It is assumed that thick filaments (myosin) get compressed or folded when fibres reach short lengths during contraction. Neglected phenomena associated with amphibian muscle fibres contracting to very short lengths, e.g., substantial active force production in long-lasting activation and not restoring fibre resting length after activation, speak against the compression of thick filaments. Here we show that such contraction phenomena are not amphibian artefacts but also occur in mammalian skeletal muscle fibres. This study provides ultrastructural and kinetic evidence that sarcomeres contract down to a small fraction of myosin filament length while keeping a regular myofilament arrangement. These findings are consistent with the hypothesis that thick filaments continue sliding at short sarcomere lengths. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) – project number 405834662 – under grants SI841/17-1, RO 5811/1-1, as well as partially funded by the DFG as part of the German Excellence Strategy – EXC 2075–390 740 016.
