Finger Abduction Research Articles

Exploring the advantages of multiband fMRI with simultaneous EEG to investigate coupling between gamma frequency neural activity and the BOLD response in humans.

We established an optimal combination of EEG recording during sparse multiband (MB) fMRI that preserves high-resolution, whole-brain fMRI coverage while enabling broad-band EEG recordings which are uncorrupted by MRI gradient artefacts (GAs). We first determined the safety of simultaneous EEG recording during MB fMRI. Application of MB factor = 4 produced <1°C peak heating of electrode/hardware during 20min of GE-EPI data acquisition. However, higher SAR sequences require specific safety testing, with greater heating observed using PCASL with MB factor = 4. Heating was greatest in the electrocardiogram channel, likely due to it possessing longest lead length. We investigated the effect of MB factor on the temporal signal-to-noise ratio for a range of GE-EPI sequences (varying MB factor and temporal interval between slice acquisitions). We found that, for our experimental purpose, the optimal acquisition was achieved with MB factor = 3, 3mm isotropic voxels, and 33 slices providing whole head coverage. This sequence afforded a 2.25s duration quiet period (without GAs) in every 3s TR. Using this sequence, we demonstrated the ability to record gamma frequency (55-80Hz) EEG oscillations, in response to right index finger abduction, that are usually obscured by GAs during continuous fMRI data acquisition. In this novel application of EEG-MB fMRI to a motor task, we observed a positive correlation between gamma and BOLD responses in bilateral motor regions. These findings support and extend previous work regarding coupling between neural and hemodynamic measures of brain activity in humans and showcase the utility of EEG-MB fMRI for future investigations.

Motor fatigability in persons with multiple sclerosis: Relation between different upper limb muscles, and with fatigue and the perceived use of the arm in daily life

BackgroundMotor fatigability is increasingly acknowledged in persons with MS (pwMS). It is unknown whether fatigability is generalized across upper limb muscles and relates to fatigue and perceived difficulties in upper limb use. MethodsThis observational case-controlled study included twenty PwMS (median EDSS = 3, range 1.5–6.5) and twenty healthy controls who performed 30″ sustained maximal muscle contractions for index finger abduction, hand grip, elbow flexion and shoulder abduction. A static fatigue index (SFI) was calculated to assess motor fatigability for each muscle group. PwMS completed the Fatigue Severity Scale (FSS) and Modified Fatigue Index Scale (MFIS), to quantify severity and perceived impact of fatigue and the Manual Ability Measure (MAM-36) reflecting perceived difficulty in using the upper limbs. Comparisons between groups and muscles was made by t-tests. Associations between outcomes were calculated with correlation coefficients. ResultsFatigue was highest in pwMS. PwMS showed preserved muscle strength and a greater motor fatigability in elbow flexors compared to healthy controls. SFI of elbow flexors and shoulder abductors were associated, and contributed to FSS and MFIS. SFI of elbow flexors and finger abductors predicted half of the variation in MAM-36. ConclusionIncreased motor fatigability was only present in elbow flexors of PwMS, indicating that expression of motor fatigability is not generalized. Fatigability was associated with perceived fatigue (impact) and daily life upper limb use. Results are preliminary given the small sample size with predominantly persons with mild MS.

Changing Artificial Playback Speed and Real Movement Velocity Do Not Differentially Influence the Excitability of Primary Motor Cortex during Observation of a Repetitive Finger Movement.

