Lift your legs on your back, your abdomen is still strong, but your thighs can't be lifted. This is why your abdominal muscles are not strong enough.

There are two reasons why thigh muscles are tense:

First, improper exertion, supine leg lifting, is abdominal exertion, driving thighs and calves to lift. Force is applied to the abdomen. If you apply force to the thigh, it will cause tension in the thigh muscles and the leg cannot be lifted.

Second, the abdominal muscles do not have enough strength to pull up, and the thighs and calves are slipping, which makes the muscles of the thighs indirectly gain strength and tension.