Purpose: To demonstrate the feasibility of in vivo calf muscle fiber tracking in human subjects.Materials and Methods: An EPI‐based diffusion tensor imaging (DTI) sequence with six‐direction diffusion gradient sensitization was implemented, and DT images were acquired at 3 Tesla on five subjects using an extremity coil. The mean diffusivity, fractional anisotropy (FA), and fiber angle (with respect to the magnet z‐axis) were measured in different muscles, and fibers were tracked from several regions of interest (ROIs).Results: The fiber orientations in the current DTI studies agree well with those determined in previous spectroscopic studies. The orientation angles ranged from 13.4° in the lateral gastrocnemius to 48.5° in the medial soleus. The diffusion ellipsoid in muscle tissue is anisotropic and approximates a prolate model, as shown by color maps of the anisotropy. Fibers were tracked from the different muscle regions, and the unipennate and bipennate structure of muscle fibers was visualized.Conclusion: The study clearly shows that in vivo fiber tracking of muscle fibers is feasible and could potentially be applied to study muscle structure function relationships. J. Magn. Reson. Imaging 2006.

In human skeletal muscles, the aging process causes a decrease of contractile and a concomitant increase of intramuscular adipose (IMAT) and connective (IMCT) tissues. The accumulation of non-contractile tissues may contribute to the significant loss of intrinsic muscle strength typically observed at older age but their in vivo quantification is challenging. The purpose of this study was to establish MR imaging-based methods to quantify the relative amounts of IMCT, IMAT and contractile tissues in young and older human cohorts, and investigate their roles in determining age-associated changes in skeletal muscle strength.

Velocity-encoded phase-contrast magnetic resonance (MR) imaging techniques and a computer-controlled MR-compatible foot pedal device were used to investigate the medial gastrocnemius muscle and aponeurosis deformations during passive and active eccentric movements of the plantarflexors. Intrafascicular strain, measured as the ratio of strain in the fascicle segment at its insertion to strain at its origin, was nonuniform along the proximodistal axis of the muscle (P < 0.01), progressively increasing from the proximal to distal direction. The high intrafascicular strain regions appeared to correlate with the muscle regions that are likely to encounter high stress concentrations, i.e., the regions where the muscle physiological cross section decreases close to the tendons. The architectural gear ratio, i.e., the mechanical amplification ratio of fascicle length displacement to that of tendon/aponeuroses in a pennate muscle, also exhibited significant regional differences, with the highest ratios in the proximal region of the muscle accompanied by a higher initial pennation angle and a larger range of fascicular rotation about the origin. Values close to unity in the distal region of the muscle suggest that the aponeurosis separation may decrease in this region. Fascicle length and pennation angle changes were significantly influenced by force generation in the muscle, probably due to a shortening of the loaded muscle fibers relative to a passive condition. Overall, our data illustrate significant proximodistal intramuscular heterogeneity as supported by a regionally variable end-to-end strain ratio of fascicles and angle changes in the medial gastrocnemius muscle during passive and active ankle movements. These observations emphasize the need to reassess current conceptual models of muscle-tendon mechanics.

The orientation of muscle fibers influences the physiological cross-sectional area, the relationship between fiber shortening and aponeurosis shear, and the total force produced by the muscle. Such architectural parameters are challenging to determine particularly in vivo in multicompartment structures such as the human soleus with a complex arrangement of muscle fibers. The objective of this study was to map the fiber architecture of the human soleus in vivo at rest in both neutral and plantarflexed ankle positions using an MRI-based method of diffusion tensor imaging (DTI). Six subjects were imaged at 3 Tesla with the foot at rest in the two ankle positions. Eigenvalues, fractional anisotropy (FA), and eigenvector orientations of fibers in the different soleus subcompartments were evaluated after denoising of the diffusion tensor. The fiber architecture from DTI was similar to earlier studies based on a 3D fiber model from cadavers. The three eigenvalues of the diffusion tensor increased by ∼14% on increasing the joint plantarflexion angle in all of the soleus subcompartments, whereas FA showed a trend to decrease in the posterior and marginal soleus and to increase in the anterior soleus. The angle change in the lead eigenvector between the two foot positions was significant: ∼41° for the posterior soleus and ∼48° for the anterior soleus. Fibers tracked from the subcompartments support these changes seen in the eigenvector orientations. DTI-derived, subject-specific, muscle morphological data could potentially be used to model a more complete description of muscle performance and changes from disease.

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.