Force-matching Task Research Articles

The senses of active and passive forces at the human ankle joint.

The traditional view of the neural basis for the sense of muscle force is that it is generated at least in part within the brain. Recently it has been proposed that force sensations do not arise entirely centrally and that there is a contribution from peripheral receptors within the contracting muscle. Evidence comes from experiments on thumb flexor and elbow flexor muscles. Here we have studied the sense of force in plantar flexor muscles of the human ankle, looking for further evidence for such a mechanism. The active angle-torque curve was measured for muscles of both legs, and for each muscle, ankle angles were identified on the ascending and descending limbs of the curve where active forces were similar. In a plantar flexion force matching task, subjects were asked to match the force in one foot, generated on the ascending limb of the curve, with force in the other foot, generated on the descending limb. It was hypothesised that despite active forces being similar, the sensation generated in the more stretched muscle should be greater because of the contribution from its peripheral stretch receptors, leading to an overestimation of the force in the stretched muscle. It was found that provided that the comparison was between active forces, there was no difference in the forces generated by the two legs, supporting the central hypothesis for the sense of force. When total forces were matched, including a component of passive force due to muscle stretch, subjects seemed to ignore the passive component. Yet subjects had an acute sense of passive force, provided that the muscles remained relaxed. It was concluded that subjects had two senses, a sense of active force, generated centrally, and a sense of passive force, or perhaps muscle stretch, generated within the muscle itself.

Bimanual elbow robotic orthoses: preliminary investigations on an impairment force-feedback rehabilitation method.

Modern rehabilitation practices have begun integrating robots, recognizing their significant role in recovery. New and alternative stroke rehabilitation treatments are essential to enhance efficacy and mitigate associated health costs. Today’s robotic interventions can play a significant role in advancing rehabilitation. In addition, robots have an inherent ability to perform tasks accurately and reliably and are typically well suited to measure and quantify performance. Most rehabilitation strategies predominantly target activation of the paretic arm. However, bimanual upper-limb rehabilitation research suggests potential in enhancing functional recovery. Moreover, studies suggest that limb coordination and synchronization can improve treatment efficacy. In this preliminary study, we aimed to investigate and validate our user-driven bimanual system in a reduced intensity rehab practice. A bimanual wearable robotic device (BWRD) with a Master–Slave configuration for the elbow joint was developed to carry out the investigation. The BWRD incorporates position and force sensors for which respective control loops are implemented, and offers varying modes of operation ranging from passive to active training. The proposed system enables the perception of the movements, as well as the forces applied by the hemiparetic arm, with the non-hemiparetic arm. Eight participants with chronic unilateral stroke were recruited to participate in a total of three 1-h sessions per participant, delivered in a week. Participants underwent pre- and post-training functional assessments along with proprioceptive measures. The post-assessment was performed at the end of the last training session. The protocol was designed to engage the user in an assortment of static and dynamic arm matching and opposing tasks. The training incorporates force-feedback movements, force-feedback positioning, and force matching tasks with same and opposite direction movements. We are able to suggest identification of impairment patterns in the position-force plot results. In addition, we performed a proprioception evaluation with the system. We set out to design innovative and user immersive training tasks that utilize the BWRD capabilities, and we demonstrate that the subjects were able to cooperate and accomplish the protocol. We found that the Fugl–Meyer and Wolf Motor Function Test (pre to post) measured improvements (15 and 19%, respectively). Recognizing the brevity of the training, we focus our report primarily on the proprioception testing (32% significant improvement, pprop = 0.033) and protocol distinctive features and results. This paper presents the electromechanical features and performance of the BWRD, the testing protocol, and the assessments utilized. Outcome measures and results are presented and demonstrate the successful application and operation of the system.

Neuromuscular deficits after peripheral joint injury: a neurophysiological hypothesis.

In addition to biomechanical disturbances, peripheral joint injuries (PJIs) can also result in chronic neuromuscular alterations due in part to loss of mechanoreceptor-mediated afferent feedback. An emerging perspective is that PJI should be viewed as a neurophysiological dysfunction, not simply a local injury. Neurophysiological and neuroimaging studies have provided some evidence for central nervous system (CNS) reorganization at both the cortical and spinal levels after PJI. The novel hypothesis proposed is that CNS reorganization is the underlying mechanism for persisting neuromuscular deficits after injury, particularly muscle weakness. There is a lack of direct evidence to support this hypothesis, but future studies utilizing force-matching tasks with superimposed transcranial magnetic stimulation may be help clarify this notion.

