Bimanual 1:1 with 90° continuous relative phase: difficult or easy!

The purpose of the present experiment was to observe the performance of participants attempting to produce a 1:1 bimanual coordination pattern with 90° relative phase between the arms when feedback concerning the movement of the two limbs was integrated within a Lissajous plot and when this information was withdrawn. One group was paced with an auditory metronome and the other was encouraged to increase frequency when they fell below the goal frequency. We predicted that providing a salient integrated feedback display without a metronome would allow participants to effectively tune-in the goal relative phase pattern within several minutes; instead of several days as typically found in the literature when the metronome was used. The data indicated remarkably effective performances after 5 min of practice when the metronome was not used, with motion of both limbs harmonic in nature, and continuous relative phase errors (~10°) and standard deviation of continuous relative phase (~10°) relatively small. This seems remarkable given that this coordination pattern has proven relatively difficult to perform under normal and Lissajous feedback conditions even after several days of practice. As predicted relative phase errors and variability increased substantially when the metronome was used. When the extrinsic feedback was withdrawn all participants tended to drift from the required 90° relative phase, but the cycle duration variability in the two limbs remained stable and limb motion remained harmonic in nature. The current findings suggest that some of the difficulty typically associated with producing various relative phase patterns is due to the less than optimal perceptual information available in the various testing situations and the use of pacing metronomes.

Observational practice of relative but not absolute motion features in a single-limb multi-joint coordination task

The learning of relative and absolute motion features as a function of physical (actor group) and observational (observer group) practice was examined in a rhythmic single limb multi-joint coordination task. The task required the participants to learn a 90° relative phase pattern between the elbow and wrist in combination with an absolute elbow joint angle of 80° and a wrist joint angle of 48°. Each actor practiced the required relative and absolute motion features for 2 days while being watched by an observer. Overall, the actor group was characterized by an improvement in performance on the relative phase component and showed a clear differentiation in joint amplitudes. In a 24-h retention test, the observer group more closely matched the performance of the actors on the relative phase component in comparison to a control group that was not exposed to physical or observational practice. However, the observer and control groups did not demonstrate a clear differentiation in required joint amplitudes. In agreement with Scully and Newell (1985), we conclude that relative phase may be classified as a relative motion feature that may be picked through observation and benefit initial physical performance, whereas the joint amplitudes may be classified as absolute motion features that require physical practice to achieve the appropriate scaling.

Interjoint coordination during handwriting-like movements

The present study investigates intrinsic preferences and tendencies in coordination of the wrist and finger movements during handwriting-like tasks. Movement of the inkless pen tip in nine right-handed subjects was registered with a digitizer. One circle-drawing task and four line-drawing tasks were included in the experiment. The line-drawing task included: (1) drawing with the wrist only, (2) drawing with the fingers only, (3) an equivalent pattern consisting of the simultaneous flexion/extension of the wrist and fingers, and (4) a nonequivalent pattern in which wrist flexion was accompanied by finger extension and wrist extension was accompanied by finger flexion. Both the line and circle drawing were performed repetitively at four speed levels, ranging from slow to "as fast as possible" movements. The analysis of the line drawing revealed differential variability and temporal characteristics across the four movement patterns. While the equivalent pattern had characteristics of performance similar to those observed in the wrist-only and fingers-only pattern, the nonequivalent pattern was more variable and was executed slower when as fast as possible movement was required, compared to the other three patterns. The circle-drawing task also revealed intrinsic tendencies in coordination of the wrist and fingers. These tendencies were manifested by a spontaneous transition of the circular path of the pen tip to a tilted oval with increases in movement speed. The transition to the oval shape was accompanied by decreases in relative phase between the wrist and finger movements, whereas amplitudes of these movements were not affected by movement speed manipulations. The results suggest that subjects did not display a tendency to decrease the number of joints involved when executing the patterns that required simultaneous wrist and finger movements. Instead, there were preferences during these patterns to integrate wrist and finger movements with low relative phase. The findings are interpreted in terms of biomechanical constraints imposed on the wrist-finger linkage. This interpretation was further examined by testing two left-handed subjects. The data obtained showed symmetrical preferences in joint coordination. Collectively, the findings support a supposition that the shape of cursive letters may have been adjusted to the biomechanical structure of the hand to facilitate the motor act of handwriting.     

