The purpose of the present study was to confirm the relationship between isolated frog muscle tension and muscle hardness by conducting physiological evaluation in vivo. Two different mounting forms of the muscle were adopted. One form placed the gastrocnemius muscle (GA) on a base plate; this dented the muscle as a "mass". The other form tightened the sartorius muscle (SA) between holders in Ringer's solution; this bent the muscle as a "string". The first experimental method allowed testing of muscle hardness during stretching up to 140% (experiment 1) and the other method allowed testing of hardness during tetanic muscle contraction (experiment 2). The response force to vertical distortion, measured as muscle hardness, increased linearly with resting tension increase and this relationship was not influenced by the hysteresis (experiment 1). The response force increments at each level of tetanic muscle tension were proportional to the contracting tension (experiment 2). Although the muscle mounting forms were different, the response force increment to muscle tension in GA and SA showed quite similar relationships in both tests. It seems likely that muscle hardness evaluated by the response force must depend on the amplitude of the tension at the instant of the hardness measurement, regardless of the mounting form or the stretching phase (ascending or descending). In conclusion, muscle hardness measured by perpendicular distortion has physiological significance related to the changes in passive and active muscle tension.
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