Developmental profile of slow hand movement oscillation coupling in humans.

In adults, slow hand and finger movements are characterized by 6- to 12-Hz discontinuities visible in the raw records and spectra of motion signals such as acceleration. This pulsitile behavior is correlated with motor unit synchronization at 6-12 Hz as shown by significant coherence at these frequencies between pairs of motor units and between the motor units and the acceleration recorded from the limb part controlled by the muscle, suggesting that it has a central origin. In this study, we examined the correlation between this 6- to 12-Hz pulsatile behavior and muscle activity as a function of childhood development. Sixty-eight participants (ages 4-25 yr) performed static wrist extensions against gravity or slow wrist extension and flexion movements while extensor carpi radialis muscle electromyographic (EMG) and wrist acceleration signals were simultaneously recorded. Coherence between EMG and acceleration within the 6- to 12-Hz frequency band was used as an index of the strength of the relation between central drive and the motor output. The main findings of the study are 1) EMG-acceleration coherence increased with increases in age, with the age differences being greater under movement conditions and the difference between conditions increasing with age; 2) the EMG signal showed increases in normalized power with increases in age under both conditions; and 3) coherence under movement conditions was moderately positively correlated with manual dexterity. These findings indicate that the strength of the 6- to 12-Hz central oscillatory drive to the motor output increases through childhood development and may contribute to age-related improvements in motor skills.

Tremor frequency profile as a function of level of mental retardation.

The characteristic slowness of movement initiation and execution in adult individuals with mental retardation may be driven by the slower frequency profile of the dynamics of the system. To investigate this hypothesis, we examined the resting and postural finger tremor frequency profile (single and dual limb) of adults as a function of level of mental retardation (moderate, severe, profound). There was a progressive increase in the contribution of slow frequency components to the enhanced amplitude of tremor as a function of mental retardation, particularly in the group with profound mental retardation. Findings support the hypothesis of mental retardation inducing a slower frequency to the system dynamics that may fundamentally drive the characteristic slowness of movement behavior.

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.

Age-related changes in the frequency profile of children's finger tremor.

Little consensus exists as to the age-related pattern of change in the frequency characteristics of postural tremor through childhood. We investigated postural finger tremor of children (6 and 10 years) and adults (18-22 years) using accelerometers under dual and single limb conditions (10s trials). The postural tremor of the children exhibited proportionally more power below 10 Hz and less power above 20 Hz than that of the adults. It also showed a significantly lower peak frequency and lower proportion of power at the peak frequency than the adults in the 15-30 Hz frequency band but did not differ significantly from the adults in peak frequency or proportion of power at the peak frequency in the 5-15 Hz frequency band. The greater relative contribution of fast time scales over the 1-30 Hz frequency band in the organization of the postural tremor of the adults in comparison to the children may be a contributing factor to adult's typically observed reduced motor skill performance variability.

The intralimb coordination of the flexor reflex response is altered in chronic human spinal cord injury.

The current study compared the intralimb coordination of flexor reflex responses in spinal intact and complete chronic spinal cord injured (SCI) individuals. Noxious electrocutaneous stimulation was applied at the apex of the medial arch of the foot (50 mA, 500 Hz, 1 ms pulse width, 20 ms) in 21 complete chronic SCI and 19 spinal intact volunteers and the flexor reflex response was quantified by measuring the isometric joint torques at the ankle, knee and hip. The results showed that SCI individuals had significantly smaller peak knee and hip joint flexion torques, often exhibited a net knee extension torque, and produced a much smaller hip joint flexion torque during the flexor reflex response in contrast to the spinal intact individuals. The latency of the reflex response, measured from the tibialis anterior electromyogram, was comparable in both test populations. These findings indicate that the intralimb coordination of the flexor reflex response of chronic complete SCI individuals is altered, possibly reflecting a functional reorganization of the flexion pathways of the spinal cord.

Intra-limb segmental influences on random-like movements in humans.

The experiment investigated the influence of segmental coupling on the ability to produce random-like movements in individual limb segments. Adult participants were instructed to move randomly (2 min trials) in the sagittal plane their index finger, hand, and lower arm as "frozen" effector units or where the individual links within the upper limb complex were free to move independently. The findings showed that the distal finger movements were more random-like when the proximal joints were also free to vary, but the reverse directional segmental effects were not present. Analysis of the movement frequency structure of the coordination between limb segments showed that patterns of modal frequencies were preserved even though the participants were trying to produce with equal probability a wide range of frequencies. These findings provide further evidence that: (1) the boundary conditions on the degrees of freedom of the neural output of an effector are relatively restrictive; (2) inter-limb reactive forces can enhance the limits on the dynamical degrees of freedom; and (3) the intrinsic dynamics influence movement output even when the task goal is a random output.

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.

Task goals and change in dynamical degrees of freedom with motor learning.

In this article, the authors examined the hypothesis that the direction of the change (increase or decrease) in the dynamical degrees of freedom (dimension) regulated as a function of motor learning is task-dependent. Adult participants learned 1 of 2 isometric force-production tasks (Experiment 1: constant force output; Experiment 2: sinusoidal force output) over 5 days of practice and a 6th day with augmented information withdrawal. The results showed that over practice, the task goal induced either an increase (Experiment 1) or a decrease (Experiment 2) in the dimension of force output as performance error was reduced. These findings support the proposition that the observed increase or decrease in dimension with learning is dependent on both the intrinsic dynamics of the system and the short-term change required to realize the task goal.

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.

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

This study examined the role of sensorimotor system noise in the organization of the force output of the thumb and index finger and the coordination between the two digits in an isometric pinch grip force task as a function of age (6, 8, 10, 18-22 years), feedback condition (with and without visual feedback information), and force level (5, 15, 25, and 35% of maximal voluntary force. With increases in age under the visual feedback conditions, the signal-to-noise ratio increased, the sequential structure of the force output signals became more irregular, the degree of coherence between the digits at higher force levels was enhanced, and the digits exhibited a greater degree of coherence across the higher frequencies of the power spectrum at all force levels. However, these age differences were either minimized or eliminated under conditions without feedback. These findings show that the age-related performance differences in grip force variability are primarily due to more effective use of visual information rather than age differences in intrinsic sensorimotor noise.