Finite element modeling shows that epimuscular myofascial loads manipulate the effects of active state titin on mechanics of muscle integrated with its surroundings

Shorter sarcomere effect was shown in recent modeling of truly isolated muscle as the characteristic mechanism determining active state titin’s contribution to muscular mechanics via manipulation of muscle fiber direction strains. For muscle with extra- and epimuscular connections to surrounding tissues, epimuscular myofascial force transmission (EMFT) leads to myofascial loads, which affects mechanical equilibrium that determines local strains. Consequently, compared to truly isolated muscle, we hypothesized that for such integrated muscle active state titin’s effects on muscular mechanics are manipulated by EMFT. The aim was to test this. Isolated vs. integrated rat extensor digitorum longus muscles were modeled at long muscle lengths (λ m = 28.7-32.7mm) and three cases were studied: passive state titin (no change in titin constitutive equation in the active state), active state titin-I (constitutive equation involves a higher stiffness in the active state) and active state titin-II (constitutive equation also involves a strain shift coefficient accounting for titin’s reduced free spring length). For isolated muscle, shorter sarcomere effect i.e., limited lengthening locally along the muscle fiber direction and force enhancement (maximally 77.4%) was consistent. However, for integrated muscles, variable fiber direction strains showed even longer or shortened sarcomeres locally and consequently, force enhancement was inconsistent being even diminished (7.8%) or elevated (96.8%) at different lengths. Shift of muscle’s optimum length to a longer length in isolated muscle (λ m = 29.6mm) increased (λ m = 29.9mm) or vanished (λ m = 28.7mm) for extra- or epimuscularly connected muscles, respectively. In conclusion, in the integrated muscle context, effects of active state titin on muscular mechanics are manipulated by EMFT.