Methods: This is a prospective and quasi-experimental study in which 30 participants (15 healthy/15 patients) who were aged between 18 and 38 years were recruited through nonprobability simple sampling. Exhaustion process of hip abductors and hip external rotator muscles was performed using Delorme method in separate sessions with a rest interval of at least 48 hours. Then, the parameters of pressure center were measured on the anterior, internalexternal plates using Force Plate System. 

Results: Before the exhaustion process, parameters of the displacement length of the pressure center on the internal-external plate (P = 0.002) and the sum of the pressure center displacement length on the internal-external plate (P=0.052) were higher in the patients group. After the occurrence of fatigue in the abductor muscles of the control group, the sum of the pressure center displacement length on the internal-external plate (P = 00.1) and the velocity of the pressure center displacement length on the anterior plate (P = 0.001) and the internal-external plate (P = 0.016) were significantly increased. But in the patients group, all of the above mentioned parameters were decreased after fatigue in both the rotator muscles (P = 0.005, P = 0.054, P = 0.038) and the abductor muscles (P = 0.045, P = 0.005, P = 0.004). 

Conclusion: Before the induction of fatigue, patients had more postural swing than the control group. As the fatigue increased in the abductor and the external rotator muscles, the postural swing increased in the control group but decreased in the patients group. In healthy subjects, the fatigue of abductor muscles has a greater role in postural disturbance but there was no significant difference between the fatigue in abductor and rotator muscles in patients with PFPS.

Methods: The kind of study is applied and data were collected from the supervised exercises of 15 male athletes without injury of lower extremity from Tehran University, which participated voluntarily in this research. It was assumed that subjects expend the extreme potential during their activities. Statistical analysis was made using repeated measures ANOVA to investigate the muscle activity in feedforward and feedback phases of landing with SPSS 20. 

Results: Significant differences (P≤0.05) were found between treatments in relation to feedforward and feedback phases of muscles activities. Single leg hop in sagittal and frontal planes showed the highest activity of biceps femoris and gluteus medius in feedforward phase, respectively. Nevertheless, tuck jump demonstrated the lowest activity of these muscles in both phases. 

Conclusion: According to the results, single leg hop in sagittal and frontal planes could be the most appropriate exercises for improving strength and neuromuscular system of gluteus medius and biceps femoris. We should take special attention to these 2 plyometric exercises in exercise programs. However, tuck jump in comparison with 2 other exercises show less efectiveness.

Purpose: This study aimed to evaluate the symmetry of the isometric strength of the shoulder and scapulothoracic muscles in volleyball and futsal athletes.
Methods: In this case-control study, the strength of the scapulothoracic muscles and rotation of the shoulders in 20 adult volleyball athletes and 20 futsal adult athletes, who were randomly selected from sports clubs around Tehran, were measured using a manual dynamometer. The obtained data between the different groups and within the individual groups were compared by Independent and paired t tests, respectively.
Results: The isometric strength of the external rotator muscles, internal rotator muscles, upper trapezius and serratus anterior of the dominant side of volleyball and futsal athletes were different (P<0.05). The isometric strength of the same muscles in the dominant side of the volleyball athletes was more than the non-dominant side (P<0.05). However, the isometric strength of the middle trapezius and rhomboid muscles were less. The ratio of agonist to antagonist, in all muscle groups between two groups, in the dominant side as well as between the dominant and non-dominant side, were different in volleyball athletes.
Conclusion: In volleyball athletes, disturbances in the strength ratio of the agonist and antagonist muscles of the scapula, and weaknesses in these muscles, impair the dynamic stability of the scapula and shoulder joints and disturbances in the rhythm of the scapulohumeral muscles. It is recommended that during training, assessment and treatment of these athletes strengthening these muscles be considered.