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

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.

Precision Sensorimotor Control in Aging FMR1 Gene Premutation Carriers

BACKGROUND

Individuals with premutation alleles of the FMR1 gene are at risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS), a neurodegenerative condition affecting sensorimotor function. Information on quantitative symptom traits associated with aging in premutation carriers is needed to clarify neurodegenerative processes contributing to FXTAS.

MATERIALS AND METHODS

26 FMR1 premutation carriers ages 44-77 years and 31 age-matched healthy controls completed rapid (2 s) and sustained (8 s) visually guided precision gripping tasks. Individuals pressed at multiple force levels to determine the impact of increasing the difficulty of sensorimotor actions on precision behavior. During initial pressing, reaction time, the rate at which individuals increased their force, the duration of pressing, and force accuracy were measured. During sustained gripping, the complexity of the force time series, force variability, and mean force were examined. During relaxation, the rate at which individuals decreased their force was measured. We also examined the relationships between visuomotor behavior and cytosine-guanine-guanine (CGG) repeat length and clinically rated FXTAS symptoms.

RESULTS

Relative to controls, premutation carriers showed reduced rates of initial force generation during rapid motor actions and longer durations of their initial pressing with their dominant hand. During sustained force, premutation carriers demonstrated reduced force complexity, though this effect was specific to younger premutation carries during dominant hand pressing and was more severe for younger relative to older premutation carriers at low and medium force levels. Increased reaction time and lower sustained force complexity each were associated with greater CGG repeat length for premutation carriers. Increased reaction time and increased sustained force variability were associated with more severe clinically rated FXTAS symptoms.

CONCLUSION

Overall our findings suggest multiple sensorimotor processes are disrupted in aging premutation carriers, including initial force control guided by feedforward mechanisms and sustained sensorimotor behaviors guided by sensory feedback control processes. Results indicating that sensorimotor issues in aging premutation carriers relate to both greater CGG repeat length and clinically rated FXTAS symptoms suggest that quantitative tests of precision sensorimotor ability may serve as key targets for monitoring FXTAS risk and progression.

Force and electromyography responses during isometric force release of different rates and step-down magnitudes

This study investigated motor responses of force release during isometric elbow flexion by comparing effects of different ramp durations and step-down magnitudes. Twelve right-handed participants (age: 23.1 ± 1.1) performed trajectory tracking tasks. Participants were instructed to release their force from the reference magnitude (REF; 40% of maximal voluntary contraction force) to a step-down magnitude of 67% REF or 33% REF and maintain the released magnitude. Force release was guided by ramp durations of either 1 s or 5 s. Electromyography of the biceps brachii and triceps brachii was performed during the experimental task, and the co-contraction ratio was evaluated. Force output was recorded to evaluate the parameters of motor performance, such as force variability and overshoot ratio. Although a longer ramp duration of 5 s decreased the force variability and overshoot ratio than did shorter ramp duration of 1 s, higher perceived exertion and co-contraction ratio were followed. Force variability was greater when force was released to the step-down magnitude of 33% REF than that when the magnitude was 67% REF, however, the overshoot ratio showed opposite results. This study provided evidence proving that motor control strategies adopted for force release were affected by both duration and step-down magnitude. In particular, it implies that different control strategies are required according to the level of step-down magnitude with a relatively short ramp duration.

Effects of practice on visual finger-force control in children at risk of developmental coordination disorder

BACKGROUND

The production of finger force control is essential for a large number of daily activities. There is evidence that deficits in the mechanisms of accuracy and control of finger force tasks are associated with children's motor difficulties.

OBJECTIVE

To compare the effect of practice of an isometric finger force/torque task between children with significant movement difficulty and those with no difficulty movement.

METHODS

Twenty-four children aged between 9 and 10 years (12 at risk of developmental coordination disorder and 12 with no movement difficulty - typically developing children) were asked to produce finger force/torque control in a continuous and constant 25% of maximum voluntary torque with visual feedback during 15s. Practice was given during five consecutive days with 15 trials per day. After the practice with visual feedback, children were asked to perform five trials without visual feedback. In these trials, feedback was removed 5s after the start of the trial.

RESULTS

Typically developing children were consistently more accurate in maintaining finger force/torque control than those children at risk of developmental coordination disorder. Children from both groups improved the performance in the task according to practice sessions. Also, children at risk of developmental coordination disorder poorly performed the task without visual feedback as they did when visual feedback was available.

CONCLUSION

The present study give support to the idea that movement difficulty is associated with finger force/torque control and children at risk of developmental coordination disorder can improve finger force/torque control with practice when visual feedback is available.