The dynamics of isometric bimanual coordination

Eight right-handed subjects performed rhythmic isometric applications of torque in the directions of pronation and supination of the forearm, in single limb and bimanual conditions. Bimanual movements were executed in either in-phase (homologous muscles simultaneously active) or anti-phase (non-homologous muscles active simultaneously) modes of coordination, in self-paced and frequency-scaled conditions. In the inphase (frequency-scaled) condition, subjects were required to synchronise (applications of torque) with each beat of a metronome, either in the direction of pronation or supination. In the anti-phase (frequency-scaled) condition, subjects were required to synchronise (applications of torque) with each beat of the metronome, either to the left or to the right. Departures from the anti-phase mode of coordination were observed as pacing frequency was increased. However, these departures were of short duration and the anti-phase mode was always re-established. These findings are in marked contrast to those obtained when there is free motion of the limbs. There also existed systematic differences between the stability of the pronation and supination phases of torque application. These differences were, in turn, modified through coincidence with the pacing signal. These results are discussed with reference to the constraints imposed upon the coordination dynamics by the intrinsic properties of the neuromuscular-skeletal system.     

Binding of movement, sound and touch: multimodal coordination dynamics

Very little is known about the coordination of movement in combination with stimuli such as sound and touch. The present research investigates the hypothesis that both the type of action (e.g., a flexion or extension movement) and the sensory modality (e.g., auditory or tactile) determine the stability of multimodal coordination. We performed a parametric study in which the ability to synchronize movement, touch and sound was explored over a broad range of stimulus frequencies or rates. As expected, synchronization of finger movement with external auditory and tactile stimuli was successfully established and maintained across all frequencies. In the key experimental conditions, participants were instructed to synchronize peak flexion of the index finger with touch and peak extension with sound (and vice-versa). In this situation, tactile and auditory stimuli were delivered counter-phase to each other. Two key effects were observed. First, switching between multimodal coordination patterns occurred, with transitions selecting one multimodal pattern (flexion with sound and extension with touch) more often than its partner. This finding indicates that the stability of multimodal coordination is influenced by both the type of action and the stimulus modality. Second, at higher rates, transitions from coherent to incoherent phase relations between touch, movement and sound occurred, attesting to the breakdown of multimodal coordination. Because timing errors in multimodal coordination were systematically altered when compared to unimodal control conditions we are led to consider the role played by time delays in multimodal coordination dynamics.     

General coordination of shoulder,elbow and wrist dynamics during multijoint arm movements

Studies of multijoint arm movements have demonstrated that the nervous system anticipates and plans for the mechanical effects that arise from motion of the linked limb segments. The general rules by which the nervous system selects appropriate muscle activities and torques to best deal with these intersegmental effects are largely unknown. In order to reveal possible rules, this study examined the relationship of muscle and interaction torques to joint acceleration at the shoulder, elbow and wrist during point-to-point arm movements to a range of targets in the horizontal plane. Results showed that, in general, dynamics differed between the joints. For most movements, shoulder muscle torque primarily determined net torque and joint acceleration, while interaction torque was minimal. In contrast, elbow and wrist net torque were determined by a combination of muscle and interaction torque that varied systematically with target direction and joint excursion. This "shoulder-centered pattern" occurred whether subjects reached targets using straight or curved finger paths. The prevalence of a shoulder-centered pattern extends findings from a range of arm movement studies including movement of healthy adults, neurological patients, and simulations with altered interaction effects. The shoulder-centered pattern occurred for most but not all movements. The majority of the remaining movements displayed an "elbow-centered pattern," in which muscle torque determined initial acceleration at the elbow and not at the shoulder. This occurred for movements when shoulder excursion was <50% of elbow excursion. Thus, both shoulder- and elbow-centered movements displayed a difference between joints but with reversed dynamics. Overall, these findings suggest that a difference in dynamics between joints is a general feature of horizontal plane arm movements, and this difference is most commonly reflected in a shoulder-centered pattern. This feature fits well with other general shoulder-elbow differences suggested in the literature on arm movements, namely that: (a) agonist muscle activity appears more closely related to certain joint kinematics at the shoulder than at the elbow, (b) adults with neurological damage display less disruption of shoulder motion than elbow motion, and (c) infants display adult-like motion first in the shoulder and last at the wrist.     

