Exercises for the thigh muscles usually target more then one particular muscle group.
Remember, during these exercises, always breathe evenly using your diaphragm but not your upper chest.

• Hamstrings workout - You will need a physioball for this exercise. Lay down on your stomach, legs together. Put a physioball behind your buttocks on top of your thighs. Bend your knees to secure the ball. Press your heels down into the ball trying to bend your knees even more. Release without taking your legs away from the ball. This is a pure hamstrings exercise.
  Repeat 10 times and do 2-3 series, resting in between.
• Modified Bridge - Lay down on your back, bend your knees, and put your feet together. Engage your buttocks and hamstrings then raise your hips in the air until your body and your thighs make a straight line. You can keep your buttocks engaged to work both hamstrings and buttocks. Or, you can try to relax your buttocks to some extend. This will work the hamstrings more.
  First variant: Lift and lower your hips. Repeat 10 times and do 2-3 series, resting in between.
  Second variant: Hold the hips lifted as long as you can, building up to 30 seconds. Lower your hips and relax. Repeat a few times.
• Modified Bridge with the Ball (feet on the ball) - This exercise is not easy and requires core stability. You will need a physioball for this exercise. Lay on your back. Bend your knees and put your feet on the top of the ball. Put your arms on the floor at 45 degree angle to your body, palms down, elbows extended. Press your arms down. Raise your hips in the air by squeezing your buttocks and engaging hamstrings and the back muscles. Your hamstrings will work very hard to stabilize the ball. Do not let the ball slide away from you.
  First variant: Lift and lower your hips. Repeat 10 times and do 2-3 series, resting in between.
  Second variant: Hold the hips lifted as long as you can, building up to 30 seconds. Breathe evenly. Lower your hips and relax. Repeat a few times.
  Third variant (the most difficult): Keep your hips lifted. Extend the knees, rolling the ball away from you. Then bend them back, rolling the ball back. Keep the movement slow and controlled and do not lose the ball. Repeat 5-10 times and do 2-3 series resting in between.

• Prone buttocks exercises - Lay on your stomach, arms by your sides and legs slightly apart and bent in the knees. Lift your right leg by squeezing your right buttock. You will be able to lift it only a little bit, but this is sufficient. If you try to lift higher you will start lifting your pelvis off the floor and engaging the back muscles. Hold your leg up in the air for a moment and bring it back slowly. Alternate right and left legs. Repeat 10 times on each leg and do 2-3 series resting in between.
  Variant: Lift both legs and keep them lifted as long as you can, building up to 30 seconds. Breathe evenly. Lower your legs and relax. Repeat a few times. This exercise is a pure buttocks workout.
• Standing leg extensions - Stand by the wall or a tall furniture and use it for support. Engage thigh muscles in your left leg to stabilize yourself. Extend your right leg backward and sideways. Be careful not to go very far backwards, otherwise you will start arching your spine. Hold your leg extended for a moment, and bring it back slowly.
  Repeat 10 times on each leg and do 2-3 series, resting in between.
• Modified Bridge with the Ball (shoulders on the ball) - You will need a physioball for this exercise. Sit on the floor with your back straight and resting against the ball, and your legs bent. There should be around 30 cm (12 inches) between your feet. Create an abdominal push, engage your buttocks and lift your hips off the floor until your body and your thighs create a horizontal line. The ball will be between your shoulder blades. You will have to keep your head in line with your body. You can have the ball closer to your head for the neck support. Lower yourself down slowly.

  	If you feel discomfort or pain in your neck consult your physician before continuing with the exercise.

  Repeat 10 times and do 2-3 series, resting in between.

Rectus Femoris is a superficial muscle located on the front of your thighs. Iliopsoas are the deep muscles located to the sides and slightly above of the pubic area.
Exercises for the core strength are good exercises for the hip flexors. When you stretch the hamstrings, you should learn to engage your hip flexors for better results. For an explanation, see Hip Flexibility.

• Reversed Sit-up - Sit on the floor, legs together, and bend your knees. Either stabilize your feet under a sofa, or ask someone to hold them. Sit close to your legs, back straight and arms folded in front. Create an abdominal push, exhale and slowly lower your torso (from the hips) down just a little bit, 5-15 degrees. Bring it back on the inhale. Keep your back straight all the time.

  	Do not arch your lower back. If you only lower your upper body you have failed to create the abdominal push.

  Repeat 10 times and do 2-3 series. Lowering your body further is less important than doing more repetitions. Women can do this exercise without stabilizing their feet, because their lower body is heavier then upper body. This exercise is working the iliopsoas very hard.
• Clock Exercise - Kneel on the floor/mat. Keep your body upright with the trunk and thighs in a vertical line. Either put your arms by your side, or fold them in front of you or behind your head. Create an abdominal push and lean backwards from your knees a few degrees. You should not arch your back. Your body, from the head to the knees, should form a straight line at all times. Come back to the vertical position. To create more work for the rectus femoris muscle make sure you do not engage your buttocks.
  Move slowly and in control. Lean backwards only as far as you can handle comfortably. Repeat 10 times and do 2-3 series, resting in between.

  	If you feel discomfort or pain in your neck, back or knees consult your physician before continuing with the exercise.

• Rider's Posture - Stand sideways to the tall mirror and in front of a wall or a solid piece of furniture. Put your feet parallel to each other and at the shoulder width. Now, look into the mirror, keep your body vertical and bend your knees. Your knees must go down and forward and come in contact with the wall in front of you. Put some weight into that contact. Look in the mirror. You will see yourself in the posture of a rider. Note that your pelvis is in a neutral position and you can draw a vertical line through your ear, shoulder, hip and heel. Still watching yourself in the mirror, keep your knees against the wall, your torso vertical, and move your body down and backwards by slightly bending your knees. Come back up and forward with your body. This exercise mainly works the rectus femoris muscle.
  Repeat 10-15 times and do 2-3 series, resting in between. At first, move slowly and in control. When you get better, increase the speed without losing the control.

  	If you feel any discomfort or pain in the knees consult your physician before continuing with the exercise.

• Modified Bridge (using your upper thighs) - Lay on your back, bend your knees, and put your feet slightly apart. Push your feet down and start unbending your knees. This will lift your thighs into the air and your body will follow. Do not engage your buttocks. Keep your body and thighs in line, and do not arch your back.
  First variant: Lift and lower your hips. Repeat 10 times and do 2-3 series, resting in between.
  Second variant: Hold the hips lifted as long as you can, building up to 30 seconds. Repeat a few times, resting in between.

To combine a workout for hip extensors and upper thighs, use the previous exercise - Modified Bridge. In that position you can use two different methods to lift your hips. You can either engage your upper thighs, or you can use your buttocks and hamstrings.
First variant: Move your hips up and down alternating these methods.
Second variant: Hold the hips lifted and switch between these two muscles groups without lowering your hips.

Click here for exercises on abductors, adductors and thigh rotators.