Effects of task and age on the magnitude and structure of force fluctuations: insights into underlying neuro-behavioral processes

Alpha band oscillations in common synaptic input are explanatory of the complexity of isometric knee extensor muscle torque signals

We investigated whether the strength of oscillations in common synaptic input was explanatory of knee extensor (KE) torque signal complexity during fresh and fatigued submaximal isometric contractions, in adults aged from 18 to 90 years. The discharge times of motor units were derived from the vastus lateralis muscle of 60 participants using high-density surface EMG, during 20 s isometric KE contractions at 20% of maximal voluntary contraction, performed before and after a fatiguing repeated isometric KE contraction protocol at 60% of maximal voluntary contraction. Within-muscle coherence Z-scores were estimated using frequency-domain coherence analysis, and muscle torque complexity was assessed using multiscale entropy analysis and detrended fluctuation analysis. Alpha band (5-15 Hz) coherence was found to predict 23.1% and 31.4% of the variance in the complexity index under 28-scales (CI-28) and detrended fluctuation analysis α complexity metrics, respectively, during the fresh contractions. Delta, alpha and low beta band coherence were significantly increased due to fatigue. Fatigue-related changes in alpha coherence were significantly predictive of the fatigue-related changes in CI-28 and detrended fluctuation analysis α. The fatigue-related increase in sample entropy from scales 11 to 28 of the multiscale entropy analysis curves was significantly predicted by the increase in the alpha band coherence. Age was not a contributory factor to the fatigue-related changes in within-muscle coherence and torque signal complexity. These findings indicate that the strength of alpha band oscillations in common synaptic input can explain, in part, isometric KE torque signal complexity and the fatigue-related changes in torque signal complexity.

Entropy in Electroencephalographic Signals Modulates with Force Magnitude During Grasping - A Preliminary Report

The ability to hold objects relies on neural processes underlying grip force control during grasping. Brain activity lateralized to contralateral hemisphere averaged over trials is associated with grip force applied on an object. However, the involvement of neural variability within-trial during grip force control remains unclear. We examined dependence of neural variability over frontal, central, and parietal regions of interest (ROI) on grip force magnitude using noninvasive electroencephalography (EEG). We utilized our existing EEG dataset comprised of healthy young adults performing an isometric force control task, cued to exert 5, 10, or 15% of their maximum voluntary contraction (MVC) across trials and received visual feedback of their grip force. We quantified variability in EEG signal via sample entropy (sequence-dependent) and standard deviation (sequence-independent measure) over ROI. We found lateralized modulation in EEG sample entropy with force magnitude over central electrodes but not over frontal or parietal electrodes. However, modulation was not observed for standard deviation in the EEG activity. These findings highlight lateralized and spatially constrained modulation in sequence-dependent, but not sequence-independent component of EEG variability. We contextualize these findings in applications requiring finer precision (e.g., prosthesis), and propose directions for future studies investigating role of neural entropy in behavior.

Lateralized Neural Entropy modulates with Grip Force during Precision Grasping

When holding a coffee mug filled to the brim, we strive to avoid spilling the coffee. This ability relies on the neural processes underlying the control of finger forces on a moment-to-moment basis. The brain activity lateralized to the contralateral hemisphere averaged over a trial and across the trials is known to be associated with the magnitude of grip force applied on an object. However, the mechanistic involvement of the variability in neural signals during grip force control remains unclear. In this study, we examined the dependence of neural variability over the frontal, central, and parietal regions assessed using noninvasive electroencephalography (EEG) on grip force magnitude during an isometric force control task. We hypothesized laterally specific modulation in EEG variability with higher magnitude of the grip force exerted during grip force control. We utilized an existing EEG dataset (64 channel) comprised of healthy young adults, who performed an isometric force control task while receiving visual feedback of the force applied. The force magnitude to be exerted on the instrumented object was cued to participants during the task, and varied pseudorandomly among 5, 10, and 15% of their maximum voluntary contraction (MVC) across the trials. We quantified neural variability via sample entropy (sequence-dependent measure) and standard deviation (sequence-independent measure) of the temporal EEG signal over the frontal, central, and parietal electrodes. The EEG sample entropy over the central electrodes showed lateralized, nonlinear, localized, modulation with force magnitude. Similar modulation was not observed over frontal or parietal EEG activity, nor for standard deviation in the EEG activity. Our findings highlight specificity in neural control of grip forces by demonstrating the modulation in sequence-dependent but not sequence-independent component of EEG variability. This modulation appeared to be lateralized, spatially constrained, and functionally dependent on the grip force magnitude. We discuss the relevance of these findings in scenarios where a finer precision is essential to enable grasp application, such as prosthesis and associated neural signal integration, and propose directions for future studies investigating the mechanistic role of neural entropy in grip force control.

