Changes in hormonal levels during the menstrual cycle are known to affect muscle electromyographic (EMG) activity, but no studies have investigated the effect of hormonal changes on the EMG activity of muscles around the knee in patients with patellofemoral pain syndrome (PFPS). The aim of this study was to evaluate the EMG activity of the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles during toe rise and heel rock tasks in different phases of the menstrual cycle in women with PFPS.Ten women with PFPS between 18 and 40 yr old participated in this study. The VL/VMO amplitude ratio and differences between onset (onset VL - onset VMO) of vastii muscle activity were measured in the affected limb during rise and rock tasks in the follicular and ovulatory phases of the menstrual cycle. The signals were analyzed with MATLAB software.During both tasks, there were no significant changes in onset differences in vastii muscles between the ovulatory and follicular phases. However, the amplitude ratio (VL/VMO) was significantly lower in the ovulatory phase compared to the follicular phase (P = 0.035 for rise tasks and 0.010 for rock tasks).The menstrual cycle may affect some EMG parameters. This article is protected by copyright. All rights reserved.