Final answer:
Contractility is the most important factor influencing myocardial contraction, affected by preload and enhanced by positive inotropic factors like autonomic nervous signals and thyroid hormones.
Step-by-step explanation:
The most important factor that increases myocardial contraction is contractility. This parameter is influenced by a series of factors, with preload being a significant one due to Starling's Law of the Heart. Contractility refers to the heart muscle's force of contraction that controls stroke volume (SV) and is influenced by positive inotropic factors. An increased ventricular filling leads to a greater stretch of the cardiac muscle fibers, contributing to contractility enhancement as per the Frank-Starling mechanism. Additionally, autonomic innervation, thyroid hormones, and catecholamines can also increase heart rate and force of contraction, acting as positive inotropic influences.