Background & Objectives: Based on the evidence, motor skills in children with Down syndrome usually develop similarly to normal children, but at a slower rate and with some delay. People with Down syndrome show some setbacks in developing motor skills and balance, which is evident during childhood and adolescence. Postural stability plays an essential role in daily activities and motor independence. Compared to the general population and other people with intellectual disabilities, people with Down syndrome gain lower scores in equilibrium tests. Various tests have been used in different studies to assess the balance in Down syndrome subjects, and few studies have been conducted to measure the balance using functional tests. One of the benefits of functional tests is their generalization ability compared to laboratory tests. This study aimed to investigate the inter–class and intra–class reliability of the motor performance tests of static and dynamic balance in people with Down syndrome in Tehran.
Methods: The present research used a descriptive survey method. The study participants were 50 people (both genders, 24.4±1.6 years old, IQ=76.8±3.2) with Down syndrome who were purposefully selected with an available sampling method after applying the inclusion and exclusion criteria. The inclusion criteria included the ability to walk without assistance and follow at least three unrelated instructions, with an IQ range of 60 to 85, without severe visual, cognitive, behavioral, or language problems, no vestibular and hearing impairments, or ear infections. Also, they should not have used drugs related to muscle function and systems involved in maintaining balance in the last six months, with no lower limb fractures, lower limb injuries, or movement problems. All samples were checked through their medical records. The exclusion criteria were dissatisfaction with continuing the study, performing physical activities outside the care plan, and not performing correctly in two test measurements. The study motor performance tests of the static and dynamic balance were the Balance Error Scoring System (BESS), Romberg, Sharpened Romberg, Stork Stand, Timed Up and Go (TUG), and tandem walk. Each participant should perform two attempts in each test and has 15 seconds of rest after each trial. The timing of both correct attempts in each session was recorded with a timer. Those who did not perform the test correctly should repeat it. All tests were taken within seven consecutive days by all participants, and one week after taking the test from the last sample, the next testing round was performed in the same manner and with the same testers. The order of the tests was the same for all participants. Inter–class and intra–class correlation coefficients were used to verify the tests' reliabilities. All statistical tests were performed in SPSS–23 software at a significant level of α=0.05. 
Results: The reliabilities of balance performance tests by examining the inter–class correlation (in one session) and intra–class correlation (between the sessions; between two measurements with an interval of one week) were confirmed (p<0.05) in the BESS test, double–leg stance on a firm surface ( 0.950 and 0.857, respectively), the BESS test, single–leg stance on a foam surface ( 0.962 and 0.785, respectively), tandem stance on a firm surface (0.938 and 0.823, respectively), double–leg stance on a foam surface (0.936 and 0.863, respectively), single–leg stance on a foam surface (0.968 and 0.777, respectively), tandem stance on a foam surface ( 0.932 and 0.787, respectively), Romberg test (0.739 and 0.724, respectively), Sharpened Romberg test (0.941 and 0.643, respectively), Stork test (0.779 and 0.839, respectively), Timed Up and Go (TUG) test (0.904 and 0.700, respectively) and tandem walk test (0.878 and 0.634, respectively).
Conclusion: According to the research findings, motor performance tests for the balance of BESS, Romberg, Sharpened Romberg, Stork, TUG, and tandem walk are reliable tests for adults with Down syndrome.