Isometric Force Production Task Research Articles

Changes in corticospinal excitability during motor imagery by physical practice of a force production task: Effect of the rate of force development during practice

Transcranial magnetic stimulation studies have indicated that the physical practice of a force production task increases corticospinal excitability during motor imagery (MI) of that task. However, it is unclear whether this practice-induced facilitation of corticospinal excitability during MI depends on a repeatedly practiced rate of force development (RFD). We aimed to investigate whether corticospinal excitability during MI of an isometric force production task is facilitated only when imagining the motor task with the same RFD as the physically practiced RFD. Furthermore, we aimed to examine whether corticospinal excitability during MI only occurs immediately after physical practice or is maintained. Twenty-eight right-handed young adults practiced isometric ramp force production using right index finger abduction. Half of the participants (high group) practiced the force production with high RFD, and the other half (low group) practiced the force production with low RFD. Questionnaire scores indicating MI ability were similar in the two groups. We examined the force error relative to the target force during the force production task without visual feedback, and motor evoked potential (MEP) amplitudes of the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) muscles during the MI of the force production task under practiced and unpracticed RFD conditions before, immediately after, and 20 min after physical practice. Our results demonstrated that the force error in both RFD conditions significantly decreased immediately after physical practice, irrespective of the RFD condition practiced. In the high group, the MEP amplitude of the FDI muscle during MI in the high RFD condition significantly increased immediately after practice compared to that before, whereas the MEP amplitude 20 min after practice was not significantly different from that before practice. Conversely, the MEP amplitude during MI in the high RFD condition did not change significantly in the low group, and neither group had significant changes in MEP amplitude during MI in the low RFD condition. The facilitatory effect of corticospinal excitability during MI with high RFD observed only immediately after physical practice in the high RFD condition may reflect short-term functional changes in the primary motor cortex induced by physical practice.

Motor unit-based synergies in a non-compartmentalized muscle.

The concept of synergies has been used to address the grouping of motor elements contributing to a task with the covariation of these elements reflecting task stability. This concept has recently been extended to groups of motor units with parallel scaling of the firing frequencies with possible contributions of intermittent recruitment (MU-modes) in compartmentalized flexor and extensor muscles of the forearm stabilizing force magnitude in finger pressing tasks. Here, we directly test for the presence and behavior of MU-modes in the tibialis anterior, a non-compartmentalized muscle. Ten participants performed an isometric cyclical dorsiflexion force production task at 1Hz between 20 and 40% of maximal voluntary contraction and electromyographic (EMG) data were collected from two high-density wireless sensors placed on the skin over the right tibialis anterior. EMG data were decomposed into individual motor unit frequencies and resolved into sets of MU-modes. Inter-cycle analysis of MU-mode magnitudes within the framework of the uncontrolled manifold (UCM) hypothesis was used to quantify force-stabilizing synergies. Two or three MU-modes were identified in all participants and trials accounting, on average, for 69% of variance and were robust to cross-validation measurements. Strong dorsiflexion force-stabilizing synergies in the space of MU-modes were present in all participants and for both electrode locations as reflected in variance within the UCM (median 954, IQR 511-1924) exceeding variance orthogonal to the UCM (median 5.82, IQR 2.9-17.4) by two orders of magnitude. In contrast, MU-mode-stabilizing synergies in the space of motor unit frequencies were not present. This study offers strong evidence for the existence of synergic control mechanisms at the level of motor units independent of muscle compartmentalization, likely organized within spinal cord circuitry.