Altered Bimanual Kinetic and Kinematic Motor Control Capabilities in Older Women

Older women may experience critical neuromuscular impairments interfering with controlling successful bimanual motor actions. Our study aimed to investigate altered bimanual motor performances in older women compared with younger women by focusing on kinetic and kinematic motor properties. Twenty-two older women and 22 younger women performed bimanual kinetic and kinematic motor tasks. To estimate bimanual kinetic functions, we calculated bimanual maximal voluntary contractions (i.e., MVC) and force control capabilities (i.e., mean force, accuracy, variability, and regularity of the total force produced by two hands) during bimanual hand-grip submaximal force control tasks. For bimanual kinematic performances, we assessed the scores of the Purdue Pegboard Test (i.e., PPT) in both hands and assembly tasks, respectively. For the bimanual MVC and PPT, we conducted an independent t-test between two groups. The bimanual force control capabilities were analyzed using two-way mixed ANOVAs (Group × Force Level; 2 × 2). Our findings revealed that the older women showed less bimanual MVC (p = 0.046) and submaximal force outputs (p = 0.036) and greater changes in bimanual force control capabilities as indicated by a greater force variability (p = 0.017) and regularity (p = 0.014). Further, the older women revealed lower scores of PPT in both the hands condition (p < 0.001) and assembly task condition (p < 0.001). The additional correlation analyses for the older women showed that lower levels of skeletal muscle mass were related to less bimanual MVC (r = 0.591; p = 0.004). Furthermore, a higher age was related to lower scores in the bimanual PPT assembly task (r = -0.427; p = 0.048). These findings suggested that older women experience greater changes in bimanual motor functions compared with younger women.

Unilateral hand force control impairments in older women

Older women may experience deficits in sensorimotor control at their upper limb because of aging progress compromising the motor system. This study aimed to investigate whether younger and older women differ in sensorimotor capabilities assessed by unilateral force control performances at a lower targeted force level. Twenty-one older and 21 younger women performed isometric unilateral force control tasks at 10 % of maximum voluntary contraction for each hand, respectively. Purdue Pegboard Test (PPT) was used to measure unilateral hand dexterity. Five force control variables (i.e., maximal and submaximal force, force error, variability, and regularity) and PPT scores were analyzed in two-way mixed ANOVAs (Group × Hand Condition), respectively. The absolute force power was analyzed in three-way mixed ANOVA (Group × Hand Condition × Frequency Band). The findings revealed that older women produced less maximal and submaximal unilateral forces than in younger women. Greater variability, regularity, and force frequency oscillations below 4 Hz were observed in older women as compared with those in younger women. Force error in the dominant hand was greater in older women than those in younger women. Finally, older women showed lower PPT scores than younger women. These findings suggested that older women may have deficits in unilateral force control capabilities as well as motor dexterity.

Low-Frequency Oscillations and Force Control Capabilities as a Function of Force Level in Older Women

Force variability is potentially related to altered low-frequency oscillations in motor outputs. This study examines the contributions of low-frequency oscillations in force to altered force control performances from lower to higher targeted force levels in older women. Fourteen older women executed unilateral hand-grip force control tasks at 10% and 40% of maximum voluntary contraction (MVC). Force control performances were estimated by calculating force accuracy (root-mean-square-error), force variability (standard deviation), and force regularity (approximate entropy). We additionally quantified low-frequency oscillations in force using absolute powers across four different frequency bands: (a) 0–0.5 Hz, (b) 0.5–1.0 Hz, (c) 1.0–1.5 Hz, and (d) 1.5–2.0 Hz. The findings reveal that from lower to higher targeted force level older women show greater force error, force variability, and force regularity with increased values of absolute power in force across the four frequency bands. The multiple regression models identified a significant relationship between greater force frequency power below 0.5 Hz and more impairments in force control performances. These findings suggest that force frequency oscillation below 0.5 Hz is a key predictor indicating altered stability of task performances across different targeted force levels in older women.

Establishing Task-Relevant MVC Protocols for Modelling Sustained Isometric Force Variability: A Manual Control Study

The present study examined how prevalent methods for determining maximal voluntary contraction (MVC) impact the experimentally derived functions of graded force-force variability. Thirty-two young healthy subjects performed continuous isometric force tracking (20 s trials) at 10 target percentages (5-95% MVC) normalized to a conventional discrete-point (n = 16), or sustained (n = 16) MVC calculation. Distinct rates and magnitudes of change were observed for absolute variability (standard deviation (SD), root mean squared error (RMSE)), tracking error (RMSE, constant error (CE)), and complexity (detrended fluctuation analysis (DFA)) (all p < 0.05) of graded force fluctuations between the MVC groups. Differential performance strategies were observed beyond ~65% MVC, with the discrete-point group minimizing their SD at force values below that of the criterion target (higher CE/RMSE). Moreover, the sustained group's capacity to minimize SD/RMSE/CE corresponded to a more complex structure in their force fluctuations. These findings reveal that the time component of MVC estimation has a direct influence on the corrective strategies supporting near-maximal manual force control. While discrete MVC protocols predominate in the study of manual strength/endurance/precision, a 1:1 MVC-task mapping appears more to be ecologically valid if visuo-motor precision outcomes are of central importance.

