Isometric Force Control Research Articles

Muscle activities in similar arms performing identical tasks reveal the neural basis of muscle synergies

Are the muscle synergies extracted from multiple electromyographic signals an expression of neural information processing, or rather a by-product of mechanical and task constraints? To address this question, we asked 41 right-handed adults to perform a variety of motor tasks with their left and right arms. The analysis of the muscle activities resulted in the identification of synergies whose activation was different for the two sides. In particular, tasks involving the control of isometric forces resulted in larger differences. As the two arms essentially have identical biomechanical structure, we concluded that the differences observed in the activation of the respective synergies must be attributed to neural control.

Effects of mechanical assistance on muscle activity and motor performance during isometric elbow flexion

Mechanical assistance on joint movement is generally beneficial; however, its effects on cooperative performance and muscle activity needs to be further explored. This study examined how motor performance and muscle activity are altered when mechanical assistance is provided during isometric force control of ramp-down and hold phases. Thirteen right-handed participants (age: 24.7 ± 1.8 years) performed trajectory tracking tasks. Participants were asked to maintain the reference magnitude of 47 N (REF) during isometric elbow flexion. The force was released to a step-down magnitude of either 75% REF or 50% REF and maintained, with and without mechanical assistance. The ramp-down durations of force release were set to 0.5, 2.5, or 5.0 s. Throughout the experiment, we measured the following: (1) the force output using load cells to compute force variability and overshoot ratio; (2) peak perturbation on the elbow movement using an accelerometer; (3) the surface electromyography (sEMG) from biceps brachii and triceps brachii muscles; and (4) EMG oscillation from the biceps brachii muscle in the bandwidth of 15–45 Hz. Our results indicated that mechanical assistance, which involved greater peak perturbation, demonstrated lower force variability than non-assistance (p < 0.01), while EMG oscillation in the biceps brachii muscle from 15 to 45 Hz was increased (p < 0.05). These findings imply that if assistive force is provided during isometric force control, the central nervous system actively tries to stabilize motor performance by controlling specific motor unit activity in the agonist muscle.

Age-related deficits in bilateral motor synergies and force coordination

BackgroundAgeing may cause impairments in executing bilateral movement control. This study investigated age-related changes in interlimb force coordination across multiple trials by quantifying bilateral motor synergies based on the uncontrolled manifold hypothesis. Participants completed the trials with and without visual feedback.MethodsTwenty healthy individuals (10 older adults and 10 young adults) performed 12 isometric force control trials for the two vision conditions at 5% of maximal voluntary contraction. All dependent variables were analyzed in two-way mixed model (Group × Vision Condition; 2 × 2) ANOVAs with repeated measures on the last factor.ResultsThe analyses revealed that older adults had greater mean force produced by two hands in both vision conditions (i.e., yes and no visual feedback). Across both vision conditions, the older adult group showed greater asymmetrical force variability (i.e., standard deviation of non-dominant hand > standard deviation of dominant hand) and revealed more positive correlation coefficients between forces produced by two hands as compared with the young adult group. Finally, an index of bilateral motor synergies was significantly greater in young adults than older adults when visual feedback was available.ConclusionThe current findings indicate that deficits in interlimb force coordination across multiple trials appeared in older adults.

Functional motor control deficits in older FMR1 premutation carriers.

Individuals with fragile X mental retardation 1 (FMR1) gene premutations are at increased risk for fragile X-associated tremor/ataxia syndrome (FXTAS) during aging. However, it is unknown whether older FMR1 premutation carriers, with or without FXTAS, exhibit functional motor control deficits compared with healthy individuals. The purpose of this study, therefore, was to determine whether older FMR1 premutation carriers exhibit impaired ability to perform functional motor tasks. Eight FMR1 premutation carriers (age: 58.88 ± 8.36 years) and eight age- and sex-matched healthy individuals (60.13 ± 9.25 years) performed (1) a steady isometric force control task with the index finger at 20% of their maximum voluntary contraction (MVC) and; (2) a single-step task. During the finger abduction task, firing rate of multiple motor units of the first dorsal interosseous (FDI) muscle was recorded. Compared with healthy controls, FMR1 premutation carriers exhibited (1) greater force variability (coefficient of variation of force) during isometric force (1.48 ± 1.02 vs. 0.63 ± 0.37%; P = 0.04); (2) reduced firing rate of multiple motor units during steady force, and; (3) reduced velocity of their weight transfer during stepping (156.62 ± 26.24 vs. 191.86 ± 18.83cm/s; P = 0.01). These findings suggest that older FMR1 premutation carriers exhibit functional motor control deficits that reflect either subclinical issues associated with premutations independent of FXTAS, or prodromal markers of the development of FXTAS.

