Development of externally guided grip force modulation in man

Force and Directional Force Modulation Effects on Accuracy and Variability in Low-Level Pinch Force Tracking

The authors investigated how varying the required low-level forces and the direction of force change affect accuracy and variability of force production in a cyclic isometric pinch force tracking task. Eighteen healthy right-handed adult volunteers performed the tracking task over 3 different force ranges. Root mean square error and coefficient of variation were higher at lower force levels and during minimum reversals compared with maximum reversals. Overall, the thumb showed greater root mean square error and coefficient of variation scores than did the index finger during maximum reversals, but not during minimum reversals. The observed impaired performance during minimum reversals might originate from history-dependent mechanisms of force production and highly coupled 2-digit performance.

The brain adjusts grip forces differently according to gravity and inertia: a parabolic flight experiment

In everyday life, one of the most frequent activities involves accelerating and decelerating an object held in precision grip. In many contexts, humans scale and synchronize their grip force (GF), normal to the finger/object contact, in anticipation of the expected tangential load force (LF), resulting from the combination of the gravitational and the inertial forces. In many contexts, GF and LF are linearly coupled. A few studies have examined how we adjust the parameters–gain and offset–of this linear relationship. However, the question remains open as to how the brain adjusts GF regardless of whether LF is generated by different combinations of weight and inertia. Here, we designed conditions to generate equivalent magnitudes of LF by independently varying mass and movement frequency. In a control experiment, we directly manipulated gravity in parabolic flights, while other factors remained constant. We show with a simple computational approach that, to adjust GF, the brain is sensitive to how LFs are produced at the fingertips. This provides clear evidence that the analysis of the origin of LF is performed centrally, and not only at the periphery.

Circadian rhythm in force tracking and in dual task costs

The present study determined a circadian rhythm in force control during a visually guided tracking task under single task conditions (i.e., tracking task presented alone) and dual task conditions (i.e., tracking task together with a memory task). Nine healthy young male subjects participated in a Constant Routine protocol involving a week of regular bedtimes, a baseline night of 8 h sleep, and subsequent wakefulness of 40 h. Subjects performed an eye-hand coordination task that required tracking an unpredictable target (presented on a computer screen) by using grip force to adjust a visual feedback to the changing target. Tracking performance (both in precision and delay) were time-of-day-specific with worst performance at around 04:00 h. The dual task costs, as an index of interference of two tasks performed simultaneously, only showed a significant effect of the memory task on tracking precision during the circadian minimum. In contrast, dual task costs were close to zero during midday and absent in tracking delay. Tracking precision descriptively revealed inter-individual differences: half of the subjects maintained fairly stable performance during the 40 h of wakefulness, whereas the other half showed a clear circadian rhythmicity in tracking precision. Thus, tracking precision seems to be a sensitive parameter for conditions of divided attention and inter-individual variability during the circadian minimum, whereas tracking delay revealed neither a dichotomy of task conditions nor inter-individual differences in performance-amplitude over sessions. Nonetheless, both tracking precision and delay showed a comparable circadian rhythmicity.

Visuomotor training improves stroke-related ipsilesional upper extremity impairments

BACKGROUND

Unilateral middle cerebral artery infarction has been reported to impair bilateral hand grasp.

METHODS

Individuals (5 males and 5 females; age 33-86 years) with chronic unilateral middle cerebral artery stroke (4 right lesions and 6 left lesions) repeatedly lifted a 260-g object. Participants were then trained to lift the object using visuomotor feedback via an oscilloscope that displayed their actual grip force (GF) and a target GF, which roughly matched the physical properties of the object.

RESULTS

The subjects failed to accurately modulate the predictive GF when relying on somatosensory information from the previous lifts. Instead, for all the lifts, they programmed excessive GF equivalent to the force used for the first lift. The predictive GF was lowered for lifts following the removal of the visual feedback. The mean difference in predictive GF between the lifts before and after visual training was significant (4.35 +/- 0.027 N; P <or= .001; 95% confidence interval [CI] = 3.80-4.88). After removal of visual feedback, there was also a significant mean difference in the applied predictive GF between the "early" and "late" lifts (0.78 +/- 0.029 N; P <or= .006; 95% CI = 0.22-1.35), demonstrating continued increase in predictive GF accuracy.

CONCLUSION

Predictive or feedforward fingertip force generation is impaired in the ipsilesional hand when lifting a novel object with precision grip. Reacquisition of the motor forces for the grasp of objects is possible after stroke. Potentially, retraining grasp control for the ipsilesional hand may translate to improved function and motor learning within the contralesional hand.

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.

Basic motor capacity in relation to object manipulation and general manual ability in young children with spastic cerebral palsy

OBJECTIVE

Limited resources in terms of elementary functions may be a limiting factor for functional activities. The objective of the study was to examine basic hand motor capacities in young children with bilateral spastic cerebral palsy (BSCP) and to compare with deficits in functional activities.

METHOD

Eighty-eight children with BSCP, 3-6 years of age, manipulated a grip object (200g) equipped with a uniaxial force sensor. Basic motor capacity was assessed based upon (1) maximal grip strength and (2) production of fast repetitive grip force changes (FFC) while holding the object on the table. Subjects' performance on this task was compared to the grip force amplitude and force rate assessed while the subject was lifting the same object. Results were compared between different degrees of manual ability according to the Manual Ability Classification System (MACS).

RESULTS

In children with BSCP, even in high-functioning children with MACS 1, fast grip force changes and grip strength were 2 SDs and more below the mean of controls. Differences increased from MACS 2 to 4 but not between MACS 1 and 2. During lifting children with BSCP used considerable proportions of their maximum grip strength (40-90%) and of their grip force rates during 70% vs. 86%. In some children with low manual abilities (MACS 3/4), grip force rates during lifting were higher than during FFC.