Visual–spatial and anatomical constraints interact in a bimanual coordination task with transformed visual feedback

There is a debate in the literature about the influence of spatial and anatomical constraints on bimanual coordination dynamics. In the present experiment, participants swung hand-held pendulums about the wrist while attending to visual feedback about relative phase (superimposed phase plots of each pendulum) that was displayed on a screen. Participants were instructed to maintain in-phase or anti-phase coordination in the visual display. Visual–spatial and anatomical constraints were dissociated by introducing a phase shift in the visual display so that visual feedback differed from the movements being performed by the participants in 15° increments from −180° to +180°. Analysis of mean relative phase and its variability suggested that visual–spatial and anatomical constraints interact in bimanual coordination dynamics.     

Head movements destabilize cyclical in-phase but not anti-phase homologous limb coordination in humans

The present study addressed the role of head movements in the coordination of the homologous upper or lower limbs in supine normal subjects. Consistent with previous research, in-phase mirror symmetrical movements were performed more accurately and consistently than anti-phase movements. However, inclusion of head movements destabilized in-phase but not anti-phase homologous limb coordination, in contrast to previous work demonstrating a higher vulnerability of anti-phase than in-phase coordination to various experimental perturbations. It was observed that the head moved in the same direction as the limbs during anti- but not during in-phase coordination. Furthermore, the interlimb patterns also affected the head rotations that were lower in spatiotemporal consistency and less consistently coupled with the limbs during in-phase than during anti-phase coordination. These findings provide new insights into the coalition of egocentric and allocentric constraints during interlimb coordination.     

Characteristics of instructed and uninstructed interpersonal coordination while walking side-by-side

We examined how people synchronize their leg movements while walking side-by-side on a treadmill. Walker pairs were either instructed to synchronize their steps in in-phase or in antiphase or received no coordination instructions. Frequency and phase analysis revealed that instructed in-phase and antiphase coordination were equally stable and independent of walking speed and the difference in individually preferred stride frequencies. Without instruction we found episodes of frequency locking in three pairs and episodes of phase locking in four pairs, albeit not always at (or near) 0 degrees or 180 degrees. Again, we found no difference in the stability of in-phase and antiphase coordination and no systematic effects of walking speed and the difference in individually preferred stride frequencies. These results suggest that the Haken-Kelso-Bunz model for rhythmic interlimb coordination does not apply to interpersonal coordination during gait in a straightforward manner. When the typically involved parameter constraints are relaxed, however, this model may largely account for the observed dynamical characteristics.     

Using objective markers to assess participant behavior in HIV prevention trials of vaginal microbicides

The need to verify participant behavior exists in any study in which behavior may affect outcomes. In vaginal microbicide trials, the act of having sex and the use of study products and condoms all affect the risk of acquiring HIV/sexually transmitted infections (STIs). Until now, these behaviors have been assessed using self-reports. But self-reports are limited by participant cooperation in answering questions, imperfect recall, and social desirability biases. Biomarkers are increasingly being used in medicine to reduce the time and resources needed to bring a drug to market. The use of biomarkers in vaginal microbicide trials has been proposed as a means of assessing factors that affect the risk of sexual acquisition of HIV/STIs, namely, the presence of preexisting infection, cervicovaginal inflammation, and the presence of HIV/STIs. Biomarkers for some of these already exist. What are needed are validated markers of behaviors that might affect risk, namely, markers for sexual behavior and for the use of study products and condoms. Validating and working out the logistics of collecting such markers in large trials will be a challenge. But finding objective markers for behavior may help improve adherence measurement during a trial and is a rate-limiting step in the field of vaginal microbicides. Resources and funding should be mobilized to develop and validate markers of sexual behavior and product use as a high priority in vaginal microbicide research.     

Using conditional FCM to mine event-related brain dynamics

We introduce a framework for mining event related dynamics based on conditional FCM (CFCM). For a given set of responses, the variation in the data is summarized by means of a small set of meaningful prototypes accompanied with a low-dimensional graph capturing their relative relationships. CFCM enables prototyping in a principled manner. User-defined constraints, which are imposed by the nature of experimental data and/or dictated by the neuroscientist's intuition, direct the process of knowledge extraction and can robustify single-trial analysis. The method is introduced using simulated data and demonstrated using actual encephalographic data.     

A simple device to assess and train motor coordination

Purpose: The purpose of this project was to develop a computer program which can be used on a laptop or other IBM-based computer to assess and train motor coordination in children with closed head trauma or cerebral palsy.