Visual feedback improves bimanual force control performances at planning and execution levels

The purpose of this study was to determine the effect of different visual conditions and targeted force levels on bilateral motor synergies and bimanual force control performances. Fourteen healthy young participants performed bimanual isometric force control tasks by extending their wrists and fingers under two visual feedback conditions (i.e., vision and no-vision) and three targeted force levels (i.e., 5%, 25%, and 50% of maximum voluntary contraction: MVC). To estimate bilateral motor synergies across multiple trials, we calculated the proportion of good variability relative to bad variability using an uncontrolled manifold analysis. To assess bimanual force control performances within a trial, we used the accuracy, variability, and regularity of total forces produced by two hands. Further, analysis included correlation coefficients between forces from the left and right hands. In addition, we examined the correlations between altered bilateral motor synergies and force control performances from no-vision to vision conditions for each targeted force level. Importantly, our findings revealed that the presence of visual feedback increased bilateral motor synergies across multiple trials significantly with a reduction of bad variability as well as improved bimanual force control performances within a trial based on higher force accuracy, lower force variability, less force regularity, and decreased correlation coefficients between hands. Further, we found two significant correlations in (a) increased bilateral motor synergy versus higher force accuracy at 5% of MVC and (b) increased bilateral motor synergy versus lower force variability at 50% of MVC. Together, these results suggested that visual feedback effectively improved both synergetic coordination behaviors across multiple trials and stability of task performance within a trial across various submaximal force levels.

Different unilateral force control strategies between athletes and non-athletes

This study investigated continuous visuomotor tracking capabilities between athletes and non-athlete controls using isometric force control paradigm. Nine female athletes and nine female age-matched controls performed unilateral hand-grip force control tasks with their dominant and non-dominant hands at 10% and 40% of maximal voluntary contraction (MVC), respectively. Three conventional outcome measures on force control capabilities included mean force, force accuracy, and force variability, and we additionally calculated two nonlinear dynamics variables including force regularity using sample entropy and force stability using maximal Lyapunov exponent. Finally, we performed correlation analyses to determine the relationship between nonlinear dynamics variables and conventional measures for each group. The findings indicated that force control capabilities as indicated by three conventional measures were not significantly different between athlete and non-athlete control groups. However, the athletes revealed less force regularity and greater force stability across hand conditions and targeted force levels than those in non-athlete controls. The correlation analyses found that increased force regularity (i.e., less sample entropy values) at 10% of MVC and decreased force regularity (i.e., greater sample entropy values) at 40% of MVC were significantly related to improved force accuracy and variability for the athlete group, and these patterns were not observed in the non-athlete control group. These findings suggested that the athletes may use different adaptive force control strategies as indicated by nonlinear dynamics tools.

Complexity of Knee Extensor Torque: Effect of Aging and Contraction Intensity

ABSTRACT

Fiogbé, E, Vassimon-Barroso, V, Catai, AM, de Melo, RC, Quitério, RJ, Porta, A, and Takahashi, ACdM. Complexity of knee extensor torque: effect of aging and contraction intensity. J Strength Cond Res 35(4): 1050-1057, 2021-Assessing the knee extensors' torque complexity in older adults is relevant because these muscles are among the most involved in functional daily activities. This study aimed to investigate the effects of aging and isometric contraction intensity on knee extensor torque complexity. Eight young (24 ± 2.8 years) and 13 old adults (63 ± 2.8 years) performed 3 maximal (maximum voluntary contraction [MVC], duration = 10 seconds) and submaximal isometric contractions (SICs, targeted at 15, 30, and 40% of MVC, respectively) of knee extensors. Torque signals were sampled continuously, and the metrics of variability and complexity were calculated basing on the SIC torque data. The coefficient of variation (CV) was used to quantify the torque variability. The torque complexity was determined by calculating the corrected approximate entropy (CApEn) and sample entropy (SampEn) and its normalized versions (NCApEn and NSampEn). Young subjects produced greater isometric torque than older adults, and the CV was similar between both groups except at the highest force level (40% MVC) where young subjects' value was higher. The major novel finding of this investigation was that although the knee extensor torque complexity is reduced in older adults, its relationship with contraction intensity is similar to young subjects. This means that despite the age-related decrease of the interactions between the components of the neuromuscular system, the organization of force control remains preserved in older adults, at least up to just below the force midrange.

