Infant Behavior and Development published research on the attainment of sitting and walking and the development of productive vocabulary between ages 16 and 28 months. This longitudinal study found that the earlier the age of unsupported sitting predicted a larger productive vocabulary. In addition, an earlier age of walking predicted a faster rate of growth in productive vocabulary. The researchers concluded that the achievement of motor milestones enhance linguistic development.
It may help to add this research to justify adaptive equipment such as supportive seating or gait trainers.
Reference: Ora Oudgenoeg-Paz, M. (Chiel) J.M. Volman, Paul P.M. Leseman. Attainment of sitting and walking predicts development of productive vocabulary between ages 16 and 28 months. Infant Behavior and Development. Volume 35, Issue 4, December 2012, Pages 733–736.