Apparatus: Muscle coordination was assessed by the child's ability to track a series of lines of increasing complexity. A stylus was used by the child to trace lines on a computer screen. Two different line tests were used. In the first, lines of various complexities were drawn on the screen at various angles. The child was asked to trace the line and the accuracy with which the line was traced was determined, providing a score. In a second test, a line was drawn on the screen and the child was asked to trace the line as it was drawn. The error in tracking the line and the time to track the line both were used to evaluate and score the child's performance. Finally, a ‘Winnie the Pooh’ character was flashed on the screen with distracters such as trees and other animals, and the child was asked to touch the Pooh. By increasing the speed of movement and decreasing the duration that the characters appeared on the screen, the child could be challenged. Successful touching of a character resulted in a positive score.

Experimental testing: Six children with cerebral palsy were compared to five children who did not have cerebral palsy, to evaluate the device. While both groups of children showed an increase in motor skills using the program, the increase seen in the children with cerebral palsy was 5-fold greater than that of the control group.

Conclusions: While only a few children were tested with the device, the device seems to prove quite useful for physical and occupational therapy for working on motor skills in children. More investigation is warranted.     

Bimanual coordination between isometric contractions and rhythmic movements: an asymmetric coupling

 Interactions between rhythmically moving limbs typically result in attraction to a limited number of coordination modes, which are distinguished in terms of their stability. In addition, the stability of coordination typically decreases with elevations in movement frequency. To gain more insight into the neurophysiological mechanisms underlying these stability characteristics, the effects of phasic voluntary muscle activation onto the movement pattern of the contralateral limb as well as onto the stability of interlimb coordination were examined. This was done in circumstances in which a minimal degree of movement-elicited afferent information was available to mediate the coupling influences. The task involved rhythmic application of isometric torque by one hand, while the other hand was moving rhythmically with unconstrained amplitude. The effects of two levels of applied torque, two coordination patterns (inphase and antiphase), and two movement frequencies were determined, both at the behavioural level (movement kinematics and kinetics) and the neuromuscular level (EMG). The isometric applications of torque clearly influenced the muscle-activation profile and movement pattern of the other limb, affecting both temporal variability and amplitude. Surprisingly, there were no differences between the two coordination patterns or between the tempo conditions. As such, the results did not conform to the Haken-Kelso-Bunz model for rhythmic movement coordination. These data suggest that the archetypal differences in stability of rhythmic bimanual coordination are contingent upon a correspondence between the limbs in terms of their respective tasks. This interpretation is elaborated in terms of the role of sensory feedback and the functional specificity of motor unit recruitment in rhythmic interlimb coordination. Received: 6 November 1998 / Accepted: 7 July 1999     

Neuromuscular-skeletal constraints upon the dynamics of perception-action coupling

Four right-handed subjects performed rhythmic flexion and extension movements of the index finger in time with an auditory metronome. On each block of trials the forearm of the response hand was placed in a prone, neutral or supine position. In the flex-on-the-beat condition, subjects were instructed to coordinate maximum excursion in the direction of finger flexion with each beat of the metronome. In the extend-on-the-beat condition, subjects were instructed to coordinate maximum excursion in the direction of finger extension with each beat of the metronome. The frequency of the metronome was increased from 1.75 Hz to 3.50 Hz in eight steps (8-s plateaus) of 0.25 Hz. During trials prepared in the extend-on-the-beat pattern, abrupt transitions to either a flex-on-the-beat pattern or to phase wandering often occurred, particularly at higher pacing frequencies. In marked constrast, during trials prepared in the flexon-the-beat pattern such transitions were never present. Both the frequency and the alacrity of these transitions were greater when the forearm was in a prone or neutral position than when the forearm was in a supine position. These results are discussed with reference to the constraints imposed on the coordination dynamics by the intrinsic properties of the neuromuscular-skeletal system.     

Speed-related changes in hindlimb intersegmental dynamics during the swing phase of cat locomotion