CONCLUSION

In children with BSCP, basic motor capacity may influence manual ability, particularly in children with MACS 3 and 4. In some of these children, the underlying processes during lifting may also differ qualitatively.

Adolescent development of motor imagery in a visually guided pointing task

The development of action representation during adolescence was investigated using a visually guided pointing motor task (VGPT) to test motor imagery. Forty adolescents (24 males; mean age 13.1 years) and 33 adults (15 males; mean age 27.5 years) were instructed to both execute and imagine hand movements from a starting point to a target of varying size. Reaction time (RT) was measured for both Execution (E) and Imagery (I) conditions. There is typically a close association between time taken to execute and image actions in adults because action execution and action simulation rely on overlapping neural circuitry. Further, representations of actions are governed by the same speed-accuracy trade-off as real actions, as expressed by Fitts' Law. In the current study, performance on the VGPT in both adolescents and adults conformed to Fitts' Law in E and I conditions. However, the strength of association between E and I significantly increased with age, reflecting a refinement in action representation between adolescence and adulthood.

The effect of age on the grip force control in lateral grip

In the paper we present the grip force tracking system for the evaluation of grip force control. We developed a grip measuring device which can be used for the computer assisted measurements of the grip force in real time. The device was used as an input to a force-tracking task where the subject applied the grip force according to the visual feedback from the computer screen. The performance of the task was evaluated by the tracking error between the target signal and the measured force. We assessed the grip force control in the groups of 10-year old children, 25- to 35-year old adults and 50- to 60-year old adults. The subjects performed a sinus tracking task which required periodic muscle activation to produce the desired output. The results of the average tracking errors show significant differences in grip force control among the three tested groups. The largest variability among subjects was observed in the group of children and older adults. No significant difference in force control was found between the dominant and non-dominant hand. The grip force tracking system presented is aimed to be used for the evaluation of grip force control in patients with different sensory-motor impairments.

Task-dependent organization of pinch grip forces

The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb (F(t)) and index finger (F(i)) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant's maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F(t) = F(i) was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F(t) = F(i) coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands.

Development of action representation during adolescence

During adolescence the body undergoes many physical changes. These changes necessitate an updating of internal models of action. Here, we tested the hypothesis that internal models undergo refinement between adolescence and adulthood. We investigated the chronometry of executed and imagined hand actions, which relies on internal models, in 40 adolescents (24 males; mean age 13.1 years) and 33 adults (15 males; mean age 27.5 years). In two different motor imagery tasks, the time it took each participant to execute a hand movement was compared with the time it took them to imagine making that movement. For all participants, movement execution time significantly correlated with movement imagery time. However, there was a significant increase in the execution-imagery time correlation between adolescence and adulthood. Cognitive-motor efficiency per se did not change as indexed by both similar execution and imagery times on both tasks for the adolescents and adults. That it was only the correlation between imagined and executed actions that changed with age suggests that the developmental change was specific to generating accurate motor images and not a result of general cognitive improvement with age. The results support the notion that aspects of internal models are refined during adolescence. We suggest that this refinement may be facilitated by the development of parietal cortex during adolescence.

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).

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.

Force tracking system for the assessment of grip force control in patients with neuromuscular diseases

BACKGROUND

The majority of hand functionality assessment methods consist of the maximal voluntary grip force measurement. Additional knowledge on sensory-motor control can be obtained by capturing functional grip force in a time frame. Tracking methods have been successfully used for the assessment of grip force control in stroke patients and patients with Parkinson's disease.

METHODS

A novel tracking system for the evaluation of grip force control is presented. The system consists of a grip-measuring device with the end-objects of different shapes which was used as input to a tracking task where the patient applied the grip force according to the visual feedback. The grip force control was assessed in 20 patients with neuromuscular diseases and 9 healthy subjects. The performance of two tracking tasks was analysed in five grips. The ramp-tracking task was designed to assess the grip strength and muscle fatigue. The sinus-tracking task was used to evaluate grip force control during periodic muscle activation.

FINDINGS

The results suggest that in some patients the disease did not affect their grip force control despite evident muscular weakness. Most patients produced larger tracking errors in precision grip while the healthy subjects showed less significant differences in performance among the grips tested.

INTERPRETATION

The current study investigated force control in patients with neuromuscular diseases where detection of small changes in motor performance is important when following the progress of disease. The presented evaluation method can provide additional information on muscle activation and fatigue as compared to traditional grip strength testing.

Development of isometric force and force control in children

Fifty-six children between 5 and 12 years of age and 15 adults performed a task (pressing on a lever with the index finger of the preferred hand), in which a force had to be maintained constant at five levels with on-line visual feedback. Since this is a simple isometric task, the hypothesis is that optimal performance (in terms of force variability control) closely matches the maturation of the corticospinal tract up to age 10. It was found that maximum voluntary contraction (MVC) matured over the full range of the examined age groups. In contrast, the coefficient of variation of force showed maturation mainly up to the age of 9-10, as hypothesised. Gender differences were found for MVC but not for the other force control parameters. Power spectral density analysis revealed developmental differences in the lower (1-6 and 7-12 Hz) and higher frequencies bands (13-18 and 19-24 Hz). In the lowest frequency range the amount of energy decreased with age, presumably because young children (5-8 years of age) rely more heavily on control from proprioceptive and visual feedback. It is argued that with increasing neural maturation the control processes become more dependent upon internal representation manifested by feed forward control that starts to substitute feedback-based control.