Action observation studies have investigated whether changing the speed of the observed movement affects the action observation network. There are two types of speed-changing conditions; one involves “changes in actual movement velocity,” and the other is “manipulation of video speed.” Previous studies have investigated the effects of these conditions separately, but to date, no study has directly investigated the differences between the effects of these conditions. In the “movement velocity condition,” increased velocity is associated with increased muscle activity; however, this change of muscle activities is not shown in the “video speed condition.” Therefore, a difference in the results obtained under these conditions could be considered to reflect a difference in muscle activity of actor in the video. The aim of the present study was to investigate the effects of different speed-changing conditions and spontaneous movement tempo (SMT) on the excitability of primary motor cortex (M1) during action observation, as assessed by motor-evoked potentials (MEPs) amplitudes induced by transcranial magnetic stimulation (TMS). A total of 29 healthy subjects observed a video clip of a repetitive index or little finger abduction movement under seven different speed conditions. The video clip in the movement velocity condition showed repetitive finger abduction movements made in time with an auditory metronome, at frequencies of 0.5, 1, 2, and 3 Hz. In the video speed condition, playback of the 1-Hz movement velocity condition video clip was modified to show movement frequencies of 0.5, 2, or 3 Hz (Hz-Fake). TMS was applied at the time of maximal abduction and MEPs were recorded from two right-hand muscles. There were no differences in M1 excitability between the movement velocity and video speed conditions. Moreover, M1 excitability did not vary across the speed conditions for either presentation condition. Our findings suggest that changing playback speed and actual differences in movement velocity do not differentially influence M1 excitability during observation of a simple action task, such as repetitive finger movement, and that it is not affected by SMT. In simple and meaningless observational task, people might not be able to recognize the difference in muscle activity of actor in the video.

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).

Transfer of the Motor Branch of the Abductor Digiti Quinti for Thenar Muscle Reinnervation in High Median Nerve Injuries.

In high median nerve repairs, thenar muscle reinnervation is impossible because of the long distances over which axons must regenerate. To overcome this obstacle, we propose transferring the abductor digiti quinti motor branch (ADQMB) to the thenar branch of the median nerve (TBMN). We used 10 embalmed hands for anatomical and histological studies. Thereafter, 5 patients with a high median nerve injury underwent surgical reconstruction within 8 months of their accident and were followed for at least 10 months after surgery (mean, 13.2 months). We transferred the ADQMB to the TBMN. The median nerve was grafted in 4 patients and the motor branch of the extensor carpi radialis brevis was transferred to the anterior interosseous nerve in 3. Patients had pre- and postoperative evaluations of thumb range of motion and strength. In cadaveric hands, the ADQMB was the first branch of the ulnar nerve to arise near the pisiform bone. The TBMN arose from the anterior surface of the median nerve, underneath the flexor retinaculum. Retrograde dissection of the TBMN allowed tension-free coaptation with the ADQMB. Both branches contained approximately 650 myelinated fibers. After surgery, all our patients improved thumb pronation, thenar eminence bulk, and abductor pollicis brevis British Medical Research Council score. They recovered approximately 75% of their normal-side grasp and pinch strength. No patient lost little finger abduction. Transfer of the ADQMB to the TBMN reinnervated the thenar muscles, whichimproved thumb range of motion and strength. Therapeutic V.

The Relative Contribution to Small Finger Abduction of the Ulnar Versus Radial Slip of the EDM: Implications for Tendon Transfers.

The extensor digiti minimi (EDM) tendon is commonly divided into a radial slip (EDM-R) and an ulnar slip (EDM-U). To our knowledge, the degree to which each EDM slip concomitantly abducts the small finger with active extension has not been formally tested. This study sought to characterize the comparative contributions of finger abduction inherent to each slip of the EDM to observe the sequelae of active small finger extension following transfer of the contralateral slip. Eighteen fresh-frozen cadaveric hands were used in this study. Starting with the hand in resting position, a controlled traction of 10 N was applied to each slip of the EDM tendon. The range of small finger abduction with respect to the fixed ring finger was recorded utilizing infrared reflective markers tracked through the range of motion using a digital video camera. The mean abduction of the small finger when the radial slip of the EDM tendon was tested was 13.33° (95% confidence interval [CI]: 10.10°-16.55°), which was significantly different ( P ≤ .001) than small finger abduction produced by the ulnar slip of the EDM, with a mean of 23.72° (95% CI: 19.40°-28.04°). Given the fact that the ulnar slip of the EDM tendon is shown to be the major contributor of aberrant abduction with active small finger extension, as traction on this slip produces almost twice as much abduction as the radial slip, the EDM-U is the ideal donor graft with respect to tendon transfers of the EDM.

Reticulospinal Contributions to Gross Hand Function after Human Spinal Cord Injury.

