Iranian Journal of Health and Physical activity (2010) 1 (1), 29-34 Electromyographic Analysis of Moderate Sustained Isometric Exercises for Vastus Medialis Components Nosratollah Hedayatpour 1 , Lars Arendt-Nielsen *1 Centre for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University 1 Received 22 June 2010 Accepted 24 August 2010 Abstract Strengthening of the oblique portion of the vastus medialis muscle (VMO) by moderate isometric exercises, particularly at the early phase of rehabilitation program for acute knee injury is an important part of knee rehabilitation protocols. The goal of this study was to investigate whether moderate sustained isometric exercises at level of 20% and 30% MVC activate the oblique portion of the VMO to a significantly greater extent than longus part (VML) of this muscle. Nine healthy men with no history of knee injury were tested; Multi-channel surface EMG signals were recorded from vastus medialis oblique and vastus medialis longus of the right leg during 70-s isometric contractions at 10%, 20% and 30% of the maximal force using controlled laboratory study. Average Rectified Value (ARV) and Mean Power Frequency (MPF) computed from the surface EMG were significantly increased with contraction intensity for both VMO and VML muscles (P < 0.05). However, the rate of change over time in ARV and MPF were significantly greater for VMO than VML (P < 0.05). Moderate sustained isometric exercise at 20% and 30% MVC was effective in activating the medial stabilizer of the patella, and therefore, it may be used at the early phase of rehabilitation program to establish patella control as stable base for knee rehabilitation. Keywords: vastus medialis oblique, EMG, muscle activation, moderate sustained isometric contraction; fatigue Introduction  The oblique portion of the vastus medialis muscle (VMO) is anatomically positioned to act primarily as an active medial stabilizer of the patella [1]. From the biomechanical point of view, a normal alignment and function of the patellofemoral joint depends on an appropriate balance of medial and lateral forces exerted on the patella by passive structures (e.g., the patellar retinacula) and by active muscular forces [2]. Studies of muscle fiber orientations suggest that the vastus medialis muscle (VM) force component is directed medially by VMO at 40 to 55 degrees and by vastus medialis longus (VML) at 15 to 18 degrees with respect to the femoral shaft [1]. Because VMO muscle fibers place more profound medial force on the patella to offset the strong lateral pull of vastus lateralis muscle (VL), therefore, the muscle is often targeted in rehabilitation protocols to treat patellofemoral tracking disorders [3,4]. Treatment protocols for  Coresponding author E-mail: lan@hst.aau.dk patella disorders often incorporate exercises to strengthen the VMO muscle preferentially [3,4,5]. Many studies have challenged selective activation of the VMO muscle in comparison with the vastus medialis longus and vastus lateralise muscles during isometric knee extension [6,7]. These studies have often examined exercises that require higher activation amplitudes that may be dangerous to be used during the early phases of rehabilitation for both post operative and acute non operative patella injury. The goal of the treatment following knee injuries is the restoration of maximum knee function while preventing recurrent injury. Muscle activation at levels of 20%-30% maximal voluntary isometric contraction (MVIC) are clinically considered to be safe and effective for retraining neuromuscular control and moderate muscle strengthening [8]. A moderate strengthening of the VMO muscle such that it stabilizes patella at normal anatomical position would act as a starting point for rehabilitation of knee function. The main purpose of the present study was to analyze muscle activity in two components of the VM muscle over 70s-long sustained fatiguing isometric contractions at 10%, 20% and 30% MVC.