Objective: We aim to survey the reliabilities of intra- and inter-rater for using the new muscle stiffness testing system in assessing the hardness of the erector spinae under different support plane. Method: A total of 10 healthy adult males and 10 healthy adult females were recruited into the study. The bilateral erector spinae hardness was evaluated on different support plane (a stable-surface chair and a Swiss ball) by therapist A, then, all participants were retested using the same method by therapist B. After five days interval, all participants were retested using the same method by therapist A. Result: The intra- and inter-rater reliabilities for assessing the hardness of the erector spinae on a Swiss ball and a stable-surface chair were excellent. A significant increase in the hardness of the erector spinae was discovered on the Swiss ball when compared with a stable-surface chair. Conclusion: Applying a new muscle stiffness testing system for measuring erector spinae stiffness on different support plane is a feasible method and has excellent reliability. The erector spinae can be activated on a Swiss ball significantly.