I was stuck on the same topic for a while and I think I've figured it out, and hopefully this will help. Please someone do correct me if I'm wrong.
Force of contraction or stroke volume is dependent on two INDEPENDENT factors, preload and contractility. Preload is better understood if you look at the Frank-Starling curve which essentially says that increase in preload will stretch the cardiac muscle and put it in a more ideal positioning in terms of its individual sarcomeres and produce a greater contraction (stroke volume)
Contractility depends on how much Calcium is available INTRA-cellularly. Drugs that increase this availability such as digoxin increase contractility of the heart.
The two lines shown on the frank-starling graph show this relationship between contractility and preload. Each line represents systolic performance with the SAME contractility, so when the preload (venous return) increases assuming no changes to contractility, the force of contraction or systolic performance increases. If given drugs that increase Intracellular availability of Calcium such as Digoxin, then the contractility increases, so the force of contraction is higher for the same amount of preload in tissue without Digoxin therapy.
