BACKGROUND and AIMS. Patellofemoral pain syndrome (PFPS) is complex and challenging musculoskeletal disorder. Maltracking of the patella is considered to be one of the primary causative factors. Vastus Medialis Oblique (VMO) and Vastus Lateralis (VL) muscle imbalance in terms of EMG magnitude and timed onset is implicated in either initiating or perpetuating the patellofemoral pain (Cowan et al, 2002, Witvrouw et al, 1996). Many physiotherapeutic treatments are aimed at addressing this muscle imbalance despite a lack of evidence confirming or refuting it exists and it's association with pain and function. The ultimate aim of the study was therefore to establish if it is appropriate to continue treating muscle imbalance in patients with clinically defined PFPS.OBJECTIVES. The overall objectives of the study were to establish:1. If VMO - VL muscle imbalance exists in PFPS patients and if so is it specific to this condition or does a similar VMO - VL muscle imbalance exists in a healthy population?2. If muscle imbalance does exist is it related to clinical symptoms used as indications of pain syndrome in clinical practice?3. Is muscle imbalance associated with lower limb muscle physiology i.e. lower limb and quadriceps muscle strength in both fresh and fatigued states.METHODS. The study employed a cross-sectional design. 63 patients with patellofemoral pain syndrome (PFPS) and 63 age/sex matched healthy subjects were recruited and VMO & VL normalised EMG RMS amplitude and time onset differences were assessed during functional and experimental tasks. Additionally, correlations with pain level, functional status, muscular flexibility and biomechanical characteristics of the lower limb were explored.RESULTS. The results revealed that the VMO-VL activation patterns are task specific and most significantly related to functional stepping down task at a fast speed of execution (p=0.000). This interesting link between the type of muscle contraction, the speed of execution and the recruitment pattern of the VMO-VL was also confirmed by the non-functional isokinetic eccentric contraction (p=0.000). Additionally, it is the timing of the VMO-VL activation rather the intensity that is important. Also, a correlation appears to exist between activation pattern and duration of symptoms and knee functional performance (p=0.03) but not with the level of pain.CONCLUSION. The findings of the study suggest that the VMO-VL muscle imbalance does exist in a clinically defined PFPS population. Unlike previous studies however, this thesis suggests that specificity of the functional activities and speed of execution have a significant role to play in the muscular performance and it could be argued that this translates to a role in PFPS. It would therefore seem appropriate to continue addressing and treating this complex and challenging issue with physiotherapeutic interventions but this may need to be targeted to interventions that are tailored to addressing issues in relation to stepping down and at fast speed.