Static stretching is typically performed as an easy method for reducing muscle tone in several fields of sports and rehabilitation. Stretching intensity is important for realizing effective static stretching. However, its instruction is ambiguous; therefore, people sometimes get injured owing to excessive intensity. To perform static stretching with appropriate intensity that results in positive effects, the intensity and effect of static stretching should be quantified. In this study, we investigated the relation between the stretching intensity based on the maximum tolerance joint torque and the change in the muscle viscoelasticity of forearm muscles based on the spring-pot viscoelastic model for evaluating the change in muscle condition during static stretching. We set the stretching intensity using a wrist joint torque adjusting device and measured the forearm muscle hardness using a palpation robot. The results showed that the muscle viscoelasticity decreased significantly during 1-min constant-angle stretching, and the amount of decrease was the largest for 60% of the maximum tolerance joint torque at the maximum joint angle. This suggests that the optimized stretching intensity can be derived from the joint torque measurement at the maximum joint angle.