Individuals with autism spectrum disorder show abnormalities during initial and subsequent phases of precision gripping

Sensorimotor impairments are common in autism spectrum disorder (ASD), but they are not well understood. Here we examined force control during initial pulses and the subsequent rise, sustained, and relaxation phases of precision gripping in 34 individuals with ASD and 25 healthy control subjects. Participants pressed on opposing load cells with their thumb and index finger while receiving visual feedback regarding their performance. They completed 2- and 8-s trials during which they pressed at 15%, 45%, or 85% of their maximum force. Initial pulses guided by feedforward control mechanisms, sustained force output controlled by visual feedback processes, and force relaxation rates all were examined. Control subjects favored an initial pulse strategy characterized by a rapid increase in and then relaxation of force when the target force was low (Type 1). When the target force level or duration of trials was increased, control subjects transitioned to a strategy in which they more gradually increased their force, paused, and then increased their force again. Individuals with ASD showed a more persistent bias toward the Type 1 strategy at higher force levels and during longer trials, and their initial force output was less accurate than that of control subjects. Patients showed increased force variability compared with control subjects when attempting to sustain a constant force level. During the relaxation phase, they showed reduced rates of force decrease. These findings suggest that both feedforward and feedback motor control mechanisms are compromised in ASD and these deficits may contribute to the dyspraxia and sensorimotor abnormalities often seen in this disorder.

Increases in cognitive and linguistic processing primarily account for increases in speaking rate with age

Age-related increases of speaking rate are not fully understood, but have been attributed to gains in biologic factors and learned skills that support speech production. This study investigated developmental changes in speaking rate and articulatory kinematics of participants aged 4 (N = 7), 7 (N = 10), 10 (N = 9), 13 (N = 7), 16 (N = 9) years, and young adults (N = 11) in speaking tasks varying in task demands. Speaking rate increased with age, with decreases in pauses and articulator displacements but not increases in articulator movement speed. Movement speed did not appear to constrain the speaking. Rather, age-related increases in speaking rate are due to gains in cognitive and linguistic processing and speech motor control.

Verbal Regulation of Grip Force in Preschoolers

The purpose of this study was to clarify the developmental processes in verbal regulation by preschool children. Participants were 152 typically developing children (74 boys, 78 girls) between 4 and 6 years of age ( M = 5.3, SD =.8), and 30 healthy adults (15 men, 15 women) between 19 and 26 years of age ( M = 20.8, SD = 1.4). In Exp. 1, the task was to regulate grip force based on quantitative instruction which implies using a scale for regulation. Participants were required to produce a half-grip force of the maximum (Task 1). In Exp. 2, the task was grip-force regulation based on nonquantitative instruction. The participants were asked to respond with a slightly weaker grip force than the maximum (Task 2) and then a further weaker grip force (Task 3) than that used on Task 2. The regulation rates produced the extent of regulation and suggest regulation by quantitative instruction may develop earlier than by nonquantitative instruction. Also, precise grip-force regulation based on the semantic aspect of instruction may be difficult for young children. The developmental changes in the rate of performance especially observed in children of 4 to 6 years indicate that the tendency to use too much grip force disappears during this preschool period. In addition, too little grip force in regulation may reflect the developmental process toward fine grasping movements.

Grip strength in children: test-retest reliability using Grippit

AIM

To examine the reliability of peak and sustained grip strength in children using the Grippit. We compared the reliability of one trial, the best and mean of three trials and the reliability within 6-, 10- and 14-year-old groups.

DESIGN

Test-retest study.

METHODS

Grip strength with the Grippit instrument was evaluated at two test occasions in 58 children. Intraclass correlation coefficient 2.1 (ICC), standard error of measurement (SEM) also expressed as a percentage value (SEM%), coefficient of repeatability and coefficient of variation were calculated.

RESULTS

The test-retest reliability was good for both peak and sustained grip strength. The mean and best of three trials were equally reliable. Peak grip strength best of three trials, for example, was more reliable for the 6-year-olds (ICC 0.96, SEM% 6.3) and 14-year-olds (ICC 0.96, SEM% 5.2) compared to the 10-year-olds (ICC 0.78, SEM% 12.5). For peak grip strength in the whole sample, there was a systematic bias with better performance at the second measurement.

CONCLUSION

Grip strength assessment in children with Grippit results in good relative and absolute reliability for peak and sustained grip strength. We recommend that three test trials are used. The mean of and the best of three trials are comparable.

Age-related changes in multi-finger interactions in adults during maximum voluntary finger force production tasks

This study aimed to continue our characterization of finger strength and multi-finger interactions across the lifespan to include those in their 60s and older. Building on our previous study of children, we examined young and elderly adults during isometric finger flexion and extension tasks. Sixteen young and 16 elderly, gender-matched participants produced maximum force using either a single finger or all four fingers in flexion and extension. The maximum voluntary finger force (MVF), the percentage contributions of individual finger forces to the sum of individual finger forces during four-finger MVF task (force sharing), and the non-task finger forces during a task finger MVF task (force enslaving), were computed as dependent variables. Force enslaving during finger extension was greater than during flexion in both young and elderly groups. The flexion-extension difference was greater in the elderly than the young adult group. The greater independency in flexion may result from more frequent use of finger flexion in everyday manipulation tasks. The non-task fingers closer to a task finger produced greater enslaving force than non-task fingers farther from the task finger. The force sharing pattern was not different between age groups. Our findings suggest that finger strength decreases over the aging process, finger independency for flexion increases throughout development, and force sharing pattern remains constant across the lifespan.

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.