Isometric exercises can be difficult and I do not recommend them except for experienced exercises to do about one time a week or less. Here are two basic isometric leg exercises used in Pilates mat classes.
1. Lie down on your back. Use your lower abdominal muscles to keep your lower back on the floor. Lift your legs and spread them apart. Close your legs in a "scissors" motion while using one hand on each inner thigh as resistant when you close your legs. Exhale when you spread your legs and inhale when you bring your legs to the inner line of your body. Do five sets of ten repetitions.
2. Lie down on your right side. Put one leg on top of the other. Your head can be on your outstretched left arm, or you can bend your left arm and put your head on your palm. Lift and lower your right leg. Keep your right hand on your right leg for resistance as you lift your leg. Exhale when you bring your leg up and inhale when you bring it down. Do five sets of ten repetitions. Repeat on your left side for your left leg, using your left hand for resistance.
Disclaimer: None of the above information can be taken as a substitute for advice from a medical professional such as a physician.
My third book, Pocket Guide to Fitness, is available on www.louizapatsis.com, http://www.authorhouse.com, www.bn.com and http://www.amazon.com. If you look up my name on those Web sites, you will find my other books The Boy in a Wheelchair and Life, Work and Play: Poems and Short Stories.