Contributions of flexor hallucis longus and brevis muscles to isometric toe flexor force production

Purpose Morphological differences between the two primary great toe flexors – flexor hallucis longus (FHL) and flexor hallucis brevis (FHB) – likely drive differences in how these muscles contribute to functional toe flexion torque production. The aim of the study was to investigate FHL and FHB activation in two isometric toe flexion tasks – one called a “toe-pushing” task with the metatarsophalangeal (MTP) joints extended and the interphalangeal (IP) joints in neutral and another called a “toe-gripping” task with the MTP joints in neutral and flexed IP joints. Methods Twenty participants’ FHL and FHB muscles were instrumented with intramuscular electromyography electrodes. Muscle activation was normalized to a maximum voluntary contraction and compared between the two isometric toe flexor force production tasks. Results Overall, participants utilized these two toe flexors completely differently in the two tasks. In the toe-gripping task, the FHL was activated to a much greater extent than the FHB. In fact, 18 our of 20 participants activated FHL at more than seventy percent maximum voluntary contraction and half of participants activated FHB at less than ten percent. In contrast, muscle activation during the toe-pushing task appeared more reliant on the FHB for most participants. Conclusions Different contributions from the FHL and FHB to toe flexor force production in these two tasks are potentially driven by differences in muscle functional length among other factors. These findings help to inform the selection of rehabilitation and training exercises meant to preferentially target intrinsic or extrinsic foot musculature.

Role of Post-Trial Visual Feedback on Unintentional Force Drift During Isometric Finger Force Production Tasks.

A reduction in fingertip forces during a visually occluded isometric task is called unintentional drift. In this study, unintentional drift was studied for two conditions, with and without "epilogue." We define epilogue as the posttrial visual feedback in which the outcome of the just-concluded trial is shown before the start of the next trial. For this study, 14 healthy participants were recruited and were instructed to produce fingertip forces to match a target line at 15% maximum voluntary contraction. The results showed a significant reduction in unintentional drift in the epilogue condition. This reduction is probably due to the difference in the shift in λ, the threshold of the tonic stretch reflex, the hypothetical control variable that the central controller can set.

Bilateral Deficits during Maximal Grip Force Production in Late Postmenopausal Women

The purpose of this study was to investigate bilateral deficit patterns during maximal hand-grip force production in late postmenopausal women. Twenty late postmenopausal and 20 young premenopausal women performed maximal isometric grip force production tasks with dominant and nondominant hands and both hands, respectively. For late postmenopausal women, pulse wave analysis was used for identifying a potential relationship between maximal hand-grip strength and risk factors of cardiovascular disease. The findings showed that late postmenopausal women produced significantly decreased maximal hand-grip strength in dominant and nondominant and bilateral hand conditions compared to those of premenopausal women. Bilateral deficit patterns appeared in late postmenopausal women. For late postmenopausal women, decreased dominant and bilateral hand-grip forces were significantly related to greater bilateral deficit patterns. Further, less maximal hand-grip strength in unilateral and bilateral hand conditions correlated with greater central pulse pressure. These findings suggested that age-related impairments in muscle strength and estrogen deficiency may interfere with conducting successful activities of bilateral movements. Further, assessing maximal dominant hand-grip strength may predict bilateral deficit patterns and risk of cardiovascular disease in late postmenopausal women.

History-Dependence of Muscle Excitation and Oxygenation During Isometric Knee Extension Force Production

PURPOSE:This study examined the issue of history-dependence on muscle excitation and oxygenation by using surface electromyography (EMG) and near-infrared spectroscopy (NIRS).METHODS: Eight male participants performed isometric knee extension force production tasks that started at four levels of initial force production (30, 40, 60, 70% of maximal voluntary contraction, MVC) and commonly converged to the production of 50% of MVC by increasing or decreasing knee extension forces. We quantified the integrated electromyogram (iEMG), total hemoglobin (tHb), and the desaturation rate of muscle tissue (TSI<sub>SLOPE</sub>) of the vastus lateralis and compared the differences in the quantified variables between the experimental conditions.RESULTS: For the four levels of initial forces production, all the variables showed significant linear relationships with outcome forces. However, the magnitudes of the variables were varied depending on the contraction history at the terminal phase where the identical level of force production (50% of MVC) was required. At the terminal phase, the iEMG was affected by the time history of both mode (e.g., increment or decrement) and magnitude (e.g., 10% or 20% change of MVC), while the tHb and TSI<sub>SLOPE</sub> showed significant differences between the time history of mode only.CONCLUSIONS:The main result of the current study demonstrates the history-dependence of the changes in physiological demands in muscle activation. In particular, the indices of muscle excitation and oxygenation at the same level of force were varied depending on the time history of contraction, which implies the indices of muscle excitation and oxygenation may not be invariant component to specify the levels of outcome forces.

