A model of cytoskeletal reorientation in response to substrate stretching

Living adherent cells change their orientation in response to substrate stretching such that their cytoskeletal components reorganize in a new direction. To study this phenomenon, we model the cytoskeleton as a planar system of elastic cables and struts both pinned at their endpoints to a flat flexible substrate. Tensed (pre-strained) cables represent acting stress fibers, whereas compression-bearing struts represent microtubules. We assume that in response to uniaxial substrate stretching the model reorients and deforms into a new configuration that minimizes its total potential energy. Using the Maxwell's global stability criterion, we find global minima configurations during static extension and compression of the substrate. Based on these results, we predict reorientation during cyclic stretching of the substrate. We find that in response to cyclic stretching cells either reorient transversely to the direction of stretching, or exhibit multiple configurations symmetrically distributed relative to the direction of stretching. These predictions are consistent with experimental data on living cells from the literature.

Muscular and Postural Components of Foot Force during Quasi-Static Extension Efforts

The forces acting within and upon a limb are derived from three sources: postural (gravitational), inertial, and muscular. A method for decomposition has been established for free limb movements (Hoy & Zernicke, 1986); however, that method does not apply to kinematically constrained tasks whereby the limb exerts force on the environment. Presented here is a method for calculating the muscular and postural components for a quasi-static limb during a kinematically constrained task. It is a modified form of the inverse dynamic method reported by Kautz and Hull (1993) combined with the technique of Gruben and López-Ortiz (2000). This method stabilizes the limb against gravity with moments at each joint of the limb. Data from quasi-static lower limb extension efforts in one individual were analyzed to compare predictions of our method with those of the Kautz and Hull (1993) method. Differences in the postural component of foot force between the two methods increased with knee extension. The novelty of the method presented here was the use of an experimentally derived direction for the muscle component of foot force and the inclusion of a physiologically-based criterion for determining the support of the limb against gravity.