Altered force perception in stroke survivors with spastic hemiplegia.

To investigate the effect of spasticity and involuntary synergistic activation on force perception during voluntary activation of spastic paretic muscles. Eleven stroke subjects with spastic hemiparesis performed various isometric elbow-flexion force-matching tasks. Subjects were instructed to generate a target reference force with visual feedback using one arm (impaired or non-impaired) and then to produce a force with the other arm to match the magnitude of the reference force without visual feedback. The reference arm was at rest in unilateral exertion trials and maintained contraction in bilateral exertion trials during the matching force-production period. Both force and effort mismatches occurred in most conditions, and there were asymmetries in force perception. When the non-impaired arm was the matching arm, effort and force overestimation occurred, but effort was matched better than force. When the impaired arm was the matching arm, force underestimation and effort overestimation occurred, but force was matched better than effort. No difference in matching performance was found between unilateral and bilateral exertion-matching tasks. Overall, both force and effort misperceptions occur in stroke survivors with spasticity. Spasticity and spastic synergistic activation probably contribute to force and effort misperception during voluntary activation in chronic stroke.

The effect of force sensation on the ability to control muscle force during fatigue condition

BACKGROUND AND OBJECTIVE: Proprioceptive information from muscle receptors is important in force control and disturbed sensation due to fatigue may reduce the precision of force control. The aim of this study was to clarify the relationship between force sensation and force control ability in knee extensor muscles during fatigue. METHODS: Eighteen healthy young volunteers participated in this study. Force matching tasks with and without feedback were performed immediately before and after the fatigue protocol consisting of 20 s isometric contractions. The target force was 10% and 30% of the maximal voluntary contraction force in the pre-fatigue protocol. The accuracy of force control ability was estimated by the root mean square error (RMS error) and average difference (average error) between the target and output force. Surface electromyograms were recorded simultaneously from the quadriceps. RESULTS:In the 30% task, there was a significant interaction effect of fatigue and feedback condition in RMS error and average error. RMS error and average error were degenerated significantly by fatigue in the feedback removal condition, but not in the feedback condition. Though the activation of quadriceps was increased by fatigue, increase of muscle activation was lower in the feedback removal condition than that in the feedback conditions. CONCLUSION: Since participants controlled their force output depending on the force sensation in the feedback removal condition, these results indicate that the decline in force sensation may be a significant factor in the decreases of accuracy in force output during fatigue.

Inhibition of the primary motor cortex can alter one's “sense of effort”: Effects of low-frequency rTMS

Studies using force-matching tasks have suggested that when we feel a “sense of effort,” cortical regions may act to increase motor commands, and thus recruit additional motor units, in order to compensate for the exerted force. We hypothesized that suppressing activity in the primary motor cortex (M1), which is the source of the motor commands, would initiate the same process, and induce the same sense of effort. In a force-matching task, grip force was applied to ‘right’ hand and 10 healthy participants were asked to try to exert the same amount by using ‘left’ hand, with no visual feedback. On some trials, low-frequency, repetitive transcranial magnetic stimulation (lf-rTMS) was used to suppress the M1 and the primary somatosensory cortex (SI) in the left hemisphere, separately. Results showed that participants tended to overestimate the level of exerted force by up to 24%. In contrast, sham stimulation of the M1 and lf-rTMS over the SI did not significantly affect participants’ estimations. Further, the M1 suppression resulted in a 42% reduction in motor-evoked potentials. Thus, the M1 suppression can affect our sense of effort, suggesting that compensatory neural mechanisms that increase the MI activity may play an important role in producing senses of effort.

Loss of sensory attenuation in patients with functional (psychogenic) movement disorders

Functional movement disorders require attention to manifest yet patients report the abnormal movement to be out of their control. In this study we explore the phenomenon of sensory attenuation, a measure of the sense of agency for movement, in this group of patients by using a force matching task. Fourteen patients and 14 healthy control subjects were presented with forces varying from 1 to 3 N on the index finger of their left hand. Participants were required to match these forces; either by pressing directly on their own finger or by operating a robot that pressed on their finger. As expected, we found that healthy control subjects consistently overestimated the force required when pressing directly on their own finger than when operating a robot. However, patients did not, indicating a significant loss of sensory attenuation in this group of patients. These data are important because they demonstrate that a fundamental component of normal voluntary movement is impaired in patients with functional movement disorders. The loss of sensory attenuation has been correlated with the loss of sense of agency, and may help to explain why patients report that they do not experience the abnormal movement as voluntary.

