Document Type : Original Articles


1 Department of Physical Therapy, School of Rehabilitation Sciences, Shiraz University of Medical Sciences, Shiraz, Iran.

2 MSc. of Physical Therapy, Rehabilitation Sciences Research Center, Shiraz University of Medical Sciences, Shiraz, Iran

3 Department of Physical Therapy,School of Rehabilitation Sciences, Shiraz University of Medical Sciences, Shiraz, Iran.



Background: Delayed activation of medial hamstrings (MH) relative to lateral hamstrings (LH) could lead to external tibial rotation. Also, it is a long-held belief that altered force sharing between vastus medialis obliquus (VMO) and vastus lateralis (VL) plays a main role in the pathophysiology of PFP. It was presumed that patients with patellofemoral pain (PFP) exhibit altered muscular activation pattern of MH and LH during functional tasks. The aim of this study was to compare the electromyography (EMG) activity of hamstrings and quadriceps in patients with PFP and healthy subjects during stair descending.
Methods: Twenty-four women with PFP and 24 non-symptomatic individuals, aged 18-40 years, recruited through convenience sampling. The EMG activity of MH and LH, VMO and VL was recorded during stair descent. The main outcome measures were onset latency and amplitude of muscle activity measured relative to the moment of foot contact measured by foot switch.
Results: There was statistically significant difference in the onset of hamstrings heads between groups (p=0.014). The LH activated before MH in PFP group. Normalized muscular activity was significantly reduced for VMO (p=0.002), VL (p=.045), and LH (p=0.019) in patients with PFP compared to control group.
Conclusions: Difference in temporal activation patterns of LH and MH may result in lack of rotational knee stabilization and lead to an increased patellofemoral joint pressure. Earlier activation of LH rotates tibia externally, and likely produced lateral patellar tracking.