Introduction: The aim of the current study was to measure ground reaction force and center of pressure (COP) during lower extremity perturbation in single leg standing whit knee flexion and extension in healthy subjects, in addition to revealing reliability of the device used for the related perturbation for using another studies, reliability is one of the characteristic of measuring instrument , this indicates that measuring instrument to what extent in the same conditions gives the same results. Materials and method: perturbation device was built firstly, rotational perturbation (trunk and femur rotation on weigh bearing tibia ) were applied to the dominant leg of 13 healthy subjects during single leg standing . Four models of rotational perturbation (external and internal tibial rotation with knee flexion and extension) were applied to the lower extremities to collect ground reaction force and centre of pressure in these subjects. Subjects attached to lower extremity perturbation device and standing whit single leg on force plate. Subjects were then asked to repeat the tests procedure after a week’s interval. Results: The intra class correlation coefficient and Cronbanch's Alpha values during within and between sessions tests for four models of rotational distributions showed the relatively high reliability of these measurements during single leg standing , the intra class correlation coefficient exceeded 0.7 in all dominants. Conclusion: The relatively high reliability of ground reaction force and centre of pressure measurements during four different perturbations of external and internal tibial rotation with knee flexion and extension in healthy subjects suggests that these measurements can be used during single leg standing in future studies.

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.