Methodological considerations in estimating active muscle stiffness index

Accurate quantification of muscle stiffness using shear wave elastography is essential for evaluating neuromuscular function. The active muscle stiffness index, derived from the relationship between shear modulus and force, can be estimated using either continuous ramp or discrete step approaches, which are often considered equivalent. This study aimed to directly compare these two approaches. Thirty healthy women performed isometric knee extensions up to 50% of maximal voluntary contraction under both ramp and step contractions, while shear wave elastography measurements were obtained from the vastus lateralis. The active muscle stiffness index was calculated for each protocol. Agreement between methods was evaluated using paired t-tests, Lin's concordance correlation coefficient, and Bland-Altman analysis. Results revealed a significant difference between protocols (p < 0.001). Although a very strong correlation was observed (r ≈ 0.88), concordance was lower (CCC ≈ 0.81). Bland-Altman analysis indicated a systematic bias of -0.20 kPa/Nm (∼20%) without proportional bias. These findings demonstrate that ramp and step approaches yield systematically different estimates and should not be considered interchangeable.