Multiple descending motor pathways likely contribute to the recovery of hand motor function following spinal cord injury (SCI). Reticulospinal neurons project to spinal motor neurons controlling hand muscles and extensively sprout into gray matter structures after SCI; therefore, it has been proposed that the reticulospinal tract is one of the descending motor pathways involved in recovery of hand function after injury. To test this hypothesis, we examined the StartReact response, an involuntary release of a planned movement via a startling stimulus that engages the reticulospinal tract, by measuring reaction times from electromyographic activity in an intrinsic finger muscle during three motor tasks requiring different degrees of hand dexterity: index finger abduction, a precision grip, and a power grip. Males and females with and without incomplete chronic cervical SCI were tested. We found that although SCI participants voluntarily responded to all tasks, reaction times were shorter during a startle cue while performing a power grip but not index finger abduction or precision grip. Control subjects had similarly shorter reaction times during a startle cue in all motor tasks. These results provide the first evidence for a contribution of the reticulospinal tract to hand control in humans with SCI during gross finger manipulations and suggest that this contribution is less pronounced during fine dexterous finger movements.SIGNIFICANCE STATEMENT It has been long proposed that brainstem pathways contribute to the recovery of hand function in humans with spinal cord injury (SCI). Here, we show that individuals with anatomically incomplete chronic cervical SCI responded to a startle stimulus, a test that engages the reticulospinal tract, while performing a power grip but not during index finger abduction or precision grip. Control subjects responded to a startle stimulus similarly across tasks. These observations suggest that reticulospinal outputs after SCI contribute to hand motor tasks involving gross finger movements. Interestingly, this contribution is less pronounced during fine dexterous finger movements.

Anconeus Epitrochlearis Muscle as a Cause of Ulnar Nerve Compression at the Elbow

AbstractUlnar nerve entrapment is the second most common compressive neuropathy in the upper limb, after carpal tunnel syndrome (Dellon, 1986). One of the causes that must be considered is the accessory anconeus epitrochlearis muscle, which is present in 4% to 34% of the general population (Husarik et al, 2010; Vanderpool et al, 1968; Nellans et al, 2014).We describe a patient with symptoms of compression of the left ulnar nerve at the elbow and the result of the surgical treatment.The patient presented with hypoesthesia in the fourth and fifth fingers of the left hand, and reduction of strength in the fifth finger abduction. No alterations were found in the thumb adduction.Initially, the treatment was conservative (splint, physiotherapy, analgesics); surgical treatment was indicated due to the continuity of the symptoms.The ulnar nerve was surgically released and transposed, with complete recovery after 6 months of follow-up.Ulnar nerve entrapment at the elbow by the anconeus epitrochlearis muscle is not common, but it must not be ignored (Chalmers, 1978). Ultrasonography (Jung et al, 2013; Bargalló et al, 2010), elbow magnetic resonance imaging (MRI) (Jeon, 2005), and electromyography (Byun, 2011) can help establish the proper diagnosis.

Abnormal Temporal Coupling of Tactile Perception and Motor Action in Parkinson's Disease.

Evidence shows altered somatosensory temporal discrimination threshold (STDT) in Parkinson’s disease in comparison to normal subjects. In healthy subjects, movement execution modulates STDT values through mechanisms of sensory gating. We investigated whether STDT modulation during movement execution in patients with Parkinson’s disease differs from that in healthy subjects. In 24 patients with Parkinson’s disease and 20 healthy subjects, we tested STDT at baseline and during index finger abductions (at movement onset “0”, 100, and 200 ms thereafter). We also recorded kinematic features of index finger abductions. Fifteen out of the 24 patients were also tested ON medication. In healthy subjects, STDT increased significantly at 0, 100, and 200 ms after movement onset, whereas in patients with Parkinson’s disease in OFF therapy, it increased significantly at 0 and 100 ms but returned to baseline values at 200 ms. When patients were tested ON therapy, STDT during index finger abductions increased significantly, with a time course similar to that of healthy subjects. Differently from healthy subjects, in patients with Parkinson’s disease, the mean velocity of the finger abductions decreased according to the time lapse between movement onset and the delivery of the paired electrical stimuli for testing somatosensory temporal discrimination. In conclusion, patients with Parkinson’s disease show abnormalities in the temporal coupling between tactile information and motor outflow. Our study provides first evidence that altered temporal processing of sensory information play a role in the pathophysiology of motor symptoms in Parkinson’s disease.

