Downward Dog

Downward dog is a yoga pose that is great for strengthening the body particularly the quadricep muscles.  It also stretches the calves and hamstrings so you can increase your flexibility as you strengthen. 

Downward dog is the english translation of it's proper name Adho Mukha Svanasana.

It can look deceptively easy, but don't be fooled ... if you haven't done this before you might be in for a bit of a suprise.  As with many yoga poses it can be quite easy to do with bad form but you'll be missing out on most of the benefits and possible causing excessive stress to the back.  So it's important to do it right.  The quadriceps should be working hard to stretch out the hamstrings and keep the back from rounding forwards.  If you are very flexible in the hamstrings you may need to watch that your back isn't flexing the other way with the rib cage sinking towards the floor.  The back should be pretty close to straight if possible.

Check out this video for a good demonstration.