Effects of force level and task difficulty on force control performance in elderly people

As the proportion of people over 60 years of age rises continuously in westernized societies, it becomes increasingly important to better understand aging processes and how to maintain independence in old age. Fine motor tasks are essential in daily living and, therefore, necessary to maintain. This paper extends the existing literature on fine motor control by manipulating the difficulty of a force maintenance task to characterize performance optima for elderly. Thirty-seven elderly (M = 68.00, SD = 4.65) performed a force control task at dynamically varying force levels, i.e. randomly changing every 3 s between 10%, 20%, and 30% of the individual's maximum voluntary contraction (MVC). This task was performed alone or with one or two additional tasks to increase task difficulty. The force control characteristics accuracy, variability, and complexity were analyzed. Lowest variability was observed at 20%. Accuracy and complexity increased with increasing force level. Overall, increased task difficulty had a negative impact on task performance. Results support the assumption, that attention control has a major impact on force control performance in elderly people. We assume different parameters to have their optimum at different force levels, which remain comparably stable when additional tasks are performed. The study contributes to a better understanding of how force control is affected in real-life situations when it is performed simultaneously to other cognitive and sensory active and passive tasks.

Longitudinal Assessment and Functional Neuroimaging of Movement Variability Reveal Novel Insights Into Motor Dysfunction in Clinical High Risk for Psychosis

Motor dysfunction in youth at clinical high risk (CHR) for psychosis is thought to reflect abnormal neurodevelopment within cortical-subcortical motor circuits and may be important for understanding clinical trajectories of CHR individuals. However, to date, our perspective of brain-behavior relationships has been informed solely by cross-sectional correlational studies linking behavior in the lab to brain structure or respective resting-state network connectivity. Here, we assess movement dysfunction from 2 perspectives: study 1 investigates the longitudinal progression of handwriting variability and positive symptoms in a sample of 91 CHR and healthy controls during a 12-month follow-up and study 2 involves a multiband functional magnetic resonance imaging task exploring the relationship between power grip force stability and motor network brain activation in a subset of participants. In study 1, we found that greater handwriting variability was a stable feature of CHR participants who experienced worse symptom progression. Study 2 results showed that CHR individuals had greater variability in their grip force and greater variability was related to decreased activation in the associative cortico-striatal network compared to controls. Motor variability may be a stable marker of vulnerability for psychosis risk and possible indicator of a vulnerable cortico-striatal brain network functioning in CHR participants, although the effects of antipsychotic medication should be considered.

How Age, Cognitive Function and Gender Affect Bimanual Force Control

Coordinated bimanual control depends on information processing in different intra- and interhemispheric networks that differ with respect to task symmetry and laterality of execution. Aging and age-related cognitive impairments, but also sex can have detrimental effects on connectivity of these networks. We therefore expected effects of age, cognitive function and sex on bimanual force coordination. We furthermore expected these effects to depend on the characteristics of the task (i.e., difficulty and symmetry). 162 right handed participants (19 younger adults [YA], 21–30 years, 9 females; 52 cognitively healthy older adults [HOA], 80–91 years, 32 females; and 91 older adults with mild cognitive impairments [MCI] 80–91 years, 37 females) performed isometric bimanual force control tasks that required following constant or alternating (cyclic sine-wave) targets and varied in symmetry, i.e., (i) constant symmetric, asymmetric [with constant left and alternating right (ii) or vice versa (iii)], (iv) alternating in- and (v) alternating antiphase (both hands alternating with 0° or 180° relative phase, respectively). We analyzed general performance (time on target), bimanual coordination as coupling between hands (linear correlation coefficient) and structure of variability (i.e., complexity measured through detrended fluctuation analysis). Performance and coupling strongly depended on task symmetry and executing hand, with better performance in symmetric tasks and in asymmetric tasks when the left hand produced a constant and the right hand an alternating force. HOA and MCI, compared to YA, showed poorer performance (time on target) and reduced coupling in in- and antiphase tasks. Furthermore, both groups of OA displayed less complex structure in alternating force production tasks, a marker of reduced control. In addition, we found strong sex effects with females displaying reduced coupling during in- and antiphase coordination and less complex variably structure in constant force production. Results of this study revealed strong effects of age, but also sex on bimanual force control. Effects depended strongly on task symmetry and executing hand, possibly due to different requirements in interhemispheric information processing. So far, we found no clear relationship between behavioral markers of bimanual force control and age-related cognitive decline (compared to healthy aging), making further investigation necessary.

Effects of dual-task demands on the complexity and task performance of submaximal isometric handgrip force control

PURPOSE

To determine the effect of cognitive-motor dual-task load on temporal structure irregularity (complexity) of motor output and task performance of submaximal isometric contractions.

METHODS

Twelve young, sedentary subjects performed handgrip isometric contractions until failure at 50% of maximal voluntary contraction under mathematical self-regulated dual-task (own pace; SDT), regulated dual-task (imposed pace; RDT), and control. Force signal complexity was calculated by sample entropy at the initial, middle, and final thirds. Task performance was assessed by muscle fatigue (time to failure and rate of median frequency of the radial flexor of the carpus), force and math task error, and self-perceived difficulty.

