Posture Maintenance of the Human Upper Extremity; Identification of Intrinsic and Reflex Based Contributions

Neuromuscular stabilization has been investigated for the human upper extremity. Force perturbation methods have been applied to characterize the human joint resistance. Non-parametric identification was used to derive transfer functions describing arm admittance and neural feedback. Using parametric identification the intrinsic and reflexive contributions could be unraveled into physical model parameters. The intrinsic resistance is represented as joint stiffness and damping attributed to muscle co-contraction. The reflexive feedback is represented by neural feedback delays and gains.

Results show that the intrinsic and reflexive joint resistance vary strongly with experimental conditions and depend in particular on the task instructions, the perturbations, the posture and on the manipulator (payload) mass and damping. Within conditions consistent results are found within and between subjects.

In most cases a considerable part of the motion resistance was attributed to the intrinsic joint resistance apparently caused by muscle co-contraction. Reflexive feedback is primarily effective in the lower frequency range.