A novel cortical target to enhance hand motor output in humans with spinal cord injury.

A main goal of rehabilitation strategies in humans with spinal cord injury is to strengthen transmission in spared neural networks. Although neuromodulatory strategies have targeted different sites within the central nervous system to restore motor function following spinal cord injury, the role of cortical targets remain poorly understood. Here, we use 180 pairs of transcranial magnetic stimulation for ∼30 min over the hand representation of the motor cortex at an interstimulus interval mimicking the rhythmicity of descending late indirect (I) waves in corticospinal neurons (4.3 ms; I-wave protocol) or at an interstimulus interval in-between I-waves (3.5 ms; control protocol) on separate days in a randomized order. Late I-waves are thought to arise from trans-synaptic cortical inputs and have a crucial role in the recruitment of spinal motor neurons following spinal cord injury. Motor evoked potentials elicited by transcranial magnetic stimulation, paired-pulse intracortical inhibition, spinal motor neuron excitability (F-waves), index finger abduction force and electromyographic activity as well as a hand dexterity task were measured before and after both protocols in 15 individuals with chronic incomplete cervical spinal cord injury and 17 uninjured participants. We found that motor evoked potentials size increased in spinal cord injury and uninjured participants after the I-wave but not the control protocol for ∼30 to 60 min after the stimulation. Intracortical inhibition decreased and F-wave amplitude and persistence increased after the I-wave but not the control protocol, suggesting that cortical and subcortical networks contributed to changes in corticospinal excitability. Importantly, hand motor output and hand dexterity increased in individuals with spinal cord injury after the I-wave protocol. These results provide the first evidence that late synaptic input to corticospinal neurons may represent a novel therapeutic target for improving motor function in humans with paralysis due to spinal cord injury.

A Two-Axis Goniometric Sensor for Tracking Finger Motion.

The study of finger kinematics has developed into an important research area. Various hand tracking systems are currently available; however, they all have limited functionality. Generally, the most commonly adopted sensors are limited to measurements with one degree of freedom, i.e., flexion/extension of fingers. More advanced measurements including finger abduction, adduction, and circumduction are much more difficult to achieve. To overcome these limitations, we propose a two-axis 3D printed optical sensor with a compact configuration for tracking finger motion. Based on Malus’ law, this sensor detects the angular changes by analyzing the attenuation of light transmitted through polarizing film. The sensor consists of two orthogonal axes each containing two pathways. The two readings from each axis are fused using a weighted average approach, enabling a measurement range up to 180 and an improvement in sensitivity. The sensor demonstrates high accuracy (±0.3), high repeatability, and low hysteresis error. Attaching the sensor to the index finger’s metacarpophalangeal joint, real-time movements consisting of flexion/extension, abduction/adduction and circumduction have been successfully recorded. The proposed two-axis sensor has demonstrated its capability for measuring finger movements with two degrees of freedom and can be potentially used to monitor other types of body motion.

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.

Cortical and reticular contributions to human precision and power grip.

The corticospinal tract contributes to the control of finger muscles during precision and power grip. We explored the neural mechanisms contributing to changes in corticospinal excitability during these gripping configurations. Motor evoked potentials (MEPs) elicited by cortical, but not by subcortical, stimulation were more suppressed during power grip compared with precision grip and index finger abduction. Intracortical inhibition was more reduced during power grip compared with the other tasks. An acoustic startle cue, a stimulus that engages the reticular system, suppressed MEP size during power grip to a lesser extent than during the other tasks at a cortical level and this positively correlated with changes in intracortical inhibition. Our findings suggest that changes in corticospinal excitability during gross more than fine finger manipulations are largely cortical in origin and that the reticular system contributed, at least in part, to these effects. It is well accepted that the corticospinal tract contributes to the control of finger muscles during precision and power grip in humans but the neural mechanisms involved remain poorly understood. Here, we examined motor evoked potentials elicited by cortical and subcortical stimulation of corticospinal axons (MEPs and CMEPs, respectively) and the activity in intracortical circuits (suppression of voluntary electromyography) and spinal motoneurons (F-waves) in an intrinsic hand muscle during index finger abduction, precision grip and power grip. We found that the size of MEPs, but not CMEPs, was more suppressed during power grip compared with precision grip and index finger abduction, suggesting a cortical origin for these effects. Notably, intracortical inhibition was more reduced during power grip compared with the other tasks. To further examine the origin of changes in intracortical inhibition we assessed the contribution of the reticular system, which projects to cortical neurons, and projects to spinal motoneurons controlling hand muscles. An acoustic startle cue, which engages the reticular system, suppressed MEP size during power grip to a lesser extent than during the other tasks and this positively correlated with changes in intracortical inhibition. A startle cue decreased intracortical inhibition, but not CMEPs, during power grip. F-waves remained unchanged across conditions. Our novel findings show that changes in corticospinal excitability present during power grip compared with fine finger manipulations are largely cortical in origin and suggest that the reticular system contributed, at least in part, to these effects.