RESULTS

Only RDT decreased complexity with respect to control (17.4% ± 12.6%, p = 0.005), all conditions decreased complexity by the final third (Control: 52.8% ± 18.7%, p < 0.001; SDT: 41.1% ± 32.1%, p = 0.003; RDT: 19.1% ± 21.9%, p = 0.035). Conditions did not affect time to failure, and only RDT decreased the rate of median frequency (0.1%/s ± 0.1%/s, p = 0.020). Inferior force error rate was increased by conditions (SDT: 1.5% ± 0.8%, p < 0.001; RDT: 2% ± 1.5%, p = 0.002). Math error was only augmented by RDT (from 9.9 ± 6.7 to 51.7 ± 18.8, p < 0.001), categorized as "very hard" in 85.7% of participants (p = 0.015).

CONCLUSION

Only the RDT condition reduced complexity and neuromuscular fatigue while increasing force error rate of the handgrip's motor output, without affecting time to failure. A highly demanding dual-task may become a strategy to modify the organization of the hand force motor output, which may contribute to its motor adaptations.

Directed Connectivity in Large-scale Brain Networks for Precision Grip Force Control

Precision grip requires accurate but dynamic control of the magnitude and direction of fingertip forces. It is still not well known whether directed information flow across activated cortical regions could change with different force outputs during precision grip. This study aimed to investigate the directed connectivity in large-scale brain networks for precision grip force control. Eight healthy volunteers participated in the experiment. Totally 32 channel electroencephalography (EEG) signals were recorded during a precision grip task that requires accurate force control following a dynamically changed visual target. The force target changed between 10% and 1% Maximal Voluntary Contraction (MVC). Horizontal visibility graph transfer entropy (HVG-TE) was used to measure the directed connectivity between pairs of EEG channels. In addition, the relative HVG-TE (rHVG-TE) was applied to evaluate the activation of each EEG channel. Results showed relatively higher rHVG-TE values in the posterior brain region but lower rHVG-TE values in the anterior brain region within the whole spectrum (4-70Hz), indicating a posterior-to-anterior information flow. This study revealed that the activation of posterior region is higher than the anterior region. There is a directed functional connectivity of cortex from posterior to anterior region for precision grip force control. This study shed light on how to quantify activation and large-scale connectivity of cortical regions, and reveal the in-depth central mechanisms for peripheral fine motor control.

Digit Force Controls and Corresponding Brain Activities in Finger Pressing Performance: A Comparison Between Older Adults and Young Individuals

This study aims toward an investigation and comparison of the digital force control and the brain activities of older adults and young groups during digital pressing tasks. A total of 15 young and 15 older adults were asked to perform force ramp tasks at different force levels with a custom pressing system. Near-infrared spectroscopy was used to collect the brain activities in the prefrontal cortex and primary motor area. The results showed that the force independence and hand function of the older adults were worse than that of the young adults. The cortical activations in the older adults were higher than those in the young group during the tasks. A significant hemodynamic between-group response and mild negative correlations between brain activation and force independence ability were found. Older adults showed poor force independence ability and manual dexterity and required additional brain activity to compensate for the degeneration.

Performance, complexity and dynamics of force maintenance and modulation in young and older adults

The present study addresses how task constraints and aging influence isometric force control. We used two tasks requiring either force maintenance (straight line target force) or force modulation (sine-wave target force) around different force levels and at different modulation frequencies. Force levels were defined relative the individual maximum voluntary contraction. A group of young adults (mean age ± SD = 25 ± 3.6 years) and a group of elderly (mean age = 77 ± 6.4 years) took part in the study. Age- and task-related effects were assessed through differences in: (i) force control accuracy, (ii) time-structure of force fluctuations, and (iii) the contribution of deterministic (predictable) and stochastic (noise-like) dynamic components to the expressed behavior. Performance-wise, the elderly showed a pervasive lower accuracy and higher variability than the young participants. The analysis of fluctuations showed that the elderly produced force signals that were less complex than those of the young adults during the maintenance task, but the reverse was observed in the modulation task. Behavioral complexity results suggest a reduced adaptability to task-constraints with advanced age. Regarding the dynamics, we found comparable generating mechanisms in both age groups for both tasks and in all conditions, namely a fixed-point for force maintenance and a limit-cycle for force modulation. However, aging increased the stochasticity (noise-driven fluctuations) of force fluctuations in the cyclic force modulation, which could be related to the increased complexity found in elderly for this same task. To our knowledge this is the first time that these different perspectives to motor control are used simultaneously to characterize force control capacities. Our findings show their complementarity in revealing distinct aspects of sensorimotor adaptation to task constraints and age-related declines. Although further research is still needed to identify the physiological underpinnings, the used task and methodology are shown to have both fundamental and clinical applications.

