Effects of Upper-Limb Posture on Endpoint Stiffness during Force Targeting Tasks

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Endpoint stiffness is characterized as the relationship between externally imposed displacements of the hand and the elastic forces generated in response and can be estimated during the application of short-term perturbations to endpoint based on measured force and position data. This research was aimed to investigate biomechanical properties via estimation of upper limb endpoint stiffness and joint torques during force targeting tasks. Sixteen able-bodied subjects were recruited in the study and the informed consent was signed by each subject. A 10-N force task was conducted at right and left directions for three upper limb postures. Hand endpoint trajectories and the response forces were recorded simultaneously for each trial. The 2x2 endpoint stiffness matrix and corresponding stiffness ellipse were determined by least squares error method. Resultant shoulder and elbow torques were also calculated by inverse kinetics. Kruskal-Wallis test was used to analyze the effect of postures and movement directions on endpoint stiffness and joint torques. The results of this research indicated that the postures have significant effects on endpoint stiffness ellipse of upper limb, the shoulder and elbow joint torques during force production tasks (p<0.05). These results demonstrate the importance of upper-limb posture when performing force targeting tasks. The clinical implementation suggests that endpoint stability can be more effectively affected by different upper-limb postures.