Dissociation of gravity and limb dynamics in motor control.

In nine participants performing visually guided pointing in parabolic flight, gravity-independent muscle-torque components stayed invariant across 0g, 1g, and 2g, while activation-derived moments scaled with load.

• Why it matters: The nervous system’s computations for anticipating and compensating intrinsic versus external forces during muscle command generation remain unresolved, especially under nonlinear body dynamics.
• What they did: Using kinematics and shoulder muscle activity, inverse dynamics separated gravity-independent torque from gravity-dependent effects across microgravity (0g), normogravity (1g), and hypergravity (2g), and regression tested how activation related to mechanical variables.
• The result: Activation-derived muscle moments showed systematic scaling of static and phasic components, especially during deceleration, and activation could be reconstructed from a compact, low-dimensional set dominated by gravity-dependent torque and movement velocity.

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