Proportionality between force and muscle cross-sectional area (CSA) is a foundational principle in muscle mechanics. However, CSA-normalized force (known as specific force) is often lower in fibres with large CSAs compared to fibres with small CSAs from the same sample population. Many physiological mechanisms proposed to account for CSA-dependence of specific force converge on the requirement for fibre CSA to impact the relationship between force and the concentration of force-activating calcium. To determine if features of the force-calcium relationship exhibited CSA-dependence in mammalian skinned muscle fibres, force-calcium relationships were generated for 85 skinned slow soleus fibres of male Sprague-Dawley rats (n = 54 rats, 1-5 fibres per rat, age = 24 weeks, experimental temperature = 18 °C) and fit using the Hill equation. Fibres were separated into quartiles based on their CSA and then compared. Despite specific force being 46 % higher (P <
0.01) in the smallest (160 ± 51 mN∙mm