Background: Muscle hardness indicates muscle condition, and its measurement before and after resistance exercise is essential for preventing resistance training-induced muscle injury. Purpose: To investigate muscle hardness of the triceps brachii (TB) before and immediately after a resistance exercise session involving the elbow extensors. Material and Methods: In 18 young men, muscle hardness of the long head of TB was measured at 50%, 60%, and 70% point along the length of the upper arm from the acromial process of the scapula to the lateral epicondyle of the humerus by using shear wave ultrasound elastography. At the same sites, muscle thickness of the long head of TB was also measured by ultrasonography. Resistance exercise was performed using a dumbbell with a mass adjusted to 80% of the one-repetition maximum. Results: Although the exercise-induced increase in muscle hardness was significant at all the regions, muscle hardness was significantly higher at 70% of the upper arm length than at the other regions before and after resistance exercise. The exercise-induced increase in muscle thickness was also significant, but the relative changes in muscle hardness before and after resistance exercise were not correlated with the corresponding relative changes in muscle thickness at each region. These results indicate the small effect of exercise-induced muscle swelling on exercise-induced changes in muscle hardness. Conclusion: We suggest that muscle damage and/or injury, particularly at the distal region of TB, should be carefully considered to safely perform resistance exercise.
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