Constructive criticism
Simplify the menu structure so you don't have to click around, ie, add menu popups on mouseover or something like that so you don't have to go into each section and choose individual parts. Being able to skip steps will be a huge time saver.
Make adding workout parameters as easy and fast as possible, this will make or break you, I certainly wouldn't spend as long as it would probably take selecting an exercise, filling in the details and then clicking through. That means you probably need to be adding multiple exercises on each page, its cumbersome to go through the same thing 5 times or whatever is required for your workout. Possibly have two modes of entry addition, basic and advanced.
Consider adding time under tension parameters (concentric, eccentric etc) for the hardcore people out there, superset flags. .that type of thing.
Back buttons ! Because you're using post you're making my browser complain every time I want to go back. So I would need a back button to avoid this.
Black and white might have seemed like a good idea at the time but after browsing the site for a few minutes it immediately gets quite dull.
