Objective functions in adjusting model parameters have been widely used to minimize the variance of joint moments, but it may be insufficient to estimate reasonable muscle forces. The purpose of this study was to introduce a novel objective function based on a correlation coefficient for predicting reliable muscle forces, and compare its performance to the existing objective function. A man with right-sided hemiparesis after stroke participated in the study, and performed the maximum voluntary isometric contractions with a dynamometer at an angular velocity of 30°/s. To compare the effects of the existing and the new objective functions on prediction of muscle forces, the relative root-mean-square error and correlation coefficient were calculated for joint moments and individual muscle forces. The new objective function yielded promising results, implying that it could potentially be used to estimate reliable muscle forces. In the future, this approach will be applied to various movements to determine the reliability of muscle forces and to understand mechanisms from the reliable muscle forces.