Abstract
Infants explore their environments often using repetitive movements of multiple degrees of freedom to create a motor sequence. Previous research suggests that infants suspended in a commercial spring-mounted harness bounce at two frequencies using distinct mechanisms of control (Vallis, 1998). We determined how typically developing infants bounced when the system natural frequency was experimentally modified. Methods Infants aged 5 to 12 months bounced while suspended over two AMTI force platforms from four spring systems with natural frequencies of 1.0 (C1), 1.3 (C2), 1.5 (C3) and 2.0 (C4) Hz. Kinematic data and vertical ground reaction forces (VGRFs) were recorded continuously. Results Across conditions, infants spent a relatively equal percent of time on the ground and in the air during each bounce cycle with the peak VGRFs decreasing as the bounce frequency increased. Bounce heights varied as a function of bounce strategy: a) infants who bounced at similar frequencies across conditions (C1 to C4 from 2.01 to 2.28 Hz) generated similar bounce heights in all conditions; while b) bounce height decreased across conditions in infants whose bounce frequency changed as a function of natural spring frequency (C1: 1.05 Hz; C2: 1.25-1.29 Hz; C3: 1.42-1.43 Hz; C4: 1.75-1.76 Hz). Discussion Infants adapt to different system natural frequencies by either maintaining a fixed frequency across conditions or by adapting the bounce frequency to closely match the system frequency.
