Isovolumetric Contraction.
It has been documented that during this phase there is a mechanical shortening of the transverse fibers belonging to the free wall of the RV in continuity with the LS, conditioning the narrowing of the cavity. This forms an ”external armor” keeping the base of the ventricles fixed, and compressing the AL (the cross-sectional diameters of both ventricles decrease) causing a brief temporary longitudinal lengthening of the LV apex during the pre-ejection interval (increase in the longitudinal diameter of the LV)19,20,23. The global translation of these movements of BL is the counterclockwise rotation of the entire heart as seen from the apex as reported by MRI studies23,24. LV elongation during the pre-ejection phase and compression secondary to BL contraction can generate a ”loading” effect on the AL to produce a Starling effect through the titin mechanism for posterior ejection26. Until this moment, the contraction of the subendocardial DS is prepared, which is continuous to the LS but directionally opposite, which during the isovolumetric contraction phase does not produce enough force to cause the longitudinal shortening movement as it happens in the next phase (ejection), the lack of chamber shortening during the isovolumetric phase implies that the counterclockwise rotation of the heart is due to circumferential muscle shortening of the BL.