Regression-based reconstruction of human grip force trajectories with noninvasive scalp electroencephalography

Objective. Robotic devices show promise in restoring motor abilities to individuals with upper limb paresis or amputations. However, these systems are still limited in obtaining reliable signals from the human body to effectively control them. We propose that these robotic devices can be controlled through scalp electroencephalography (EEG), a neuroimaging technique that can capture motor commands through brain rhythms. In this work, we studied if EEG can be used to predict an individual’s grip forces produced by the hand. Approach. Brain rhythms and grip forces were recorded from able-bodied human subjects while they performed an isometric force production task and a grasp-and-lift task. Grip force trajectories were reconstructed with a linear model that incorporated delta band (0.1–1 Hz) voltage potentials and spectral power in the theta (4–8 Hz), alpha (8–13 Hz), beta (13–30 Hz), low gamma (30–50 Hz), mid gamma (70–110 Hz), and high gamma (130–200 Hz) bands. Trajectory reconstruction models were trained and tested through 10-fold cross validation. Main results. Modest accuracies were attained in reconstructing grip forces during isometric force production (median r = 0.42), and the grasp-and-lift task (median r = 0.51). Predicted trajectories were also analyzed further to assess the linear models’ performance based on task requirements. For the isometric force production task, we found that predicted grip trajectories did not yield static grip forces that were distinguishable in magnitude across three task conditions. For the grasp-and-lift task, we estimate there would be an approximate 25% error in distinguishing when a user wants to hold or release an object. Significance. These findings indicate that EEG, a noninvasive neuroimaging modality, has predictive information in neural features associated with finger force control and can potentially contribute to the development of brain machine interfaces (BMI) for performing activities of daily living.

Effects of spastic cerebral palsy on multi-finger coordination during isometric force production tasks.

In this study, we quantified changes in finger interdependence (enslaving), multi-finger synergies, and feedforward modulation of synergy properties (i.e., anticipatory synergy adjustment) during single- and multi-finger force production tasks in individuals with cerebral palsy (CP). Spastic diplegic CP and healthy control subjects performed sets of finger force production tasks by each of the hands, including maximal force production and submaximal quick pulse force production in an isometric condition. The framework of the uncontrolled manifold hypothesis was used to quantify the indices of multi-finger synergies and the anticipatory synergy adjustment (ASA). The CP group showed lower maximal forces and higher indices of finger interdependence (enslaving), while the indices of multi-finger synergies stabilizing total finger forces during stable force production were not different significantly compared to the controls. Further, the time of ASA for the CP group was not delayed. The CP group showed a significantly less drop in the synergy indices during the anticipatory and quick pulse phase compared to the control group, which was accompanied by larger co-contraction indices of the forearm muscles. These findings suggest that the function of assembling motor synergies for stable force production is not affected by CP, while the ability to modulate synergy properties may be impaired with CP partially due to spasticity. The spasticity presumably hampers the purposeful feedforward destabilization of the performance. The results suggest that quantification of multi-digit synergies may provide an alternative tool for quantitative assessment of impaired coordination in the CP individuals.

Emergence of anticipatory actions in a novel task.

Humans normally adapt earlier segments of multistep motor actions in such a way that the execution of later segments is facilitated. For example, the kinematics of grasping movements are adapted to the requirements of the intended subsequent object manipulations. Here we studied which factors foster adaptation of earlier action segments to later ones in a novel task for which no prior experience existed. Participants executed a two-step isometric force production task, in which the force produced in the first segment determined the difficulty of the second segment. Adaptation of the first segment to the second one benefited from explicit knowledge of the dependency between both segments but not from extensive prior experience with the second segment. These observations show that adaptation of motor actions to subsequent actions demands the construction of a task representation that allows to plan the first action segment with respect to its successor. How specifically the first segment is tailored to the second one does not depend on prior experience with the second segment but depends on experience from performing the interdependent two-step action sequence.

