Trajectories of arm pointing movements on the sagittal plane vary with both direction and speed.

Five subjects performed arm upward and
downward movements at different speeds (movement
duration ranged from 0.26 to 1.2 s). Fingertip paths,
velocity profiles and muscle activation patterns of arm
and forearm were computed. Inspection of the electromyograph
(EMG) revealed that for relatively slow speeds
(>0.7 s) and for both directions, only the flexor muscles
were active, mainly the anterior deltoid, for motor
(upward) and braking action (downward) respectively.
However, where gravity was no longer sufficient to
accelerate downward and decelerate upward movements
(<0.7 s), both flexors and extensors muscles were active.
Path curvature and position of maximum deviation from
straightness were lower for downward than for upward
movements. In addition, the position of maximum
deviation from straightness became progressively higher
with increase in duration for both upward and downward
movements. The ratio of acceleration duration to total
movement duration was greater for downward than
upward directions for all the range of speeds. The ratio
of maximum to mean velocity was similar for upward and
downward movements but decreased with decrease in
speed. The results indicate that the brain accomplishes
arm movements in the vertical plane with different
planning processes for movements with or against gravity.
Furthermore, they provide evidence that both gravitational
and inertial forces are determinant for arm trajectory
generation in the vertical plan.