Children's coordination of force output in a pinch grip task

Visual feedback and motor memory contributions to sustained motor control deficits in autism spectrum disorder across childhood and into adulthood

BACKGROUND

Autistic individuals show deficits in sustained fine motor control which are associated with an over-reliance on visual feedback. Motor memory deficits also have been reported during sustained fine motor control in autism spectrum disorders (ASD). The development of motor memory and visuomotor feedback processes contributing to sustained motor control issues in ASD are not known. The present study aimed to characterize age-related changes in visual feedback and motor memory processes contributing to sustained fine motor control issues in ASD.

METHODS

Fifty-four autistic participants and 31 neurotypical (NT) controls ages 10-25 years completed visually guided and memory guided sustained precision gripping tests by pressing on force sensors with their dominant hand index finger and thumb. For visually guided trials, participants viewed a stationary target bar and a force bar that moved upwards with increased force for 15s. During memory guided trials, the force bar was visible for 3s, after which participants attempted to maintain their force output without visual feedback for another 12s. To assess visual feedback processing, force accuracy, variability (standard deviation), and regularity (sample entropy) were examined. To assess motor memory, force decay latency, slope, and magnitude were examined during epochs without visual feedback.

RESULTS

Relative to NT controls, autistic individuals showed a greater magnitude and a trend for a steeper slope of force decay during memory guided trials. Across conditions, the ASD group showed reduced force accuracy (β = 0.41, R2 = 0.043, t79.3=2.36, p = .021) and greater force variability (β=-2.16, R2 = 0.143, t77.1=-4.04, p = .0001) and regularity (β=-0.52, R2 = 0.021, t77.4=-2.21, p = .030) relative to NT controls at younger ages, but these differences normalized by adolescence (age x group interactions). Lower force accuracy and greater force variability during visually guided trials and steeper decay slope during memory guided trials were associated with overall autism severity.

CONCLUSIONS

Our findings that autistic individuals show a greater magnitude and tendency for a greater rate of force decay than NT individuals following the removal of visual feedback indicate that motor memory deficits contribute to fine motor control issues in ASD. Findings that sensorimotor differences in ASD were specific to younger ages suggest delayed development across multiple motor control processes.

Visual feedback and motor memory contributions to sustained motor control deficits in autism spectrum disorder across childhood and into adulthood

Background

Autistic individuals show deficits in sustained fine motor control which are associated with an over-reliance on visual feedback. Motor memory deficits also have been reported during sustained fine motor control in autism spectrum disorders (ASD). The development of motor memory and visuomotor feedback processes contributing to sustained motor control issues in ASD are not known. The present study aimed to characterize age-related changes in visual feedback and motor memory processes contributing to sustained fine motor control issues in ASD.

Methods

Fifty-four autistic participants and 31 neurotypical (NT) controls ages 10-25 years completed visually guided and memory guided sustained precision gripping tests by pressing on force sensors with their dominant hand index finger and thumb. For visually guided trials, participants viewed a stationary target bar and a force bar that moved upwards with increased force for 15s. During memory guided trials, the force bar was visible for 3s, after which participants attempted to maintain their force output without visual feedback for another 12s. To assess visual feedback processing, force accuracy, variability (standard deviation), and regularity (sample entropy) were examined. To assess motor memory, force decay latency, slope, and magnitude were examined during epochs without visual feedback.

Results

Relative to NT controls, autistic individuals showed a greater magnitude and steeper slope of force decay during memory guided trials. Across conditions, the ASD group showed reduced force accuracy (β = .41, R2 = 0.043, t79.3=2.36, p = 0.021) and greater force variability (β=-2.16, R2 = .143, t77.1=-4.04, p = 0.0001) and regularity (β=-.52, R2 = .021, t77.4=-2.21, p = 0.030) relative to controls at younger ages, but these differences normalized by adolescence (age × group interactions). Lower force accuracy and greater force variability during visually guided trials and steeper decay slope during memory guided trials were associated with overall autism severity.

Conclusions

Our findings that autistic individuals show a greater rate and magnitude of force decay than NT individuals following the removal of visual feedback indicate that motor memory deficits contribute to fine motor control issues in ASD. Findings that sensorimotor differences in ASD were specific to younger ages suggest delayed development across multiple motor control processes.

