Final answer:
Inotropic factors, which can be positive or negative, are the primary determinants of the strength of myocardial contraction. These, alongside the Frank-Starling mechanism and various other factors such as autonomic innervation and endocrine influences, govern how forcefully the heart contracts.
Step-by-step explanation:
The factors that affect the strength of myocardial contraction are known as inotropic factors. These can be either positive or negative inotropic factors. Positive inotropic factors increase the contractility of the heart, leading to a greater stroke volume (SV) and a smaller end-systolic volume (ESV), whereas negative inotropic factors decrease the heart's contractility, resulting in a smaller SV and a larger ESV. The Frank-Starling mechanism also plays a role, where an increase in ventricular muscle stretch (preload) within physiological limits leads to a more forceful contraction and thus increased contractility and SV.
Many factors can influence strength of myocardial contraction, including autonomic innervation, endocrine control, and environmental factors such as electrolytes, metabolic products, temperature, and the afterload, which is the resistance in the vessels that the ventricles must overcome to pump blood.
Moreover, the heart's own pacemaker, the SA node, generates the sinus rhythm that initiates heart contractions. The depolarization of the atria represented by the P wave on the electrocardiogram is crucial for coordinating the timing of these contractions.