Investigation of absolute intra-rater and inter-rater reliabilities during the muscle hardness estimation.

[Purpose] This study aimed to investigate the absolute intra-rater and inter-rater reliabilities during the measurement of muscle hardness, which is used to evaluate physical therapy. Moreover, we examined the effects of using different equipment types and their positioning on the intra-rater and inter-rater reliabilities. [Participants and Methods] Participants of this study comprised 12 healthy adult male individuals. Two experts and two beginners measured the muscle hardness of the lumbar erector spinae and rectus femoris using three types of hardness meters at two positions, including when the muscle was relaxed and stretched. [Results] Intra-rater fixed bias was observed during some measurements by both experts and beginners. Inter-rater fixed bias was observed during measurements by some experts and not the beginners. [Conclusion] In this study, the measurement of muscle hardness demonstrated a need to reconsider the measurement position and acclimation time. These examinations require the consideration of relative and absolute reliabilities.