Human elbow motor learning skills of varying loads: Proof of internal model generation using joint stiffness estimation

Abstract

This study presents a human elbow motor learning strategy responding to varying loads. Inspired by Kawato's internal model theory, we suggest hypothesis that human minimize the internal model error by updating the joint stiffness to generate stable and robust motion during repetitive voluntary action with varying weight of load condition. We designed experimental robotics device to verify our hypothesis and the device is capable of precisely measuring human elbow joint stiffness very accurately. The subject was instructed to perform the prescribed elbow motion without notifying the weight of the load for neutral experimental condition and we recorded joint position, perturbation torque of actuator, reaction torque from torque sensor, and mean absolute value (MAV) of the surface EMG (sEMG) in forearm muscles and upper arm muscles as a reference criterion for elbow joint impedance modulation during motor learning. Modified ensemble-based system identification was applied to characterize the dynamic elbow mechanical impedance in transient state of moving loads. Experimental results show that subjects utilized high joint stiffness initially, but it decreases gradually and saturated to the level of 20%~60% of initial value after repetitive motion tests. The degree of saturation of motor learning varied with the weight of loads, this result supports the hypothesis that motor learning reduces joint stiffness by providing accurate internal model.

Elbow Joint Stiffness Functional Scales Based on Hill’s Muscle Model and Genetic Optimization

“…The maximum values of the estimated stiffness for task E1 are in the range [1.75 Nm/rad–7 Nm/rad], while those for task E2 are in the range [2.2 Nm/rad–5.25 Nm/rad]. Obtained maximal values of joint stiffness are aligned with the values of elbow joint stiffness presented in papers [ 15 ] (from 3 to 20 Nm/rad), [ 46 ] (from 2.5 to 9 Nm/rad), [ 47 ] (from 9 to 14 Nm/rad), and [ 48 ] (~10 Nm/rad).…”

“…A possible reason for this phenomenon is the saturation of stiffness in the elbow joint with an increase in weights greater than 0.75 kg. This saturation property of elbow joint stiffness can be explained by the compensatory ability of motor behavior [ 47 ]. Based on the obtained results for different weights, the functional scale of stiffness change regarding weight change can be defined and is given in Equation (31).…”