A muscle tendon unit complex has traditionally been modeled as a series/parallel arrangement of contractile and passive elastic elements. However, the actual in vivo, measurement of the movement of these elements has been limited. PURPOSE The aim of this study was to develop a model of the human triceps surae complex in terms of the soleus (Sol) and medial gastrocnemius (MG) and how they move relative to one another based on measuring their velocities of movement during a submaximal isometric contraction using MR imaging techniques. METHODS Seven normal subjects performed cyclic isometric contractions at 35% of maximal voluntary contraction (MVC) force level while in a 1.5Tesla Magnetic Resonance Imaging (MRI) scanner (GE, Milwaukee). Subjects performed the contractions timed to an audio metronome. A velocity-encoded cine phase contrast pulse sequence was used to acquire images of the leg in the sagittal plane during the contraction. Velocity was encoded in S/I (superior/inferior) direction. The sagittal images contained MG and Sol tissue velocity information in the form of phase contrast images in which the pixel signal intensity was proportional to the velocity of tissue. RESULTS MG and Sol had significant differences in their movement patterns (p < 0.01, paired t-test). Mean MG contraction velocity was −0.40 cm/s (+/- 0.44) and mean Sol contraction velocity was 0.87 cm/s (+/- 1.1) during the force development phase (see graphs). CONCLUSION During submaximal isometric contraction, the Sol muscle belly lengthened (positive contraction velocity) during the force development phase of the contraction while the MG shortened (negative contraction velocity) during the same phase. These data demonstrate that within the triceps surae muscle complex there are substantial differences in the temporal and regional patterns of shortening and lengthening.Figure: No Caption Available