Raising a straight leg builds tremendous hip flexor strength and lengthens the hamstring actively. This is an actively mobility drill and will only be as hard as you make it. In other words, you get out of it what you put into it.
Cues:
- Shift your weight to one side and slowly raise your other leg as high as possible without leaning back
- Keep the standing leg locked
Mistakes:
- Failing to keep both legs locked
- Leaning back