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.

Le conflit fémoro-acétabulaire (femoroacetabular impingement, FAI) est une pathologie mécanique de la hanche qui peut causer des douleurs et limitations fonctionnelles. Le but de cette thèse était d’étudier la fonction musculaire au niveau de la hanche chez des patients présentant un FAI symptomatique. La fonction musculaire de la hanche a été évaluée, dans un premier temps, chez des patients avant qu’ils ne subissent une opération. Ces patients démontraient un déficit de force qui pourrait être expliqué par de l’inhibition musculaire. Cependant, ce déficit de force n’était pas associé à une plus grande fatigabilité musculaire. Dans un deuxième temps, les altérations de force musculaire ont été évaluées chez des patients ayant subi une arthroscopie de la hanche. Après l’opération, les patients récupéraient un niveau de force normal au niveau de tous les groupes musculaires de la hanche excepté les fléchisseurs. Le cas d’un joueur de hockey sur glace ayant subi une chirurgie ouverte aux deux hanches pour traiter un FAI bilatéral a aussi été décrit. On a démontré que la déhiscence de la bandelette iléo-tibiale pouvait survenir après chirurgie, empêcher l’augmentation de force musculaire des abducteurs de la hanche, et retarder la reprise du sport. Enfin, un protocole d’évaluation du taux de développement de la force normalisé, variable permettant d’estimer l’inhibition musculaire de la hanche, a été proposé chez des sujets sains. La fiabilité et reproductibilité des résultats ont été montrées au niveau des adducteurs, rotateurs externes, et fléchisseurs de la hanche. Ces résultats montrent que ces patients ont une fonction musculaire altérée au niveau de la hanche, qui est toutefois récupéré après une opération. / Femoroacetabular impingement (FAI) is a pathomechanical process of the hip joint, which could lead to hip pain and functional disability. Aim of this thesis was to investigate hip muscle function in patients with a symptomatic FAI. Hip muscle function was first investigated before patients underwent any surgical treatment for managing FAI. It was shown that they present with reduced hip muscle strength (i.e., muscle weakness), probably due to hip muscle inhibition. Nevertheless, hip muscle weakness was not associated with exaggerated hip muscle fatigue. Hip muscle strength recovery was then evaluated in a series of patients after hip arthroscopy to treat FAI. These patients demonstrated a good recovery for all hip muscle groups, except for hip flexors. The case of a professional ice hockey player who underwent bilateral hip open surgeries for treating bilateral FAI was also documented. This report showed that iliotibial band dehiscence could occur after hip open surgery, thereby preventing hip abductor strength increase during rehabilitation and delaying the return to sport. In addition, the assessment of the rate of force development scaling factor for the hip muscles was evaluated in a group of healthy adults. This parameter seems to be promising for the evaluation of hip muscle inhibition. The testing protocol was feasible and reproducible for hip adductors, external rotators and flexors. Taken as a whole, these findings show that patients with symptomatic FAI demonstrate an impaired hip muscle function, which is however mainly resolved after surgical treatment.