Autism Spectrum Disorders (ASD) is a diagnosis that describes kids with significant social, communicative, and behavioral challenges. While the diagnosis is mostly associated with difficulty with communication, there are also common physical issues experienced by children with ASD.
They may have low muscle tone, or have a tough time with gross motor coordination (running, kicking, throwing, etc.). These issues can interfere with basic day-to-day functioning -- and they're almost certain to interfere with social and physical development.
Children with autism often develop typically for a short period of time, and then present symptoms as toddlers. Physical symptoms range from difficulty with coordination to lack of muscular strength.
Perhaps most significantly, autistic children are likely to have difficulty with "motor planning." In other words, they may have the skills to climb onto a swing and be able to hang on -- but they may have a very difficult time coordinating their bodies to "pump" and get the swing moving.
New research on those with autism found that children with better motor skills are more proficient at socialization and communication than those who have physical deficits.