The relationship between muscle-tendon unit stiffness and muscle hardness of the gastrocnemius muscle using ultrasound real-time tissue elastography

Aim. Muscle hardness is important for the objective evaluation of muscle condition. Muscle hardness may be related to muscle-tendon unit (MTU) stiffness on the longitudinal axis, but the relationship remains unclear. This study aimed to examine the relationship between MTU stiffness and muscle hardness of the gastrocnemius muscle using ultrasound real-time tissue elastography (RTE). Methods. Participants were 20 healthy young men. MTU stiffness and muscle hardness were assessed 3 times at 15-minute intervals. MTU stiffness of the final area of the ankle joint angle was compared with muscle hardness. Muscle hardness was measured by RTE, and was identified at the position of the proximal 30% of the line connecting the popliteal line and the ankle joint lateral malleolus. In addition, RTE enabled digitizing of the hardness distribution of semiquantitative evaluations of muscle hardness by calculating the strain ratio (SR; muscle-to-coupler ratio). MTU stiffness was measured using an isokinetic torque machine, and was calculated as the slope of the polynomial fit passive torque-angle curves (the slope at a 10% final curve). Both measurements were taken while the left ankle was secured on an isokinetic machine with the knee joint fully extended. Results. Although there were no significant changes in the value of MTU stiffness and muscle hardness across the 15-minute intervals, muscle hardness was significantly correlated with the MTU stiffness at all intervals (range of r coefficients = -0.82 to -0.84). Conclusion. Muscle hardness is likely to reflect MTU stiffness showing longitudinal axis direction.