Coronal as well as Sagittal Fascicle Dynamics Can Bring About a Gearing Effect in Muscle Elongation by Passive Lengthening

The amount of muscle belly elongation induced by passive lengthening is often assumed to be equal to that of fascicles. But these are different if fascicles shorter than the muscle belly rotate around their attachment sites. Such discrepancy between fascicles and muscle belly length changes can be considered as gearing. As the muscle fascicle arrangement is three-dimensional, the fascicle rotation by passive lengthening may occur in the coronal as well as the sagittal planes. Here we examined the fascicle 3D dynamics and resultant gearing during passive elongation of human medial gastrocnemius in vivo. For 16 healthy adults, we reconstructed fascicles three-dimensionally using diffusion tensor imaging and evaluated the change in fascicle length and angles in the sagittal and coronal planes during passive ankle dorsiflexion (from 20° plantar flexion to 20° dorsiflexion). Whole muscle belly elongation during passive ankle dorsiflexion was 38% greater than the fascicle elongation. Upon passive lengthening, the fascicle angle in the sagittal plane in all regions (-5.9°) and that in the coronal plane in the middle-medial (-2.7°) and distal-medial (-4.3°) regions decreased significantly. Combining the fascicle coronal and sagittal rotation significantly increased the gearing effects in the middle-medial (+10%) and distal-medial (+23%) regions. The gearing effect by fascicle sagittal and coronal rotations corresponded to 26% of fascicle elongation, accounting for 19% of whole muscle belly elongation. Fascicle rotation in the coronal and sagittal planes is responsible for passive gearing contributing to the whole muscle belly elongation. Passive gearing can be favorable for reducing fascicle elongation for a given muscle belly elongation.