Effect of verbal cues on the coupling and stability of anti-phase bimanual coordination pattern in children with probable developmental coordination disorder

The study of the emergence and stability of bimanual and interlimb coordination patterns in children with Developmental Coordination Disorder (DCD) has shown that they encounter greater difficulties in coupling their limbs compared to typically developing (TD) children. Verbal cues have been identified as strategies to direct children's attention to more relevant task information, thus potentially improving motor performance. Consequently, this study investigated the effect of providing verbal cues on the execution of bimanual tasks in children with and without probable DCD. Twenty-eight children aged 9-10, matched by age and gender, were divided into two groups: pDCD [n = 14] and TD. The children performed bilateral trajectory movements with both hands (horizontal back-and-forth), holding a pen on a tablet, in anti-phase (180°) coordination pattern, in two conditions: No cues and Verbal cues. In the last condition, children received verbal cues to maintain the anti-phase pattern even with an increase in hand oscillation frequency. Relative phase and variability of relative phase between the hands were calculated for analysis of pattern coupling and stability. Hand cycles, movement amplitude, and tablet pressure force were calculated to analyze pattern control parameters. All these variables were compared between groups and conditions. The results indicated that despite the pDCD group showing greater variability in the anti-phase coordination pattern compared to the TD group, both groups performed better in the Verbal cues than the No cues condition. Furthermore, the pDCD group exhibited more hand movement cycles and applied greater pressure force compared to the TD group, suggesting different motor control strategies during the bimanual task. It is suggested that the use of verbal cues during bimanual task execution improves children's performance, potentially by promoting interaction between attention, as a cognitive function, and intrinsic coordination dynamics, thereby reducing variability in the perceptual-motor system.

Motor Unit Firing Properties During Force Control Task and Associations With Neurological Tests in Children

The present study aimed to clarify the development of motor unit (MU) firing properties and the association between those neural properties and force steadiness (FS)/neurological tests in 6- to 12-year-old children. Fifty-eight school-aged children performed maximal voluntary knee extension contraction, a submaximal FS test at 10% of maximal voluntary knee extension contraction, knee extension reaction time to light stimulus test, and single-leg standing test, and data from 38 children who passed the criteria were subject to analysis. During the FS test, high-density surface electromyography was recorded from the vastus lateralis muscle to identify individual MU firing activity. FS was improved with an increase in age (r = -.540, P < .001). The MU firing rate (MUFR) was significantly decreased with an increase in age (r = -.343, P = .035). MUFR variability was not associated with age. Although there was no significant correlation between FS and MUFR, FS was significantly correlated with MUFR variability even after adjustment for the effect of age (r = .551, P = .002). Neither the reaction time nor the single-leg standing test was correlated with any MU firing properties. These findings suggest that MUFR variability makes an important contribution to precise force control in children but does not naturally develop with age.

Adolescent boys who participate in sports exhibit similar ramp torque control with young men despite differences in strength and tendon characteristics

PURPOSE

The goal of this paper was to determine if sports participation influences torque control differently for adolescent boys and young men during a slow ramp task.

METHODS

Twenty-one adolescent boys (11 athletes) and 31 young men (16 athletes) performed a slow ramp increase in plantar flexion torque from 0 to maximum. We quantified torque control as the coefficient of variation (CV) of torque during the ramp and quantified the Achilles tendon mechanical properties using ultrasonography.

RESULTS

Relative to adolescent boys, young men were taller, heavier, stronger, and had a longer and stiffer Achilles tendon. However, these characteristics were not different between athletes and non-athletes in adolescent boys. For the CV of torque, there was a significant interaction with sports participation, indicating that only adolescent boys who were non-athletes had greater variability than young men. The CV of torque of all participants was predicted from the maximum torque and torque oscillations from 1 to 2 Hz, whereas the CV of torque for adolescent boys was predicted only from torque oscillations from 1 to 2 Hz.

CONCLUSION

These findings suggested that adolescent boys who participate in sports exhibited lower torque variability during a slow ramp task, which was not explained by differences in Achilles tendon properties or strength.

Influence of central and peripheral motor unit properties on isometric muscle force entropy: A computer simulation study

Approximate entropy of isometric force is a popular measure to characterize behavioral changes across muscle contraction conditions. The degree to which force entropy characterizes the randomness of the motor control strategy, however, is not known. In this study, we used a computational model to investigate the correlation between approximate entropy of the synaptic input to a motor neuron pool, the neural drive to muscle (cumulative spike train; CST), and the force. This comparison was made across several simulation conditions, that included different synaptic command signal bandwidths, motor neuron pool sizes, and muscle contractile properties. The results indicated that although force entropy to some degree reflects the entropy of the synaptic command to motor neurons, it is biased by changes in motor unit properties. As a consequence, there was a low correlation between approximate entropy of force and the motor neuron input signal across all simulation conditions (r² = 0.13). Therefore, force entropy should only be used to compare motor control strategies across conditions where motor neuron properties can be assumed to be maintained. Instead, we recommend that the entropy of the descending motor commands should be estimated from CSTs comprising spike trains of multiple motor units.

