New hypothesis for mechanism of sliding filament theory of skeletal muscle contraction

Background: Current understanding of skeletal muscle contraction is based on the sliding filament theory proposed independently by A.F. Huxley and H.E. Huxley (1954). The sliding filament theory very well explained shortening of skeletal muscle during contraction with reference to a single sarcomere with movement of Z discs toward center of that particular sarcomere. However, when we consider shortening of multiple sarcomeres in a myofibril arranged in series, the sliding filament theory fails to justify the movement of Z discs as each Z disc is being pulled in opposite direction by myosin heads of adjacent sarcomeres. Aims & Objectives: To find out alternate possible mechanism of Z disc movement on either side of A band if movement toward center by both sides is not possible. Materials and Methods: We have prepared a simulating model of sliding filament theory and observed mechanism of contraction on it. Results: The sliding filament theory fails to explain the movement of Z discs when all sarcomeres in a myofibril are contracting simultaneously. Therefore, we proposed a new hypothesis that Z discs and actin filaments in the half part of each sarcomere, which is toward origin end of muscle fiber, are not pulled toward center of myosin filament instead myosin filament slide toward origin end, and in the other half of each sarcomere (which is toward insertion end) actin filaments slide over myosin filament toward origin end of the muscle. Conclusion: This study adds a new insight into the sliding filament theory of muscle contraction and needs further confirmation on electron microscopic studies.