Understanding isotonic contraction of muscles.

Could some one explain for me how the muscle shortens and keeps a constant tension (case of isotonic contraction).

I'm very confused, I mean since the muscle gets short then there must be more tension generated, right?

could someone explain it for me, as I read many articles about it and still don't understand it, I fell that there are something messing and it will help me understand it, but I don't know what exactly is it?

Thanks!