The width of 'A bands' in muscle fibres remains constant during contraction suggesting a 'sliding filament' model in which myosin filaments run the length of the A band and actin filaments slide into the A band. © 2004 Nature Publishing Group.
Two ground-breaking papers, published back-to-back in Nature 54 years ago, independently showed that muscle shortens as a result of the sliding between the thick and thin filaments of the fibres (muscle cells, each of which consists of many parallel myofibrils). Although these papers reported work that was done independently, both sets of authors discussed their results before submitting their manuscripts, and referred to each other's studies in the Nature papers.
In the experiments reported in the first of these two papers, Andrew Huxley and Rolf Niedergerke used interference microscopy to show that the width of 'A bands' (thick filaments consisting of the protein myosin) in muscle fibres remains constant during contraction, implying that during muscle contraction, the actin-containing thin filaments of the 'I band' are drawn into the A band. In order to perform their desired experiments, the authors had to design an interference microscope and have it built to their specifications. In interference microscopy, the reference beam does not cross through the specimen, allowing the striations within it (A and I bands) to be identified unambiguously and their lengths to be measured when the fibres are stretched, stimulated or otherwise manipulated; the resultant changes in the width of the striations can be measured. In muscle cells, the A and I bands are organized into basic structural units (called sarcomeres), which repeat along the length of the myofibril, so measurements of a single sarcomere can be extrapolated to a whole muscle's action.
Huxley and Niedergerke found that the ratio of widths of the A and I band depend simply on the length of the fibre, and are unaffected by tension development. The length of the A band is constant. The natural conclusion from these results is that the myosin in the A bands is in the form of submicroscopic (in 1950s terms) rods of definite length. During contraction, the actin filaments are drawn into the A bands, between the rodlets of myosin.
In their accompanying paper, Hugh Huxley and Jean Hanson, who had previously shown that myosin is located in the thick filaments of the A band and that actin is located in the thin filaments of the I band, reported their studies of myofibrils using light microscopy. In their experiments, they mounted a suspension of myofibrils on a microscope slide under a coverslip. When they found a fibril with one end embedded in a fibre fragment adhering to the coverslip and the other end in a fragment attached to the slide, they moved the coverslip slightly to make the fibril stretch, observing the behaviour of the A and I bands during the process. During this stretching of the single myofibrils, the A band remained at constant length and the actin filaments were pulled or 'folded-up' into the A band. They photographed myofibrils contracting either freely or while held at both ends, without or with the addition of various concentrations of ATP. They found that the I bands shortened from 
0.8 
m at resting length to zero during contraction, whereas the A bands remained at a constant length of 1.5 m.
The authors suggested, on the basis of these and other results described in the paper, that the driving force for the process of contraction is the formation of actin–myosin linkages when ATP is split by the myosin enzyme. The enzymatically generated movement of the linkages or crossbridges represents the molecular 'working stroke' that drives muscle contraction.
In his retrospective essay "A personal view of muscle and motility mechanisms", Hugh Huxley recalls how he and Hanson estimated at the time that under maximum load, the actin filaments needed to be pulled along by 100 Å (0.01 m) each time about one-third of the myosin molecules split ATP, to create the sliding force needed to explain muscle contraction.
Hence, the 'sliding filament' proposal emerged from the work described in these two 1954 papers: the force between actin and myosin is generated in the region of overlap between the thick and thin filaments, sliding the filaments together and shortening the muscle fibre.
See also Nature web focus (2004): Muscle crossbridges: 50-year anniversary
