Variability in development of overarm throwing: a longitudinal case study over the first 6 months of throwing.

One female subject of 15 months of age, at the onset of over-arm-throwing behaviour, was tested on a longitudinal study of throwing development. Data were collected at the onset of throwing and monthly thereafter, producing 6 sets of data. Kinematic variables were obtained using 3-dimensional video analysis and digitization. Qualitative observations showed that both arm-dominated and sequentially linked throws, right- and left-handed throws, and homolateral and contralateral forward steps were generated in an array of inconsistent throwing. Sequentially linked throws were generally 'interrupted', whereby the child paused briefly after the Back swing to focus externally, then the child executed the propulsion. The throwing elbow remained flexed at ball release. Angles of ball release (referred to the horizontal) fluctuated from 2.17 degrees to 28.03 degrees for all 6 months of throwing development, and the speed of ball release varied from 2.08 m/sec. to 4.32 m/sec. Height of ball release oscillated between 91.5% and 103.3% of the child's height. Horizontal and vertical components of the velocity of the ball while in the hand differed amongst both arm-dominated and sequentially linked throws. The time of the Push up phase in arm-dominated throws varied from 0.14 sec. to 0.50 sec. In sequentially linked throws the time of the Back swing ranged from 0.18 sec. to 0.22 sec., and the Propulsion varied from 0.06 sec. to 0.14 sec. This work in identifying such variability is important, therefore, in the understanding of the motor skill of throwing.