Summary To determine speed-related changes in hindlimb motion that might account for the mutability of bifunctional (hip extensor/knee flexor) muscle activity during the E1 phase of swing, we studied hip and knee joint kinematics and kinetics during swing over a ten-fold increase in locomotor speed (0.35 to 3.5 m/s). Three cats were filmed (100 frames/s) while locomoting on a motorized treadmill; kinematics were analyzed for the entire step cycle and kinetics for the swing phase. During swing, angular excursions at the hip and knee joints were similar for walking and trotting, but hip flexion and extension were significantly less after the transition from trot to gallop, while knee-angle range of motion increased during gallop phases E1, E2, and E3. During swing, knee-extension velocity peaked early in E1 and increased linearly with speed, while hip-flexion velocity peaked late in the flexion (F) phase and also increased linearly, but decreased precipitously at the trotgallop transition and remained constant as speed of galloping increased. Muscle torque directions during E1, flexor at the knee and extensor at the hip, were consistent with the proposed role of bifunctional posterior thigh muscles to decelerate thigh and leg segments for paw contact. At the knee joint, muscle torque during E1 counteracted a large interactive torque due to leg angular acceleration; the magnitudes of both torques were speed related with maximal values at the fastest speed tested (3.5 m/s). At the hip joint, muscle torque during E1 also counteracted a large interactive torque due to leg angular acceleration; the magnitudes of these two torques were speed related during the walk and trot, and like hip flexion velocity, decreased at the trot-gallop transition. Our data on speed-related changes in hindlimb dynamics suggest that the E1 burst amplitude (and perhaps duration) of posterior thigh muscles will be speed related during the walk and trot. After the trot-gallop transition at about 2.5 m/s, the recruitment of these bifunctional muscles may decline due to the changes in hindlimb dynamics. Because activity of these muscles counteracts interactive torques primarily related to leg angular acceleration, we suggest that motion-related feedback decoding this action may be important for regulating recruitment during E1.     

Relative phase destabilization during interlimb coordination: the disruptive role of kinesthetic afferences induced by passive movement

The disruption of three patterns of two-limb coordination, involving cyclical flexion-extension movements performed in the same or in different directions, was investigated through application of passive movement to a third limb by the experimenter. The three patterns referred to the homologous, homolateral, and heterolateral (diagonal) limb combinations which were performed in the sagittal plane. The passive movement involved a spatiotemporal trajectory that differed from the movements controlled actively. Even though subjects were instructed to completely ignore the passive limb movement, the findings of experiment 1 demonstrated a moderate to severe destabilization of the two-limb patterns, as revealed by analyses of power spectra, relative phase, cycle duration, and amplitude. This disruption was more pronounced in the homolateral and heterolateral than in the homologous effector combinations, suggesting stronger coupling between homologous than nonhomologous limb pairs. Moreover, passive mobilization affected antiphase (nonisodirectional) movements more than inphase (isodirectional) movements, pointing to the differential stability of these patterns. Experiment 2 focused on homolateral coordination and demonstrated that withdrawal of visual information did not alter the effects induced by passive movement. It was therefore hypothesized that the generation of extra kinesthetic afferences through passive limb motion was primarily responsible for the detriment in interlimb coordination, possibly conflicting with the sensory information accompanying active movement production. In addition, it was demonstrated that the active limbs were more affected by their homologous passive counterpart than by their non-homologous counterpart, favoring the notion of specific interference. The findings are discussed in view of the potential role of kinesthetic afferences in human interlimb coordination, more specifically the preservance of relative phasing through a kinesthetic feedback loop.     

Using urbanization profiles to assess screening performance

The large Dutch data sets acquired as a result of population-based cervical smear screening programs can be further exploited to obtain an urbanization-weighted score to gain insight into the quality of the performance of the individual cytology laboratories. Based on the first four digits of the postal code of the screenees, the data are stratified according to urbanization. Urb 1 corresponds to (semi)rural, which includes villages and small townships with less than 20,000 inhabitants; Urb 2, to towns with between 20,000 and 250,000 inhabitants; and Urb 3, to big cities, in this case, The Hague. From the postal code data of the screenees, the urbanization profiles of the laboratories can be calculated. The urbanization degree proved to have a substantial effect on the cytologic scores in the four laboratories. The number of expected, urbanization-weighted patient cases is calculated. Accordingly, the laboratories could be compared with respect to performance. We conclude that laboratories in our screening program were quite similar in performance for the cytologic diagnosis leading to referral to the hospital, with little difference between the actual and the expected, urbanization-weighted number of cases. It is evident that the equation for calculating the expected scores for S5-S9 is relevant for control of quality of care provided by laboratories and regions, but also for the quality of these assessments.     

