Squats and lunges are great exercises that build leg mass, especially if you can slowly add some weight on your shoulders to add to the build up. On the machines, you can do leg presses, quadriceps raises and leg curls for specific muscles. For your lower part of the legs, you can do toe curls (for the shins), while heel raises which are specific to the calves. Both have to be balanced, so as to prevent injury to the weaker parts.