Multiple congenital myopathies, including nemaline myopathy, can arise due to mutations in the ACTA1 gene encoding skeletal muscle a-actin. The main characteristics of ACTA1 null mutations (absence of skeletal muscle a-actin) are generalized skeletal muscle weakness and premature death. A mouse model (ACTC^Co/KO) mimicking these conditions has successfully been rescued by transgenic over-expression of cardiac a-actin in skeletal muscles using the ACTC gene. Nevertheless, myofibres from ACTC^Co/KO animals generate less force than normal myofibres (220 to 25%). To understand the underlying mechanisms, here we have undertaken a detailed functional study of myofibres from ACTCCo/KO rodents. Mechanical and X-ray diffraction pattern analyses of single membrane-permeabilized myofibres showed, upon maximal Ca21 activation and under rigor conditions, lower stiffness and disrupted actin-layer line reflections in ACTC^Co/KO when compared with age-matched wild-types. These results demonstrate that in ACTCCo/KO myofibres, the presence of cardiac a-actin instead of skeletal muscle a-actin alters actin conformational changes upon activation. This later finely modulates the strain of individual actomyosin interactions and overall lowers myofibre force production. Taken together, the present findings provide novel primordial information about actin isoforms, their functional differences and have to be considered when designing gene therapies forACTA1-based congenital myopathies.