<P> According to the sliding filament theory, the myosin (thick) filaments of muscle fibers slide past the actin (thin) filaments during muscle contraction, while the two groups of filaments remain at relatively constant length . Before the 1950s there were several competing theories on muscle contraction, including electrical attraction, protein folding, and protein modification . The novel theory directly introduced a new concept called cross-bridge theory (classically swinging cross-bridge, now mostly referred to as cross-bridge cycle) which explains the molecular mechanism of sliding filament . Cross-bridge theory states that actin and myosin form a protein complex (classically called actomyosin) by attachment of myosin head on the actin filament, thereby forming a sort of cross-bridge between the two filaments . These two complementary hypotheses turned out to be the correct description, and became a universally accepted explanation of the mechanism of muscle movement . </P> <P> The first muscle protein discovered was myosin by a German scientist Willy Kühne, who extracted and named it in 1864 . In 1939 a Russian husband and wife team Vladimir Alexandrovich Engelhardt and Militsa Nikolaevna Lyubimova discovered that myosin had an enzymatic (called ATPase) property that can breakdown ATP to release energy . Albert Szent - Györgyi, a Hungarian physiologist, turned his focus on muscle physiology after winning the Nobel Prize in Physiology or Medicine in 1937 for his works on vitamin C and fumaric acid . He demonstrated in 1942 that ATP was the source of energy for muscle contraction . He actually observed that muscle fibres containing myosin B shortened in the presence of ATP, but not with myosin A, the experience which he later described as "perhaps the most thrilling moment of my life ." With Brunó Ferenc Straub, he soon found that myosin B was associated with another protein, which they called actin, while myosin A was not . Straub purified actin in 1942, and Szent - Györgyi purified myosin A in 1943 . It became apparent that myosin B was a combination of myosin A and actin, so that myosin A retained the original name, whereas they renamed myosin B as actomyosin . By the end of the 1940s Szent - Györgyi's team had postulated with evidence that contraction of actomyosin was equivalent to muscle contraction as a whole . But the notion was generally opposed, even from the likes of Nobel laureates such as Otto Fritz Meyerhof and Archibald Hill, who adhered to the prevailing dogma that myosin was a structural protein and not a functional enzyme . However, in one of his last contributions to muscle research, Szent - Györgyi demonstrated that actomyosin driven by ATP was the basic principle of muscle contraction . </P> <P> By the time Hugh Huxley earned his PhD from the University of Cambridge in 1952 on his research on the structure of muscle, Szent - Györgyi had turned his career into cancer research . Huxley went to Francis O. Schmitt's laboratory at the Massachusetts Institute of Technology with a post-doctoral fellowship in September 1952, where he was joined by another English post-doctoral fellow Jean Hanson in January 1953 . Hanson had a PhD in muscle structure from King's College, London in 1951 . Huxley had used X-ray diffraction to speculate that muscle proteins, particularly myosin, form structured filaments giving rise to sarcomere (a segment of muscle fibre). Their main aim was to use electron microscopy to study the details of those filaments as never done before . They soon discovered and confirmed the filament nature of muscle proteins . Myosin and actin form overlapping filaments, myosin filaments mainly constituting the A band (the dark region of a sarcomere), while actin filaments traverse both the A and I (light region) bands . Huxley was the first to suggest the sliding filament theory in 1953, stating: </P> <P> "...(I) f it is postulated that stretching of the muscle takes place, not by an extension of the filaments, but by a process in which the two sets of filaments slide (emphasis added) past each other; extensibility will then be inhibited if the myosin and actin are linked together ." </P>

Which of the following is a component of the sliding filament theory