Many tasks require the arm to move from its initial position to a spec
ified target position, but leave us free to choose the trajectory betw
een them. This paper presents and tests the hypothesis that trajectori
es are chosen to minimize metabolic energy costs. Costs are calculated
for the range of possible trajectories, for movements between the end
points used in previously published experiments. Calculated energy mi
nimizing trajectories for a model with biarticular elbow muscles agree
well with observed trajectories for fast movements. Good agreement is
also obtained for slow movements if they are assumed to be performed
by slower muscles. A model in which all muscles are uniarticular is le
ss successful in predicting observed trajectories. The effects of load
s and of reversing the direction of movement are investigated.