Development of motor programming capacity during lifting a task with a precision grip in infants

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This study investigated the capacity to generate programmed isometric forces in infants (N = 49, age = 10 month old - 6 years) during lifting varied object weights using a precision grip. Grip force and the vertical lifting force, and vertical position of the object were recorded. The subjects lifted the object of varied weights (200 g and 400 g) 20 - 30 times per day over 5 days. Object weight was changed in a random manner. It was found that for the 1st day data regardless of the object weight younger children below 5 years compared to adults were significantly slow in sequencing the task, and their grip forces were higher. Grip force rate (dGF/dt) curves in 0 and 1 year old infants contained multiple peaks, while the curves with a unimodal peak were revealed from 2 year old infants. Grip force lifting force coupling action was poor in infants below 5 years old. For the 5th day data, similar results to the 1st day data were also found, but compared to the 1st day results, there was improvement of the performance in both movement time and force magnitude particularly in infants aged 2 - 4 years old. Motor programming capacity in relation to object weight was examined in the data of unexpected weight change. It was found that with an increase in age the effect of unexpected weight change became more evident; longer timing and force magnitude correction were revealed as age of the infants increased. After five days of lifting practice, the effect of unexpected weight change became more clear even in 10 month old infants. Improvement was quite evident in 2 - 4 year old infants, suggesting that these periods were important in the acquisition of programming capacity.