Eccentric contractions are of interest since many physiological functions occur when the muscle is actively lengthening. Further, eccentric contractions have been shown to be most effective for muscle strengthening.¹ This study focuses on effect of atrophy, induced by unilateral limb suspension (ULLS), on muscle strain rate (SR) tensor indices during eccentric contractions of the Medial Gastrocnemius (MG) derived from velocity encoded phase contrast (VE-PC) images.Acute atrophy was induced by the ULLS model² (4 weeks of chronic unloading of the dominant leg using crutches and a raised shoe for the non-dominant leg) in six subjects with IRB approval. Dynamic imaging was performed pre- and post-ULLS during eccentric contractions at 60% Maximum Voluntary Contraction (MVC). A computer controlled foot pedal device rotated the foot during eccentric contraction, with visual feedback guidance enabling consistent force generation.³ The parameters of the gated, 3-directionally encoded VE-PC sequence were: 7-8 oblique-sagittal slices (1.7x1.7x5 mm) to cover the MG and 22 temporal phases. Muscle fascicles were manually contoured from water suppressed FSE images acquired in the same orientation and tracked through the dynamic cycle for fiber orientation. The 2D SR tensor (symmetric part of the spatial gradient of the velocity) of the MG was calculated from the VE-PC images to extract: (i) positive and negative eigenvalues, (ii) the out-of-plane component calculated as the negative of the (sum of the in-plane) eigenvalues, (iii) SR-muscle fiber angles corresponding to the angle between the eigenvector corresponding to the positive eigenvalue and the muscle fiber. Statistical analysis compared pre- to post-ULLS cohorts based on SR indices at the force value corresponding to the post-ULLS for each subject.The eigenvalues extracted at each voxel were sorted into negative and positive eigenvalue images (Figure 1). During isometric contraction, the local deformation is negative and thus the negative eigenvalue image extracted at peak contraction is denoted as strain rate along the muscle fiber. In eccentric contraction, the dynamic cycle starts with the foot in a plantarflexed position and pushed to the dorsiflexed position by the pedal while the foot is exerting a resisting force (phases 1-12 in Fig. 2) with the latter half of the cycle (12-24) being passive plantarflexion with the foot following the pedal. Since the net deformation is active lengthening in the first half of the cycle, the positive eigenvalue (local elongation) is denoted as the SR along the muscle fiber (Fig. 1). The average SR indices (positive, negative, sum eigenvalues and SR-muscle fiber angles) as a function of the dynamic cycle are shown in Figure 2. No significant changes were seen in any of the SR values between the pre- and post-ULLS subjects.Overall, SR values are much lower for eccentric contractions compared to our findings on isometric contractions for the same level of MVC. The lower values of SR arise from the fact the muscles are contracting while there is an active lengthening during dorsiflexion caused by external force from the foot pedal. The net result is a smaller extent of tissue deformation due to simultaneous local contraction and elongation. The net deformation is lengthening as the foot is pushed to the dorsiflexed position (Fig. 2, 2nd column). The local elongation of tissue is also confirmed by the low values of the SR-fiber angle where the SR eigenvector corresponds to the positive eigenvalue (SR-fiber angles: pre-ULLS: -200 and post-ULLS: -300; the negative sign indicates that the SR is rotated distally compared to the muscle fiber). Surprisingly, there are no significant differences in the SR indices between pre- and post-ULLS cohorts. This is in contrast to our findings on SR indices during isometric contractions which showed significant differences in SR-fiber angles and in fiber cross section SR. A potential explanation for minimal changes in SR with acute atrophy in eccentric contraction comes from our modeling studies simulating extracellular matrix (ECM) material stiffness. Modeling predicts that during eccentric contraction, the muscle tissue with stiffer ECM (as may occur in atrophy) generates more force, as the ECM is in strained state tending to enhance force generation (Figure 3). But atrophy induced reduction in specific muscle fiber force will reduce the force generation. The balance between loss of muscle force due to reduced specific force and increase in force due to increase in matrix ECM stiffness may account for minimal changes in SR in eccentric contractions between pre- and post-ULLS subjects.Dynamic studies using eccentric contraction may provide physiological insights into acute atrophy reflecting changes in both contractile elements and ECM material properties.