A cross-linguistic preference for torso stability in the lexicon

When the arms move in certain ways, they can cause the torso to twist or rock. Such extraneous torso movement is undesirable, especially during sign language communication, when torso position may carry linguistic significance, so we expend effort to resist it when it is not intended. This so-called “reactive effort” has only recently been identified by Sanders and Napoli (2016), but their preliminary work on three genetically unrelated languages suggests that the effects of reactive effort can be observed cross-linguistically by examination of sign language lexicons. In particular, the frequency of different kinds of manual movements in the lexicon correlates with the amount of reactive effort needed to resist movement of the torso. Following this line of research, we present evidence from 24 sign languages confirming that there is a cross-linguistic preference for minimizing the reactive effort needed to keep the torso stable.

Locating Separatrices and Basins of Stability in Biodynamic Systems

In biomechanics, a separatrix or recovery envelope exists between standing and falling. Standing with postural sway is a distinctly different type of motion than falling. A comparable problem to standing postural sway is the challenge of maintaining torso stability. In this case, a separatrix exists delineating stable torso sway from unstable and potentially injurious motion. An approach is presented for identifying separatrices in state space generated from noisy time series data sets representative of those generated from experiments. We demonstrate how Lagrangian coherent structures (LCS), ridges in the state space distribution of finite-time Lyapunov exponents (FTLE), can be used to locate these separatrices. As opposed to previous approaches which required an entire vector field, this method can be performed using a single trajectory that evolves over time.