The metabolic cost of in vivo constant muscle force production at zero net mechanical work.

The metabolic cost per unit force is generally thought to increase with the mechanical work done by the muscle fibres. It is currently unclear how the metabolic cost of doing alternating positive and negative muscle fibre mechanical work relates to the metabolic cost of doing zero muscle fibre mechanical work at similar muscle force. The current study aimed to investigate this issue by comparing in vivo metabolic power between a dynamic and an isometric near-constant force production task. In both tasks, participants performed periodic movement about the knee joint in the gravitational field. Therefore, net external mechanical work was constrained to be zero. The tasks mainly differed from each other in average positive knee joint mechanical power, which was 4.3±0.5 W per leg during the dynamic task and 0.1±0.1 W per leg during the isometric task. Knee extension torque was near-constant around 15.2±1.7 N m during the dynamic task and around 15.7±1.7 N m during the isometric task. Owing to near-constant knee extension torque, quadriceps tendon length was presumably nearly constant during both tasks. Therefore, knee joint mechanical work was predominantly done by the muscle fibres in both tasks. Average gross metabolic power was 3.22±0.46 W kg-1 during the dynamic task and 2.13±0.36 W kg-1 during the isometric task. Because tasks differed mainly in the amount of positive muscle fibre mechanical work, these results imply that the metabolic cost of near-constant force production in vivo at zero net mechanical work can be reduced by minimizing positive muscle fibre mechanical work.

Expectation of movement generates contrasting changes in multifinger synergies in young and older adults.

Anticipatory synergy adjustment (ASA) is a feed-forward control mechanism that describes a continuous decrease in the stability of the current motor state beginning about 150ms prior to a state transition. Recently, we described an associated phenomenon in which the system stability was reduced solely in response to a cue that generates an expectation of a state change, independent of whether the state change actually occurs. Both phenomena are of the same kind (stability reduction), but evoked by distinct antecedent conditions. Since, logically, cuing for movement must occur before the initiation of that movement, we named this new phenomenon 'Stage-1 ASA' and rechristened the well-established version 'Stage-2 ASA'. Here, we used a four-finger, isometric force production task to explore (1) the effect of healthy aging on Stage-1 ASA, and (2) if Stage-1 ASA resulted in a more rapid state change. Young and older adult participants produced 10% of their maximal force when they did not expect to produce any change in the force, and when they expected to change their force in an unknown direction and at an unknown time. In the latter condition, the 10% constant-force phase was followed by a choice reaction time task, in which the participants rapidly changed their force to follow a moving target presented on a computer monitor. Both young and older adults displayed equivalent amount of Stage-1 ASA. This was driven by a 42% reduction in finger-force variability in young adults. In contrast, it was driven by a 38% increase in finger-force variability in older adults. We speculate that the reduction in finger force variability assists the young adults in rapid state changes via two mechanisms: (1) the finger forces occupy a restricted set of states that are optimal for quick state transitions, and (2) lower variability during steady state translates into lower self-motion during state transition. Self-motion is the covariation between finger forces that fails to change the total force. The older adults are unable to adopt this strategy, and the increase in finger-force variability arises from (1) the adoption of an alternative strategy of destabilizing the attractor associated with the current state to facilitate state transitions and (2) the inability to coordinate multiple finger forces. Finally, older adults displayed longer reaction times than young adults, but a clear relation between Stage-1 ASA and consequent behavioral benefit in terms of reduced reaction time remained elusive.

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

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

A cryptography-based approach for movement decoding.

Brain decoders use neural recordings to infer the activity or intent of a user. To train a decoder, one generally needs to infer the measured variables of interest (covariates) from simultaneously measured neural activity. However, there are cases for which obtaining supervised data is difficult or impossible. Here, we describe an approach for movement decoding that doesn’t require access to simultaneously measured neural activity and motor outputs. We use the statistics of movement—much like cryptographers use the statistics of language—to find a mapping between neural activity and motor variables, and then align the distribution of decoder outputs with the typical distribution of motor outputs by minimizing their Kullback-Leibler divergence. By using datasets collected from the motor cortex of three non-human primates performing either a reaching task or an isometric force-production task, we show that the performance of such a distribution-alignment decoding algorithm is comparable with the performance of supervised approaches. Distribution-alignment decoding promises to broaden the set of potential applications of brain decoding.

