The authors combined precise mechanical measurements of individual muscle fibers with real-time x-ray diffraction, allowing them to measure myosin's force and velocity during contraction while imaging changes in the highly regular myosin array. They showed that only a proportion of myosins remained attached, reducing the total force generated by the fiber. Those myosins remaining attached to actin continued to exert a steady force even as they changed shape. This previously described shape change is an active process that continuously maintains the motor force.
These results are counter to a model proposed 50 years ago by Andrew Huxley, who suggested that the force reduction of a contracting muscle fiber was due to reduced force from individual myosins. But later in his career, Huxley also suggested that conformational changes in myosin would allow it to generate active force. Irving notes that the new model supports that concept.
Reducing the number of active myosins during low-load contraction makes sense, Irving says, since it matches ATP expenditure to muscle output. Since less force is needed, the cell can save on ATP by reducing the number of active myosins.