Preferential loss of the motor protein myosin, as observed in patients with acute quadriplegic myopathy (AQM) or cancer cachexia, causes generalized muscle wasting, muscle weakness and a decrease in muscle fibre force normalized to cross-sectional area. It remains unclear, however, whether this myosin loss influences other important features of muscle fibre function, such as Ca ²⁺ activation of the contractile proteins. To address this question, we have studied Ca²⁺ sensitivity of force generation using skinned muscle fibres from four patients with AQM or cancer cachexia and a preferential loss of myosin; and from seven healthy control individuals. Force and apparent rate constant of force redevelopment (k_tr) were assessed in solutions with varying Ca²⁺ concentrations (pCa), allowing construction of relative force-pCa and k_tr-pCa relationships. Results showed a rightward shift of the relative force-pCa relationship and a leftward shift of the relative k_tr-pCa curve in muscle fibres with a preferential myosin loss. To improve the understanding of the mechanisms underlying these alterations, the relative stiffness-pCa relationship was evaluated. A rightward shift of this curve was observed, suggesting that the changes in the Ca ²⁺ activation of force and k_tr were predominantly due to a decrease in the relative number of attached cross-bridges at different pCa values. Thus, a change in Ca²⁺ activation of the contractile apparatus in patients with preferential myosin loss is proposed as an additional factor contributing to the muscle function impairment in these patients.