Visual and somatosensory feedback mechanisms of precision manual motor control in autism spectrum disorder

BACKGROUND

Individuals with autism spectrum disorder (ASD) show deficits processing sensory feedback to reactively adjust ongoing motor behaviors. Atypical reliance on visual and somatosensory feedback each have been reported during motor behaviors in ASD suggesting that impairments are not specific to one sensory domain but may instead reflect a deficit in multisensory processing, resulting in reliance on unimodal feedback. The present study tested this hypothesis by examining motor behavior across different visual and somatosensory feedback conditions during a visually guided precision grip force test.

METHODS

Participants with ASD (N = 43) and age-matched typically developing (TD) controls (N = 23), ages 10-20 years, completed a test of precision gripping. They pressed on force transducers with their index finger and thumb while receiving visual feedback on a computer screen in the form of a horizontal bar that moved upwards with increased force. They were instructed to press so that the bar reached the level of a static target bar and then to hold their grip force as steadily as possible. Visual feedback was manipulated by changing the gain of the force bar. Somatosensory feedback was manipulated by applying 80 Hz tendon vibration at the wrist to disrupt the somatosensory percept. Force variability (standard deviation) and irregularity (sample entropy) were examined using multilevel linear models.

RESULTS

While TD controls showed increased force variability with the tendon vibration on compared to off, individuals with ASD showed similar levels of force variability across tendon vibration conditions. Individuals with ASD showed stronger age-associated reductions in force variability relative to controls across conditions. The ASD group also showed greater age-associated increases in force irregularity relative to controls, especially at higher gain levels and when the tendon vibrator was turned on.

CONCLUSIONS

Our findings that disrupting somatosensory feedback did not contribute to changes in force variability or regularity among individuals with ASD suggests a reduced ability to integrate somatosensory feedback information to guide ongoing precision manual motor behavior. We also document stronger age-associated gains in force control in ASD relative to TD suggesting delayed development of multisensory feedback control of motor behavior.

Atypical neural processing during the execution of complex sensorimotor behavior in autism

Stereotyped behavior is rhythmic, repetitive movement that is essentially invariant in form. Stereotypy is common in several clinical disorders, such as autism spectrum disorders (ASD), where it is considered maladaptive. However, it also occurs early in typical development (TD) where it is hypothesized to serve as the foundation on which complex, adaptive motor behavior develops. This transition from stereotyped to complex movement in TD is thought to be supported by sensorimotor integration. Stereotypy in clinical disorders may persist due to deficits in sensorimotor integration. The present study assessed whether differences in sensorimotor processing may limit the expression of complex motor behavior in individuals with ASD and contribute to the clinical stereotypy observed in this population. Adult participants with ASD and TD performed a computer-based stimulus-tracking task in the presence and absence of visual feedback. Electroencephalography was recorded during the task. Groups were compared on motor performance (root mean square error), motor complexity (sample entropy), and neural complexity (multiscale sample entropy of the electroencephalography signal) in the presence and absence of visual feedback. No group differences were found for motor performance or motor complexity. The ASD group demonstrated greater neural complexity and greater differences between feedback conditions than TD individuals, specifically in signals relevant to sensorimotor processing. Motor performance and motor complexity correlated with clinical stereotypy in the ASD group. These findings support the hypothesis that individuals with ASD have differences in sensorimotor processing when executing complex motor behavior and that stereotypy is associated with low motor complexity.

Motor Control and Achilles Tendon Adaptation in Adolescence: Effects of Sport Participation and Maturity

An important but unresolved research question in adolescent children is the following: “Does sport participation interact with maturation to change motor control and the mechanical and morphological properties of tendons?” Here, we address this important research question with a longitudinal study around the age of peak height velocity (PHV). Our purpose was to characterize the interactive effects of maturation and sports participation on motor control and the mechanical and morphological properties of the Achilles tendon (AT) in adolescent athletes and non-athletes. Twenty-two adolescent athletes (13.1 ± 1.1 years) and 19 adolescent non-athletes (12.8 ± 1.1 years) volunteered for this study. We quantified motor control as the coefficient of variation of torque during a ramp task. In addition, we quantified the AT morphological and mechanical properties using ultrasonography from 18 months before to 12 months after PHV. We found that motor control improved with maturation in both athletes and non-athletes. We found that athletes have a greater increase in body mass with maturation that relates to greater plantarflexion peak force and AT peak stress. Also, athletes have a thicker and longer AT, as assessed with resting cross-sectional area and length. Although the rate of increase in the morphological change with maturation was similar for athletes and non-athletes, the rate of increase in normalized AT stiffness was greater for athletes. This increased AT stiffness in athletes related to peak force and stress. In summary, maturation improves motor control in adolescent children. Further, we provide novel longitudinal evidence that sport participation interacts with maturation in adolescents to induce adaptive effects on the Achilles tendon morphology and mechanical properties. These findings have the potential to minimize the risk of injuries and maximize athletic development in talented adolescents.

