This isometric exercise is a great hip and hamstring strengthener. It also teaches you to use your hips and hamstrings rather than your low back to extend your hips.
Muscles involved: Erector Spinae (low back), Gluteus Maximus (buttocks), and Hamstrings.
Action: Lie on your back with your arms at your sides. Bend your left knee to 90 degrees with your left foot flat on the floor. Then, while keeping your right leg straight, raise your hips so that you create a straight line from your knee through your hip to your shoulder. Maintain this position by pushing your left foot into the ground while squeezing your glutes. Hold for 30 seconds then repeat on the other side.
Comments: Be sure to brace your mid-section (abdominals) while squeezing your glutes.