4) Leg presses.

This exercise is done with the help of the leg-press machine. It aims for overall leg-development with a special emphasis on the quadriceps.
Since squats can put quite an amount of pressure on the lower back, executing this exercise on the leg-press machine eliminates that strain.
The subject lays back on the leg-press machine feet close together against the mobile piece of the machine. Pulling the safety pin he/she slowly lowers the weight down till the knees get close to his/her shoulders, then he/she pushes the weight back to starting position.
Extra considerations:
- load the machine up well. Since the lower back is cut from the equation it is safer to work with bigger weights than the squats.
- take full advantage of the range of motion the machine provides.