多层螺旋CT对活体人脾体积的评估

目的 运用16层螺旋CT测量成人正常脾体积及容积相关参数,探讨脾体积与年龄、身高和体重等参数的关系,提供一个脾体积的简易估测方法。 方法 对无明确脾疾病的510例患者进行16层螺旋CT腹部增强扫描,针对1mm层厚、0.5mm重建间隔的门脉期图像进行分析。采用东芝公司Volume体积测量软件测定脾的体积、脾上下径、左右径、前后径、厚度和最大截面积,并对测量结果进行统计分析。 结果 510例患者的平均脾体积为(203±79)cm³。平均脾上下径、左右径、前后径、厚度和最大截面积分别为(9.0±1.7)cm、(9.1±1.0)cm、(9.5±1.9)cm、(3.8±0.7)cm及(35.6±9.5)cm²。体积与身高、体重存在较弱的正相关关系,与年龄间存在较弱的负相关关系。消除了体重因素的影响后,两性间脾体积无显著差异。脾体积与脾上下径及脾最大截面积均具有很好的正相关性(r=0.787 6, P<0.01;r=0.865 1, P<0.01)。简化的回归方程为：脾体积＝0.616×脾上下径×脾最大截面积(t=109.7, r²=0.990 6, P<0.01)。诊断脾肿大的标准是:脾体积大于358cm³为脾增大。 结论 本研究与传统的脾体积估算方法相比,可简单、迅速而较准确地估测脾体积,操作方法便于掌握,有较好的临床应用价值。     

Using exposomics to assess cumulative risks and promote health

Under the exposome paradigm all nongenetic factors contributing to disease are considered to be ‘environmental’ including chemicals, drugs, infectious agents, and psychosocial stress. We can consider these collectively as environmental stressors. Exposomics is the comprehensive analysis of exposure to all environmental stressors and should yield a more thorough understanding of chronic disease development. We can operationalize exposomics by studying all the small molecules in the body and their influence on biological pathways that lead to impaired health. Here, we describe methods by which this may be achieved and discuss the application of exposomics to cumulative risk assessment in vulnerable populations. Since the goal of cumulative risk assessment is to analyze, characterize, and quantify the combined risks to health from exposures to multiple agents or stressors, it seems that exposomics is perfectly poised to advance this important area of environmental health science. We should therefore support development of tools for exposomic analysis and begin to engage impacted communities in participatory exposome research. A first step may be to apply exposomics to vulnerable populations already studied by more conventional cumulative risk approaches. We further propose that recent migrants, low socioeconomic groups with high environmental chemical exposures, and pregnant women should be high priority populations for study by exposomics. Moreover, exposomics allows us to study interactions between chronic stress and environmental chemicals that disrupt stress response pathways (i.e., ‘stressogens’). Exploring the impact of early life exposures and maternal stress may be an interesting and accessible topic for investigation by exposomics using biobanked samples. Environ. Mol. Mutagen. 56:715–723, 2015. © 2015 Wiley Periodicals, Inc.     

Changes in finger coordination and responses to single pulse TMS of motor cortex during practice of a multifinger force production task

We investigated the changes in finger coordination and in finger force responses to transcranial magnetic stimulation (TMS) applied over the motor cortex associated with a single practice session of an accurate ramp force production task. Subjects pressed with their index, middle and ring fingers onto three force transducers fixed to a rigid platform that was balanced on a narrow pivot under the middle finger. The task was to produce a smoothly increasing ramp of total force from 0 to 25 N over 4 s following a visual target. Subjects performed three brief series of trials without TMS (12 trials each) in the beginning, in the middle, and in the end of the experiment. The main part of the experiment involved 173 trials, and in each of them at random times in the ramp a suprathreshold TMS pulse was applied over the hand area of the contralateral motor cortex in order to evoke a twitch in the finger flexor muscles. At the end of the experiment the subjects also performed 12 constant force production trials, and TMS was unexpectedly applied in each trial. During the ramp force trials the amplitude of the response to TMS was largely independent of the force exerted at the time of stimulation, whereas in static holding trials the amplitude of the response increased with higher levels of background contraction. Over time subjects improved their overall tracking performance: the variance of the force trajectory (VarF_TOT), as computed over sets of unperturbed trials, declined by 60% after the first 100 trials, but there was little additional improvement after the second 100 trials. Variance in the force finger space related to the total moment with respect to the pivot also showed a decline during the first half of practice and minimal further changes during the second half. In contrast, finger force variance that did not affect either total force or total moment showed no changes after the first 100 trials and a decline during the second 100 trials. This variance component quantified per finger was significantly larger than those related to the total force and total moment. The mean size of the TMS-induced phasic force increment decreased by 12% over the course of the 200 trials. The forces evoked in the index and ring fingers gradually became more equal, reducing the total moment with respect to the pivot and improving balance. We speculate that development of a relatively low twitch force with low total moment on the pivot made it easier for subjects to continue tracking after the TMS pulse. Such changes could well be correlated with the degree of corticospinal involvement in the task. The results suggest task specific, practice-related plastic changes in neural structures involved in the responses to TMS.