Isometric Force Task Research Articles

Young and middle-aged rats exhibit isometric forelimb force control deficits in a model of early-stage Parkinson's disease

Deficits in manual motor control often accompany the early stages of Parkinson's disease (PD), and are often revealed through isometric force tasks. In order to determine whether similar deficits occur in a rat model of early-stage PD, young (8 months) and middle-aged (18 months) rats were trained to produce sustained press-hold-release isometric forelimb responses that allowed for analyses of force output and spectral analysis of forelimb stability and tremor. Rats then received a 6-hydroxydopamine (6-OHDA) infusion into the striatum contralateral to the trained forelimb and were tested for 4 weeks post-lesion. The resulting partial striatal dopamine depletions (which at 41 ± 12% and 43 ± 6% in young and middle-aged rats, respectively, did not differ between the two groups) resulted in isometric forelimb deficits. Specifically, rats exhibited significantly diminished force stability and increased high frequency (10–25 Hz) tremor, indicating potential postural disturbances and increased postural tremor, respectively. Durations of press-hold-release bouts were also increased post-lesion, suggesting difficulty in task disengagement. Despite pre-lesion differences in some of the force measures, the effects of partial nigrostriatal DA depletion did not differ between the two age groups. These results support the use of the press-while-licking task in preclinical studies modeling isometric force control deficits in PD.

Combined measures of movement and force variability distinguish Parkinson’s disease from essential tremor

Objective To examine whether behavioral and electrophysiological measures of motor performance accurately differentiate Parkinson’s disease (PD) and essential tremor (ET). Methods Twenty-four patients (12 PD; 12 ET) performed isometric force, ballistic movements, and tremor tasks. Receiver operating characteristic (ROC) analyses were conducted on all dependent measures that were significantly different between the two patient groups. Results Patients with PD were more impaired on measures of movement deceleration than ET. Patients with ET were more impaired on measures of force variability than PD. ROC analyses revealed that sensitivity and specificity were excellent when combining measures during the isometric force task (torque rise time and force variability; 92% sensitivity and 92% specificity; AUC = 0.97). When combining measures across the force and movement tasks, the ROC analysis revealed improved sensitivity and specificity (force variability and peak deceleration; 92% sensitivity and 100% specificity; AUC = 0.99). Conclusions Combining measures of force variability and movement deceleration accurately differentiate patients with PD from those with ET with high sensitivity and specificity. Significance If validated in a larger sample, these measures can serve as markers to confirm the diagnosis of PD or ET and thus, enhance decision making for appropriate treatments for patients with these respective diseases.

Dependence of asymmetrical interference on task demands and hand dominance in bimanual isometric force tasks

This study investigated the asymmetry of bilateral interference in a bimanual isometric force pulse task and the relation of the degree of interference with the asymmetry of the force levels and the hand dominance. One hand produced force pulses with the same peak force target, while the other hand produced different peak forces in blocked conditions with force target ratios between the hands that ranged from 1:1 to 16:1. There was asymmetric interference between the hands in that the hand performing the same peak forces showed stronger (i.e., higher bias and variable error) interference with the hand performing the different peak force than vice versa. The force-time properties also correlated more strongly when the different peak forces were generated by the left non-dominant than the right dominant hand. With increasing peak force ratios, the extent of interference became stronger and plateaued around the force ratio of 8:1 indicating a boundary condition to the asymmetric interference between hands. The results extend to bimanual isometric force control the dependence of bilateral asymmetric interference on task demands and hand dominance and provide further evidence on the degree of bilateral interference with task asymmetry.

Visual information gain and the regulation of constant force levels

Visual information is essential in human motor control, and especially in the continuous modulation of isometric force. The gain of visual feedback, that is, the amount of space used to represent change in force, has been shown to affect both the magnitude and time-dependent properties of variability in the force output. However, little is known regarding the interacting effects of visual gain and target force level on force variability and whether the effects of force level can be mediated by a gain that is adjusted to force level. We examined the effect of different types and levels of visual feedback gain and target force level (1, 2, 4, 8, and 12 N) on the magnitude (standard deviation, SD) and regularity (approximate entropy, ApEn) of isometric force variability. Young adults performed an isometric force task with high and low levels of constant (same gain level for all forces) and scaled (proportional to force level) gain. The magnitude of force variability increased exponentially as a function of force level once the SD was corrected for the limits of the display area. The time-dependent properties of force variability remained constant across force levels when gain was adjusted to force level. These findings suggest that the time-dependent properties of force variability are the result an interaction between visual feedback and task force level demands, while the increases in SD over force levels are primarily due to the invariant properties of human muscle and the motor system.

