Measurement of Muscle Hardness: A Methodological Study

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Abstract
The aim of the present study was to determine the intra- and interobserver variation during measurement of muscle hardness with a new instrument, a so-called hardness meter. In addition, we investigated the factors which may influence the recording of muscle hardness and whether the hardness differs within the same muscle. Hardness was measured at a standardized point on the trapezius muscle in 20 volunteers by 2 observers in random order and again by 1 of the observers after 30 min. Muscle hardness was then measured at 3 different locations on the trapezius muscle. In addition, the muscle hardness at the standard point was measured in 10 volunteers by both observers at 3 different speeds of the applied pressure. The intraobserver variation was 10% and the interobserver variation was 12% for recording of muscle hardness. There was no significant difference in hardness values from time to time within the same observer (p=0.12), while there was a significant difference in hardness measurements between observers (p=0.007). Muscle hardness differed significantly among the 3 anatomical locations (p=0.03). For both observers, there was a significant difference among hardness recordings obtained at 3 different speeds (p=0.01 and p=0.0001). In conclusion, we have shown that the hardness meter can measure muscle hardness reliably if the same observer is used throughout a study. The speed factor was a major source of variability. In addition, we have demonstrated that muscle hardness differs within the same muscle. We suggest that the muscle hardness meter will be an important tool in future research of the mechanisms leading to myofascial pain.