Cortical and spinal mechanisms of task failure of sustained submaximal fatiguing contractions.

In this and the subsequent companion paper, results are presented that collectively seek to delineate the contribution that supraspinal circuits have in determining the time to task failure (TTF) of sustained submaximal contractions. The purpose of this study was to compare adjustments in supraspinal and spinal excitability taken concurrently throughout the performance of two different fatigue tasks with identical mechanical demands but different TTF (i.e., force-matching and position-matching tasks). On separate visits, ten healthy volunteers performed the force-matching or position-matching task at 15% of maximum strength with the elbow flexors to task failure. Single-pulse transcranial magnetic stimulation (TMS), paired-pulse TMS, paired cortico-cervicomedullary stimulation, and brachial plexus electrical stimulation were delivered in a 6-stimuli sequence at baseline and every 2–3 minutes throughout fatigue-task performance. Contrary to expectations, the force-matching task TTF was 42% shorter (17.5±7.9 min) than the position-matching task (26.9±15.11 min; p<0.01); however, both tasks caused the same amount of muscle fatigue (p = 0.59). There were no task-specific differences for the total amount or rate of change in the neurophysiologic outcome variables over time (p>0.05). Therefore, failure occurred after a similar mean decline in motorneuron excitability developed (p<0.02, ES = 0.35–0.52) coupled with a similar mean increase in measures of corticospinal excitability (p<0.03, ES = 0.30–0.41). Additionally, the amount of intracortical inhibition decreased (p<0.03, ES = 0.32) and the amount of intracortical facilitation (p>0.10) and an index of upstream excitation of the motor cortex remained constant (p>0.40). Together, these results suggest that as fatigue develops prior to task failure, the increase in corticospinal excitability observed in relationship to the decrease in spinal excitability results from a combination of decreasing intracortical inhibition with constant levels of intracortical facilitation and upstream excitability that together eventually fail to provide the input to the motor cortex necessary for descending drive to overcome the spinal cord resistance, thereby contributing to task failure.

Asymmetries of bilateral isometric force matching with movement intention and unilateral fatigue

During bilateral coordination, some level of inter-hemispheric remapping (i.e., the congruency between afferent and efferent force signals from both hemispheres) is required. In this case, sensory-motor information is exchanged between the two hemispheres, but it remains unclear whether this information exchange is always equivalent or not, especially in a bilateral isometric force-matching task. We used unilateral fatigue applied to one arm in order to determine whether inter-hemispheric remapping can vary asymmetrically during a bilateral isometric matching task. Because fatigue is considered to bias the sensory-motor system, we hypothesized that if bimanual coordination is modulated solely in function of the state of the sensory-motor system (motor efferences, inter-hemispheric inhibitions, and sensory reafferences), we should not observe any asymmetric effect of fatigue with movement intention (leading vs. matching arm). However, if any other process could participate in the modulation of inter-hemispheric remapping, we should observe an interaction between movement intentions and fatigue on the force produced. We found that, when the leading arm was the non-fatigued arm, participants succeeded in reproducing the force level with their fatigued arm. By contrast, when the leading arm was fatigued, subjects over-estimated the force level produced with their non-fatigued arm. Hence, lateralized fatigue exacerbates an asymmetric behavior that seems modulated by movement intention (leading vs. matching). In other words, when unilateral fatigue is introduced in a bilateral isometric force-matching task, inter-hemispheric remapping is asymmetrical. Intensity levels of motor commands sent to both arms (directly or modulated through inter-hemispheric inhibitions) and sensory reafferences alone cannot explain these observations. Some attentional focus may be not balanced continuously between both arms but may be mainly directed toward the matching arm.

Two sensory channels mediate perception of fingertip force.

