  When moving a piano or dancing tango with a partner, how should I control my
arm muscles to best feel their movements and follow or guide them smoothly?
Here we observe how physically connected pairs tracking a moving target with
the arm modify muscle coactivation with their visual acuity and the partners
performance. They coactivate muscles to stiffen the arm when the partners
performance is worse, and relax with blurry visual feedback. Computational
modelling show that this adaptive sensing property cannot be explained by
movement error minimization proposed in earlier models. Instead, individuals
skilfully control the arms stiffness to guide it on the planned motion while
minimizing effort and extracting useful haptic information from the partners
movement. The central nervous system regulates muscles' activation to guide
motion with accurate task information from vision and haptics while minimizing
the metabolic cost.
