Moving a generalised limb:a simulation with consequences for theories on limb control

The movement control of articulated limbs in vertebrates has been explained in terms of equilibrium points and moving equilibrium points or virtual trajectories. These hypotheses state that the nervous system makes the control Of multi-segment limbs easier by simply planning in terms of these equilibrium points and trajectories. The present paper elaborates on a planar computer simulation of an articulated three-segment limb, controlled by pairs of muscles. The nature of the virtual trajectory is analysed when the limb is required to make fast movements with endpoint movements along a straight line with bell-shaped velocity profiles. It appears that the faster the movement, the more the virtual trajectory deviates from the real trajectory, becoming up to eight times longer. The complexity of the shape of the virtual trajectories in these fast movements makes it unlikely that the nervous system plans to use these trajectories. It seems simpler to set up the required bursts of muscle activation, coupled in the nervous system to the direction of movement, the speed and the place in workspace. Finally, it is argued that the two types of explanation do not contradict each other: when a relation is established in the nervous system between muscle activation and movements, equilibrium points and virtual trajectories are implicitly part of that relation.