In two experiments we examined the ability of humans to exert forces accurately with the fingertips, and to perceive those forces. In experiment 1 participants used visual feedback to apply a range of fingertip forces with the distal pad of the thumb. Participants made magnitude discriminations regarding these forces, and their just noticeable differences were calculated at a series of standards by means of a two-interval, forced-choice tracking paradigm. As the standard increased, participants demonstrated a relative improvement in force discrimination; and the presence of a possible inflection point, at approximately 400 g, suggested that two sensory channels may contribute to performance. If this is the case, the operative channel at low forces is almost certainly the slowly adapting type I (SA-I) channel, while another mechanoreceptor class, the SA-II nail unit, is a plausible mediator of the more accurate performance seen at high force levels. To test this two-channel hypothesis in experiment 2, we hydrated participants' thumbnails in order to reduce nail rigidity and thus prevent stimulation of underlying SA-II mechanoreceptors. This technique was found to reduce sensory accuracy in a force-matching task at high forces (1000 g) while leaving low force matching (100 g) unimpaired. Taken together, these results suggest that two sensory channels mediate the perception of fingertip forces in humans: one channel predominating at low forces (below approximately 400 g) and another responsible for perceiving high forces which is likely mediated by the SA-II nail unit.

Relations among Visual Strategies, Force Fluctuations, and Attention during a Force-Matching Task

Greater understanding of how people use visual information to minimize force fluctuations provides critical insight into visuomotor processing. Visual strategies were examined during a force-matching task with different feedback displays. When only vertical feedback was provided, young healthy participants (N = 20, 9 men) fixated their gaze centrally. When vertical and horizontal visual feedback was provided, participants performed saccades to maintain gaze near the leading edge of the force trace. Performance on a separate attention task assessed visual and motor attention capabilities in the same participants. Selecting the correct saccade trajectory on the attention task was positively correlated with measures predicting performance on the force-matching task. Optimal visual strategies, combined with motor attention, may contribute to minimizing pinch force variability at low force.

Impaired force control in writer's cramp showing a bilateral deficit in sensorimotor integration

Abnormal cortical processing of sensory inputs has been found bilaterally in writer's cramp (WC). This study tested the hypothesis that patients with WC have an impaired ability to adjust grip forces according to visual and somatosensory cues in both hands. A unimanual visuomotor force-tracking task and a bimanual sense of effort force-matching task were performed by WC patients and healthy controls. In visuomotor tracking, WC patients showed increased error, greater variability, and longer release duration than controls. In the force-matching task, patients underestimated, whereas controls overestimated, the force applied in the other hand. Visuomotor tracking and force matching were equally impaired in both the symptomatic and nonsymptomatic hand in WC patients. This study provides evidence of bilaterally impaired grip-force control in WC, when using visual or sense of effort cues. This suggests a generalized subclinical deficit in sensorimotor integration in WC.

Larger plantar flexion torque variability implies less stable balance in the young: An association affected by knee position

The present study examined the association between plantar flexion torque variability during isolated isometric contractions and during quiet bipedal standing. For plantar flexion torque measurements in quiet stance (QS), subjects stood still over a force plate. The mean plantar flexion torque level exerted by each subject in QS (divided by 2 to give the torque due to a single leg) served as the target torque level for right leg force-matching tasks in extended knee (KE) and flexed knee (KF) conditions. Muscle activation levels (EMG amplitudes) of the triceps surae and mean, standard deviation and coefficient of variation of plantar flexion torque were computed from signals acquired during periods with and without visual feedback. No significant correlations were found between EMG amplitudes and torque variability, regardless of the condition and muscle being analyzed. A significant correlation was found between torque variability in QS and KE, whereas no significant correlation was found between torque variability in QS and KF, regardless of vision availability. Therefore, torque variability measured in a controlled extended knee plantar flexion contraction is a predictor of torque variability in the anterior-posterior direction when the subjects are in quiet standing. In other words, larger plantar flexion torque variability in KE (but not in KF) implies less stable balance. The mechanisms underlying the findings above are probably associated with the similar proprioceptive feedback from the triceps surae in QS and KE and poorer proprioceptive feedback from the triceps surae in KF due to the slackening of the gastrocnemii. An additional putative mechanism includes the different torque contributions of each component of the triceps surae in the two knee angles. From a clinical and research standpoint, it would be advantageous to be able to estimate changes in balance ability by means of simple measurements of torque variability in a force matching task.

