Load emphasizes muscle effort minimization during selection of arm movement direction

Area of Science:

• Biomechanics
• Motor Control
• Human Movement Analysis

Background:

• Directional preferences in shoulder-elbow movements exist for both dominant and non-dominant arms.
• Previous studies suggested simplification of interaction torque control, not muscle effort minimization, influenced these preferences.
• The role of muscle effort minimization in directional preferences remained unclear.

Purpose of the Study:

• To investigate if distal load influences directional preferences in arm movements.
• To determine if load enhances the effect of muscle effort minimization on movement direction selection.
• To explore the independence of muscle effort minimization from arm dominance.

Main Methods:

• A free-stroke drawing task was used with dominant and non-dominant arms, with and without wrist load.
• Arm movements were restricted to horizontal plane rotations of the shoulder and elbow.
• Directional histograms and cost function optimization were employed to analyze movement preferences and underlying causes.

Main Results:

• Diagonal directional preferences were observed in both arms under both load conditions.
• A tendency to accelerate one joint while passively moving the other was confirmed.
• Increased load enhanced movement in transverse diagonal directions, suggesting greater influence of muscle effort minimization.

Conclusions:

• Muscle effort minimization, minor without load, became apparent and significant with added load.
• The tendency to minimize muscle effort was independent of arm dominance.
• Load is a key factor in revealing the influence of muscle effort minimization on movement direction selection.