Effects of aging on motor control strategies during bimanual isometric force control

Aging-induced degeneration of the neuromuscular system would result in deteriorated complex muscle force coordination and difficulty in executing daily activities that require both hands. The aim of this study was to provide a basic description of how aging and dual-task activity would affect the motor control strategy during bimanual isometric force control in healthy adults. In total, 17 young adults (aged 25.1 ± 2.4 years) and 14 older adults (aged 72.6 ± 3.4 years) participated in the study. The subjects were instructed to press both hands simultaneously to match the 1 Hz sine curve force under two conditions (with or without calculation) with continuous visual feedback. Differences in bimanual motor synergy, bimanual coordination, force accuracy, force variability, and calculation speed were compared. This study found that the specific motor control strategy of older adults involved a decreased bimanual force control ability with both increased VUCM and VORT, and was not influenced by dual tasking. These findings might have implications for establishing interventions for aging-induced hand force control deficits.

Force Control of Ankle Dorsiflexors in Young Adults: Effects of Bilateral Control and Leg Dominance

We investigated whether bilateral lower-limb control and leg dominance affect force control ability in 15 healthy young adults (9 males and 6 females, age =26.8 ± 4.1 years). Participants performed isometric ankle dorsiflexion force control tasks, matching a visual target (10% of maximal effort) as quickly and precisely as possible in ballistic and tonic tasks. Performance was evaluated using force error, force steadiness, amount of muscle activity of the tibialis anterior, and response time characteristics. Results showed no significant effects of leg dominance during both ballistic and tonic tasks, while bilateral condition resulted in significantly larger error, less force steadiness, compared to unilateral condition, and only during the tonic task. Consequently, bilateral control, specifically in tasks utilizing feedback control (i.e., tonic task) might affect force control ability, possibly because of the interhemispheric inhibition to meet bilateral task complexity and integrate afferent bilateral sensory information from both right and left legs.

Noninvasive brain stimulation over M1 and DLPFC cortex enhances the learning of bimanual isometric force control

Motor learning plays an important role in upper-limb function and the recovery of lost functionality. This study aimed to investigate the relative impact of transcranial direct current stimulation (tDCS) on learning in relation to the left primary motor cortex (M1) and left dorsolateral prefrontal cortex (DLPFC) during bimanual isometric force-control tasks performed with both hands under different task constraints. In a single-blind cross-over design, 20 right-handed participants were randomly assigned to either the M1 group (n = 10; mean age, 22.90 ± 1.66 years, mean ± standard deviation) or the DLPFC group (n = 10; mean age, 23.20 ± 1.54 years). Each participant received 30 min of tDCS (anodal or sham, applied randomly in two experiments) while performing the bimanual force control tasks. Anodal tDCS of the M1 improved the accuracy of maintenance and rhythmic alteration of force tasks, while anodal tDCS of the DLPFC improved only the maintenance of the force control tasks compared with sham tDCS. Hence, tDCS over the left M1 and DLPFC has a beneficial effect on the learning of bimanual force control.

