Keeping legs straight !

IzzyBooNZ1

Hi there
Exercises that require you to lay on the floor on your back and then you have your legs straight up in the air :

I find this incredibly hard to do. I lay on the floor, legs in the air, point toes but I still have a bend at the knees. The only way I can have my legs straight is if I lower them near the floor and even then it is hard to have them dead straight.

Any pointers or tips to help overcome this?

thanks