Sara Fereydounnia 1 Azadeh Shadmehr 1 Saeed Talebian Moghadam 1 Gholamreza Olyaei 1 Shohreh Jalaie 1 Zeinab Shiravi 1 Saba Salemi 2 1 Physical Therapy Department, School of Rehabilitation, Tehran University of Medical Sciences, Tehran, Iran 2 Rehabilitation Faculty, Shiraz University of Medical Sciences, Shiraz, Iran Corresponding author: Azadeh Shadmehr School of Rehabilitation Tehran University of Medical Sciences PicheShemiran, Enghelab Street, Tehran, Iran Tel.: 0098-21-77528468 E-mail: shadmehr@tums.ac.ir Summary Introduction: The objectives of this study were to compare muscle activation time and dynamic postural variables in athletes with and without chronic ankle instability during jump-landing, fol- lowed by a choice reaction time task which was provided by the visual stimulus. Methods: Nineteen athletes [11 healthy athletes and 8 athletes with chronic ankle instability (CAI)] participated in this cross-sectional study. After informing them about the procedure and goals of the study, they started jump-landing protocol in response to the visual stimulus. Muscle activation time and dynamic postural control data were tak- en using an electromyographic (EMG) machine and force plate, respectively. Results: The results of the two-way repeated mea- surement analysis of variance (ANOVA) showed that there were significant differences in athletes with and without CAI for medial/lateral stability in- dex (MLSI; tested leg effect: p=0.006); the pre-mo- tor time of the gastroc-soleus, peroneus longus, and peroneus brevis (interaction effects of the tested leg and the tested group: p=0.001, p=0.015 and p=0.006, respectively) and the pre-motor time of the tibialis anterior (tested group effect: p=0.036). Conclusion: More attention should be on the mus- cle activation time because changes in this para- meter may be one of the preliminary risk factors for instability, although cohort studies are re- quired to prove it. A more challenging task with more sensitivity for differentiating between stable and unstable ankle is needed. Level of evidence: IIIb. KEY WORDS: chronic ankle instability, dynamic pos- tural control, muscle activation time. Introduction Ankle sprain is one of the most common sport in- juries 1 . About 70 to 85% of sprains are of the inver- sion type. It has been reported that 10 to 30% of peo- ple with the inversion type of sprain will develop chronic mechanical instability and almost 80% of these people will suffer from recurrent ankle sprain 2 . Therefore, the evaluation and treatment of chronic ankle instability (CAI) is a significant challenge in ath- letic health care. Fundamental and clinical studies have been conduct- ed to investigate the preliminary risk factor of recur- rence. Despite the recent progress in prevention pro- tocols for the improvement of strength, flexibility, pro- prioception, and neuromuscular control, the rate of recurrence is still very high and its causes are still un- known 2-4 . Sensory-motor impairments include weakness and muscle dysfunction, changes in static and dynamic postural control, changes in information integration in the central nervous system (CNS) and muscle spin- dles sensitivity 5 . Proprioception impairments sec- ondary to ankle sprain may disturb the required feed- back for suitable functioning of the central motor pro- gram. Reduced proprioception may reduce the mus- cle activity around the ankle joint, especially the ever- tor muscles; thereby causing failure in the correction of ankle position 6 . The distal leg muscles, such as ankle muscles, are important for walking, maintaining balance, avoiding falls, and sprains 7 . Peroneus longus dysfunction with regard to strength, activity onset, and duration of con- Muscles, Ligaments and Tendons Journal 2018;8 (4):552-559 552 The comparison of dynamic postural control and muscle activity in time domain in athletes with and without chronic ankle instability Original article © CIC Edizioni Internazionali