The linear co-variance between joint muscle torques is not a generalized principle

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Previous research proposed a principle of linear co-variance for joint torques during multijoint movements.
• This principle suggested synchronized, linear changes in joint muscle torques.

Purpose of the Study:

• To investigate the validity of the principle of linear co-variance for voluntary multijoint movements.
• To determine the torque characteristics underlying accurate and fast human movements.

Main Methods:

• Neurologically normal subjects performed unconstrained elbow and shoulder movements in the sagittal plane.
• Movement kinematics were captured using marker coordinates.
• Inverse dynamics calculated joint muscle, interaction, and net torques.

Main Results:

• Joint muscle torques and interaction torques were not linearly correlated during the analyzed movements.
• Net torques at the elbow and shoulder joints demonstrated a linear correlation.
• Decoupling joint muscle torques while maintaining linear net torques facilitated straight fingertip paths.

Conclusions:

• The principle of linear co-variance does not universally apply to joint muscle torques in voluntary multijoint movements.
• Linear correlation of net torques, rather than joint muscle torques, appears crucial for generating efficient and accurate movements.
• The central nervous system may decouple joint muscle torques to achieve straight-line movements.