Motor control strategies during bimanual isometric force control among healthy individuals

This study aimed to provide a basic description of the motor control strategy during bimanual isometric force control in healthy young adults. Thirty healthy young adults (mean age: 27.4 ± 3.7 years) participated in the study. The subjects were instructed to press both hands simultaneously to match the target force level of 5%, 25%, and 50% bimanual maximum voluntary force using continuous visual feedback. Bimanual motor synergy and bimanual coordination, as well as force asymmetry, force accuracy, and force variability were compared. This study identified the specific motor control strategy of healthy young adults during bimanual isometric force control, indicating that they proportionally increased “good” and “bad” variabilities, resulting in comparable bimanual motor synergy as the target force level increased.

Right Hemisphere Contributions to Bilateral Force Control in Chronic Stroke: A Preliminary Report

BackgroundBilateral motor control deficits poststroke may be lateralized by hemisphere damage. This preliminary study investigated bilateral force control between left and right hemisphere-damaged groups at baseline and after coupled bilateral movement training with neuromuscular stimulation. MethodsStroke participants (8 left hemisphere and 6 right hemisphere cerebrovascular accidents) performed a bilateral isometric force control task at 3 submaximal force levels (5%, 25%, and 50% of maximum voluntary contraction [MVC]) before and after training. Force accuracy, force variability, and interlimb force coordination were analyzed in 3-way mixed design ANOVAs (2 × 2 × 3; Group × Test Session × Force Level) with repeated measures on test session and force level. ResultsThe findings indicated that force accuracy and variability at 50% of MVC in the right hemisphere-damaged group were more impaired than lower targeted force levels at baseline, and the impairment at the highest target level was improved after coupled bilateral movement training. However, these patterns were not observed in the left hemisphere-damaged group. ConclusionsCurrent findings support a proposition that the right hemisphere presumably contributes to controlling bilateral force production.

Alterations in jaw clenching force control in people with myogenic temporomandibular disorders

Isometric bite force control, via measures of force accuracy, force steadiness and force proprioception, was assessed in patients with myogenic temporomandibular disorders (TMDs) compared to healthy controls. Twelve people with myogenic TMDs and twelve age- and gender-matched asymptomatic controls performed maximal voluntary contractions (MVC) of unilateral jaw clenching followed by submaximal isometric contractions, with and without visual feedback of force, at 10, 30 50% and 70% MVC. Force performance was assessed with indices of accuracy (mean distance, MD) and precision (standard deviation, SD) and reported as a percentage of the MVC. A mixed-effect model was used to evaluate differences in MVC, MD and SD. The MVC was lower in the TMD group when clenching either ipsilateral or contralateral to the side of greatest pain (p < 0.05). No difference in MD was observed between groups. The SD depended on the interaction between group and painful side (p = 0.04) with the TMD group displaying higher SD when executing the task with the most painful side when compared to the ipsilateral or contralateral sides of the control group. The reduced maximal bite force and force steadiness observed in people with myogenic pain may interfere with masticatory function and should be considered when planning therapeutic interventions for TMDs.

Influence of a self-regulated cognitive dual task on time to failure and complexity of submaximal isometric force control.

To determine the effects of performing a self-regulated cognitive dual task on time to failure and neuromuscular force control during submaximal isometric contractions. Fifteen young sedentary males performed isometric contractions at 50% of each individual's maximal voluntary contraction (MVC) under single-task (without cognitive load) and dual-task (with self-regulated mathematical task) conditions. Force signal complexity and biceps brachialis muscle activity were determined at the start, middle, and end of each trial. The slope of the linear regression of median frequency determined the rate of muscle fatigue. Force-task error was established as any amplitude percentage greater or less than 50% MVC. The dual-task condition resulted in a 42s longer time to failure than the single-task condition. EMG amplitude did not differ between conditions. The rate of muscle fatigue was higher in the single-task (- 0.35%/s) than the dual-task (- 0.2%/s) condition. Force signal complexity was, on average, 22% lower in the dual-task condition. The dual-task condition, as compared to the single-task condition, elicited a higher rate of force-task error below (6.37 versus 4.76%) and over (2.11 versus 1.68%) the force threshold. The dual-task condition resulted in a longer time to failure and decreased motor output complexity and fatigue rate when performing a submaximal force task. As the dual task also increased the force-task error, we suggest cognitive dual tasks as a possible strategy for delaying fatigue in sedentary young males when exerting submaximal isometric force, contributing to neuromuscular training when error in force control can be ignored.

