Its not about the amount of weight on the bar, (you could use a broomstick if you like, it's there to keep your posture straight). From a standing position, do a deep lunge at a 45 degree angle, then your next lunge is a 45 degree angle on the other side. Do not return to the center in between, just deep lunge side to side. The deeper the lunge, the more difficult it is. Do 20 at a time as part of a circuit. If you really do it right, you'll feel it in your inner thighs the next day!