Twist mechanics of the left ventricle. Following electrical and mechanical activation in the apical subendocardial region, there follows a period of left ventricular isovolumic contraction (IVC), during which (A), the subendocardial myofibers (right-handed helix) shorten with stretching of the subepicardial myofibres (left-handed helix) to effect clockwise rotation of the apex and counterclockwise rotation of the base. During ejection (B), there is simultaneous shortening of the subendocardial and subepicardial layers. The larger arm of moment of the subepicardial fibers dominates the direction of twist, causing counterclockwise and clockwise rotation of the apex and base, respectively. During isovolumic relaxation (IVR) (C), subepicardial fibres lengthen from base to apex and subendocardial fibres lengthen from apex to base. In diastole, there is relaxation in both layers, with minimum untwisting (D). Reproduced with permission from Sengupta PP, et al.