There's a lot of forward folds in a Vinyasa practice. Take a typical Sun Salutation for example. It's simply a series of folding and unfolding the body. For some students forward folds feel great! The sensation of surrendering forward and the stretch of the hamstrings is why so many yogis carve out time for their practice. On the other hand, many practitioners feel discomfort when folding and feel inadequate when they're unable to touch their toes. No matter where you lie on the forward fold spectrum please remember that your ability or inability to touch your toes in a forward fold says nothing about who you are as a human being. It's simply a comment on the range of motion in your legs, hips, and low back. That's it!
When exploring forward folds it can be helpful to understand the movement of your pelvis over your femurs, thigh bones. When you move into a seated forward fold it is best to move your sitting bones back in space. Even if it's subtle, moving the sitting bones back initiates an anterior tilt of your pelvis and allows your pelvis to tip forward over your femur bones. When the pelvis tips forward the hamstrings lengthen and the quadriceps shorten. There you have a forward fold!
If you find that your lower spine is very rounded in your forward folds experiment with sticking your butt out way behind you. As you can see below, sticking the butt out helps the lower spine lengthen. When you do the opposite and you curl your butt under (posterior tilt) you cause the lower back to round. The position of the pelvis is everything as it informs the rest of the spine.
Forcing my pelvis to anteriorly tilt is quite easy. So what to do if your butt is stuck under you? You are not alone! For many it is hard to anteriorly tilt the pelvis and the lower spine is constantly in a state of roundness. Don't you worry! Sure you can experiment with bending your knees and then anteriorly tilting your pelvis. However, I've been encouraging a lot of my students with rounded spines to sit up on a block. When you sit forward on a block you are assisting your pelvis into an anterior tilt which could help you lengthen the rest of your spine.
Your forward fold will probably never be featured on the cover of Yoga Journal if you're sitting on a block, but who cares?! Again, you are not more or less of a good person if you prop up your posture. We need to question our drive to put our face on our shins in deep forward folds. Yoga shouldn't make you feel inferior. Yoga shouldn't make you feel inadequate. Your yoga practice is there for you to be in your body and better understand who you are physically and mentally.
I have so much more to say about forward folds and their relationship to the pelvis, but I'll add one last thing on alignment. I'm fairly mobile in my hamstrings. Due to my mobility I can get very lazy in forward folds. Unfortunately too many disengaged forward folds has caused me a lot of pain and I have developed a little something called Yoga Butt. Yoga Butt is where the attachment points of the hamstrings into the pelvis are overworked and irritated. For so long yoga teachers, including myself, would encourage students to bend their knees in forward folds if they were dealing with Yoga Butt. I am not ashamed to say I now know better and would not advise a student with hamstring attachment issues to bend their knees in forward folds. I'm actually moving in the direction of encouraging all students to move away from bending their knees in forward folds. In my own practice I used to bend my knees all the time in forward folds and the issues I was facing never got better. My teacher then pointed out to me that bending the knees actually increases the stretch at the top of the hamstrings which just aggravates my injury even more. I am very grateful I learned this piece of information. I now no longer deal with my Yoga Butt symptoms and I'm always happy to share this tip with my students who have similar issues.
I once took a workshop with Bryan Kest. He had so many inspiring things to share, but I took away one important nugget. He was discussing how yoga asana can be looked at as simple shapes. Sure the shapes are powerful and you can go as deep as you want with the practice, but the shapes alone can keep you healthier, longer. He took Pyramid Pose as an example. Rather than calling the big hamstring stretching posture by the name we all know it as, he called it Tying Your Shoe Pose. I liked that. You aren't just stretching your hamstrings in a forward fold. You're keeping your body mobile and agile. Age will slow us all down, but a consistent yoga practice will probably make things like tying our shoes easier to tackle. The ability to perform basic tasks unassisted will most likely help us stay happier and more fulfilled.