Having good leg muscles and a strong core will improve your rucks, scrums, and tackles.
I’m not sure if other teams use this term exactly, but one of the exercises my team does to strengthen the core is the constipated puppy.
It is a good way to practice getting low for tackles and avoiding being too high, which can lead to a penalty. It also reinforces strong body positions in rucks and scrums, thereby making them safer and less likely to collapse or bridge.
A constipated puppy, as shown doing by the woman in the photo above, is where your legs and knees are at 90 degree angles, your hands are about shoulder width apart, and your back is flat. If another person were to go up and shove you and you have a strong body position, you will not move. Imagine that you are sucking your belly button into your back and puff your chest out to help remember to keep your head up.
Doing a plank (propped up on elbows, forearms and toes with a flat back and straight legs) or a superman (balancing on your stomach with arms and legs extended in front and behind you) can also help improve core strength.
Doing these during commercials while watching TV is a great way to exercise without altering your schedule.