Visual feedback during motor performance is associated with increased complexity and adaptability of motor and neural output

Complex motor behavior is believed to be dependent on sensorimotor integration - the neural process of using sensory input to plan, guide, and correct movements. Previous studies have shown that the complexity of motor output is low when sensory feedback is withheld during precision motor tasks. However, much of this research has focused on motor behavior rather than neural processing, and therefore, has not specifically assessed the role of sensorimotor neural functioning in the execution of complex motor behavior. The present study uses a stimulus-tracking task with simultaneous electroencephalography (EEG) recording to assess the effect of visual feedback on motor performance, motor complexity, and sensorimotor neural processing in healthy adults. The complexity of the EEG signal was analyzed to capture the information content in frequency bands (alpha and beta) and scalp regions (central, parietal, and occipital) that are associated with sensorimotor processing. Consistent with previous literature, motor performance and its complexity were higher when visual feedback was provided relative to when it was withheld. The complexity of the neural signal was also higher when visual feedback was provided. This was most robust at frequency bands (alpha and beta) and scalp regions (parietal and occipital) associated with sensorimotor processing. The findings show that visual feedback increases the information available to the brain when generating complex, adaptive motor output.

A Cohesive Framework for Motor Stereotypy in Typical and Atypical Development: The Role of Sensorimotor Integration

Stereotyped motor behavior manifests as rhythmic, repetitive movements. It is common in several neurologic and psychiatric disorders where it is considered maladaptive. However, it also occurs early in typical development where it serves an adaptive function in the development of complex, controlled motor behavior. Currently, no framework accounts for both adaptive and maladaptive forms of motor stereotypy. We propose a conceptual model that implicates sensorimotor mechanisms in the phenomenology of adaptive and maladaptive stereotypy. The extensive structural and functional connectivity between sensory and motor neural circuits evidences the importance of sensory integration in the production of controlled movement. In support of our model, motor stereotypy in normative development occurs when the sensory and motor brain regions are immature and the infant has limited sensory and motor experience. With maturation and experience, complex movements develop and replace simple, stereotyped movements. This developmental increase in motor complexity depends on the availability of sensory feedback indicating that the integration of sensory information with ongoing movement allows individuals to adaptively cater their movements to the environmental context. In atypical development, altered neural function of sensorimotor circuitry may limit an individual's ability to integrate sensory feedback to adapt movements to appropriately respond to environmental conditions. Consequently, the motor repertoire would remain relatively simple, resulting in the persistence of motor stereotypy. A framework that considers motor stereotypy as a manifestation of low motor complexity resulting from poor sensorimotor integration has many implications for research, identification and treatment of motor stereotypy in a variety of developmental disorders.

Physiological tremor (8-12Hz component) in isometric force control

The experiment investigated the influence of physiological tremor (8-12Hz band) on the variability of isometric force control as a function of force level and hand dominance. Subjects were instructed to match a constant force level target line on a computer screen and minimize error in a uni-manual isometric finger abduction task at 5%, 25%, 45%, 65%, and 85% of their maximal voluntary contraction (MVC). The experimental protocol was performed independently with the left and right hands in separate blocks of performance. Tremor amplitude was enhanced at an increasing rate with increments of force level and was correlated with both performance outcome (Root mean square error - RMSE) and time-dependent regularity (Sample Entropy) of the force signal. No significant findings in force variability (dispersion or irregularity) were found between the dominant and non-dominant hands. Physiological tremor has a small but direct influence on the dispersion and time dependent structure of the variability of isometric force control but its relative influence on force amplitude decreases with increments of force level.

Effects of Tactile Sensitivity on Structural Variability of Digit Forces during Stable Precision Grip

This study investigated the effects of fingertip tactile sensitivity on the structural variability of thumb and index finger forces during stable precision grip. Thirty right-handed healthy subjects participated in the experiment. Transient perturbation of tactile afferents was achieved by wrapping up the distal pads of the thumb or index finger with transparent polyethylene films. The time-dependent structure of each digit force and the variability of interdigit force correlation were examined by detrended fluctuation analysis (DFA) and detrended cross-correlation analysis (DCCA), respectively. Results showed that the tactile sensitivity affected α_DFA of the vertical shear force Fx (F_3,239 = 6.814, p < 0.001) and α_DCCA of Fx (χ² = 16.440, p < 0.001). No significant difference was observed in α_DFA or α_DCCA of the normal forces produced by the thumb or index finger. These results suggested that with blurred tactile sensory inputs the central nervous system might decrease the vertical shear force flexibility and increase the interdigit shear force coupling in order to guarantee a stable grip control of an object against gravity. This study shed light on the feedback and feed-forward strategies involved in digit force control and the role of SA-II afferent fibers in regulation of vertical shear force variability for precision grip.