Functional differences between anatomical regions of the anconeus muscle in humans

This study sought to resolve a longstanding debate of the function of anconeus. Intramuscular and surface electromyography electrodes recorded muscle activity from two regions of anconeus and from typical elbow flexion and extension muscles. Eleven participants performed pronation–supination around the medial and lateral axes of the forearm, elbow flexion–extension in pronation, supination and neutral positions of the forearm, and gripping. Maximal voluntary contractions (MVC) and submaximal (10% MVC) force-matching tasks were completed. Activity varied between longitudinal (AL) and transverse (AT) segments of anconeus. Although both muscle regions were active across multiple directions (including opposing directions), AL was more active during pronation than supination, whereas AT showed no such difference. During pronation, activity of AL and AT was greatest about the lateral forearm axis. AT was more active during elbow extension with the forearm in pronation, whereas AL did not differ between pronated and neutral forearm alignment. These findings are consistent with the proposal that AL makes a contribution to control of abduction of the ulna during forearm pronation. Different effects of forearm position on AL and AT activity during elbow extension may be explained by the anatomical differences between the regions. These data suggest anconeus performs multiple functions at the elbow and forearm and this varies between anatomically distinct regions of the muscle.

The Effect of Transcranial Direct Current Stimulation on Perception of Effort in an Isolated Isometric Elbow Flexion Task

The purported ergogenic actions of transcranial direct current stimulation (tDCS) applied to motor cortex (M1) on force production and perception of effort were investigated using a 10-item numerical rating scale (0-10 NRS) in nonfatiguing bouts of a force-matching task utilizing isometric elbow flexion. Using a crossover design, 12 healthy volunteers received sham, anodal, and cathodal tDCS randomly for 10 min (1.5 mA, 62 μA/cm2) to the left M1 in a double-blind manner. Corticospinal excitability changes were also monitored using transcranial magnetic stimulation (TMS) with surface electromyography (sEMG) to monitor both motor evoked potentials (MEPs) and force-EMG from right m. biceps brachii and m. brachioradialis brachii. No significant differences between the verum and sham stimulation were obtained for elbow flexion maximum voluntary force, perception of effort, or sEMG. There were also no significant differences in MEP changes for the types of tDCS, which is consistent with reports that tDCS excitability effects are diminished during ongoing cognitive and motor activities.

The senses of force and heaviness at the human elbow joint

The present-day view of the neural basis for the senses of muscle force and heaviness is that they are generated centrally, within the brain, from copies of motor commands. A corollary of the motor discharge generates a sense of effort which underlies these sensations. In recent experiments on force and heaviness sensations using thumb flexor muscles, a rather different explanation has been invoked: Subjects were proposed to rely predominantly on inputs of a peripheral origin, in particular, the signals of muscle spindles. The present experiments have been carried out at the elbow joint to determine whether these new ideas apply more widely. The effects of fatigue of elbow flexor muscles have been studied in force and heaviness matching tasks using three exercise regimes, a sustained maximum voluntary contraction (MVC), a maintained contraction of 35 % MVC, and a maintained contraction of 35 % MVC combined with muscle vibration at 80 Hz. In force-matching experiments, subjects were required to contract both arms and while the reference arm generated the target force under visual control, it was matched by the indicator arm without visual feedback. During the 100 % MVC exercise, force in the exercising reference arm fell rapidly to almost a half of its original value over 90 s while force in the indicator did not fall, leading to a significant overestimation of the reference force. During the 35 % MVC exercise, subjects also overestimated the reference force and this persisted at 5 and 10 min after the exercise. When 35 % MVC was combined with vibration, the amount by which the indicator arm overestimated the reference force was significantly reduced. In heaviness matching experiments, subjects could move their arms through a small range. The reference arm was loaded with a weight, and weights were added or removed from the indicator until heaviness felt the same in the two arms. There was a small, but significant fall in the matching weight used after 100 % MVC exercise, that is, the weight held by the fatigued arm felt lighter. The 35 % exercise did not alter heaviness sensation while 35 % MVC exercise with vibration led to a significant reduction in perceived heaviness. To conclude, while the results of these experiments on elbow flexors are not as clear cut as for thumb flexors, the central effort hypothesis falls short, in a number of respects in explaining the data which are able to be interpreted in terms of a peripheral afferent contribution to the senses of force and heaviness.

Interaction of poststroke voluntary effort and functional neuromuscular electrical stimulation

Functional electrical stimulation (FES) may be able to augment functional arm and hand movement after stroke. Poststroke neuroprostheses that incorporate voluntary effort and FES to produce the desired movement must consider how forces generated by voluntary effort and FES combine, even in the same muscle, in order to provide an appropriate level of stimulation to elicit the desired assistive force. The goal of this study was to determine whether the force produced by voluntary effort and FES add together independently of effort or whether the increment in force depends on the level of voluntary effort. Isometric force matching tasks were performed under different combinations of voluntary effort and FES. Participants reached a steady level of force, and while attempting to maintain a constant effort level, FES was applied to augment the force. Results indicate that the increment in force produced by FES decreases as the level of initial voluntary effort increases. Potential mechanisms causing the change in force output are proposed, but the relative contribution of each mechanism is unknown.

