Muscles in Focus: Quadriceps for Stronger Sexier Legs
Stand in front of a wall just far enough away so your arms can reach it comfortably. Put both hands on the wall, neutralise your spine by straightening your back and drop your knee towards the floor. If you then slightly lean back with your shoulders this exercise will work the entire quadricep and your abs as well. To make it much harder, try doing this exercise one leg at a time!
Please be very careful doing this not to bend your knees too much since the knee is very vulnerable to strain in this position. I don't recommend this exercise for beginners or people who are not used to simple balance-based body weight exercises without the supervision of a trainer.