at rest (100ms ISI) and during index finger abduction (150ms ISI), but not during precision grip. Our results support age-related differences in pre- and post-synaptic motor cortex inhibition, which may contribute to impaired hand function during task performance in older adults.

Timing and specificity of early changes in motor excitability during movement observation.

Motor resonance is typically considered a modulation of motor activity during action observation, which mirrors the pattern of muscle activation occurring during execution of the observed action. There is preliminary evidence that motor activity accompanying action observation comprises two stages: an initial non-specific modulation, followed by a later change in excitability that is specific to the muscles involved in the action. We built on this previous work, with the aims of (1) clarifying the time-course of muscle specificity during action observation and (2) establishing whether modulation at each time point reflects significant changes (facilitation or suppression) from baseline. Motor-evoked potentials were elicited in two muscles-the first dorsal interosseous (FDI) and abductor digiti minimi (ADM)-while participants watched stimuli showing index or little finger abduction. Our results provide partial support for previous findings of muscle-specific modulation after, but not before, 200 ms. Interestingly, however, this was shown only by facilitation of the FDI during index finger observation; there was no significant facilitation of the ADM when little finger movement was observed. Furthermore, we found no facilitation of corticospinal excitability (specific or non-specific) at time points earlier than 200 ms (100-150 ms). We discuss these results in the context of previous work showing a non-specific increase in excitability at 90 ms during action observation. Together, these findings suggest a fast and short-lived facilitation of corticospinal excitability that occurs at around 60-90 ms and ends by 100-150 ms.

Motor cortical plasticity and corticospinal tract diffusion tensor image in patients with Parkinson’s disease and essential tremor

s / Brain Stimulation 8 (2015) 326e342 342 Background: Non-invasive brain stimulation (NIBS) protocols can interfere with or even facilitate motor learning. However, it is still unclear whether the response to NIBS protocols, such as theta-burst stimulation (TBS), can predict its ability to interfere with or enhance learning of a motor task. We aim to investigate this question using TBS and a ballistic finger movement task. Methods: Eight right handed healthy human volunteers participated in the study. Experiment 1: We investigated the effects of continuous TBS (cTBS) and intermittent TBS (iTBS) on cortical excitability. Thirty motor evoked potentials (MEP) were measured before and after intervention (cTBS or iTBS) for 30 min. Experiment 2: In separate sessions, we tested the effects of cTBS or iTBS on a ballistic finger movement task. Subjects were required to make rapid index finger abduction movements of the right hand paced at a rate of 0.5 Hz. One block consists of 75 movements for 2.5 min and 4 blocks were tested in total. Between blocks 2 and 3, either cTBS or iTBS was applied over left M1. We quantified the learning effect by measuring the increase the mean peak acceleration (ACC) for each set of 30 consecutive movements. Results: In experiment 1, there was a non-significant trend for MEPs to get smaller after cTBS and larger after iTBS. In experiment 2, cTBS or iTBS did not change ACC immediately after application of TBS. However, ACC increased significantly more in the final block of trials after cTBS than it did after iTBS. The normalized ACC in the last block was negatively correlated with excitability changes induced by cTBS in experiment 1 (Spearman’s r 1⁄4 0.881, p 1⁄4 0.002). Conclusions: This preliminary result suggest that cTBS did not affect early consolidation in M1, but facilitated further performance gains in model-free learning. The effect on performance was correlated with the effect of cTBS on MEPs.

Reversal of Practice-related Effects on Corticospinal Excitability has no Immediate Effect on Behavioral Outcome.

Motor training usually increases the excitability of corticospinal outputs to the trained muscles. However, it is uncertain to what extent the change in excitability is a critical component of behavioral learning or whether it is a non-specific side effect. We used a depotentiation protocol to abolish the training-induced increase of corticospinal excitability and tested whether this had any immediate effect on the improved motor performance. We used an index finger abduction task in which behavioral improvement is known to be associated with M1 excitability changes as monitored by the amplitude of motor-evoked potentials produced by single-pulse transcranial magnetic stimulation (TMS). These effects could be reversed by a depotentiation protocol using a short form of continuous theta-burst stimulation (cTBS150). Participants underwent three experimental interventions: 'motor training', 'motor training plus cTBS150' and 'cTBS150'. M1 excitability and TMS-evoked finger movements were assessed before the experimental interventions and 5 min, 15 min, and 30 min thereafter. Motor retention was tested 45 min after the experimental interventions. During training, acceleration of the practiced movement improved. At the end of training, M1 excitability and the acceleration of TMS-evoked index finger movements in the direction of training had increased and the enhanced performance was retained when tested 45 min later. The depotentiation protocol, delivered immediately after the end of training, reversed the excitability changes in M1 but did not affect the acceleration of the TMS-evoked finger movement nor the retention of performance. The depotentiation protocol alone did not modify M1 excitability. The present study indicates that in the short term, increases in corticospinal excitability are not related to immediate changes in behavioral motor outcome.

