Age differences in the magnitude of associated movement.

Using a quantitative measure of unintended mirror-movements in the contralateral limb during a unimanual task, the magnitude of associated movement across the ages from six to 16 years was determined. Male children in five age-groups (means 6.5, 8.5, 10.4, 12.4 and 16.5 years) were asked to squeeze their index finger and thumb together to various percentages of their own maximal volitional force. Results indicate that the 6.5-year-old group differ from all other age-groups, exhibiting significantly greater associated movements at all levels of force. The results are discussed in terms of the development of inhibitory control over innate neuromotor synergies.