Research report
Development of isometric force and force control in children

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Abstract

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.

Introduction

Many factors underlie the improvement in manual force control in children, including structural maturation of the neuromotor system and changes in the use of sensorimotor pathways. Muscles grow and become stronger until adulthood [23], [27]. Muscle cross-sectional area doubles between 5 and 20 years of age. This increment is caused by an increase in mean fibre diameter and fibre density, accompanied by a functional development of the fibre population. Fibre type development is strongly influenced by intrinsic genetic programs, gender, neuronal input, function of the muscle and physical activity [13]. Although there are differences between the rate of fibre growth with age, muscle, fibre type and between individuals, the common trend during development is that the proportion of type II fibres increases significantly from approximately 35% at the age of 5 to approximately 50% at the age of 20 [21], [33]. Moreover, not only the contractile properties of muscles change but so does the central nervous system and its control over the increasing force in the growing muscles. Maturation of the peripheral efferent and afferent pathways reaches adult values around the age of 3. Myelinisation in spinal root and peripheral nerves is complete by the age of 2 and axon diameters attain adult values by 2–5 years [15], [28].

Corticomotorneuronal connections constitute an essential anatomical substrate for relatively independent finger movements [20]. In contrast to the fast peripheral changes, both efferent and afferent central pathways show a prolonged maturational pattern. Adult values are reached by the age of 5–7 years for the afferent pathways while the developmental profile of the cortico-motorneuronal pathways reach maturity around 8–10 years of age [26], [25]. However, Eyre et al. [9] for example, has argued that the maximum fibre diameters in motor and somatosensory central pathways increase in proportion to height and thus leading to constant central conduction delays with growth from 2 years of age. After measuring latency differences for motor evoked potentials between relaxed and contracted muscles, Caramia et al. [5] indicated that the maturation of the corticospinal pathway proceeds until 10–12 years of age. Possibly related to this, Lin et al. [22] found that muscle relaxation time halved between the age of 3 (about 90 ms) and 10 (40 ms). Additionally, the level of synchronisation of the motor units increases until around 11 years of age [14]. These findings related to the development of the corticospinal tract suggest that one should find equivalent maturation in motor behaviour. However, little is known how central development is expressed in behavioural tasks. In a comprehensive study, Fietzek et al. [10] have tried to correlate maturation of the corticospinal tract, as evaluated through magnetic stimulation, to the development of motor functions in the same group of children. They found many parallels but also a discrepancy between the time course of development of corticospinal tracts (saturation at age 10) and the performance in tracking tasks (yielding a saturation at about 15 years of age). Furthermore, tracking is a complex motor skill relying not only on the corticospinal tract but also on other motor areas such as the parietal cortex.

These age-related changes seem to go from a feed forward strategy with intermittent use of sensorimotor feedback towards a fairly parallel and well-integrated feedback and feed forward processing [16]. Tasks in which children had to lift an object have been used successfully to describe sensorimotor development and specifically the adaptation of grip and load force to mass, shape and surface friction [11], [12]. However, to test force generation and optimisation of force control per se, there is a need to use a simpler task. Moreover, since variability of force is related to absolute levels of force [19]. In the present study force control development is, contrary to other studies [4], [11], [12], described in terms of CV, coefficient of variation of force, an outcome variable relative to MVC and over a large range of force levels. An experiment was designed in which isometric force had to be maintained at five relative force levels. Since this is a simple isometric task, the hypothesis is that optimal performance (in terms of force variability control) is expected to match more closely the maturation of the corticospinal tract than does tracking performance. This variability control should show an increase mainly up to the age of 10. Power spectral density analysis (PSDA) will be used to investigate the relationship between variability in motor performance and the underlying motor control mechanisms of the recorded signal in terms of its frequency content [6], [29]. Developmental changes of the underlying mechanisms have been studied before by comparing the relative contribution of the underlying oscillations to the recorded signal [8], [31], [32]. As children age it is expected that they improve their ability to utilise available feedback information more effectively to realize task demands [7].

No gender differences in force development are expected until the unset of puberty, when the hormonal differences in the growth of muscle fibres between boys and girls are beginning to show [2], [33]. With respect to the development of the maximum force production our prediction is that there should be an increase over the full range of the examined age groups since we speculate that this parameter is primarily related to muscle growth.

Section snippets

Materials and methods

Sixty children of three primary schools were approached to take part in the study. The inclusion criteria consisted of confirmed right-handedness [1] and no developmental delay, motor problems or muscle diseases (confirmed by scores above the 5th percentile on the Manual dexterity subscore of the Movement ABC) [17], [30]. After checking for the inclusion criteria, 56 children (mean age 9.1 year, S.D. 2.08) entered the study, 24 boys (43%) and 32 girls (57%) belonging to four age groups. Age

Results

First the data were checked for gender differences. Gender differences were only found with respect to the MVC. There were no interaction effects of gender and generated force with age, force percentage or repetitions. It illustrates that difference in force stayed the same, as the girls grew older and that girls were not more fatigued or showing a different learning curve. None of the other measures (force compliance, CV, time to peak and to release) showed significant differences between the

Discussion

The present results show a continuous increase of maximum force with development, which is very gradual between 5 and 10 years of age and becomes more pronounced after the age of 11. The time needed to contract the muscle develops already early (7–8 years), consistent with the findings of others showing that ballistic movements mature quite fast [10]. The principle new finding of the present study is that for a simple force control task using a single finger, the maturation of force regulation

Acknowledgments

We wish to thank all the children and their parents for their commitment and willingness to participate in this study and Mr Peter de Jong for developing the OASIS software needed in this experiment.

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