The purposes of this study were 1) to quantify the volume of activated parts within a whole muscle and 2) to examine activated area distributions along the length of muscle. Seven male subjects performed five sets of 10 repetitions of a single-leg calf-raise exercise with the knee fully extended. Transverse relaxation time (T2)-weighted spin echo images were acquired before and immediately after the exercise. A range of pixels with a T2 greater than the mean +1 SD of the region of interest (ROI) from the preexercise image and pixels with a T2 lower than the mean + SD of the ROI from the postexercise image were defined as “active” muscle. The active muscle images were three dimensionally reconstructed, from which the volume of the activated muscle was determined for individual triceps surae (TS) muscles. Our data indicate that ∼46% of the medial gastrocnemius (MG) muscle was activated during the exercise, with activation of the lateral gastrocnemius (LG) and soleus (Sol) muscles being ∼35%. In the MG, distal portions had a greater percentage area of activated muscle than the proximal portions (P < 0.05), which was consistent with the results regarding electromyogram activity. In contrast, regional activation differences were not observed in the LG and Sol. These findings suggest that the amounts of activated muscle and its distribution would be different among TS muscles.

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.

The behavior of the entire medial gastrocnemius (MG) superficial and deep aponeurosis structure was investigated with velocity-encoded phase-contrast, spin-tag, and three-dimensional morphometric magnetic resonance imaging. The displacements and strain of both these aponeuroses, muscle length, and the cross-sectional segment length of the deep aponeurosis were measured during isometric plantarflexion at 20% and 40% of maximal voluntary contraction (MVC). The length of the entire MG shortened during 20% and 40% MVC. All regions of interest in both aponeuroses moved proximally. Positive strain (lengthening) occurred in both ends of the deep aponeurosis and in the proximal region of the superficial aponeurosis. In contrast, negative strain (shortening) was observed in the middle region of the deep aponeurosis and in the distal region of the superficial aponeurosis. Consistent with this shortening of the deep aponeurosis length along the proximal-distal axis was expansion of the aponeuroses in the medial-lateral and anterior-posterior directions in the cross-sectional plane. It is concluded that at low to moderate force levels of isometric contraction, regional differences in strain occur along the proximal-distal axis of both aponeuroses, and some regions of both aponeuroses shorten.

Purpose:The aim of the present study was to clarify the neuromuscular activation patterns among individual triceps surae (TS) muscles during a repetitive plantarflexion using muscle functional magnetic resonance imaging (MRI) (mfMRI) and electromyography (EMG).Methods:Six healthy men partici