The reserve of joint torque determines movement coordination

Area of Science:

• Biomechanics
• Motor Control
• Human Movement Science

Background:

• Human movement relies on coordinating multiple joints to achieve tasks efficiently, minimizing energy expenditure and motion jerk.
• Under fatigue or neurological conditions like stroke, altered movement patterns emerge, often involving increased trunk motion, but the underlying cost functions are not fully understood.

Purpose of the Study:

• To investigate the biomechanical cost functions underlying human motor coordination during reaching tasks.
• To determine if trunk recruitment serves to increase shoulder torque reserve when torque is near maximal capacity.

Main Methods:

• 26 healthy participants performed reaching tasks from a seated position.
• Trunk contribution to hand displacement was measured under normal and loaded conditions.
• Changes in shoulder torque reserve were calculated based on trunk flexion and rotation.

Main Results:

• Trunk contribution to hand displacement increased significantly from 11% to 27% when an additional load was introduced.
• Participants' trunk movements increased shoulder anti-gravitational torque reserve from 25% to 40% of maximal voluntary torque.

Conclusions:

• Trunk recruitment is a strategy employed by healthy individuals to augment shoulder torque reserve when facing limitations.
• Findings suggest that optimal control models should incorporate torque reserve as a cost factor for understanding human coordination in fatigue and post-stroke conditions.