In Janu Sirsasana B one knee is straight while the other knee is bent. To get into the pose you lift your hips and place your anus or perineum (which is just in front of the anus) on top of your heel. And then you bend forwards, towards your straight leg making this a yoga pose a seated hamstring stretch.
In the picture you can see the position of my foot prior to me sitting on it. Some teachers advocate turning the foot so that the toes point in the same direction as the straight leg. Try both options to see which suits you best. I prefer this position because it stretches the top of my foot. However, you may find that pointing the foot forwards makes it easier to position your anus relative to your heel.
Also notice that my right leg is at ninety degrees to my left (if viewed from above.) I've recently experimented with closing this angle to about 70 degrees so that my knees are closer together. This too can make it easier for you to position your anus or perineium directly on top of your heel.
Why might you want to do this? One possibility is to help you become more aware of this area. With your weight on your heel you can practice drawing your perineum up away from your heel. If you have difficulty finding or maintaining this contraction you could try contracting (or lifting) each time you inhale and then relax it while exhaling.
As mentioned, to get into Janu Sirsasana B (B for pain in the Butt), bend one knee and have your other knee straight. Open your legs so the thighs are 90 degrees apart.
Note that if you have knee problems, move into this hamstring stretching yoga pose slowly. Or miss it out all together until your knee is able to handle the pressure.
To move into janu sirsasana b, slowly use your arms to lift your pelvis, and then slowly bend your elbows to positon your pelvis on top of your foot. You can just hold this position, using your arms to support your weight and then work at slowly (over the course of several sessions) let your hips sink deeper.
As you can see in the middle picture, my pelvis is tilted towards my straight leg side. You can try making your pelvis level by moving your bent knee forwards or back slightly or by adjusting the position of your foot. I'd suggest that having the pelvis level in this posture is not critical.
While bending forwards, especially when first learning this pose, continue to use your arms to support the weight of your body.
Keep your knee straight without pressing it down. Keep your spine long. Focus on drawing your head away from your ribcage to lengthen your neck and open the top of your chest. Then draw your chest away from your pelvis to make your spine long. You may also find it helpful to pull your lower belly inwards.
As you reach your hands forwards you can rest them on the floor. If you can reach beyond your foot then you can also grab one wrist. Try both options, grabbing the wrist of your straight leg arm and then your bent leg arm. See how each option affects the way janu sirsasana B feels.
To come up out of the pose you can continue reaching your arms forwards and up as you sit up or you can place them on the floor and use them to help support your ribcage when sitting up. You might want to straighten your bent knee leg and and give it a few moments to rest before moving into your next pose.
If you are using this as a prep for half bound lotus you might choose to move from this pose, into janu sirsasana C and then from there into half bound lotus.
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