1. Introduction
Frequent instability, leading to repeated ankle sprains, is defined as chronic ankle instability [1]. Some major problems that people with chronic ankle instability face are impaired balance, trunk instability, and ultimately postural instability. One of the most important factors in maintaining postural stability and balance in the body is the function of core muscles. Since the core muscles are at the center of all kinetic chains, the control of strength, balance, and movement in the center of body maximizes the connection between the upper and lower kinetic chains. According to the evidence, core stability plays a major role in the prevention of lower limb injuries in athletes. In addition, core muscle activity, by increasing intra-abdominal pressure as well as applying tension to the thoracolumbar fascia, can cause a strong cylinder and ultimately increased postural stability. Therefore, it is necessary to evaluate the function, role and efficiency of core muscles in people with chronic ankle instability to improve postural stability and other kinematic and muscular factors of proximal segments.
2. Participants and Methods
In this review study, search was conducted in three online databases of PubMed, Scopus, and Google scholar, to find articles that have examined the function of core muscles, motor and kinematic patterns of proximal segments, and the role of core muscles and their related exercises in people with chronic ankle instability during 2000-2019. Keywords were selected based on Patient, Intervention, Comparison, Outcome (PICO) search strategy (Table 1). In the end of searching, 146 articles were obtained. By checking the relevance of these articles to the purpose of current study, finally 7 related articles were selected for the final review.
3. Results
There are many studies and evidence on changes and adaptations of the recalling patterns of proximal segments in people with functional instability. Most of these studies have examined the muscles around the hip joint. Few studies have examined the contraction pattern and factors related to the activity of core muscles, including the abdominal and vertebral muscles, in people with chronic ankle instability. Marshal et al. examined the difference between impaired trunk and ankle stability between healthy individuals and those with functional ankle instability. They found a significant delay in the activity of the trunk and abdomen muscles in patients compared to healthy individuals.

Rios et al. observed a decrease in distal muscle activity and increased proximal muscle activity in people with chronic ankle instability, following perturbations on various surfaces. This may indicate a change in the function of these muscles in order to provide compensatory strategies to maintain body stability. Terada et al. compared diaphragm contractility in individuals with chronic ankle instability and healthy peers using ultrasound method. Their results showed lower diaphragm contractility in patients. In a study by de la Motte et al., they compared trunk-rotation differences under a star excursion balance test in people with chronic ankle instability and healthy peers. 
They reported that people with chronic ankle instability use proximal strategies to maintain balance. These compensatory strategies are created due to impaired hip function or core dysfunction. Hoch et al. compared the kinematics of the lower extremity joints, trunk, and hip under a star excursion balance test in people with chronic ankle instability compared to healthy peers. 
They showed that people with chronic ankle instability had a greater traveled distance, trunk movement on the frontal plate, hip adduction and ankle eversion on the frontal plate. Dastmanesh et al. examined the effect of core stability training on postural stability in people with chronic ankle instability and observed an increase in the consequences of posture control in the intervention group. Calicchio et al. reported no any relationship between core muscular strength and objective and theoretical consequences of chronic ankle instability such as pain, dysfunction and impaired balance.
4. Conclusion
Existence of the evidences regarding the changes in the diaphragm contractility, movement pattern of the trunk/hip, and muscular activity during electromyography of core muscles can be an indicative of a defect in the normal functioning of the core muscles in providing postural stability in people with chronic ankle instability. The core muscles act synergistically, and a malfunction in each of its components can affect the activity of the other components. Therefore, it is likely that providing normal function to these muscles can be effective in relieving postural disorders and compensatory strategies in these patients.
Ethical Considerations
Compliance with ethical guidelines
This is a review study; no experiments on humans or animals were conducted; hence, no ethical considerations were needed.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Authors' contributions
Investigation and Resource: Navid Mohammadpour; Writing, Editing & Review: all authors.
Conflicts of interest
The authors declared no conflict of interest.