Early independent walking: a longitudinal study of load perturbation effects.

This study investigated infants' ability to adapt to experimentally induced changes in their body dimensions at walk onset, and how this ability is affected by increased walking experience. Fifteen infants were studied over their first 6 months of independent walking with a load perturbation design. They traversed a walkway with loads symmetrically placed around the shoulders, waist, or ankles, and without loading. At walk onset, infants fell more with shoulder and ankle loads than with waist or no loads. Shoulder loads further resulted in higher walking speed and longer steps, while waist loads resulted in increased walking speed and larger foot rotation. Ankle loads disrupted walking proficiency the most, as indicated by lower walking speed, shorter steps, larger foot rotation, and smaller step-to-step angle. Step width was not differentially affected by the conditions. With increased experience, walking proficiency increased across all conditions, but ankle loads lagged behind the other conditions. Loading effects are discussed with respect to walking experience and position of the loads on the body.