Age-related differences in strain rate tensor of the medial gastrocnemius muscle during passive plantarflexion and active isometric contraction using velocity encoded MR imaging: Potential index of lateral force transmission.

Purpose The strain rate (SR) tensor measures the principal directions and magnitude of the instantaneous deformation; this study aims to track age-related changes in the 2D SR tensor in the medial gastrocnemius during passive joint rotation and active isometric contraction. Methods SR tensors were derived from velocity encoded magnetic resonance phase-contrast images in nine young (28 years) and eight senior (78 years) women. Strain rates along and in the cross-section of the fiber were calculated from the SR tensor and used to derive the out-plane SR. Age-related and regional differences in the SR eigenvalues, orientation, and the angle between the SR and muscle fiber (SR-fiber angle) were statistically analyzed. Results SR along the fiber was significantly different between the cohorts during isometric contraction with higher values in the young ( P < 0.05). The SR-fiber angle was larger in the young for both motion types but this difference was not statistically significant. Significant regional differences in the SR indices was seen in passive joint rotation ( P < 0.05) for both cohorts. Conclusion SR mapping reflects age-related and regional differences during active and passive motion respectively; this may arise from differences in contractility (active motion) and elastic properties (active and passive motion). Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc. Magn Reson Med 73:1852-1863, 2015. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]