I don't have an exercise ball, is the advantage of owning one worth the space it takes up?
Absolutely worth it.
You don't have to go for the most expensive ball--most cost between 15 and 30 dollars. It's a one-time cost for a simple piece of equipment that will last a long time and which you can use every day. (I have a real bias--I'm not into complex machines but I do like inexpensive and effective simple pieces of equipment that really help with a workout.)
Using the ball adds a great twist, can add range of motion, increases the difficulty to usual exercises, and helps develop strong core muscles. Doing a push up against the ball or with your feet on the ball destabilizes you and means that you have to recruit extra muscles to maintain balance. Doing a sit up on a ball adds range of motion and also makes a sit up a full core exercise. There are some exercises that are unique to the ball.
Recommendations vary depending on how you use the ball, but you should get a ball where when you sit on it with feet flat on the floor, you should able to sit on the ball with your thighs parallel to the floor and your knees at a 90 degree angle or slightly greater. If you can bend your knees to less than 90 degrees [with your feet still flat on the floor and your thighs parallel] than the ball is too small for you. If your feet can't touch the floor with your thighs parallel to the floor, the ball is too big. (Some make recommendations by height which are pretty good, but these don't always work for everyone: something like 4'11" to 5'4" height: 55 cm ball; 5'4" to 5'11" height: 65 cm ball; 5'11" to 6' 7" height: 75 cm ball...)
I went out and bought one because I found a video that requires one, but now it won't suggest that video to me! So frustrating. But I've done some crunches on it on my own and I like it, I can tell they are more intense than regular crunches.
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