Purpose To date, elastographic ultrasound has not been used to quantify the spatial stiffness changes of the muscle-tendon unit following concentric, isometric and eccentric contractions. Methods A total of 16 participants were enrolled in this study to perform three exercise protocols including concentric, isometric, and eccentric contractions at a load of 50% of maximal voluntary contraction. Strain elastographic ultrasound was used to measure stiffness changes of 11 regions of the biceps brachii. Generalized Estimating Equation model with false discovery rate correction was applied to analyze the interaction between different types of contractions and regions of the biceps brachii. Results Results demonstrated that an increase in stiffness in the medial area of proximal muscle was significantly greater after eccentric contractions compared with both concentric and isometric contractions ( p =0.022). Concentric contractions lead to a significant increase in stiffness in the middle part of biceps brachii than proximal and distal sites ( p =0.01). In addition, sixteen comparisons exhibited significant differences ( p <0.05) in stiffness at the muscle-tendon unit following exercise, with 11 regions showing decreased stiffness and 5 regions showing increased stiffness. Conclusion This study reveals that muscle stiffness within the biceps brachii exhibits significant spatial variations in response to different types of muscle contractions. Our findings suggest that the proximal muscle region exhibits increased stiffness in response to eccentric contractions. In contrast, concentric contractions produce a more pronounced increase in stiffness at the mid-belly region compared with the proximal and distal regions.
Guo et al. (Mon,) studied this question.