Climb a Tree is a wonderful move done on the Reformer Long Box as well as the Ladder Barrel, but I also love replicating it on the mat. It gives a great stretch to the hamstring as well as beautiful abdominal control when walking up and down the leg. Pelvic stability and precise core flow are also players here. This may feel challenging, especially if you are tight in the hamstring, but stick with it. You will feel rewarded and rejuvenated!
How to do this exercise:
From a chest lift position, grab the back of one leg. Stretch that leg straight up to the ceiling. Your hands will begin at the thigh. Exhale as you walk your hands up the leg and bring yourself to a seated position. Inhale at the top. Exhale to walk back down placing your spine down on the floor. Stop just where your shoulders are about to touch the floor. Do 5-8 reps on one side then switch to the other leg.
Some things to think about:
If your hamstrings are tight, work with a bent leg at first. Try to get to a point where you can feel a slight stretch through the back of the leg and take it a little more each time. Never go past your end range. I love moves that also work in dynamic stretching, of which this is one, as opposed to just holding a static stretch.
Just one new move a day builds your toolbox immensely. Figure out what your body likes and where it can be challenged. Most importantly, keep on moving!