Muscle Fatigability During a Sustained Index Finger Abduction and Depression Scores Are Associated With Perceived Fatigue in Patients With Relapsing-Remitting Multiple Sclerosis

Introduction. Fatigue is a common and debilitating symptom in patients with multiple sclerosis (MS). Self-reported levels of perceived fatigue are associated with both patient characteristics and clinical measures. Pilot analysis indicated that muscle fatigability combined with depression scores was highly associated with perceived fatigue in patients with MS. Studies that combine physiological and psychological constructs to explain MS-related fatigue are scarce. Therefore, the present study aimed to evaluate the robustness of the association between perceived fatigue, muscle fatigability, and depression scores in MS. Methods. Eighty-six patients with relapsing-remitting MS completed 2 fatigue questionnaires (Fatigue Severity Scale [FSS] and Modified Fatigue Impact Scale [MFIS]) and a depression questionnaire (Hospital Anxiety and Depression Scale [HADS]). Maximal index finger abduction force (maximum voluntary contraction [MVC]) was measured, as well as muscle fatigability during a 2-minutes sustained maximal contraction. Multivariable regression analyses were used to analyze the association between perceived fatigue, and muscle fatigability and depression scores. Results. Perceived fatigue was associated with depression, muscle fatigability, and, depending on the questionnaire, to sex or to MVC. The model explained 40% and 48% of the variation in perception of fatigue as indexed with FSS questionnaire (rpartial: HADS 0.45, muscle fatigability 0.45, MVC −0.14, sex 0.32), and MFIS physical questionnaire (rpartial: HADS 0.59, muscle fatigability 0.49, MVC −0.38), respectively. Conclusions. The found association accentuates the importance of including both physiological fatigability-related and psychological mood–related constructs in models to explain perceived fatigue in patients with MS. The model also directs future research toward applying effortful conditions and emphasizes the importance of assessing different constructs when evaluating rehabilitation strategies to reduce MS-related fatigue.

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.

Task-related changes in intracortical inhibition assessed with paired- and triple-pulse transcranial magnetic stimulation.

Recent research has demonstrated a task-related modulation of postsynaptic intracortical inhibition within primary motor cortex for tasks requiring isolated (abduction) or synergistic (precision grip) muscle activation. The current study sought to investigate task-related changes in pre- and postsynaptic intracortical inhibition in motor cortex. In 13 young adults (22.5 ± 3.5 yr), paired-pulse transcranial magnetic stimulation (TMS) was used to measure short (SICI)- and long-interval intracortical inhibition (LICI) (i.e., postsynaptic motor cortex inhibition) in first dorsal interosseous muscle, and triple-pulse TMS was used to investigate changes in SICI-LICI interactions (i.e., presynaptic motor cortex inhibition). These measurements were obtained at rest and during muscle activation involving isolated abduction of the index finger and during a precision grip using the index finger and thumb. SICI was reduced during abduction and precision grip compared with rest, with greater reductions during precision grip. The modulation of LICI during muscle activation depended on the interstimulus interval (ISI; 100 and 150 ms) but was not different between abduction and precision grip. For triple-pulse TMS, SICI was reduced in the presence of LICI at both ISIs in resting muscle (reflecting presynaptic motor cortex inhibition) but was only modulated at the 150-ms ISI during index finger abduction. Results suggest that synergistic contractions are accompanied by greater reductions in postsynaptic motor cortex inhibition than isolated contractions, but the contribution of presynaptic mechanisms to this disinhibition is limited. Furthermore, timing-dependent variations in LICI provide additional evidence that measurements using different ISIs may not represent activation of the same cortical process.

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.

Reduced Dual-Task Performance in MS Patients Is Further Decreased by Muscle Fatigue

Background. Multiple sclerosis (MS) can be accompanied by motor, cognitive, and sensory impairments. Additionally, MS patients often report fatigue as one of their most debilitating symptoms. It is, therefore, expected that MS patients will have difficulties in performing cognitive-motor dual tasks (DTs), especially in a fatiguing condition. Objective. To determine whether MS patients are more challenged by a DT than controls in a fatiguing and less-fatiguing condition and whether DT performance is associated with perceived fatigue. Methods. A group of 19 MS patients and 19 age-, sex-, and education-matched controls performed a cognitive task (2-choice reaction time task) separately or concurrent with a low-force or a high-force motor task (index finger abduction at 10% or 30% maximal voluntary contraction). Results. MS patients performed less well on a cognitive task than controls. Cognitive task performance under DT conditions decreased more for MS patients. Moreover, under high-force DT conditions, cognitive performance declined in both groups but to a larger degree for MS patients. Besides a decline in cognitive task performance, MS patients also showed a stronger decrease in motor performance under high-force DT conditions. DT costs were positively related to perceived fatigue as measured by questionnaires. Conclusions. Compared with controls, MS patients performed less well on DTs as demonstrated by a reduction in both cognitive and motor performances. This performance decrease was stronger under fatiguing conditions and was related to the sense of fatigue of MS patients. These data illustrate problems that MS patients may encounter in daily life because of their fatigue.