Physiological tremor (8–12 Hz component) in isometric force control

The experiment investigated the influence of physiological tremor (8–12Hz band) on the variability of isometric force control as a function of force level and hand dominance. Subjects were instructed to match a constant force level target line on a computer screen and minimize error in a uni-manual isometric finger abduction task at 5%, 25%, 45%, 65%, and 85% of their maximal voluntary contraction (MVC). The experimental protocol was performed independently with the left and right hands in separate blocks of performance. Tremor amplitude was enhanced at an increasing rate with increments of force level and was correlated with both performance outcome (Root mean square error – RMSE) and time-dependent regularity (Sample Entropy) of the force signal. No significant findings in force variability (dispersion or irregularity) were found between the dominant and non-dominant hands. Physiological tremor has a small but direct influence on the dispersion and time dependent structure of the variability of isometric force control but its relative influence on force amplitude decreases with increments of force level.

Quantifying the history dependency of muscle recovery from a fatiguing intermittent task

Muscle fatigue and recovery are complex processes influencing muscle force generation capacity. While fatigue reduces this capacity, recovery acts to restore the unfatigued muscle state. Many factors can potentially affect muscle recovery, and 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. We examined the dependency of muscle recovery subsequent to four different histories of fatiguing muscle contractions, imposed using two cycle times (30 and 60s) during low to moderate levels (15% and 25% of maximum voluntary contraction (MVC)) of intermittent static exertions involving index finger abduction. MVC and low-frequency electrical stimulation (LFES) measures (i.e., magnitude, rise and relaxation rates) of muscle capacity were used, all of which indicated a dependency of muscle recovery 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. These results imply that the post-exercise rate of recovery is primarily influenced by the immediate post-exercise muscle contractile status (estimated by MVC and LFES measures). Such results may help improve existing models of muscle recovery, facilitating more accurate predictions of localized muscle fatigue development and thereby helping to enhance muscle performance and reduce the risk of injury.

Adopting an external focus of attention alters intracortical inhibition within the primary motor cortex.

AimAlthough it is well established that an external (EF) compared to an internal (IF) or neutral focus of attention enhances motor performance, little is known about the underlying neural mechanisms. This study aimed to clarify whether the focus of attention influences not only motor performance but also activity of the primary motor cortex (M1) when executing identical fatiguing tasks of the right index finger (first dorsal interosseous). Transcranial magnetic stimulation (TMS) at intensities below motor threshold was applied over M1 to assess and compare the excitability of intracortical inhibitory circuits.MethodsIn session 1, 14 subjects performed an isometric finger abduction at 30% of their maximal force to measure the time to task failure (TTF) with either an IF or EF. In session 2, the same task was performed with the other focus. In sessions 3 and 4, subthreshold TMS (subTMS) and paired‐pulse TMS were applied to the contralateral M1 to compare the activity of cortical inhibitory circuits within M1 during EF and IF.ResultsWith an EF, TTF was significantly prolonged (P = 0.01), subTMS‐induced electromyographical suppression enhanced (P = 0.001) and short‐interval intracortical inhibition (SICI) increased (P = 0.004).ConclusionThe level of intracortical inhibition was previously shown to influence motor performance. Our data shed new light on the ability to instantly modulate the activity of inhibitory circuits within M1 by changing the type of attentional focus. The increased inhibition with EF might contribute to the better movement efficiency, which is generally associated with focusing externally.