Strength Training Does Not Affect the Accuracy of Force Gradation in an Isometric Force Task in Young Men

The aim of this study is to investigate potential differences in fine motor control between strength trained (ST) and non-strength trained (NT) individuals. By use of an isometric force production task, two groups, 20 ST (mean age 25.6, SD 4.9) and 19 NT (mean age 24.1, SD 2.9) male individuals, were measured on the ability to control forces with the muscles of their index finger. The maximum voluntary contraction (MVC) was higher in ST than in NT group. Error and SD of the signal increased with increasing force levels in both groups. Despite higher absolute force levels of the ST group, SD and Error were the same in both groups for a given MVC percentage. Signal to noise ratio values showed inverted U-shapes for both groups, with no significant differences between groups on various force levels. The power spectral density analysis disclosed significant differences between groups, with more power in the lowest frequency band (1 - 6 Hz) for the ST group. It is concluded that strength training of arm muscles has no negative effect on the stability of sustained contractions of finger muscles. No evidence was found to support the notion of a loss in accuracy as a result of strength training.

Contrasting Interpretations of the Nonuniform Distribution of Preferred Directions Within Primary Motor Cortex

to the editor: A recent paper by Georgopoulos and colleagues ([Naselaris et al. 2006][1]) revisits the influential and controversial issue of population coding within primary motor cortex (M1). Although researchers uniformly recognize that neural activity in M1 often covaries with the direction of

Task-dependent organization of pinch grip forces

The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb (F(t)) and index finger (F(i)) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant's maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F(t) = F(i) was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F(t) = F(i) coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands.

Neural Correlates of Internal-Model Loading

Skilled object manipulation requires knowledge, or internal models, of object dynamics relating applied force to motion , and our ability to handle myriad objects indicates that the brain maintains multiple models . Recent behavioral studies have shown that once learned, an internal model of an object with novel dynamics can be rapidly recruited and derecruited as the object is grasped and released . We used event-related fMRI to investigate neural activity linked to grasping an object with recently learned dynamics in preparation for moving it after a delay. Subjects also performed two control tasks in which they either moved without the object in hand or applied isometric forces to the object. In all trials, subjects received a cue indicating which task to perform in response to a go signal delivered 5-10 s later. We examined BOLD responses during the interval between the cue and go and assessed the conjunction of the two contrasts formed by comparing the primary task to each control. The analysis revealed significant activity in the ipsilateral cerebellum and the contralateral and supplementary motor areas. We propose that these regions are involved in internal-model recruitment in preparation for movement execution.

Pathological muscle activation patterns in patients with massive rotator cuff tears, with and without subacromial anaesthetics

A mechanical deficit due to a massive rotator cuff tear is generally concurrent to a pain-induced decrease of maximum arm elevation and peak elevation torque. The purpose of this study was to measure shoulder muscle coordination in patients with massive cuff tears, including the effect of subacromial pain suppression. Ten patients, with MRI-proven cuff tears, performed an isometric force task in which they were asked to exert a force in 24 equidistant intervals in a plane perpendicular to the humerus. By means of bi-polar surface electromyography (EMG) the direction of the maximal muscle activation or principal action of six muscles, as well as the external force, were identified prior to, and after subacromial pain suppression. Subacromial lidocaine injection led to a significant reduction of pain and a significant increase in exerted arm force. Prior to the pain suppression, we observed an activation pattern of the arm adductors (pectoralis major pars clavicularis and/or latissimus dorsi and/or teres major) during abduction force delivery in eight patients. In these eight patients, adductor activation was different from the normal adductor activation pattern. Five out of these eight restored this aberrant activity (partly) in one or more adductor muscles after subacromial lidocaine injection. Absence of glenoid directed forces of the supraspinate muscle and compensation for the lost supraspinate abduction torque by the deltoideus leads to destabilizating forces in the glenohumeral joint, with subsequent upward translation of the humeral head and pain. In order to reduce the superior translation force, arm adductors will be co-activated at the cost of arm force and abduction torque. Pain seems to be the key factor in this (avoidance) mechanism, explaining the observed limitations in arm force and limitations in maximum arm elevation in patients suffering subacromial pathologies. Masking this pain may further deteriorate the subacromial tissues as a result of proximal migration of the humeral head and subsequent impingement of subacromial tissues.

