The paper concerns a framework of an estimation of multijoint human arm viscoelasticity in a small sufficient time period. The uncertainties have to be considered in estimating the viscoelasticity of the multijoint human arm. In general, the uncertainties existing in the structure of the human arm and the motor command from the central nervous system are subject to the non-Gaussian noises. A generalized Gaussian ratio function is brought in to deal with the non-Gaussian noises. The momotonicity of the generalized Gaussian ratio function is studied based on the approximation formula of Gamma functions, then a robust condition is proposed for the computation of even moments using shape parameters. That is, we can guarantee the accuracy of the simulation results and experimental results by the robust condition. The effectiveness of the proposed method is confirmed by the experimental results.