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Construct validity of myotonometric measurements of muscle compliance as a measure of strength

Myotonometric measurement of muscle compliance represents new technology that quantifies muscle tone. Compliance change during muscle contraction might provide an indirect measure of strength. The purpose of this study was to determine relationships among myotonometric measurements of muscle compliance, surface electromyographic (sEMG) measurements of muscle activation and joint force production during voluntary isometric knee extensions. The level of relationship will contribute to the construct validity of use of muscle compliance as an indirect strength measurement. Thirteen male subjects, mean age 25 ± 1.5 years, participated. Simultaneous recordings of myotonometric, sEMG of the rectus femoris and isometric knee extension force measurements were taken at rest, during maximal voluntary contraction (MVC), and during 33% and 66% MVC contractions. Relationships among the three measurement procedures were calculated using correlation and regression analyses. Myotonometric measures of muscle compliance, sEMG and force measurements were highly correlated. Myotonometric measurements were best represented by a curvilinear (quadratic) relationship to sEMG (r = 0.82, p < 0.001) and joint force (r = 0.83, p < 0.001). The present experiments establish the construct validity of myotonometric measurements of muscle compliance as an indirect means of quantifying muscle strength and activation levels. This method, therefore, offers a possible alternative for cases in which direct measurement of joint force or sEMG is difficult or inappropriate.