Parietal Area 5 Activity Does Not Reflect the Differential Time-Course of Motor Output Kinetics During Arm-Reaching and Isometric-Force Tasks

Many single-neuron recording studies have examined the degree to which the activity of primary motor cortex (M1) neurons is related to the kinematics and kinetics of various motor tasks. This has not been explored as extensively for arm movement-related neurons in posterior parietal cortex area 5. We recorded the activity of 78 proximal arm-related neurons in area 5 of two monkeys while they used their whole arm to make reaching movements toward eight targets on a horizontal plane against an inertial load or to generate isometric forces at the hand in the same eight horizontal directions. The overall range of measured output forces was similar in the two tasks. The forces increased monotonically in the desired direction in the isometric task. In the movement task, in contrast, they showed a rapid initial increase in the direction of movement, followed by a transient reversal of forces as the hand approached the target. Many task-related area 5 neurons were tuned for the direction of motor output in the tasks, but most area 5 neurons were more strongly active or exclusively active in the movement task than in the isometric task. Furthermore, their activity at either the single cell or population level did not reflect the transient reversal of output forces during movement. In contrast, M1 neuronal activity was typically strong in both tasks and showed task-related changes that reflected the differences in the time course and directionality of force outputs between both tasks, including the transient reversal of forces in the movement task. These results show that area 5 neurons are less strongly related to the time-course of task kinetics than M1 during isometric and arm-movement tasks.

Functional Activity of Superior Head of Human Lateral Pterygoid Muscle during Isometric Force

There is controversy as to the jaw tasks for which the superior head of the human lateral pterygoid muscle (SHLP) becomes active. The aim was to describe the functional activities of SHLP single motor units (SMUs) during horizontal isometric force tasks. In 11 subjects, 48 SMUs were recorded from computer-tomography-verified SHLP sites during generation of horizontal isometric force in the contralateral (CL), protrusive (P), and ipsilateral (IL) directions and intermediate directions (CL-P, IL-P). In eight subjects, SHLP SMUs were active in CL, CL-P, and P. Qualitatively, SHLP EMG activity increased with increased isometric force. Forty-two SMUs were active in directions other than IL; 6 exhibited activity at IL and other directions. The similarity of these data to previous human lateral pterygoid (IHLP) data supports the notion that SHLP and IHLP should be regarded as a single muscle, with activities shaded according to the biomechanical demands of the task.

Children with spastic hemiplegia are equally able as controls in maintaining a precise percentage of maximum force without visually monitoring their performance

In this study the hypothesis was tested that children with spastic hemiplegia rely more on externally guided visual feedback when trying to keep force constant with their affected hand (AH) as compared to their non-affected hand (NAH) and as compared to controls. An isometric force task in which a cursor had to be moved to a visually specified target that disappeared half way the task, was performed by 19 children with cerebral palsy (CP), spastic hemiplegia, aged between 5 and 16 years and an aged matched control group. It was found that the absolute deterioration of performance after withdrawal of target visualization did differ between AH, NAH and controls. The absolute error was smaller and the variability was larger in the hemiplegic hand. However, the normalized force error and co-efficient of variation increased similarly between groups. Furthermore, power spectrum density analysis of the force signal showed that both hands in both groups had a similar loss in the energy in the 2–3 Hz range when target visualization was removed. These results suggest that CP children are equally able to produce stable force without visually monitoring their performance than children without CP, provided they are allowed to operate within their own force range.

Development of unimanual versus bimanual task performance in an isometric task

Sixty-three children between 5 and 12 years of age and 15 adults performed a unimanual and a bimanual isometric force task. The performance of the preferred hand in the unimanual task was compared to the performance of the preferred hand in the bimanual task. It was hypothesized that in the bimanual task the absolute error will be higher, there will be more irregularity and the participants will need more time due to the additional effort from the central nervous system, especially with respect to the communication between the hemispheres. Furthermore, in younger children bimanual force variability was expected to be higher due to developmental aspects concerning callosal maturation and attention. It was found that with respect to force generation the preferred hand was not affected by bilateral isometric force generation, but with respect to force regulation it was. The coefficient of variation (CV) of the force was 34% larger in the bimanual task as compared to the unimanual task. For the time to target force, the increase was 28%. With repetition of the trials the CV decreased in the bimanual task, but only in the youngest age group. During development there was no change in absolute error, yet there was a major reduction in force variability in the bimanual task. It is suggested that improvement in interhemispheric communication and in the ability to focus attention plays a role in the decrease in variability with age.

