[Chivalrous (OP)]

I'd like to delete that topic as I haven't received more replies concerning my questions.
But Dr. Chris,
Thanks a lot for your reply, I really appreciate it. You have clarified some points for me.  []

[chris]

The force increases because, as the graph states, a positive inotrope is used, which increases muscle contracility and therefore force delivered for the same preload.

Tachycardia is just a change in rate, without necessarily a change in contracility. At progressively higher rates there is insufficient time for complete filling. Noradrenaline is positively inotropic and positively chronotropic, so it increases contracility, which is why the curve is not the same as for tachycardia alone.