Significant Impact of Environment Stiffness and Joint Proximity on Sensory Integration of Position and Force Feedback

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During human motor control, sensory information is integrated, and the influence of each cue depends on its variance. Sensory weighting within the proprioceptive system, specifically between force and position feedback, including its stiffness dependency, has been demonstrated for the shoulder and digits. Both studies encompass upper limb joints, but the impact of joint proximity on sensory integration remains unknown. In this study, we analyzed isolated vertical reaching movements of the wrist (palmar flexion), elbow (flexion), and shoulder (retroflexion) using a haptic robotic manipulator. We varied the stiffness constant of a virtual spring (15 N/rad, 50 N/rad, 80 N/rad) to assess the influence of environment stiffness on participants' ability to blindly reproduce a target force. A nonlinear spring was introduced to reveal the weighting strategy between force and position feedback. Differences in force and position when participants unknowingly operated in the nonlinear environment, compared to the linear environment, allowed us to calculate the weights of both feedback cues. Repeating the experiment for the three joints allowed us to assess the influence of joint location on the weighting strategy between force and position feedback. Ten participants performed a total of 720 reaches, covering three different stiffness conditions, three distinct joints, and ten separate runs, each consisting of eight reaches. We hypothesized that both environment stiffness and joint location have a significant impact on the weighting strategy. We expanded on existing findings that show a tendency toward up-weighting force feedback in the integration process as environment stiffness increases. Additionally, we examined whether force or position feedback is favored when the joint is more proximal. The hypothesis is supported by experimental evidence, as both stiffness and joint type revealed statistically significant effects on the weighting strategy (p $<$ 0.001, p = 0.020, respectively). As environment stiffness increased, a shift from higher position weighting factors to higher force weighting factors was observed for all three joints. When comparing our findings to the Maximum Likelihood Estimation model predictions, we observed that the more proximal joints exhibited higher force weighting factors. Additionally, normalizing the experimental environment stiffness conditions over the maximal voluntary force of each joint times the moment arm and over the joint stiffness showed a similar trend. More distal joints, on the other hand, favored higher position weighting factors.