Reliability and Validity of a Device to Measure Muscle Hardness

Although clinicians judge muscle tenseness by palpation, methods to quantify this have not become established. We examined the reliability and validity of a commercially available myotonometer to assess the biceps brachii and brachioradialis. Torques ranging from 0-72% of maximum effort were isometrically resisted in five men and five women, all healthy and 21-23 years of age. Root-mean-square of surface electromyographic response was also measured. Intraclass correlations were 0.80 or higher within sessions but decreased to 0.57 between sessions except for myotonometry of the biceps brachii. Correlation with isometric load was 0.79 or higher for both myotonometry and electromyography, but these relations were consistently curvilinear. The myotonometer is more useful for fleshy muscles like the biceps brachii than for thin muscles like the brachioradialis. Because hardness has a curvilinear relation with tension in the muscle, hardness cannot be considered strictly equivalent to tension