Isometric and isotonic exercises both build muscle. The difference is that isotonic exercises involve contracting muscles while moving the joints around. Isometric exercises involve contracting the muscles while keeping the joints still. You can find examples of both in yoga, and that's what makes the practice such a beneficial way to gain fitness and build muscle.
The plank position in yoga is a good example of an isometric exercise. Place your hands shoulder-width apart directly below your shoulders on a mat. Keep the rest of your body stretched out straight behind you. Use your abdominal muscles to keep your back completely flat. If you look in a mirror, your body should form a straight line from the tip of your head to your feet. Hold this position for several deep breaths. For more intensity, try the one-armed plank position, where you place one arm on the mat, under your shoulder, and rotate your other arm up off the mat until both arms are sticking straight out from your torso (one in the air and one on the mat) and you are facing a side wall. Hold for several breaths and then perform this move on the other side.
Lower Body Isometrics
The warrior poses in yoga are all examples of isometric exercises that work the lower body. To perform the pose known as "warrior 2" in yoga, stand on a mat with your feet spread out wider than shoulder-width underneath you. Rotate your left foot 90 degrees to face a side wall, and rotate your right foot 45 degrees to face your left foot. Bend your left leg until your knee is directly over your left ankle. Do not bring your knee past your ankle or you risk injury. Keep your right leg straight and your right heel on the ground. Extend your arms out to the sides and look to the left. Hold this position for several breaths, then repeat on the right side. According to Yoga Journal magazine, this pose strengthens the legs and ankles, and can relieve back pain.
You can also find isotonic exercises in the practice of yoga. One example is the yoga push-up, which actually combines isotonic and isometric work. To do the yoga push-up, start in the plank pose discussed earlier. Slowly begin to lower your body toward the mat, keeping your elbows tucked against your sides and keeping your back flat. Go halfway down and hold for a long breath. Come back to plank pose. Hold for another breath. Repeat two or three times.