Perceived effort in force production as reflected in motor-related cortical potentials

Objective: The perceived effort in force production was investigated in a series of experiments where subjects performed isometric force tasks with the index finger while the nominal force, the rate of force development and signal gain were controlled and rating of effort for each task was obtained. The hypotheses tested were that: (1) force-related perceived effort may selectively influence the amplitude of motor-related cortical potentials (MRCP); and (2) the MRCP may directly reflect the intensity of perceived effort associated with force production. Methods: The force trace was displayed on a computer monitor using various control-gains so that the perceived effort matched or was at odds with actual muscular effort applied to the load cell to accomplish the task. The MRCP were extracted from continuous EEG records using averaging techniques. Results: The findings showed that: (1) perceived effort proportionally increased with the increment of rate of force development and force error, but not with the actual force level; (2) the amplitude of the MRCP increased when a large amount of force was accompanied by an increased rate of force development; (3) the amplitude of early components of MRCP preceding the force initiation (MP-100 to 0) increased as a function of anticipated effort, whereas, the amplitude movement-monitoring potentials (MMP) accompanying the force production increased as a function of actual force level. Conclusions: The findings from this study provide additional insight clarifying the distinct patterns of EEG activity exhibited under various degrees of perceived effort associated with force output. The findings support the hypothesis that the early components of MRCP may reflect the perceived effort associated with achieving the required force level. Significance: The results from this study may be considered in the larger context of physical activity in terms of importance of perceived effort during prescribed exercise in rehabilitation programs.

Excitability Changes of Motor Evoked Potentials Dependent on Muscle Properties and Contraction Modes

Using transcranial magnetic stimulation (TMS), differences in the excitability changes of motor evoked potentials (MEPs) between isometric (force task) and isotonic (movement task) muscle contractions in a distal (first dorsal interosseous; FDI) and a proximal (middle deltoid; MD) muscle were studied. In the FDI muscle, the active threshold of MEP recruitment was significantly lower in the isotonic than that in the isometric muscle contraction in spite of identical background EMG activity levels. Additionally, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction at low EMG activity levels, but the difference disappeared beyond middle EMG activity levels. In the MD muscle, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction, and further this dependence was kept at all muscle contraction levels. These results indicate that the dependence of the MEP amplitude on background EMG activity is modulated not only by the different muscle contraction modes (isotonic and isometric), but also by muscle properties (distal and proximal). Thus, the present findings suggest that the task-specific extra excitation in the proximal muscle is definitely produced corresponding to task differences (task-dependent subliminal fringe), which might be explained by the predominant frequency principle if applied to the proximal muscle. On the other hand, the lack of task-dependent extra excitation in the distal muscle is explained by the predominant recruitment principle for force grading in small hand muscles.

Slow motor performance in girls with Turner Syndrome is not related to increased neuromotor noise.

This study examined the relationship between decreased speed-accuracy trade-off and increased neuromotor noise in girls with Turner Syndrome (TS). Fifteen girls with TS and 15 age-matched controls performed isometric force contractions with both index fingers separately at 5 force levels, based on their maximum voluntary contraction. The results revealed that (a) groups did not differ in speed-accuracy tradeoff or neuromotor noise, (b) output-variability increased linearly with force level, (c) signal-to-noise ratio changed according to an inverted U-shaped function, (d) broadening in the frequency profile is highest at the lower force levels, (e) with increasing force level, the power peak in the 0-4 Hz domain dominates, (f) frequency profile broadens more in the dominant hand. These findings suggest that, in girls with TS, motor performance is not diminished in an isometric force task, that motor recruitment is intact, and that neuromotor noise is not increased. The findings are discussed with respect to motor control and neuromotor noise.

Changes in movement characteristics of the spastic upper extremity after botulinum toxin injection

