The relative position of EDL muscle affects the lengths of sarcomeres within muscle fibers: experimental results and finite-element modeling

Background: Effects of extramuscular connective tissues on muscle force (experimentally measured) and lengths of sarcomeres (modeled) were investigated in rat. It was hypothesized that changes of muscle-relative position affect the distribution of lengths of sarcomeres within muscle fibers. Method of approach: The position of extensor digitorum longus muscle (EDL) relative to intact extramuscular connective tissues of the anterior crural compartment was manipulated without changing its muscle-tendon complex length. Results: Significant effects of EDL muscle relative position on proximal and distal EDL forces were found, indicating changes of extramuscular myofascial force transmission. EDL isometric force exerted at its proximal and distal tendons differed significantly. Finite-element modeling showed that the distribution of lengths of sarcomeres is altered by changes of muscle-relative position. Conclusions: It is concluded that forces exerted on a muscle via extramuscular myofascial pathways augment distributions of lengths of sarcomeres within that muscle.