Cue-induced changes in the stability of finger force-production tasks revealed by the uncontrolled manifold analysis.

A motor system configured to maximize the stability of its current state cannot dexterously transition between states. Yet, we routinely resolve the stability-dexterity conflict and rapidly change our current behavior without allowing it to become unstable before the desired transition. The phenomenon called anticipatory synergy adjustment (ASA) partly describes how the central nervous system handles this conflict. ASA is a continuous decrease in the stability of the current motor state beginning 150-400 ms before a rapid state transition accomplished using redundant sets of motor inputs (more input variables than task-specific output variables). So far, ASAs have been observed only when the timing of the upcoming transition is known. We utilized a multifinger, isometric force-production task to demonstrate that compared with a condition where no state transition is expected, the stability of the current state is lower by ~12% when a participant is cued to make a transition, even when the nature and timing of that transition are unknown. This result (stage 1 ASA) is distinct from its traditional version (stage 2 ASA), and it describes early destabilization that occurs solely in response to the expectation to move. Stage 2 ASA occurs later, only if the timing of the transition is known sufficiently in advance. Stage 1 ASA lasts much longer (~1.5 s) and may scale in response to the perceived difficulty of the upcoming task. Therefore, this work reveals a much refined view of the processes that underlie the resolution of the stability-dexterity conflict. NEW & NOTEWORTHY We compared the stability of multifinger, isometric force-production tasks for trials in which force changes of unknown direction and timing were expected with trials in which there was no expectation of any force change. Mere expectation of a change caused the stability of the current motor state to drop. This novel result provides a much refined view of the processes that facilitate dexterous switching between motor states.

Empirical Evaluation of Voluntarily Activatable Muscle Synergies.

The muscle synergy hypothesis assumes that individual muscle synergies are independent of each other and voluntarily controllable. However, this assumption has not been empirically tested. This study tested if human subjects can voluntarily activate individual muscle synergies extracted by non-negative matrix factorization (NMF), the standard mathematical method for synergy extraction. We defined the activation of a single muscle synergy as the generation of a muscle activity pattern vector parallel to the single muscle synergy vector. Subjects performed an isometric force production task with their right hand, and the 13 muscle activity patterns associated with their elbow and shoulder movements were measured. We extracted muscle synergies during the task using electromyogram (EMG) data and the NMF method with varied numbers of muscle synergies. The number (N) of muscle synergies was determined by using the variability accounted for (VAF, NVAF) and the coefficient of determination (CD, NCD). An additional muscle synergy model with NAD was also considered. We defined a conventional muscle synergy as the muscle synergy extracted by the NVAF, NCD, and NAD. We also defined an extended muscle synergy as the muscle synergy extracted by the NEX> NAD. To examine whether the individual muscle synergy was voluntarily activatable or not, we calculated the index of independent activation, which reflects similarities between a selected single muscle synergy and the current muscle activation pattern of the subject. Subjects were visually feed-backed the index of independent activation, then instructed to generate muscle activity patterns similar to the conventional and extended muscle synergies. As a result, an average of 90.8% of the muscle synergy extracted by the NVAF was independently activated. However, the proportion of activatable muscle synergies extracted by NCD and NAD was lower. These results partly support the assumption of the muscle synergy hypothesis, i.e., that the conventional method can extract voluntarily and independently activatable muscle synergies by using the appropriate index of reconstruction. Moreover, an average of 25.5% of the extended muscle synergy was significantly activatable. This result suggests that the CNS can use extended muscle synergies to perform voluntary movements.