Coordination of digit force variability during dominant and non-dominant sustained precision pinch

This study examined the effects of handedness on the inter-digit coordination of force variability with and without concurrent visual feedback during sustained precision pinch. Twenty-four right-handed subjects were instructed to pinch an instrumented apparatus with their dominant and non-dominant hands, separately. During the pinch, the subjects were required to maintain a stable force output at 5 N for 1 min. Visual feedback was given for the first 30 s and removed for the second 30 s. Coefficient of variation and detrended fluctuation analysis were employed to examine the amount and structural variability of the thumb and index finger forces. Similarly, correlation coefficient and detrended cross-correlation analysis were applied to quantify the inter-digit correlation of force amount and structural variability. Results showed that, compared to the non-dominant hand, the dominant hand had higher inter-digit difference in the amount of digit force variability. Without visual feedback, the dominant hand exhibited lower digit force structural variability but higher inter-digit force structural correlation than the non-dominant hand. These results implied that the dominant hand would be more independent, less flexible and with lower dynamic degrees of freedom than the non-dominant hand in coordination of the thumb and index finger forces during sustained precision pinch. The effects of handedness on inter-digit force coordination were dependent on sensory condition, which shed light on higher-level sensorimotor mechanisms that may be responsible for the asymmetries in coordination of digit force variability.

Age-related differences in the availability of visual feedback during bimanual pinch

PURPOSE

Previous research has indicated that older adults have significantly lower accuracy in terms of force control than young adults. In addition, accuracy of force control is known to decrease in the absence of visual feedback. However, whether the effect of visual feedback on fine motor control is similar for young adults and older adults is not clear. The purpose of this study, therefore, was to examine the effect of visual feedback on bimanual pinch force control in older adults.

METHODS

Thirty-one undergraduate students (age 19.7 ± 0.9 years) and 31 older adults (age 65.1 ± 8.1 years) participated in this study. After measuring finger-pinch maximal voluntary force (MVF), the participants were asked to maintain 10% MVF as steadily as possible in two different conditions: with visual feedback (visual feedback condition; VF condition) and without visual feedback (no visual feedback condition; NVF condition).

RESULTS

We found that older adults had significantly greater targeting error and force variability than young adults in the VF condition, but not in the NVF condition. In addition, older participants exhibited a significantly greater sum of power for the 0-4 and 4-8 Hz frequency bin than young adults (p < 0.05) in the VF condition, although there was no significant difference in the NVF condition.

CONCLUSIONS

These results suggest that older adults do not use visual information as effectively as younger adults to reduce force control error.

Carpal tunnel syndrome impairs sustained precision pinch performance

OBJECTIVE

The purpose of this study was to investigate effects of carpal tunnel syndrome (CTS) on digit force control during a sustained precision pinch.

METHODS

Eleven CTS individuals and 11 age- and gender-matched healthy volunteers participated in the study. The subjects were instructed to isometrically pinch an instrumented apparatus for 60s with a stable force output. Visual feedback of force output was provided for the first 30s but removed for the remaining 30s. Pinch forces were examined for accuracy, variability, and inter-digit correlation.

RESULTS

CTS led to a decrease in force accuracy and an increase in amount of force variability, particularly without visual feedback (p<0.001). However, CTS did not affect the structure of force variability or force correlation between digits (p>0.05). The force of the thumb was less accurate and more variable than that of the index finger for both the CTS and healthy groups (p<0.001).

CONCLUSIONS

Sensorimotor deficits associated with CTS lead to inaccurate and unstable digit forces during sustained precision pinch.

SIGNIFICANCE

This study shed light on basic and pathophysiological mechanisms of fine motor control and aids in development of new strategies for diagnosis and evaluation of CTS.

