Extended Abstract
1. Introduction
own Syndrome (DS) is the most common genetic disease in people with Intellectual Disability (ID), with a prevalence of 1 in 800-1200 live births [1-4]. Studies show that almost one-third of people with ID are physically active enough. Demographics and health profiles of these patients include congenital heart disease, respiratory diseases, deterioration of functional capacities due to Alzheimer’s, recurrent pneumonia, sensory disorders, musculoskeletal disorders, and movement disorders. Physical fitness of people with ID is much lower than of the general population. People with DS are no exception. One of the important features of these patients is low physical activity [5]. 
Various studies have shown that DS patients have less physical activity than other people in the society and even other people with ID [6-9]. They, equal to or more than healthy people, need physical activity and the health of the cardiovascular and musculoskeletal systems to be healthy and prevent from diseases and premature death [10]. Low levels of physical fitness and function may be due to an increase in the prevalence of overweight or obesity as well as a decrease in bone mass growth which may eventually lead to the exacerbation of clinical manifestations. Therefore, the goal of all health organizations is to improve the quality of life and health of citizens. Given the effects of exercise and physical activity on increasing quality of life, social skills and reducing the risk of disease, it is necessary to study the effect of physical activity on indicators related to the health of young people and children with DS.
2. Methods
In this review study, to investigate the effect of physical activity on health of young people and children with DS, search was conducted in national and international databases including EBSCO, Medline, PubMed, Elsevier, Google Scholar and SID using following keywords in Persian and English: Physical Fitness, Physical Activity, Down Syndrome, Rehabilitation, Intellectual Disability, Muscle Strength, Muscle Endurance, Aerobic Fitness, Balance, Agility, and Flexibility. Initial search yielded 130 papers. Of these, 35 were selected based on inclusion and exclusion criteria.
3. Results
Table 1 presents a systematic review of studies that have shown physical fitness, cardiovascular fitness, strength and combined exercises, postural control, and body composition of individuals with DS. The results show that the cardiovascular fitness was lower among people with DS compared to people without ID. However, a selected exercise program could have a significant effect on improving their maximal oxygen uptake, peak minute ventilation, chest fluid content, cardiac output, mean arterial blood pressure, regular vascular resistance, regular vascular resistance index, and other cardiorespiratory parameters. Spending more time in moderate physical activity is associated with greater cardiorespiratory fitness in adolescents with DS. 
The results showed an improvement in strength of lower extremity muscles and all muscle groups as well as cardiac function and agility compared to the controls. People with DS have unstable postural control and increased oscillation in both anterior-posterior and medio-lateral directions compared to normal people. In anterior-lateral plane with eyes open and closed conditions, DS people with and without visual information had more body sway than healthy individuals in the lateral and anterior-posterior plane. The prevalence of overweight is higher in these patients than in the general population; children with DS are often overweight and obese compared to the general population.

4. Conclusion
One of the reasons for the lower cardiovascular fitness in DS people can be due to less muscle mass, less muscle strength, thyroid disorders, muscle laxity, obesity or sympathetic system disorders in response to exercise, inactive lifestyle, eating habits, lack of recreational activities, lack of motor coordination, lack of sufficient motivation for physical activity. Some studies have shown that proper exercise can improve cardiovascular health. One of the most important types of exercises used for people with DS is strength training. The results have shown that the implementation of progressive strength training improves the strength of the muscles in the body [15-18], increases muscle mass and decreases weight [17], improves functional tasks and maximal aerobic capacity [15] in individuals with DS [19]. 

Decreased muscle tone and joint laxity are common in people with DS. In DS patients, due to some factors such as the number of muscle fibers or a lower percentage of slow-twitch muscle fibers, muscle strength is poorer than in normal and mentally retarded people. This muscle weakness in the lower limbs and poor postural balance increase the risk of falling in these people [1, 4]. Body mass index and body fat percentage are also higher in people with DS than in healthy people. Overweight and obesity are new concerns for DS patients. Some studies have shown that 35-50% of them are obese. The main reason for their overweight is a sedentary lifestyle, of course because these people are not able to participate in health promotion activities and programs due to many obstacles.
Ethical Considerations
Compliance with ethical guidelines
This article is a meta-analysis with no human or animal sample.
Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.
Authors' contributions
Conceptualization, methodology and supervision: all authors; Review and writing the original draft and sources: Saeed Bahiraei, Rahman Amiri; Review and editing: Hassan Daneshmandi.
Conflicts of interest
The authors declared no conflict of interest.


Acknowledgements
The authors would like to thank the research and educational officials of the University of Guilan and the Faculty of Physical Education and Sports Sciences.