Side Leg Lifts
Sets: 2 Reps: 10 (each side)
1. Lie on your side with your hips stacked vertically.
2. Lift your top leg with pointing your toe up slightly.
3. Pause for a second when you feel the muscles in your hip engaged.
4. Lower back to the starting position.
Single-Legged Bridges
Sets: 2 Reps: 10 (each side)
1. Start on your back with your feet flat on the ground and knees bent at 90 degrees. Straighten one leg but keep your thighs and knees in line.
2. Push your hips off the ground without rotating your pelvis.
3. When your hips are fully extended, pause for a second.
4. Lower back to the starting position.
Clam Shells
Sets: 2 Reps: 10 (each side)
1. Start by lying on your side with your knees bent at about 90 degrees.
2. Raise the knee of your top leg as high as you can without lifting your feet apart.
3. Pause for a second at the top of the movement.
4. Lower back to the starting position.
Pistol Squats
Sets: 2 Reps: 10 (each side)
1. Stand on one leg with your other leg straight in front of you.
2. Squat as low as you can while keeping your back straight and your knee in line with your toe.
3. Return to the starting position.
Lunge
Sets: 2 Reps: 10 (each side)
1. Stand tall and take a big step forward.
2. Touch your back knee to the ground while keeping your torso upright.
3. Repeat on the other side.
Note: Hold dumbbells to make this exercise harder.
References
http://dressageridertraining.com/blog/6-leg-exercises-to-improve-dressage-rider-leg-strength/