Gender differences in nonlinear motor performance following concussion

PURPOSE

To quantify differences in nonlinear aspects of performance on a seated visual-motor tracking task between clinically asymptomatic males and females with and without a self-reported mild traumatic brain injury history.

METHODS

Seventy-three individuals with a self-reported concussion history (age: 21.40 ± 2.25 years, mean ± SD) and 75 without completed the visual-motor tracking task (age: 21.50 ± 2.00 years). Participants pressed an index finger against a force sensor, tracing a line across a computer screen (visual-motor tracking). The produced signal's root-mean-square error (RMSE), sample entropy (SampEn, a measure of regularity), and average power (AvP) between 0 and 12 Hz were calculated.

RESULTS

Males with a history of 0 or 1 concussion had greater RMSE (worse performance) than females with 0 (p < 0.0001) and 1 concussion (p = 0.052). Additionally, females with 2+ concussions exhibited lower SampEn than females with no history (p = 0.001) or a history of 1 concussion (p = 0.026). Finally, females with 2+ concussions had lower 8-12 Hz AvP than males with 2+ concussions (p = 0.031). Few differences were observed in the male participants.

CONCLUSION

Females with a self-reported history of multiple concussions exhibited lower SampEn in the visual-motor tracking-task force output structure as compared to those with no reported history of concussion and their male counterparts. Lower SampEn and lower power between 8 and 12 Hz indicated persistent impairment in visual processing and feed-forward or predictive motor control systems.

Improved Neural Control of Movements Manifests in Expertise-Related Differences in Force Output and Brain Network Dynamics

It is well-established that expertise developed through continuous and deliberate practice has the potential to delay age-related decline in fine motor skills. However, less is known about the underlying mechanisms, that is, whether expertise leads to a higher performance level changing the initial status from which age-related decline starts or if expertise-related changes result in qualitatively different motor output and neural processing providing a resource of compensation for age-related changes. Thus, as a first step, this study aims at a better understanding of expertise-related changes in fine motor control with respect to force output and respective electrophysiological correlates. Here, using a multidimensional approach, we investigated fine motor control of experts and novices in precision mechanics during the execution of a dynamic force control task. On the level of force output, we analyzed precision, variability, and complexity. We further used dynamic mode decomposition (DMD) to analyze the electrophysiological correlates of force control to deduce brain network dynamics. Experts’ force output was more precise, less variable, and more complex. Task-related DMD mean mode magnitudes within the α-band at electrodes over sensorimotor relevant areas were reduced in experts, and lower DMD mean mode magnitudes related to the force output in novices. Our results provide evidence for expertise dependent central adaptions with distinct and more complex organization and decentralization of sensorimotor subsystems. Results from our multidimensional approach can be seen as a step forward in understanding expertise-related changes and exploiting their potential as resources for healthy aging.

Novel Behavioral and Neural Evidences for Age-Related changes in Force complexity

This study investigated age-related changes in behavioral and neural complexity for a polyrhythmic movement, which appeared to be an exception to the loss of complexity hypothesis. Young (n = 15; age = 24.2 years) and older (15; 68.1 years) adults performed low-level force-tracking with isometric index abduction to couple a compound sinusoidal target. Multiscale entropy (MSE) of tracking force and inter-spike interval (ISI) of motor unit (MU) in the first dorsal interosseus muscle were assessed. The MSE area of tracking force at shorter time scales of older adults was greater (more complex) than that of young adults, whereas an opposite trend (less complex for the elders) was noted at longer time scales. The MSE area of force fluctuations (the stochastic component of the tracking force) were generally smaller (less complex) for older adults. Along with greater mean and coefficient of ISI, the MSE area of the cumulative discharge rate of elders tended to be lower (less complex) than that of young adults. In conclusion, age-related complexity changes in polyrhythmic force-tracking depended on the time scale. The adaptive behavioral consequences could be multifactorial origins of the age-related impairment in rate coding, increased discharge noises, and lower discharge complexity of pooled MUs.

Concussion history is negatively associated with visual-motor force complexity: evidence for persistent effects on visual-motor integration

OBJECTIVES

Long-term monitoring of concussion recovery requires time- and cost-effective methods. Physiologic complexity may be useful in evaluating visual-motor integration following concussion. The purpose of this study was to quantify the extent to which prior number of concussions influenced visual-motor tracking force complexity.

METHODS

Thirty-five individuals with a self-reported concussion history (age: 20.92 ± 1.98) and 15 without (age: 20.92 ± 2.21) performed an isometric visual-motor tracking task, using index finger force to trace a straight line across a computer screen. Finger force root mean square error (RMSE), multi-scale complexity, and average power from 0 to 12 Hertz (Hz) were calculated. Individual multiple regressions were fit to these outcomes.

