A novel E41K β-tropomyosin (β-Tm) mutation, associated with congenital myopathy and muscle weakness, was recently identified in a woman and her daughter. In both patients, muscle weakness was coupled with muscle fibre atrophy. It remains unknown, however, whether the E41K β-Tm mutation directly affects regulation of muscle contraction, contributing to the muscle weakness. To address this question, we studied a broad range of contractile characteristics in skinned muscle fibres from the two patients and eight healthy controls. Results showed decreases (i) in speed of contraction at saturated Ca²⁺ concentration (apparent rate constant of force redevelopment (k_tr) and unloaded shortening speed (V₀)); and (ii) in contraction sensitivity to Ca²⁺ concentration, in fibres from patients compared with controls, suggesting that the mutation has a negative effect on contractile function, contributing to the muscle weakness. To investigate whether these negative impacts are reversible, we exposed skinned muscle fibres to the Ca²⁺ sensitizer EMD 57033. In fibres from patients, 30 μ m of EMD 57033 (i) had no effect on speed of contraction (k_tr and V₀) at saturated Ca²⁺ concentration but (ii) increased Ca²⁺ sensitivity of contraction, suggesting a potential therapeutic approach in patients carrying the E41K β-Tm mutation.