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.