RESULTS

Force complexity decreased linearly with an increasing number of concussions (R² = 0.101). Males had more complex force overall (R² = 0.219) and greater 4-8 Hz average power (R² = 0.193). The 8-12 Hz average power decreased significantly for individuals with prior loss of consciousness (LOC) and increasing numbers of concussions (R² = 0.143).

CONCLUSION

Individuals exhibited linear decreases in visual-motor tracking force complexity with increasing numbers of concussions, influenced by both gender and a history of LOC. These findings indicate cumulative changes in the ways in which previously concussed individuals process and integrate visual information to guide behaviour.

Variations in Static Force Control and Motor Unit Behavior with Error Amplification Feedback in the Elderly

Error amplification (EA) feedback is a promising approach to advance visuomotor skill. As error detection and visuomotor processing at short time scales decline with age, this study examined whether older adults could benefit from EA feedback that included higher-frequency information to guide a force-tracking task. Fourteen young and 14 older adults performed low-level static isometric force-tracking with visual guidance of typical visual feedback and EA feedback containing augmented high-frequency errors. Stabilogram diffusion analysis was used to characterize force fluctuation dynamics. Also, the discharge behaviors of motor units and pooled motor unit coherence were assessed following the decomposition of multi-channel surface electromyography (EMG). EA produced different behavioral and neurophysiological impacts on young and older adults. Older adults exhibited inferior task accuracy with EA feedback than with typical visual feedback, but not young adults. Although stabilogram diffusion analysis revealed that EA led to a significant decrease in critical time points for both groups, EA potentiated the critical point of force fluctuations [Formula: see text], short-term effective diffusion coefficients (Ds), and short-term exponent scaling only for the older adults. Moreover, in older adults, EA added to the size of discharge variability of motor units and discharge regularity of cumulative discharge rate, but suppressed the pooled motor unit coherence in the 13-35 Hz band. Virtual EA alters the strategic balance between open-loop and closed-loop controls for force-tracking. Contrary to expectations, the prevailing use of closed-loop control with EA that contained high-frequency error information enhanced the motor unit discharge variability and undermined the force steadiness in the older group, concerning declines in physiological complexity in the neurobehavioral system and the common drive to the motoneuronal pool against force destabilization.

Differences in grip force control between young and late middle-aged adults

Grip force control is a crucial function for human to guarantee the quality of life. To examine the effects of age on grip force control, 10 young adults and 11 late middle-aged adults participated in visually guided tracking tasks using different target force levels (25, 50, and 75% of the subject's maximal grip force). Multiple measures were used to evaluate the tracking performance during force rising phase and force maintenance phase. The measurements include the rise time, fuzzy entropy, mean force percentage, coefficient of variation, and target deviation ratio. The results show that the maximal grip force was significantly lower in the late middle-aged adults than in the young adults. The time of rising phase was systematically longer among late middle-aged adults. The fuzzy entropy is a useful indicator for quantitating the force variability of the grip force signal at higher force levels. These results suggest that the late middle-aged adults applied a compensatory strategy that allow allows for sufficient time to reach the required grip force and reduce the impact of the early and subtle degenerative changes in hand motor function.

Age-specific neural strategies to maintain motor performance after an acute social stress bout

Stress due to cognitive demands and fatigue have shown to impair motor performance in older adults; however, the effect of social stress and its influence on prefrontal cortex (PFC) functioning in older adults during upper extremity motor performance tasks is not known. The present study explored the after-effects of an acute social stress bout on neural strategies, measured using PFC and hand/arm muscle activation, and adopted by younger and older adults to maintain handgrip force control. Nine older [74.1 (6.5) years; three men, six women] and ten younger [24.2 (5.0) years, four men, six women] adults performed handgrip force control trials at 30% maximum voluntary contractions before and after the Trier Social Stress Test (TSST). PFC activity was measured using functional near infrared spectroscopy and muscle activity from the flexor and extensor carpi radialis (FCR/ECR) was measured using electromyography. In general, aging was associated with decreased force steadiness and force complexity with a concomitant increase in bilateral PFC activity. While motor performance remained comparable before and after the TSST stress session in both age groups, the associated neural strategies differed between groups. While the stress condition was associated with lower FCR and ECR activity in younger adults despite no change in the PFC activation, stress was associated with increases in FCR activity in older adults. This stress-related compensatory neural strategy of increasing hand/arm muscle activation, potentially via the additional recruitment of the stress-motor neural circuitry, may have played a role in maintaining motor performance in older adults.

