The geometry of actin filament-membrane associations can modify adhesive strength of the myotendinous junction.

Junctions between skeletal muscle cells and tendon collagen fibers transmit forces generated by muscle cells to the skeletal system. Since force trajectories across adhesive joints partly determine the stresses at the joint (eg, shear or tensile), the geometry of actin filament-membrane-collagen fiber associations has been modeled based on ultrastructural data, and force trajectories at the junction have thereby been established. Measurements show that in healthy twitch cells, actin filaments lie at a mean angle of 4.3 degrees (standard deviation = 0.95 degrees; 15 cells analyzed) to the plasma membrane. Calculations indicate that maximum isometric loading is seen by the junctional membrane almost entirely as a shear stress. In disuse-atrophied muscle cells, the mean angle between actin filaments and the membrane is 9.1 degrees (standard deviation = 3.3 degrees; 11 cells analyzed). The shear component of loading for the junctions of atrophied cells is only 1% less than that in healthy cells. The tensile component of the stress at atrophied junctions is more than doubled, however. These data are used to interpret patterns of myotendinous junction mechanical failure in terms of adhesive joint mechanics. An increased occurrence of failure of the atrophied junction is observed at physiological loads and can be attributed to a reduction of adhesive strength under increased tensile load component.