Availability of vision and tactile gating: vision enhances tactile sensitivity.

A multitude of events bombard our sensory systems at every moment of our lives. Thus, it is important for the sensory and motor cortices to gate unimportant events. Tactile suppression is a well-known phenomenon defined as a reduced ability to detect tactile events on the skin before and during movement. Previous experiments (Buckingham et al. in Exp Brain Res 201(3):411-419, 2010; Colino et al. in Physiol Rep 2(3):e00267, 2014) found detection rates decrease just prior to and during finger abduction and decrease according to the proximity of the moving effector. However, what effect does vision have on tactile gating? There is ample evidence (see Serino and Haggard in Neurosci Biobehav Rev 34:224-236, 2010) observing increased tactile acuity when participants see their limbs. The present study examined how tactile detection changes in response to visual condition (vision/no vision). Ten human participants used their right hand to reach and grasp a cylinder. Tactors were attached to the index finger and the forearm of both the right and left arm and vibrated at various epochs relative to a "go" tone. Results replicate previous findings from our laboratory (Colino et al. in Physiol Rep 2(3):e00267, 2014). Also, tactile acuity decreased when participants did not have vision. These results indicate that the vision affects the somatosensation via inputs from parietal areas (Konen and Haggard in Cereb Cortex 24(2):501-507, 2014) but does so in a reach-to-grasp context.

Distinct Corticocortical Contributions to Human Precision and Power Grip.

The corticospinal tract contributes to the control of finger muscles during precision and power grip. The involvement of different sets of cortical interneuronal circuits during these distinct grasping behaviors remains unknown. To examine this question in humans we used noninvasive transcranial magnetic stimulation (TMS) over the hand representation of the primary motor cortex to elicit motor evoked potentials (MEPs) in an intrinsic finger muscle during index finger abduction (control task), precision grip, and power grip. The TMS coil was oriented to induce currents in the brain in the latero-medial (LM), posterior-anterior (PA), and anterior-posterior (AP) direction to preferentially activate corticospinal axons directly and early and late synaptic inputs to corticospinal neurons, respectively. We found that AP-LM MEP latency differences were consistently longer during power grip compared with index finger abduction and precision grip, while PA-LM differences remained similar across tasks. Short-interval intracortical facilitation, targeting AP but not PA inputs, increased during power grip compared with other tasks. Our novel findings suggest that cortical structures activated by PA and AP stimuli are differentially active during precision and power grip. We propose that a preferential recruitment of late synaptic inputs to corticospinal neurons may be achieved when humans perform a power grip.

Maximal intermittent contractions of the first dorsal interosseous inhibits voluntary activation of the contralateral homologous muscle.

The aim of this study was to investigate how maximal intermittent contractions for a hand muscle influence cortical and reflex activity, as well as the ability to voluntarily activate, the homologous muscle in the opposite limb. Twelve healthy subjects (age: 24 ± 3 years, all right hand dominant) performed maximal contractions of the dominant limb first dorsal interosseous (FDI), and activity of the contralateral FDI was examined in a series of experiments. Index finger abduction force, FDI EMG, motor evoked potentials and heteronomous reflexes were obtained from the contralateral limb during brief non-fatiguing contractions. The same measures, as well as the ability to voluntarily activate the contralateral FDI, were then assessed in an extended intermittent contraction protocol that elicited fatigue. Brief contractions under non-fatigued conditions increased index finger abduction force, FDI EMG, and motor evoked potential amplitude of the contralateral limb. However, when intermittent maximal contractions were continued until fatigue, there was an inability to produce maximal force with the contralateral limb (~30%) which was coupled to a decrease in the level of voluntary activation (~20%). These declines were present without changes in reflex activity, and regardless of whether cortical or motor point stimulation was used to assess voluntary activation. It is concluded that performing maximal intermittent contractions with a single limb causes an inability of the CNS to maximally drive the homologous muscle of the contralateral limb. This was, in part, mediated by mechanisms that involve the motor cortex ipsilateral to the contracting limb.

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 &amp; 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 &amp; Nussbaum, 2015b), facilitating more accurate predictions of localized muscle fatigue development (Rashedi &amp; Nussbaum, 2015a), and thereby helping to enhance muscle performance and reduce the risk of injury