One of the earliest articles I can find looking at this topic is “Movement analysis in infancy may be useful for early diagnosis of autism” by Teitelbaum et al 1998. It’s a fascinating discussion of analysis of home videos of 17 children. While the analysis showed that movement differences varied from child to child there were some commonalities and the observations were much more in-depth than just documenting a delay in milestones. They indicated that movement differences were present at 4-6 months of age.

The observations were as follows:

1. There was persistent asymmetry in a variety of positions: in the trunk in prone and sitting, in the legs in crawling, in the arms and legs when walking.
2. Impairment in head and body righting reactions in rolling. Rather than developing a segmental rolling pattern (involving rotation around the body axis), 3 autistic children moved their body as a block, developing compensations to accomplish the task.
3. Difficulty weight bearing on extended arms during crawling.
4. Challenge with development of mature gait: arms maintained in a high guard position, decreased activation of the muscles in the shank and foot resulting in a continued foot flat gait pattern (without the development of weight transfer through the foot), delayed weight shift of the trunk during swing phase.