Karp says with recent studies, the rules are changing.
"But now this new research is coming out to show that regardless of how you weight-train, if you fatigue the muscle and go into muscular failure, that both of those induce protein synthesis, which helps people get stronger," said Karp.
But here's what you need to know and this is key: You need to lift enough weight or do enough repetitions to make that muscle fail within 90 seconds.
Again: The muscle has to fatigue, or fail.
"As long as you fatigue the muscle, you train the muscle, and that stimulates adaptation so you get stronger," said Karp.
Young men to elderly women were tested, performing exercises using relatively light weight and lots of repetitions, while other tests used heavy weights and a small number of reps. Either way the results indicate that both methods can stimulate muscle growth and strength.
One thing to note however: 90 seconds with a weight that is challenging can seem like eternity.
Regardless, the studies indicate women can lift heavier weights than they usually do without the fear of bulking up. Unless they genetically have more testosterone -- which is unusual -- they aren't going to get "hulky."
"And I have been trying to tell them for the longest time that it's not how much weight you lift that's going to bulk you up. That's really in your genetics and your DNA," said fitness expert and trainer Jill Brown.
Brown has been singing this song for years, yet so many women fail to listen.
"For me the take-home on this is that you have to lift weights to maintain good joint structure and good health, but you have options whether you want to lift heavy or whether you want to lift light," said Brown.
"Very good news because you can first of all shorten the workout if you lift heavier weights, and it's a quicker way to see adaptations if you use a heavier weight for fewer reps," said Karp.