Finite element modeling of aponeurotomy: altered intramuscular myofascial force transmission yields complex sarcomere length distributions determining acute effects

Abstract

Finite element modeling of aponeurotomized rat extensor digitorium longus muscle was performed to investigate the acute effects of proximal aponeurotomy. The specific goal was to assess the changes in lengths of sarcomeres within aponeurotomized muscle and to explain how the intervention leads to alterations in muscle length-force characteristics. Major changes in muscle length-active force characteristics were shown for the aponeurotomized muscle modeled with (1) only a discontinuity in the proximal aponeurosis and (2) with additional discontinuities of the muscles' extracellular matrix (i.e., when both myotendinous and myofascial force transmission mechanisms are interfered with). After muscle lengthening, two cut ends of the aponeurosis were separated by a gap. After intervention (1), only active slack length increased (by approximately 0.9 mm) and limited reductions in muscle active force were found (e.g., muscle optimum force decreased by only 1%) After intervention (2) active slack increased further (by 1.2 mm) and optimum length as well (by 2.0 mm) shifted and the range

Extramuscular myofascial force transmission alters substantially the acute effects of surgical aponeurotomy: assessment by finite element modeling

“…Experiments on fully dissected rat muscles in situ showed that the intervention yields a net increase in the length range of active force exertion due to a shift in muscle optimum length to a higher muscle length (Jaspers et al 1999;Brunner et al 2000). For truly isolated EDL muscle, present results and previous modeling (Yucesoy et al 2006a) show that aponeurotomy acutely leads to major inhomogeneities of lengths of sarcomeres. This explains the shift in muscle optimum length to higher lengths.…”

“…This change can be characterized by the length of the gap within the muscles' proximal aponeurosis. Our recent modeling work (Yucesoy et al 2006a) showed that if no such gap is allowed to occur (i.e., the aponeurosis is cut, but the intramuscular connective tissues remain intact) the effects of the intervention are unimportant. Therefore, the dominant mechanism of the effects of the surgery is interfering with the intramuscular myofascial force transmission mechanism.…”

“…Finite element model results showed for isolated muscle that the dominance of short sarcomeres within the distal fiber population is the major reason for the reduction of muscle active force (Yucesoy et al 2006a). However, presently we show that changes in sarcomere length distributions caused by extramuscular myofascial force transmission for both proximal and distal populations of muscle fibers allow most sarcomeres within the muscle to attain lengths favorable for exertion of higher active force.…”

“…In order to do that explicitly, we developed a model using a two-domain approach: the intracellular and the extracellular matrix domains of skeletal muscle are represented by two separate but elastically linked meshes (Yucesoy et al 2002). Using this model, we recently studied the acute effects of aponeurotomy on fully isolated rat muscle and showed that intramuscular myofascial force transmission dominates the acute effects of the intervention (Yucesoy et al 2006a). …”

Myofascial force transmission and tendon transfer for patients suffering from spastic paresis: A review and some new observations

“…However, to what extent this holds up for non-dissected muscle with intact myofascial pathways remains to be studied. In any case, as non-uniformity of sarcomere lengths is very plausible when myofascial force transmission is accounted for (Yucesoy et al, 2006), such transmission may not only affect the fraction of the force that is transmitted to the tendon, but may also affect the actual force that is exerted by the muscle.…”