Inclusion Criteria:  age between 7 and 9 years. able to follow one step motor commands. independent ambulators. not suffering from atlantoaxial joint instability as reported from their medical files.  Exclusion Criteria:  Children with un-corrected cardiac anomalies. Children with un-corrected vision or hearing loss. Children with un-corrected pulmonary disorders. Children with un-corrected neurological signs as epilepsy. Children who had a history of musculoskeletal surgery in the lower extremities through the past year.