Proximodistal gradient in the perception of delayed stiffness.

Proximal and distal muscles are different in size, maximum force, mechanical action, and neuromuscular control. In the current study we explore the perception of delayed stiffness when probing is executed using movement of different joints. We found a proximodistal gradient in the amount of underestimation of delayed stiffness in the transition between probing with shoulder, elbow, and wrist joints. Moreover, there was a similar gradient in the optimal weighting between estimation of stiffness and the inverse of estimation of compliance that predicted the perception of the subjects. These gradients could not be ascribed to differences in movement amplitude, duration, velocity, and force amplitude because these variables were not significantly modulated by the joint used for probing. Mean force did not follow a similar gradient either. Therefore we suggest that the observed gradient in perception reveals a proximodistal gradient in control, such that proximal joints are dominated by force control, whereas distal joints are dominated by position control.