Cardiac muscle health is dependent on the adequate supply of oxygenated blood and nutrients to ensure optimal cardiac function, avoiding ischemia or heart attack. The continuous supply of oxygenated blood occurs mainly through the coronary vessels tethered and embedded within the muscle. Cardiac ventricular motions involve twisting, contraction and expansion throughout a cardiac cycle, giving rise to the biomechanical behaviour of the coronary vessels. The goal of this work is to investigate the impact of cardiac motion on the flow dynamics within the left main coronary artery using a two-way fluid-structure interaction (FSI) simulation. Blood flow was modelled within an idealized three-dimensional coronary arterial structure using incompressible laminar Navier-Stokes equations. This study was conducted on a left main bifurcation in which the geometry was deformed dynamically with prescribed displacements and the structure is assumed to have an isotropic five-parameter Mooney-Rivlin hyperelastic material to simulate torsional and expansion motions. We assessed the wall shear stress (WSS) and the blood velocity of the moving structure relative to a non-moving structure. Results indicated that the WSS at the bifurcation near the flow divider was found to be lower in the non-moving vessel relative to the moving one. Further, in the moving vessel, higher velocities were produced particularly during systolic torsional motion, compared to the non-moving model. In the non-moving model, a helical-shaped pattern of secondary flow was observed as the blood flowed through the curved vessel. However, this pattern diminished in the moving model, where the arterial curvature dynamically changed throughout the cardiac cycle. Overall, the impact of dynamic cardiac motion applied to the coronary artery bifurcation has been shown to produce distinct changes in the local hemodynamics.