Final answer:
The period where cardiac muscle fibers are contracting without shortening is during the isovolumetric contraction phase of ventricular systole within the cardiac cycle.
Step-by-step explanation:
The period of time when cardiac muscle fibers are contracting and exerting force but not shortening is known as isovolumetric contraction, which occurs during the initial phase of ventricular systole.
During this phase, the ventricles are contracting against closed heart valves, leading to increased pressure within the ventricles but no change in volume, hence no shortening of muscle fibers. This is part of the cardiac cycle, which also includes diastole, the phase of relaxation and filling of the heart chambers.
The proper coordination of systole and diastole is crucial for efficient blood pumping. The muscle fiber sarcomeres do reach optimal length for forceful contraction as they stretch with ventricular filling, dictated by the end-diastolic volume (EDV) or preload. However, during isovolumetric contraction, despite the exertion of force, the muscle fibers do not actually shorten due to the closed valves.