I've done a lot of squats, front squats, leg presses and hack squats but my hip flexor are tight from soccer, bike racing, sit ups and leg raises. It causes tightness in lower back etc... Stretching helps but what really helps is running sprints. Yeah I must we are designed to run.. Definitely more athletic movements. 40-60 yard sprints with a 30-40 second walk repeated 10x's is more anaerobic and will actually build muscle... I do walking lunges in sets of 30-40 (15-20per leg) hams quads and glutes will be sore like a leg routine in gym except calves. Different post... lol