In terms of the ability of drugs like digoxin to increase cardiac contractility, their primary action on cardiac cells is

Cardiac glycosides increase contractility by inhibiting the Na/K-ATPase pump, causing an increase in intracellular Na. This, in turn, increases intracellular Ca by slowing down Na/Ca exchange. The increase in intracellular Ca leads to its binding to the troponin-tropomyosin complex, causing an allosteric change and facilitating the interaction between actin and myosin.