Dynamical signatures of isometric force control as a function of age, expertise, and task constraints

From the conceptual and methodological framework of the dynamical systems approach, force control results from complex interactions of various subsystems yielding observable behavioral fluctuations, which comprise both deterministic (predictable) and stochastic (noise-like) dynamical components. Here, we investigated these components contributing to the observed variability in force control in groups of participants differing in age and expertise level. To this aim, young (18-25 yr) as well as late middle-aged (55-65 yr) novices and experts (precision mechanics) performed a force maintenance and a force modulation task. Results showed that whereas the amplitude of force variability did not differ across groups in the maintenance tasks, in the modulation task it was higher for late middle-aged novices than for experts and higher for both these groups than for young participants. Within both tasks and for all groups, stochastic fluctuations were lowest where the deterministic influence was smallest. However, although all groups showed similar dynamics underlying force control in the maintenance task, a group effect was found for deterministic and stochastic fluctuations in the modulation task. The latter findings imply that both components were involved in the observed group differences in the variability of force fluctuations in the modulation task. These findings suggest that between groups the general characteristics of the dynamics do not differ in either task and that force control is more affected by age than by expertise. However, expertise seems to counteract some of the age effects.NEW & NOTEWORTHY Stochastic and deterministic dynamical components contribute to force production. Dynamical signatures differ between force maintenance and cyclic force modulation tasks but hardly between age and expertise groups. Differences in both stochastic and deterministic components are associated with group differences in behavioral variability, and observed behavioral variability is more strongly task dependent than person dependent.

Age-related changes in force control under different task contexts

We investigated age-related differences in motor behavior under different task contexts of isometric force control. The tasks involved rapid force production and force maintenance, either separately or in combination. For the combined context, we used Fitts-like tasks, in which we scaled either the force level (D manipulation, i.e., manipulation of the amplitude of the force to be produced) or the tolerance range (W manipulation, i.e., manipulation of the target width in which force is allowed to fluctuate). We studied two age groups and analyzed mainly variables that quantify behavioral variability (SD), information processing (signal-to-noise ratio and efficiency functions), and age-related slowing (slowing ratio). For rapid force control, age-related differences were more pronounced when preplanned processes were primarily involved, that is, in the rapid force production and Fitts-D manipulation tasks. Further, older adults were comparable to the younger adults in terms of end-point variability at the cost of being slower and more variable in timing. For force maintenance control, requiring mainly online control, age-related differences were the most visible in the stabilized phase of Fitts-D manipulation, followed by Fitts-W manipulation for SD, and then the force maintenance task. In sum, our findings reveal an age-related reorganization of how preplanned and online control processes are combined under different force control contexts. Indeed, both behavioral slowing and the overreliance on online control processes seem to be dependent on the task. In this respect, beyond the study of force control, we show the interest of investigating age effects using functionally different tasks.

Shoulder pain and time dependent structure in wheelchair propulsion variability

Manual wheelchair propulsion places considerable repetitive mechanical strain on the upper limbs leading to shoulder injury and pain. While recent research indicates that the amount of variability in wheelchair propulsion and shoulder pain may be related. There has been minimal inquiry into the fluctuation over time (i.e. time-dependent structure) in wheelchair propulsion variability. Consequently the purpose of this investigation was to examine if the time-dependent structure in the wheelchair propulsion parameters are related to shoulder pain. 27 experienced wheelchair users manually propelled their own wheelchair fitted with a SMARTWheel on a roller at 1.1m/s for 3min. Time-dependent structure of cycle-to-cycle fluctuations in contact angle and inter push time interval was quantified using sample entropy (SampEn) and compared between the groups with/without shoulder pain using non-parametric statistics. Overall findings were, (1) variability observed in contact angle fluctuations during manual wheelchair propulsion is structured (Z=3.15;p<0.05), (2) individuals with shoulder pain exhibited higher SampEn magnitude for contact angle during wheelchair propulsion than those without pain (χ(2)(1)=6.12;p<0.05); and (3) SampEn of contact angle correlated significantly with self-reported shoulder pain (rs (WUSPI) =0.41;rs (VAS)=0.56;p<0.05). It was concluded that the time-dependent structure in wheelchair propulsion may provide novel information for tracking and monitoring shoulder pain.

Performing Isometric Force Control in Combination with a Cognitive Task: A Multidimensional Assessment

Introduction

We used a multidimensional approach to study isometric force control in single and dual-task conditions.

Methods

Multiple measures of performance, efficiency, variability, and structural interference were calculated at low and higher force levels under single (force maintenance) and dual-task (force maintenance and reaction time) conditions.

Results

Reaction time and signal-to-noise ratio were larger in the dual-task conditions. They were also greater for the higher force condition, while sample entropy was lower. Perturbation analyses revealed smaller relative amplitude of downward perturbations for the higher force level.

Discussion

Attentional effort and efficiency are positively related when force level increases, and inversely related to entropy. These relations were presumably mediated by attentional investment. Behavioral perturbations show that attentional resources and structural interference models are not mutually exclusive to account for dual-task situation. Overall, the present study highlights the interest of a multidimensional assessment of force control.