Fitts' Law in the Control of Isometric Grip Force With Naturalistic Targets.

Fitts’ law models the relationship between amplitude, precision, and speed of rapid movements. It is widely used to quantify performance in pointing tasks, study human-computer interaction, and generally to understand perceptual-motor information processes, including research to model performance in isometric force production tasks. Applying Fitts’ law to an isometric grip force task would allow for quantifying grasp performance in rehabilitative medicine and may aid research on prosthetic control and design. We examined whether Fitts’ law would hold when participants attempted to accurately produce their intended force output while grasping a manipulandum when presented with images of various everyday objects (we termed this the implicit task). Although our main interest was the implicit task, to benchmark it and establish validity, we examined performance against a more standard visual feedback condition via a digital force-feedback meter on a video monitor (explicit task). Next, we progressed from visual force feedback with force meter targets to the same targets without visual force feedback (operating largely on feedforward control with tactile feedback). This provided an opportunity to see if Fitts’ law would hold without vision, and allowed us to progress toward the more naturalistic implicit task (which does not include visual feedback). Finally, we changed the nature of the targets from requiring explicit force values presented as arrows on a force-feedback meter (explicit targets) to the more naturalistic and intuitive target forces implied by images of objects (implicit targets). With visual force feedback the relation between task difficulty and the time to produce the target grip force was predicted by Fitts’ law (average r2 = 0.82). Without vision, average grip force scaled accurately although force variability was insensitive to the target presented. In contrast, images of everyday objects generated more reliable grip forces without the visualized force meter. In sum, population means were well-described by Fitts’ law for explicit targets with vision (r2 = 0.96) and implicit targets (r2 = 0.89), but not as well-described for explicit targets without vision (r2 = 0.54). Implicit targets should provide a realistic see-object-squeeze-object test using Fitts’ law to quantify the relative speed-accuracy relationship of any given grasper.

Coherence and interlimb force control: Effects of visual gain

Neural coupling across hemispheres and homologous muscles often appears during bimanual motor control. Force coupling in a specific frequency domain may indicate specific bimanual force coordination patterns. This study investigated coherence on pairs of bimanual isometric index finger force while manipulating visual gain and task asymmetry conditions. We used two visual gain conditions (low and high gain = 8 and 512 pixels/N), and created task asymmetry by manipulating coefficient ratios imposed on the left and right index finger forces (0.4:1.6; 1:1; 1.6:0.4, respectively). Unequal coefficient ratios required different contributions from each hand to the bimanual force task resulting in force asymmetry. Fourteen healthy young adults performed bimanual isometric force control at 20% of their maximal level of the summed force of both fingers. We quantified peak coherence and relative phase angle between hands at 0–4, 4–8, and 8–12 Hz, and estimated a signal-to-noise ratio of bimanual forces. The findings revealed higher peak coherence and relative phase angle at 0–4 Hz than at 4–8 and 8–12 Hz for both visual gain conditions. Further, peak coherence and relative phase angle values at 0–4 Hz were larger at the high gain than at the low gain. At the high gain, higher peak coherence at 0–4 Hz collapsed across task asymmetry conditions significantly predicted greater signal-to-noise ratio. These findings indicate that a greater level of visual information facilitates bimanual force coupling at a specific frequency range related to sensorimotor processing.

