Table I.
List of dependent measures.
| Dependent measure | Definition |
|---|---|
| Reach Phase | |
| Number of movement units (MU) |
A count of the number of speed peaks in a reach. A speed peak was defined as maximum speed between two minima where the difference between a minimum and the peak must be at least 10 mm/s (Clifton, RK, Rochat P, Robin DJ, Berthier NE. Multimodal perception in the control of infant reaching. 1994. J Exp Psychol Hum Percept Perform 29(4):876–886) The peak speed must also be greater than 20% of the maximum hand speed (Konczak J, Dichgans J. The development toward stereotypic arm kinematics during reaching in the first 3 years of life. 1997. Exp Brain Res 117(2):346–354) |
| Proportion of time the first movement unit (MU) |
During the reach, the time from movement onset until the end of the first MU. A lower proportion indicates a jerky trajectory at the onset of the reach. |
| Peak velocity (cm/s) | The highest velocity of the hand over the course of a reach (Babinsky E; Braddick O, Atkinson J. The effect of removing visual information on reach control in young children. 2012. Exp. Brain Res 222:291–302) |
| Proportion of maximum aperture |
During the reach, the child opens his thumb and pointer fingers to prepare for the grasp. Higher proportion indicates a wider aperture than is necessary to grasp the object. |
| Drop Phase | |
| Phase duration (ms) | The time of the reach from entering the drop phase (4 cm from the hole) to releasing the object |
| Number of minor MU | A count of minor speed peaks in the drop phase. A speed peak was defined as a maximum speed between two minima. The criterion of reaching greater than 20% of the maximum phase speed was not applied. |
| Straightness | Determined by dividing the distance the hand traveled (hand Path) by the straight-line distance between entering the drop zone and the hand’s position at object release. A ratio of 1.0 would represent a perfectly straight trajectory. Increasing values reflect greater deviation from the straight line (von Hofsten. Development of visually guided reaching: the approach phase. 1979. J Hum Mov Stud 5:160–178) |