Unintentional force changes in cyclical tasks performed by an abundant system: Empirical observations and a dynamical model

The study explored unintentional force changes elicited by removing visual feedback during cyclical, two-finger isometric force production tasks. Subjects performed two types of tasks at 1Hz, paced by an auditory metronome. One – Force task – required cyclical changes in total force while maintaining the sharing, defined as relative contribution of a finger to total force. The other task – Share task – required cyclical changes in sharing while keeping total force unchanged. Each trial started under full visual feedback on both force and sharing; subsequently, feedback on the variable that was instructed to stay constant was frozen, and finally feedback on the other variable was also removed. In both tasks, turning off visual feedback on total force elicited a drop in the mid-point of the force cycle and an increase in the peak-to-peak force amplitude. Turning off visual feedback on sharing led to a drift of mean share toward 50:50 across both tasks. Without visual feedback there was consistent deviation of the two force time series from the in-phase pattern (typical of the Force task) and from the out-of-phase pattern (typical of the Share task). This finding is in contrast to most earlier studies that demonstrated only two stable patterns, in-phase and out-of-phase. We interpret the results as consequences of drifts of parameters in a dynamical system leading in particular to drifts in the referent finger coordinates toward their actual coordinates. The relative phase desynchronization is caused by the right–left differences in the hypothesized drift processes, consistent with the dynamic dominance hypothesis.

Editorial: A Hand at Work: Effects of Aging.

Editorial: A Hand at Work: Effects of Aging.

Predicting hand forces from scalp electroencephalography during isometric force production and object grasping.

In this study, we demonstrate the feasibility of predicting hand forces from brain activity recorded with scalp electroencephalography (EEG). Ten able-bodied subjects participated in two tasks: an isometric force production task and a grasp-and-lift task using unconstrained and constrained grasps. We found that EEG electrodes spanning central areas of the scalp were highly correlated to force rate trajectories. Moreover, EEG grand averages in central sites resembled force rate trajectories as opposed to force trajectories. The grasp-and-lift task resulted in higher decoding accuracies than the isometric force production task: across nine subjects, median accuracies for the isometric force production task were r=0.35 whereas median accuracies for unconstrained grasping were r=0.51 and for constrained grasping were r=0.50. Such results could lead to an understanding of the neural representation behind the control of hand forces and could be implemented in the neural control of closed-loop hand-based neuroprostheses.

Increase in joint stability at the expense of energy efficiency correlates with force variability during a fatiguing task

Empirical evidence suggests that our nervous system considers many objectives when performing various tasks. With the progression of fatigue, researchers have noted increase in both joint moment variability and muscular cocontraction during isometric force production tasks. Muscular cocontraction increases joint stability, but is metabolically costly. Thus, our nervous system must select a compromise between joint stability and energy efficiency. Interestingly, the continuous increase in cocontraction with fatigue suggests there may be a shift in the relative weighting of these objectives. Here we test the notion of dynamic objective weightings. Using multi-objective optimization, we found a shift in objective weighting that favoured joint stability at the expense of energy efficiency during fatigue. This shift was highly correlated with muscular cocontraction (R2=0.78, p<0.001) and elbow moment variability in the time (R2=0.56, p<0.01) and frequency (R2=0.57, p<0.01) domains. By considering a dynamic objective weighting we obtained strong correlations with predicted and collected muscle activity (R2=0.94, p<0.001).

Task-Specific Stability of Multifinger Steady-State Action

ABSTRACTThe authors explored task-specific stability during accurate multifinger force production tasks with different numbers of instructed fingers. Subjects performed steady-state isometric force production tasks and were instructed not to interfere voluntarily with transient lifting-and-lowering perturbations applied to the index finger. The main results were (a) intertrial variance in the space of finger modes at steady states was larger within the subspace that had no effect on the total force (the uncontrolled manifold [UCM]); (b) perturbations caused large deviations of finger modes within the UCM (motor equivalence); and (c) deviations caused by the perturbation showed larger variance within the UCM. No significant effects of the number of task fingers were noted in any of the 3 indicators. The results are discussed within the frameworks of the UCM and referent configuration hypotheses. The authors conclude, in particular, that all the tasks were effectively 4-finger tasks with different involvement of task and nontask fingers.