Validation of the Human Arm Stiffness Estimation Method Developed for Overground Physical Interaction Experiments

To build a physically interactive robot for overground applications, it is crucial to first understand the biomechanics of humans underlying overground physical human-robot interaction (pHRI) tasks. Estimating human arm stiffness during overground interactive tasks is a promising first step toward this goal. For this, an arm stiffness estimation technique was developed in our previous works that consider the unique challenges involving overground pHRI, such as the need to estimate the arm stiffness from a short duration of data with fewer repetitions. In this work, our stiffness estimation method is further validated with a passive spring setup with known stiffness values, as well as with a human experiment setup that resembles the widely used seated reaching tasks. Results show that our method can estimate the passive spring stiffness within 0.5% of error. We also show that the human arm stiffness measured through our method is comparable to those reported in well-known literature. In addition, our method was able to discern experimental conditions such as early vs. late trials or differences in arm movement conditions. Implications of these results are discussed further.Clinical Relevance— This method can aid the design and development of overground interactive robots for human movement assistance and rehabilitation.