Three-dimensional head and upper arm orientations during kinematically redundant movements and at rest

Area of Science:

• Neuroscience
• Biomechanics
• Human Movement Science

Background:

• Human movements, like head and arm orientation, possess three rotational degrees of freedom.
• Donders' law suggests these degrees of freedom reduce to two during specific tasks, with one being a function of the others.
• Previous research indicated a reduction in degrees of freedom for facing and pointing tasks.

Purpose of the Study:

• To investigate if three-dimensional (3-D) head and arm orientations at rest match those during movement for pointing or facing tasks.
• To determine if Donders' law holds true for dynamic head and upper arm movements.

Main Methods:

• Two experiments were conducted, one focusing on head orientation and the other on upper arm orientation.
• Participants were instructed to direct their nose (head) or point their arm towards targets appearing randomly.
• Head and arm orientations were recorded at rest and during movement between targets.

Main Results:

• At rest, head and upper arm orientations were confined to a 2-D surface with minimal scatter (<3-4 degrees).
• During movement, orientations often deviated from the 2-D surface, though these deviations were consistent and reproducible.
• Upper arm movement deviations correlated positively with movement velocity, unlike head movements.

Conclusions:

• Donders' law is violated during dynamic head and upper arm movements.
• Observed deviations suggest that simple 2-D models may not fully capture the control of these movements.
• Findings challenge existing models of movement control for the head and upper limb.