Emergence and stability of interlimb coordination patterns in children with developmental coordination disorder

The purpose of this study was to investigate the emergence and stability of coordination patterns in children with developmental coordination disorder (DCD) when performing a rhythmic interlimb coordination task on rigid (floor) and elastic (mini-trampoline) surfaces. Twelve typically developing (TD) children and 12 children with DCD were required to clap while jumping under different conditions: in a chosen pattern - Free; when the feet touched the surface - Clapping-surface; when the body reached the maximum jumping height - Clapping-jump; and when the feet touched the surface and the body reached the maximum jumping height - Clapping-both. The results showed that the coordination pattern of children with DCD was more variable in the Free, Clapping-surface, and Clapping-jumping conditions and more variable on the mini-trampoline than on the floor under the Free condition when compared with the TD children. Clapping-jumping was more difficult to perform than Clapping-surface for both groups. These findings suggest that the children with DCD were less capable of rhythmically coordinating the jumping-clapping task because they used a type of exploratory strategy regarding the physical properties of the surfaces, whereas the TD children used a type of adaptive strategy displaying behavior that was more consistent across the tasks/environmental demands.

Developmental improvements in dynamic control of fingertip forces last throughout childhood and into adolescence

While it is clear that the development of dexterous manipulation in children exhibits dramatic improvements over an extended period, it is difficult to separate musculoskeletal from neural contributors to these important functional gains. This is in part due to the inability of current methods to disambiguate improvements in hand strength from gains in finger dexterity (i.e., the dynamic control of fingertip force vectors at low magnitudes). We adapted our novel instrumentation to evaluate finger dexterity in 130 typically developing children between the ages of 4 and 16 yr. We find that finger dexterity continues to develop well into late adolescence and musculoskeletal growth and strength are poorly correlated with the improvements in dexterity. Importantly, because these behavioral results seem to mirror the known timelines of neuroanatomical development up to adolescence, we speculate that they reflect the functional benefits of such continual neural maturation. This novel perspective now enables the systematic study of the functional roles of specific neuroanatomical structures and their connectivity, maturity, and plasticity. Moreover, the temporal dynamics of the fingertip force vectors shows improvements in stability that provide a novel way to look at the maturation of finger control. From a clinical perspective, our results provide a practical means to chart functional development of dexterous manipulation in typically developing children and could be adapted for clinical use and for use in children with developmental disorders.

Removal of visual feedback lowers structural variability of inter-digit force coordination during sustained precision pinch

This study examined the effects of visual feedback on inter-digit force coordination during a precision pinch. Sixteen healthy, right-handed subjects were instructed to pinch an instrumented apparatus for 1 min with a stable force output. Visual feedback was provided for the first 30s and withdrawn for the second 30s. Detrended fluctuation analysis (DFA) and detrended cross-correlation analysis (DCCA) methods were used to quantify the time-dependent structures of each digit's force and of the force correlation between the digits. After removing visual feedback, the DFA scaling exponent, αDFA, increased from 1.10±0.12 to 1.29±0.13 for the thumb and from 0.95±0.08 to 1.33±0.13 for the index finger (F1,95=372.47, p<0.001); the DCCA scaling exponent, αDCCA, increased from 1.00±0.08 to 1.33±0.13 (t95=20.33, p<0.001). Structural changes were observed beginning with the first 5s epoch after the removal of visual feedback. The results provide evidence that removing visual feedback lowers the structural variability of inter-digit force coordination. This change is reflected in the high-level control strategy, resulting in the two digits being more tightly coupled under somatosensory feedback without visual inputs.

Aging and motor variability: a test of the neural noise hypothesis

Experimental tests of the neural noise hypothesis of aging, which holds that aging-related increments in motor variability are due to increases in white noise in the perceptual-motor system, were conducted. Young (20-29 years old) and old (60-69 and 70-79 years old) adults performed several perceptual-motor tasks. Older adults were progressively more variable in their performance outcome, but there was no age-related difference in white noise in the motor output. Older adults had a greater frequency-dependent structure in their motor variability that was associated with performance decrements. The findings challenge the main tenet of the neural noise hypothesis of aging in that the increased variability of older adults was due to a decreased ability to adapt to the constraints of the task rather than an increment of neural noise per se.

Hand digit control in children: age-related changes in hand digit force interactions during maximum flexion and extension force production tasks

