1. Introduction
hildren with special needs are defined as children who are different from normal children in several dimensions, including physical, cognitive, and psychological, and are not able to use the facilities. Children with intellectual disabilities are children who have physical and mental disabilities and are mentally less intelligent than their normal peers [2]. They have an inability to learn or delayed reading, writing and arithmetic abilities and are associated with delayed speaking and low language development. The prevalence of intellectual disability is about 3-13% of the world’s population, and 75-90% of these people have a mild or educable intellectual disability. 4.6% of people under 18 years of age in developing countries and 0.5-2.5% of people in developed countries have intellectual disability. In terms of education, these people are divided into the group of slow learners (IQ=75-90), educable (IQ=50-70), trainable group (IQ=25-50) and custodial (IQ <25). In total, intellectual disability is more common in males [2]. Common known features in people with intellectual disability are: obesity, muscle weakness, postural deviation, impaired balance, vision problems, hearing problems, and physical disability. It should be noted that some of the mentioned features such as obesity are due to inactivity which causes a high prevalence of mortality among these people [4]. People with intellectual disability have lower scores on motor perceptual tests than normal individuals due to impaired sensory-motor information integrity, and their balance is more unstable than healthy peers [5]. There are several factors involved in the development of intellectual disability in children including hereditary factors or causes related to before the formation of the fetus, pre-delivery causes or factors related to pregnancy such as medication use, maternal infection, etc., and causes during delivery, the most common cause of which is premature birth. Some factors after birth also can lead to mental retardation of the child [6]. The educable mentally retarded children have lower physical strength, endurance level, agility, running speed, reaction time and balance compared to normal peers. 
They are 2-4 years behind normal children in doing motor activities. Hoffman in assessing children who learn things faster (both girls and boys) compared to slow learners, found that mentally retarded female children are superior in their ability to perform balance movements [6]. The ability of individuals to maintain balance is essential to the successful completion of all movements. Static and dynamic balances have been proven to be very important in a child’s life. Since the fall is a recurring event in people with intellectual disabilities, balance is more important in mentally retarded children. Movement is the most basic means of communication with the outside world and the first reaction by which a person can realize the process of adaptation. Developing motor skills through motor activities, in addition to gaining coordination, causes calmness, stability of behavior and relaxation. In people with mental disabilities, movement and physical health have a special importance and role in areas such as daily activities, educational activities, social relations, self-confidence and self-concept. Moreover, the quantity and quality of movement is directly related to the overall growth rate of the individual. Since children with intellectual disability go through the stages of motor development irregularly and with delay, it seems necessary to include motor activities in their daily routine. Given that intellectual disability is not curable, the main task of this group of patients is rehabilitation and education.
2. Methods
This is a quasi-experimental study with pre-test/post-test design using a control group. Participants were 24 educable mentally retarded female students in Dr. Sajjadi elementary school located in Mashhad, Iran aged 9-13 years (IQ=50-70). Students with obvious physical disabilities or behavioral disorders, as well as those with Down syndrome, were excluded from the study. After additional explanations on how to perform the exercise, written informed consent was obtained from the children’s parents. Stork test was used to assess static balance, TUG test for dynamic balance, 50-m walk test for walking speed, and chair stand test for lower limb power. Paired t-test was used to examine the differences between pre-test and post-test scores in the two groups separately, , For statistical analysis, ANCOVA was used in SPSS v.16 software. Significance level was set at 0.05.
3. Results
According to Tables 1 and 2, Paired t-test results showed a significant difference between pre-test and post-test scores in static and dynamic balances in the exercise group (p<0.05), while there was no significant difference in the control group. 




Therefore, it can be said that the exercise program caused a significant progress in static and dynamic balance of children.
4. Discussion and conclusion
The purpose of this study was to investigate the effect of an eight-week of strengthening and proprioceptive training program on balance, lower limb strength and gait speed of mentally retarded female children. Strength and proprioception exercises are known as one of the effective training methods in preventing damage and strengthening the proprioception system, nerves and muscles. Considering that the training program had an effect on the core stability, strengthening the muscles in this area had a great effect on the center of gravity of mentally retarded children. It improved the neuromuscular system, coordinated and strengthened the core muscles, caused the displacement of the center of gravity, brought the line of gravity closer to the axis of the joints, and reduced the torque created in the joints. As a result, it maintained the stability of the body’s center of gravity in the base of support and increased static balance. The results of the present study indicate that combined exercises increase the level of physical fitness in mentally retarded children, and cause muscle activation and increased rate of muscle contraction. On the other hand, it stimulates the proprioception system, which is one of the parameters of balance, and increases the balance capacity of female children with intellectual disabilities.

Ethical Considerations
Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed about the purpose of the research and its implementation stages. They were also assured abou the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them.

Funding
This study was extracted from the master thesis of first author approved by University of Guilan

Authors' contributions
All authors contributed equally in preparing this article

Conflicts of interest
The authors declare no conflict of interest

Acknowledgements
The authors would like to thank the parents of children participated in this study for their cooperation


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