Document Type : Original Articles


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

2 Rehabilitation Sciences Research Center, Shiraz University of Medical Sciences, Shiraz, Iran

3 MSc. of Physical Therapy, Rehabilitation Sciences Research Center, 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. It is a long-held belief that altered force sharing between the vastus medialis obliquus (VMO) and the 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 descent.
Methods: Twenty-four women with PFP and 24 non-symptomatic individuals, aged 18-40 years, were recruited through convenience sampling and participated in this observational cross-sectional study. 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 relative to the moment of foot contact measured by foot switch. Groups were compared by Mann-Whitney test. Repeatability of task was evaluated using intra-class correlation coefficient (ICC).
Results: A statistically significant difference was seen in the onset of hamstring heads between groups (p=0.014). The LH activated before the MH in the PFP group. Normalized muscular activity was significantly reduced for VMO (p=0.002), VL (p=0.045), and LH (p=0.019) in patients with PFP compared to the control group.
Conclusions: Differences in temporal activation patterns of LH and MH may result in a lack of rotational knee stabilization and lead to increased patellofemoral joint pressure. Earlier activation of LH rotates the tibia externally and likely produces lateral patellar tracking.


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