Can all barbell exercises be done with dumbbells?
Almost all of the exercises can be done. The rolling abs barbell exercise cannot be done with dumbbells if dumbbells are not round.
What is the single most important ingredient for success in weights workouts?
The feel for exceeding just a little.
What is the single most important consideration in cardio activities?
Having no weaknesses in legs and lungs and metabolism. A cardio activity is successful till your weakest of the three namely legs or lungs or metabolism can withstand.
So, will removing weaknesses and strengthening legs, lungs and metabolism individually help?
It will help till a lot of extent. But ultimately coordination is also important.
Here's what :