We studied the finger interactions during maximum voluntary force (MVF) production in flexion and extension in children and adults. The goal of this study was to investigate the age-related changes and flexion-extension differences of MVF and finger interaction indices, such as finger inter-dependency (force enslaving (FE): unintended finger forces produced by non-instructed fingers during force production of an instructed finger), force sharing (FS; percent contributions of individual finger forces to the total force at four-finger MVF), and force deficit (FD; force difference between single-finger MVF and the force of the same finger at four-finger MVF). Twenty-five right-handed children of 6-10 years of age and 25 adults of 20-24 years of age participated as subjects in this study (five subjects at each age). During the experiments, the subjects had their forearms secured in armrests. The subjects inserted the distal phalanges of the right hand into C-shaped aluminum thimbles affixed to small force sensors with 200 of flexion about the metacarpophalangeal (MCP) joint. The subjects were instructed to produce their maximum isometric force with a single finger or all four fingers in flexion or extension. In order to examine the effects of muscle-force relationship on MVF and other digit interaction indices, six subjects were randomly selected from the group of 25 adult subjects and asked to perform the same experimental protocol described above. However, the MCP joint was at 800 of flexion. The results from the 20' of MCP joint flexion showed that (1) MVF increased and finger inter-dependency decreased with children's age, (2) the increasing and decreasing absolute slopes (N/year) from regression analysis were steeper in flexion than extension while the relative slopes (%/year) with respect to adults' maximum finger forces were higher in extension than flexion, (3) the larger MVF, FE, and FD were found in flexion than in extension, (4) the finger FS was very similar in children and adults, (5) the FS pattern of individual fingers was different for flexion and extension, and (6) the differences between flexion and extension found at 20 degrees MCP joint conditions were also valid at 80 degrees MCP joint conditions. We conclude that (a) the finger strength and independency increase from 6 to 10 years of age, and the increasing trends are more evident in flexion than in extension as indexed by the absolute slopes, (b) the finger strength and finger independency is greater in flexion than in extension, and (c) the sharing pattern in children appears to develop before 6 years of age or it is an inherent property of the hand neuromusculoskletal system. One noteworthy observation, which requires further investigation, was that FE was slightly smaller in the 80 degrees condition than in the 20 degrees condition for flexion, but larger for extension for all subjects. This may be interpreted as a greater FE when flexor or extensor muscles are stretched.

Does muscular weakness account for younger children's enhanced force variability?

This study examined the degree to which younger children's greater variability in force control is associated with muscular weakness. Children aged 6, 8, and 10 years and adults aged 18-22 years produced isometric force via index finger metacarpo-phalangeal joint flexion to varying force levels (5%, 15%, 25%, and 35% maximal voluntary contraction). The force output of the younger children was more variable and had greater time dependent structure compared to that of the adults at all force levels. However, the effect of age on variability was significantly reduced, but not eliminated when absolute muscular strength was taken into account. It is concluded that age-related changes in children's force control result from a multitude of developmental processes including increases in muscular strength.

Effect of kinetic redundancy on hand digit control in children with DCD

We investigated finger strength and the ability to control digit force/torque production in children with developmental coordination disorder (DCD) using manipulative tasks with different kinetic redundancies (KNR). Age-related changes in finger strength and finger force/torque control in typically developing (TD) children were also examined to provide a developmental landscape that allows a comparison with children with DCD. Forty-eight TD children (7-, 9-, and 11- year-olds) and sixteen 9-year-old children with DCD participated in the study. Three isometric tasks with different KNR were tested: constant index finger pressing force production (KNR=0), constant thumb-index finger pinching force production (KNR=1), and constant thumb-index finger torque production (KNR=5). Each subject performed two conditions for each isometric task: maximum voluntary force/torque production and constant force/torque control (40% of maximum force/torque). The results showed that the maximum force/torque production increased and the variability of constant force/torque control decreased with age in all tasks in TD children. Children with DCD showed larger variability than TD children in the constant thumb-index finger pinching torque production. These results suggest that children with DCD, as compared to TD children, are capable of producing the same level of maximum finger force, but have poor control in manipulation tasks with a large number of kinetic redundancies.

Power and precision grip force control in three-to-five-year-old children: velocity control precedes amplitude control in development

The aim of this study was to examine the development of underlying motor control strategies in young children by characterizing the changes in performance of a visually guided force regulation task using two different grip formations; a whole-hand power grip (developmentally easier) and a thumb-index finger precision grip (developmentally more advanced). Typically developing preschool children (n=50, 3.0-5.5 years) used precision and power grips to perform a ramp and hold task with their dominant and non-dominant hands. Participants performed five trials with each hand and grip holding the force at 30% of their maximum volitional contraction for 3 s. The data were examined for both age-related and performance-related changes in motor performance. Across ages, children increased in strength, decreased in initial overshoot of the target force level, and decreased in rate of force release. Results of a cluster analysis suggest non-linear changes in the development of force control in preschool children, with a plateau in (or maturation of) velocity measures (rate of force increase and force decrease) earlier than in amplitude-related measures (initial force overshoot and force variability).

