A good and easy way to make your legs straight easily naturally is to use a straight leg lift exercise.
To do the straight leg lift exercise.
Start by lying on your back with your left leg bent upward.
Keep your right leg completely extended straight out.
Slowly raise your right leg to about a forty-five degree angle, keeping the leg locked straight.
Hold for five seconds and then slowly lower to the flat, resting position.
Why can't my legs go straight?
Tendon Injuries: Some people can not straighten their knees due to weakness or tendon injury. Injuries to the quadriceps or patella tendons will affect your ability to straighten the knee.
If one of those two strong tendons are torn, then you will not be able to straighten your knee.
Exercises to stretch hip and thigh muscles and to strengthen hip muscles have been shown to correct bow-legged deformity.
Exercises that may help improve genu varum include:
Hamstring stretches.
Groin stretches.
Piriformis stretches.
Gluteus medius strengthening with a resistance band.