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1 Department of Neuromotor Physiology, Santa Lucia Foundation, Rome, Italy; Department of Neuroscience, University of Rome Tor Vergata, Rome, Idaho, Italy
2 Department of Neuromotor Physiology, Santa Lucia Foundation, Rome, Italy
3 Department of Neuromotor Physiology, Santa Lucia Foundation, Rome, Italy; Department of Neuroscience, University of Rome Tor Vergata, Rome, Italy; Centre of Space Bio-medicine, University of Rome Tor Vergata, Rome, Italy
* To whom correspondence should be addressed. E-mail: y.ivanenko{at}hsantalucia.it.
On Earth, body weight is an inherent constraint and accordingly load-regulating mechanisms play an important role in terrestrial locomotion. How do toddlers deal with the effects of their full body weight when faced with the task of independent upright locomotion for the first time? Here we studied the effect of load variation on walking in twelve toddlers during their first unsupported steps, fifteen older children (1.3-5yr), and ten adults. To simulate various levels of body weight, an experimenter held the trunk of the subject with both hands and supplied an approximately constant vertical force during stepping on a force platform. During unsupported stepping, the shape of the foot path in toddlers (typically single-peak toe trajectory) was different from that of adults and older children (double-peak trajectory). In contrast to adults and older children, who showed only limited changes in kinematic coordination, the 'reduced-gravity' condition considerably affected the shape of the foot path in toddlers: they tended to make a high lift and forward foot overshoot at the end of swing. In addition, stepping at high levels of body unloading was characterized by a significant change in the initial direction of foot motion during early swing. Intermediate walkers (1.5-5mo after walking onset) showed only partial improvement in foot trajectory characteristics. The results suggest that, at the onset of walking, changes in vertical body loads are not compensated accurately by the kinematic controllers, compensation necessitating a few months of independent walking experience.
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