Short-term plasticity in children's speech motor systems

Speech production is a highly skilled behavior that requires rapid and coordinated movements of the orofacial articulators. Previous studies of speech development have shown that children have more variable articulatory movements compared to adults, and cross-sectional studies have revealed that a gradual transition to more stable movement patterns occurs with age. The focus of the present investigation is on the potential role of short-term changes in speech motor performance related to practice. Thus we developed a paradigm to examine the influences of phonological complexity and practice on children (9 and 10-year-olds) and adults' production of novel nonwords. Using two indices that reflect the degree of trial-to-trial consistency of articulatory movements, we analyzed the first and last five productions of the novel nonwords. Both children and adults accurately produced the novel nonwords; however, children showed a practice effect; their last five trials were more consistently produced than their first five trials. Adults did not show this practice effect. This study provides new evidence that children show short-term changes in their speech coordinative patterns with practice. In addition, the present findings support the contribution of neuromotor noise or background, inherent variability to speech motor development.

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

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

Changes in the structure of children's isometric force variability with practice

This study examined the effect of age and practice on the structure of children's force variability to test the information processing hypothesis that a reduction of sensorimotor system noise accounts in large part for age-related reductions in perceptual-motor performance variability. In the study, 6-year-olds, 10-year-olds, and young adults practiced on 5 consecutive days (15 trials/day), maintaining for 15-s trials a constant level of force (5 or 25% of maximum voluntary contraction) against an object using a pinch grip (thumb and index finger). With increasing age, the amount of force error and variability decreased, but the sequential structure of variability increased in irregularity. With practice, children reduced the amount of variability by changing the structure of the force output so as to be more similar to that of their older counterparts. The findings provide further evidence that practice-driven changes in the structure of force output, rather than a decline in the amount of white noise, largely account for age-related reductions in the amount of force variability.

Adapting to changing task demands: variability in children's response to manipulations of resistance and cadence during pedaling

Reduction in performance variability is characteristic of skill acquisition during childhood. Less understood is the role of variability in adaptive skill. The purpose of this study was to determine children's capacity for adapting to changing task requirements. Children ages 4-14 years and adults rode a stationary ergometer at different levels of cadence and resistance. Younger children were less successful in meeting task requirements. When they did succeed, the younger children were more variable. However, no interactions were found. Variability did not change with resistance, and all groups showed increasing variability as cadence increased. It was concluded that in spite of a weaker synergy (more variability), children were adept to changes in task demand within tested limits.

The relation between force magnitude, force steadiness, and muscle co-contraction in the thumb during precision grip

The aim of this study was to investigate the problem of agonist-antagonist co-contracti during a precision force task performed at different force levels. Using a precision grip, seven young adults performed a constant force matching task (10, 22.5, 35, 47.5, and 60% maximum) as accurately as possible (10 trials per force level). Muscle co-contraction in the thumb was monitored by the surface EMG activity of the extensor pollicis longus (EPL) and the flexor pollicis brevis (FPB), and the ratio between those EMG activities (EPL/FPB). Results showed that both EMG activities increased as grip force increased, but the EPL/FPB ratio decreased over the range of force investigated. Force steadiness (as expressed by the coefficient of variation, CV) appeared as a U-shape function of the force level (with maximal steadiness at 22.5%). Separate analyses at each force level showed no correlation between CV and EMG indices. In addition, the contrast between trials with high and low CV revealed no significant difference in terms of our EMG indices. We conclude that muscle co-contraction and grip force steadiness depend on grip force magnitude, but grip force steadiness does not depend on muscle co-contraction.

Late developmental deficits in force control in children with hemiplegia

Differences related to development were investigated using a finger isometric force task in children with cerebral palsy and control children. The increase in force and force control observed in controls did not take place in patients. In the younger subset of patients (< 10 years) the force produced by the non-affected hand was greater than in either hand of young controls. This effect was not observed in the older subset of patients (< 10 years). Older controls also differed from younger controls in that they used higher frequency feedback loops. In contrast, older patients failed to show this increase. Moreover, the failure occurred both in the affected and non-affected hand, indicating that abnormalities involve the force control system of both sides.

Deterministic and stochastic processes in children's isometric force variability

This study examined the influence of deterministic and stochastic processes (including white Gaussian noise) on reductions in the amount of force output variability through childhood. The structure of the force signal produced during a constant isometric pinch grip task was examined as a function of age (6, 8, and 10 years, and young adults), availability of feedback information (with and without vision), digit (thumb and index finger), and force level (5, 15, 25, and 35% of maximal voluntary contraction). The amount of white Gaussian noise in the force signals was negligible and not age related. The availability of vision led increasingly over the older age groups to lower long-range correlations with more than a single scaling range in a 1/f-like decay process. The reductions in the amount of force variability from childhood to adulthood were related in large part to deterministic organization that increased the adaptive use of higher frequency components, due to the more flexible use of information feedback and feedforward processes.