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The isometric Maximum Voluntary Contraction (MVC) of the right quadriceps femoris muscle was measured in 82 females and 58 males using a chair dynamometer and their mid-thigh quadriceps thickness was measured using real-time B-mode ultrasound. Statistically significant relationships (ranging from P < 0.001 to P < 0.0001) were found between quadriceps MVC, quadriceps thickness and body weight for both males and females. A significant inter-sex difference was present in the MVC-weight and MVC-thickness relationships, with males tending to have a higher MVC for a given body weight (P < 0.0001) and for a given muscle thickness (P < 0.0001). The inter-sex difference in the force-size relationship may relate to the previously demonstrated larger size of type 2b fibres and to the greater force generating capacity of these fibres compared with type 1 fibres.