ProTrack is listed in Joan Price and Shannon Entin's book -

"The Complete Idiot's Guide to Online Health and Fitness".

 Fit Tip of the Day for Wednesday, May 17, 2017

The proper way to do sit-ups. Don't do sit-ups with your feet hooked under a chair or some other object. The flexor muscles in your hips (iliopsoas) must work harder than your abdominal muscles when your feet are anchored. Instead, place your feet on the floor and try to hold them still.