Control of Fingertip Forces in Young and Older Adults Pressing against Fixed Low- and High-Friction Surfaces

Mobile computing devices (e.g., smartphones and tablets) that have low-friction surfaces require well-directed fingertip forces of sufficient and precise magnitudes for proper use. Although general impairments in manual dexterity are well-documented in older adults, it is unclear how these sensorimotor impairments influence the ability of older adults to dexterously manipulate fixed, low-friction surfaces in particular. 21 young and 18 older (65+ yrs) adults produced maximal voluntary contractions (MVCs) and steady submaximal forces (2.5 and 10% MVC) with the fingertip of the index finger. A Teflon covered custom-molded splint was placed on the fingertip. A three-axis force sensor was covered with either Teflon or sandpaper to create low- and high-friction surfaces, respectively. Maximal downward forces (Fz) were similar (p = .135) for young and older adults, and decreased by 15% (p<.001) while pressing on Teflon compared to sandpaper. Fluctuations in Fz during the submaximal force-matching tasks were 2.45× greater (p<.001) for older adults than in young adults, and reached a maximum when older adults pressed against the Teflon surface while receiving visual feedback. These age-associated changes in motor performance are explained, in part, by altered muscle activity from three hand muscles and out-of-plane forces. Quantifying the ability to produce steady fingertip forces against low-friction surfaces may be a better indicator of impairment and disability than the current practice of evaluating maximal forces with pinch meters. These age-associated impairments in dexterity while interacting with low-friction surfaces may limit the use of the current generation of computing interfaces by older adults.

I07 Impaired isometric force coordination in Huntington's disease

<h3>Background</h3> Assessment of motor phenotype and diagnosis of Huntington9s Disease (HD) are based on the Unified HD Rating Scale-Total Motor Score, a categorical scale. We have previously shown that impairments in tongue protrusion forces can be seen in premanifest and manifest HD and correlate with clinical severity as assessed by the Unified HD Rating Scale-Total Motor Score and the disease burden score (Reilmann <i>et al.</i> 2010). In this study we aimed to investigate whether similar impairments of force coordination can be found in different force matching tasks. <h3>Objective</h3> To assess whether impairments in isometric force coordination of the upper and lower extremities can be detected in manifest HD gene carriers compared to controls. <h3>Methods</h3> Using a precalibrated force transducer, the ability of subjects with symptomatic HD (n=31) to generate and maintain isometric TPF at different target force levels (1.5, and 5.0 N for thumb, 1.0, 5.0 and 10.0 N for big toe) was assessed and compared with age-matched controls (n=22) in a cross-sectional pilot study. <h3>Results</h3> All paradigms distinguish controls from symptomatic subjects. The static coefficient of variability (%), assessing the degree of deviation from the target force normalised to the mean force, was the most robust measure in all matching trials. Comparisons between tasks and correlations with clinical measures such as the TMS, DBS and TFC will be presented. <h3>Conclusions</h3> Assessment of force matching tasks by hand and feet may provide quantitative objective measures for severity of motor phenotype in symptomatic HD.

Association of Force Steadiness of Plantar Flexor Muscles and Postural Sway during Quiet Standing by Young Adults

This study was conducted to assess the relations of force fluctuations during isometric plantar-flexion and postural sway during quiet standing. Twelve healthy men (M age = 21 yr., SD = 1) performed unilateral plantar flexion measured by a strain gauge force transducer. Participants performed force-matching tasks; sustained plantar flexion for 20 sec. at levels corresponding to 10% and 20% of maximum voluntary contraction with the visual feedback. Also, participants were asked to stand quietly with their eyes open, and then the center of mass displacement and velocity in the anteroposterior were measured. In analysis, postural sway was associated with force fluctuation at only 10% of maximum voluntary contraction. The statistically significant correlation between variables was found only at corresponding contraction intensities for plantar-flexor muscles. From this one may infer neural strategies in plantar-flexor muscles during quiet standing may be characteristics similar to those controlling the plantar-flexion force in young adults.