Forward bending yoga poses are normally associated with hamstring stretches.
The hamstrings are the muscles that run up the back of the thigh. They cross both the knee and the hips and so are affected by both joints.
To stretch the hamstrings the knee has to be straight and the pelvis tilted forwards.
If the knee joint is bent then tilting the pelvis forwards only stretches the hip extensors, (the muscles that work on the back of the hip joint) and not the hamstrings.
If you have tight hamstrings then it can be helpful to focus on keeping the back straight when doing forward bending yoga poses so that the focus is on opening the back of the hips and stretching the hamstrings. (Read more about doing a hunchback free seated forward bend.)
To stretch just the hamstrings in a forward bending yoga pose focus on tilting the pelvis forwards in poses like standing or seated forward bend. This can be relaxed stretch (hip flexors relaxed) or active (hip flexors engaged). It could also be a resisted stretch (hamstrings and glutes active).
To stretch just the back of the spine, tilt the pelvis backwards and bend the spine forwards. This can be a relaxed stretch (abs relaxed) or it can be an active stretch (abs engaged.)
To stretch the back of the spine and the back of the legs simultaneously in either a standing or seated forward bend, you can tilt the pelvis forwards first and then bend the spine forwards from there.
Or Start by bending the spine forwards and then tilt the pelvis forwards while keeping the spinal forward bend.
To stretch the calves you can focus on moving the fronts of the feet towards the knees. You can do this with knees bent to stretch only the Soleus muscle. If you do this with knee straight then you also stretch the gastrocnemius.
One way to stretch the calves is in downward facing dog yoga pose.
You can do downward dog with knees straight or with knees bent. So that you stretch the calves with knees bent continue to reach your heels towards the floor.
If you keep the knees straight you can stretch the calves by trying to lift the fronts of your feet. (This activates the Tibialis Anterior muscle, at the front of the shin.)
To stretch the hamstrings at the same time in this forward bending yoga pose, you can tilt your pelvis forwards.
You could also use this yoga pose to bend the spine forwards. Simply tilt the pelvis back so that the lumbar spine and the bottom portion of the thoracic spine bend forwards. You then lose the hamstring stretch but you get a stretch for the lower back instead.
If you do a seated forward bending yoga pose with feet together and knees straight, and pull the fronts of your feet towards your knees you can stretch your hamstrings and your calves at the same time.
Hip extensor stretches are like hamstring stretches in that they open the back of the hip, except they are done with the knee bent. An example is low lunge, shown below.
In plough pose (below left) you can focus on stretching your hamstrings or the back of your spine.
To stretch the back of your neck, you can lift your legs into shoulder stand pose (below right). To reduce the neck stretch in shoulder stand you can move your hips forwards (away from your head.)
Note when lifting your legs from plough to shoulderstand, activate your neck first. Press your head down into the floor and then lift your legs.
A front bend that takes the stress away from the hamstrings and focuses it on the spine is bent back hero pose.
Here you can focus on tilting your pelvis back while bending your spine forwards. Push your ribcage back so that hangs over the edge of your feet. Focus on curving your spine forwards.
I like to breath into the back of my body when doing this pose. And it's a great way to counter stretch back bending yoga poses, particularly if you've been using the spinal erectors.
You can reach your arms forwards or rest them on your thighs.
You can activate the abs (on the inhales or exhales) or keep them relaxed.
Another forward bend that focuses on the spine and not the hamstrings is bound angle pose.
You can do this pose seated upright first, and then bend forwards. So that the knees go down in this yoga pose I either focus on lengthening or opening the inner thighs or contracting or squeezing the outer thighs.
You can also focus on pressing your feet (not your knees) into the floor. The knees will move downwards as a result.
Grabbing the feet to bend forwards you can contract your abs to bend your spine forwards. At the same time you can tilt your pelvis forwards to move the head and chest closer to the feet. Try contracting the abs on either the inhale or the exhale. (If you contract on the inhale, relax on the exhale. If you contract on the exhale, then focus on lengthening or opening on the inhale.)
Front bending the neck is relatively simple. You can simply tilt your head forwards while sitting or standing. However, to make it more active contract the front of your neck behind your throat (longus colli muscle.)
If you focus on using this muscle, which attaches to the front of all the cervical vertebrae and the uppermost thoracic vertebrae, you may get a smoother curve in the spine than if you activate the more external muscles (those towards the front of the throat and neck: scalenes, sternocleidomastoid.)
You may also find it interesting (and helpful) by resisting the stretch. Use your spinal erectors to resist the forward bend. Your spinal erectors will then be working against your neck flexors. However, so that you get the forward bend, make the flexors stronger. It's like a tug-o-war where one team allows the other team to slowly pull them forwards.
A slightly more passive neck stretching yoga pose option is rabbit pose where you place your crown on the floor and try to roll your head forwards.
I'd suggest supporting the weight of your body with your hands. Press them down a little or a lot to support the top of your ribcage. You can actually use your arms to help lift your ribcage giving your neck some room to move with respect to your head. Move your knees closer to your head if need be (you may find your neck feels "safer.")
Rather than letting your ribcage sink down onto your neck and head (crushing the neck) lift it using your arms, and then when it feels comfortable enough, slowly and gradually roll your head forwards, little bits at a time to stretch the back of your neck.
Imagine you are trying to do a forward somersault but keep your knees on the floor.
Then slowly come out.
A counter pose that focuses on strengthening the muscle that you've stretched is locust yoga pose.
However, to strengthen the muscles of the neck, keep your chest and shoulders on the floor and just lift your chin off of the floor. Slowly bend your neck backwards to lift your head higher.
For more on strengthening the back of the body check out yoga poses that strengthen the back of the body.
You can also counter pose forward bending yoga poses with back bending poses.