Hurvitz EA, Conti GE, Brown SH. Changes in movement characteristics of the spastic upper extremity after botulinum toxin injection. Arch Phys Med Rehabil 2003;84:444-54. Objective: To examine the longitudinal effects of botulinum toxin injection on movement characteristics of the spastic upper extremity in children by using motor control testing (MCT) techniques and standard clinical measures. Design: Open-label clinical trial. Setting: Motor control laboratory at an academic medical center. Participants: A convenience sample of 9 subjects (5 boys, 4 girls; age range, 7[ndash ]16y) with cerebral injury (stroke or cerebral palsy) and asymmetric upper-extremity function because of spasticity. Eight subjects had right-sided involvement. Interventions: Botulinum toxin injection to the involved upper extremity, involving elbow, wrist, and finger flexors, depending on clinical presentation. Main Outcome Measures: Clinical measures included range of motion (ROM), the Ashworth Scale, FIM[trade ] instrument, Pediatric Evaluation of Disability Inventory, portions of the Bruininks-Oseretsky Test of Motor Proficiency, and the Purdue pegboard. MCT consisted of visually guided reaching, bilateral finger-to-nose movements, hand tapping, and isometric pinch force tasks. Kinematic assessments were made before and at 2, 4, 6, 12, 18, and 24 weeks after botulinum toxin injection. Results: All subjects had increased ROM and decreased Ashworth values throughout the testing period. In motor control tasks, improvement typically occurred earlier in the least complex movements, such as hand tapping, with 6 of 9 subjects showing a maximum, although transient, unilateral tapping speed by 6 weeks. A similar time course was observed for pinch force tasks. Improvement in more complex, forward-reaching tasks occurred much later (week 12 or later) or did not occur at all. As with the hand tasks, improved reach performance declined toward the end of the testing period. All subject showed minimal or no improvement in bilateral finger-to-nose movements. Neither maximum changes in ROM or Ashworth values correlated with improvements in functional elbow extension during sit and reach tasks, with 3 subjects with normal active ROM showing late onset or no change in reach. Conclusions: Although botulinum toxin reduced tone and increased ROM of the spastic upper extremity, the time course and degree of motor improvement appears to depend on the complexity of the task. Future research should focus on the value of adjunct therapy, such as task-specific training, in addition to botulinum toxin treatments to facilitate functional improvement of the spastic upper extremity. [copy ] 2003 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Activation of intrinsic and extrinsic finger muscles in relation to the fingertip force vector.

Surface EMG was recorded from two intrinsic and two extrinsic muscles of the index finger during a two-dimensional isometric force task in the plane of flexion and extension. Subjects applied force isometrically at the fingertip in eight equally spaced directions, encompassing 360 degrees. Target forces spanned the range from 20% to 50% of maximum for each direction. The effect of varying the metacarpophalangeal (MCP) and interphalangeal (IP) joint angles was investigated. We found that when applying isometric force with the fingertip, the intrinsic muscles of the index finger behaved as a single unit whose region of activation overlapped that of the extrinsic flexor and extensor muscles. The activation region of the intrinsic muscles also spanned a range of force directions for which the extrinsic muscles were virtually inactive. The activation of all muscles, with the exception of the extrinsic extensor, was modified by changing the MCP and IP joint angles. Both IP flexion and MCP extension produced rotation of the resultant activity vector in the direction of MCP flexion. However, the relative rotation was much greater with IP flexion than MCP extension. The effect of IP flexion is linked to rotation of the force direction where joint torque switches from extension to flexion, while the effect of MCP extension is more likely related to changes in muscle length and MCP moment arm. Our results suggest that the primary role of intrinsic finger muscles is to precisely control the direction of fingertip force, while extrinsic muscles provide stability of the joints.

Changes in the temporal pattern of primary motor cortex activity in a directional isometric force versus limb movement task.

We recorded the activity of 75 proximal-arm-related cells in caudal primary motor cortex (MI) while a monkey generated either isometric forces or limb movements against an inertial load. The forces and movements were in eight directions in a horizontal plane. The isometric force generated at the hand increased monotonically in the direction of the target force level. The force exerted against the load in the movement task was more complex, including a transient decelerative phase during the movement as the hand approached the target. Electromyographic (EMG) activity of proximal-arm muscles reflected the task-dependent changes in dynamics, showing a ramp increase in activity during the isometric task and a reciprocal triphasic burst pattern in the movement task. A sliding 50-ms window analysis showed that the directionality of the EMG, when expressed in hand-centered spatial coordinates, remained stable throughout the isometric ramp but often showed a significant transient shift during the limb movements. Many cells in M1 showed corresponding significant changes in activity pattern and instantaneous directionality between the two tasks. This momentary dissociation of discharge from the directional kinematics of hand displacement is evidence that the activity of many single proximal-arm related M1 cells is not coupled only to the direction and velocity of hand motion.

Isometric pinch force control and learning in older adults

The learning of a dynamic isometric pinch force task was investigated in young adults (aged 19 to 29 years) and older adults (aged 64 to 75 years) through use of a visuo-motor tracking paradigm. Both groups significantly improved performance over trials, retained what was learned 1 week later, successfully transferred to a new target, and demonstrated interlimb transfer of training effects, reflecting a strong central component to this task that is apparently intact in older adults. However, performance differences between the two groups remained throughout the trials. Although it appeared that older adults were able to utilize a model-based strategy to predict the target path, as evidenced by minimal response lag, their absolute performance was inferior to that of young adults relative to an overall root mean square error score, a correlation coefficient, and their increased use of high-frequency components in the tracking signal. The age-related performance differences may be attributable to a peripheral decrement in tactile sensibility and/or muscle reorganization as well as a slowness in processing afferent information.