Coordination underlying the control of whole body momentum during sit-to-stand

The stability of linear and angular momentum of the center of mass (CM) and the underlying coordination of body segments was investigated for a sit-to-stand task to better understand how the nervous system organizes the redundant degrees of freedom available to accomplish this task. From the effector geometry, we derived a mathematical model relating body segment angles and their angular velocities (i.e. state space) to CM angular and linear momentum. We used this model to partition the variability of joint angle and joint velocity configurations into combinations that leave CM momentum invariant and combinations that do not leave CM momentum invariant. The results revealed that subjects used a range of different state-space combinations from trial to trial that were equivalent with respect to producing a stable value of angular and linear momentum. In contrast, body segment combinations that changed the value of momentum were more restricted. Most interesting was the finding that, when standing up under more challenging support surface conditions, the range of state-space combinations used to stabilize momentum was increased. That is, variability increased most strongly for those angle and angular velocity combinations that left CM momentum invariant, with smaller increases registered for combinations that affected CM momentum.