Effects of Anodal Tdcs Over Left Primary Motor Cortex on Bimanual Isometric Force Control

Effects of Anodal Tdcs Over Left Primary Motor Cortex on Bimanual Isometric Force Control

An examination of lower limb asymmetry in ankle isometric force control

While asymmetries have been observed between the dominant and non-dominant legs, it is unclear whether they have different abilities in isometric force control (IFC). The purpose of this study was to compare ankle IFC between the legs. IFC is important for stabilization rather than object manipulation, and people typically use their non-dominant leg for stabilization tasks. Additionally, studies suggested that a limb can better acquire a motor task when the control mechanism of the task is related to what the limb is specialized for. We hypothesized that the non-dominant leg would better (1) control ankle IFC with speed and accuracy, and (2) acquire an ankle IFC skill through direct learning and transfer of learning. Two participant groups practiced an IFC task using either their dominant or non-dominant ankle. In a virtual environment, subjects moved a cursor to hit 24 targets in a maze by adjusting the direction and magnitude of ankle isometric force with speed (measured by the time required to hit all targets or movement time) and accuracy (number of collisions to a maze wall). Both groups demonstrated similar movement time and accuracy between the dominant and non-dominant limbs before practicing the task. After practice, both groups showed improvement in both variables on both the practiced and non-practiced sides (p < .01), but no between-group difference was detected in the degree of improvement on each side. The ability to control and acquire the IFC skill was similar between the legs, which did not support the brain is lateralized for ankle IFC.

The level of performance stabilization influences motor adaptation on an isometric force control task.

This study compared the effects of two levels of performance stabilization on the adaptation to unpredictable perturbations in an isometric control force task with the goal of controlling 40% of the maximum force. The experiment consisted of pre-exposure and exposure phases. In the pre-exposure two levels of performance stabilization were manipulated: a stabilization group (SG) performed three trials in a row while maintaining 40% of the maximum force for three seconds and an absolute error less than or equal to 5% (i.e., the criteria of performance), and a superstabilization group (SSG) performed six blocks of the same criteria. During the exposure phase, the task was the same as that in the pre-exposure phase; however, it was inserted 9 trials of perturbations when the task goal changed to 60% of the maximum force. We measured the %RMSE, RMS from the biceps and triceps brachii and co-contraction. In the pre-exposure phase, both groups showed similar performance and muscle activity. When exposed to the perturbations, SSG performance more quickly returned to the previous level of accuracy, showed lower muscle activation and demonstrated a greater muscle co-contraction than did SG. The results give support to the adaptive process model on motor learning.

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.

Bilateral synergy as an index of force coordination in chronic stroke.

Dysfunction in bilateral coordination post-stroke is responsible for impaired bilateral movements. This study examined bilateral synergies using the uncontrolled manifold (UCM) approach while individuals in a chronic stage after stroke executed bilateral isometric force control at three submaximal force levels. Nine patients with stroke and nine age-matched healthy controls performed 24 trials of wrist and fingers extension at 5, 25, and 50% of MVC. The UCM findings revealed: (a) decreased bilateral synergies in patients with stroke as compared to controls at 50% of MVC and (b) reduced good variability and increased bad variability components in patients with stroke at 50% of MVC. Moreover, correlation analyses across both groups showed that a reduction of bilateral synergies was related to increased force error at 50% of MVC and an inconsistent force ratio between the two hands across multiple trials correlated with increased good and bad variability at 5% of MVC. Current findings indicate that quantifying bilateral synergies may provide meaningful profiles for estimating impairments as well as improvements of cooperative behaviors between two hands post-stroke.

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.

Preconditioning tDCS facilitates subsequent tDCS effect on skill acquisition in older adults

Functional motor declines that often occur with advancing age—including reduced efficacy to learn new skills—can have a substantial impact on the quality of life. Recent studies using noninvasive brain stimulation indicate that priming the corticospinal system by lowering the threshold for the induction of long-term potentiation–like plasticity before skill training may facilitate subsequent skill learning. Here, we used “priming” protocol, in which we used transcranial direct current stimulation (tDCS) applying the cathode over the primary motor cortex (M1) before the anode placed over M1 during unimanual isometric force control training (FORCEtraining). Older individuals who received tDCS with the cathode placed over M1 before tDCS with the anode placed over M1 concurrent with FORCEtraining showed greater skill improvement and corticospinal excitability increases following the tDCS/FORCEtraining protocol compared with both young and older individuals who did not receive the preceding tDCS with the cathode placed over M1. The results suggested that priming tDCS protocols may be used in clinical settings to improve motor function and thus maintain the functional independence of older adults.