Early activation of lateral hamstrings (LH) in respect of medial hamstrings (MH) may cause abnormal knee abduction and external rotation moments in individuals. This situation may increase the risk of ligament injury and may be the reason for susceptibility of patellofemoral pain (PFP). The aim of this study is to compare the time delays in electromyographic studies between medial and lateral hamstrings in patients with PFP and asymptomatic subjects.

Patellofemoral pain (PFP) is a very common complaint in younger population with still unclear and controversial etiology. It was suggested that a combination of intrinsic and extrinsic factors cause PFP. The strain of the retropatellar subcondral bone caused by excessive lateral patellar tracking is known to be the one among intrinsic factors (Thomee R.1999). Imbalance of the dynamics between knee extensor muscles, Vastus Medialis (VM) and Vastus Lateralis (VL), has also received a great research interest during last 10 years. However, uneven timing between onsets of counterpart knee flexor muscles, Lateral and Medial hamstring, were just considered as a possible etiological factor of the PFP (Patil S.2011), yet this hypothesis remains to be proven. The investigators aimed to investigate whether there is a relative time delays of the LH (biceps femoris, BF) and MH (semitendinosus, ST) during the walking at the self-selected speed in patients with patellofemoral pain. Even though dynamic EMG investigations are challenging because of cross-talks between muscles, hence difficult to determine clearly onset times, the investigators think that any information acquired during walking would be highly important because patellofemoral pain is mainly exaggerated by dynamic activities.

15 patients who were diagnosed with PFP (age 28.73±7.44, height 169.73±7.09, weight 67.47±14.31) and 15 asymptomatic subjects (age 30.47±6.22, height 167.87±7.81, weight 67.87±13.48) were recruited in the study. The inclusion criteria for PFP subjects was presence of the anterior and retropatellar pain for minimum 2 months scaled as 3 in accordance with Numeric Analog Scale during at least two of the following activities: stair descending, ascending, squatting, prolonged sitting, jumping, sitting on the knees. Surface EMG was recorded from Medial Hamstring and Lateral Hamstring muscles at 1000 Hz sampling rate during walking at participants' normal walking speed. EMG data obtained from 5 gait trials were evaluated. The EMG traces were full wave rectified and low pass filtered at 50 Hz. Delay in timing before heel strikes between MH and LH was calculated. Statistical significance between differences in time delays was analysed using "paired sample t-test".

Inclusion Criterias:

• Pain originating specifically from the patellofemoral articulation (vague or localized);
• At least 3/10 pain intensity accordingly to the Numeric Analog Scale (NAS) with at least 2 of the following functional activities commonly associated with PFP stair ascent or descent, squatting, kneeling, or prolonged sitting;
• Reports of pain greater than 2 months in duration.
• Being between 18-40 years of age

Exclusion Criterias:

• Previous history of knee surgery,
• History of patellar instability,
• Neurologic involvement that would influence gait

starting 6 months after publication

The algorithm used to calculate the onset of the EMG was identified according to the point at which the signal deviated by more than three times standard deviations, for a minimum of 25 ms from the baseline level (computing started 200 ms prior to the start of walking). MatLab program (R2011 version) was used for design a custom made program for data analysis. Additionally to automatic calculation, all data was visually checked to ensure that there was no signal noise, deteriorating the signal analysis, possibly caused by the movement artefact. A blind assessor visualy checked the onsets of the MH and LH muscles and record the onset time.