“If you have strong legs and strong arms but your core is weak, it’s as if you have two cinder blocks with a balloon between them,” Free explains. In other words, not a very stable structure.

In TODAY.com's Expert Tip of the Day, a trainer reveals how she can always tell when a client is neglecting their deep core and her top exercises to try.

To have strong knees, we have to have strong hips." How to do this knee-strengthening workout Of the four exercises recommended by Costigan, two are activation exercises that isolate a single muscle.

If you are doing any exercise at all you are likely already keeping your hip flexors strong and flexible. If you are not exercising, the exercises mentioned earlier will give you a place to start.