The only thing is, my back is very rounded in this position, not flat at all.
Does this mean I have to work more on stretching the muscles in my lower back – or is it my leg muscles that need to be more flexible?
As with all physical questions, this would be best addressed by a teacher who can see your body. However there is some information here that can help others, so I will do my best.
A completely flat spine in forward bends is not actually the goal. However, for beginners it is important to focus on staying as flat as possible in order to avoid excessive rounding while the student learns the sensitivity and awareness that are required for doing deep forward bends safely.
It may be that the rounding of your back in this position is completely normal, but without seeing you I can’t tell. As we move into deep forward bends, it is normal for the back to round. The key is that you want the rounding to be even rather than localized in either the upper or lower back.
The first thing to do is to go into the pose and just see how you feel in the pose and where exactly you feel the most sensation. Since you say that you enjoy yoga, then probably the rounding of your back is not excessive or dangerous as long as you don’t push yourself (again, caveat that I can’t actually see your body here).
If you would like to be able to go deeper into these poses safely, and with a flatter spine, then you will want to work on opening your hips and hamstrings using good biomechanical alignment. This is best learned from a teacher in person, but for those who learn well from the written word, I am providing some things to start with.
While they may appear to be easy poses, seated forward bends are actually very advanced because they require a great deal of flexibility in the hamstrings to do safely. Specifically, you need enough flexibility in your hamstrings to tilt your pelvis forward in order to keep straight at first and then evenly rounded as you go deeper into the pose.
Come onto your hands and knees do cat/cow where your spine flexes in both directions. Notice that when you are in the cat position with your spine rounded like a cat arching it’s back, your thighbones are moving forward towards your hands and your tailbone (the very bottom of your spine) is reaching towards the ground. When you are in the cow position (swayback) your thighbones move back towards your feet and your tailbone reaches up towards the sky.
Move back and forth between cat and cow and bring your attention to the way that your pelvis tips up in cat and down in cow.
Now come into an easy cross legged sitting position and move your pelvis forward and back the same way you did on your hands and knees. Depending on the flexibility of your hips, this will probably be much harder in a seated position than it was on your hands and knees.
When you are in a seated forward bend, you need to tilt your pelvis into the same direction as you do in the cow position. Imagine that your pelvis is a bowl of water. If you tilt the top forward in a seated position the water would spill out the front. Notice that when you tilt the top of your pelvis forward, it is much easier to sit up tall.
While it is easy to tilt it back (like cat position), tilting forward may be much more difficult. If this is the case for you while your knees are bent, then chances are that your hips are very tight.
Straighten your legs and try the same thing. It will be much more difficult to do with your legs straight because now the flexibility of your hamstrings are involved as well as the flexibility of your hips.
In order to prepare for seated forward folds, you should work on increasing the flexibility of your hips and hamstrings in poses where your back is stable and therefore less vulnerable.
Before moving into these poses you should warm up, either with sun salutations, a brisk walk or light jog, or any other way you like to get your heart rate going .
Big toe pose is the best preparatory pose for seated forward bends because you can use it to open your hamstrings and your hips at the same time.
Lay on your back with your legs straight on the ground. Lift one leg, loop a yoga strap or a belt around your foot, straighten your leg and use the strap to pull your foot towards you.
Press the thighbone of the lower leg firmly into the ground and push the thighbone of the higher leg away from you while keeping hold of the strap.
After 5-10 breaths move both sides of the strap into the same hand as the top leg. Keep the thigh of the bottom leg pressing firmly into the ground as you open your top leg to the side. Don’t try to get your foot all the way to the ground, just about halfway. Stay here 5-10 breaths and then return to the first position. Move your foot across your body as if you were going into a twist, but only go about a foot past neutral. 5-10 breaths here, switch the strap into your other hand and then go all the way into the twist. Repeat all that with the other leg.
After that preparatory sequence you can begin to work on your seated forward bends. First of all, sit your hips on a neatly folded blanket. This will already give you some freedom in your hips.
Start your forward bending with a one legged variation, with your other foot folded in towards your thigh. Take your hands and manually roll the flesh of your thighs in and down and pull the flesh of your buttocks back.
At first do not try to fold forward at all. Simply tilt the top of your pelvis forward and sit up tall. Keep your extended leg engaged with your thighbone pressing into the ground just like in big toe pose.
Once you feel like you are able to tilt your pelvis forward while sitting tall, you can start to fold. Once you feel your back start to round excessively, back up a bit and press the thigh of your extended leg firmly into the ground.
Do the half forward fold several times, switching back and forth between each side before moving on to the two legged forward fold. Approach the two legged forward fold in the same way as the one above. Sit on a blanket. Roll the flesh of your thighs down and pull the flesh of your buttocks back behind you one leg at a time. Press your thighs into the ground and tilt your pelvis forward as you sit up tall. Fold forward slowly as long as the curving of your spine remains even.