In these yoga poses, the hamstrings may tighten up to help control the forward tilt of the body.
In the standing hamstring stretch above, called Front Triangle or Parsvottanasana, if my arms are lifted as in the first picture my hamstrings have to activate to keep my torso horizontal.
And if I want to tilt my pelvis forwards so that my torso moves closer to my front leg then the hamstrings have to lengthen while staying active.
(This is known as eccentric contraction. But you can think of it as the hamstrings "lengthening while activated".)
With lack of experience, your hamstrings may lock and prevent you from deepening the forward bend.
They won't be able to lengthen while active. You could think of this as a fear response.
And so one goal of hamstring stretching is negating the fear response.
One way to overcome the fear response is to use the arms to help support the body when doing standing (or seated) hamstring stretches.
In the second picture above I'm resting my hands on my legs. If I press my hands down against my leg then my arms support my upper body meaning that my hamstrings can ideally relax.
If I can keep my hamstrings relaxed I can gradually tilt my pelvis forwards. So that my ribcage can lower I can either bend my elbows as I tilt my pelvis forwards or slide my hands down my leg towards the floor.
Another option for overcoming the fear response is to practice activating and relaxing the hamstrings.
Using the same standing hamstring stretch, your can start with the hands on the leg. Then, slowly lift the arms and reach them back.
The hamstrings will have to activate to support the body.
Then put the arms back down on the leg. With the arms supporting the body the hamstrings can relax again.
Repeat this a few times and then switch sides.
One you have the basic feel for this exercise a variation uses the following repeated steps:
If you practice a few times you'll learn to feel what you need to do to make your legs strong in just the right way to support the weight of your body.
With the legs strong, your spine then has a stable base (the pelvis) from which to lengthen. Your then have a firm foundation for lifting the arms.
Each time you lift the arms you practice engaging the hamstrings. You then give yourself, and your hamstrings, the confidence and experience so that you can begin deepening your hamstring stretch.
When do you stretch the hamstrings in this exercise?
When you put your arms back down on your leg or the floor.
With your hands on your leg (or on the floor) you can gradually bend your elbows as your hamstrings relax.
As your elbows bend your ribcage can move closer to your leg.
However, rather than bending the spine so that this happens, tilt your pelvis forwards so that your ribcage and pelvis tilt forwards as one unit.
You'll then be stretching your hamstrings.
Another way to support the body while doing a standing hamstring stretch is to use the butt muscles.
Standing with feet parallel and hip width (or shoulder width) apart for standing forward bending hamstring stretch, you can slowly bend forwards while keeping your butt engaged.
If you like rest your hands on your legs, or on the floor if you can reach it.
You could also focus on squeezing your butt cheeks and then relaxing them.
Each time you relax your butt cheeks see if you can also relax your hamstrings so that you can stretch them.
If you find that your lower back is really stiff when doing standing yoga hamstrings stretches, you might find that activating your gluteus maximus muscle is really helpful.
Another way to work around the fear response while doing hamstring stretches is to focus on moving your bones.
In the case of the standing wide leg hamstring stretch (prasaritta padottanasana) you can focus on lengthening your hamstrings by moving your sitting bones upwards, away from the backs of your knees.
The sitting bones are the bones that you can feel when you sit on a hard chair.
Lifting the sitting bones is simply a focused method for tilting your pelvis forwards.
This technique is taught in detail in the Simple Hip Control Exercises video.
If you can get your hands onto the floor then use your arms to pull your ribcage towards your legs (Prasaritta Padottanasana A).
You can also grab onto your big toes (Prasaritta Padottanasana D). In this case pull your arms forwards, so that your ribcage moves back, towards your legs.
In both of these versions keep your spine feeling long. Make your legs feel long also.
If you have your hands on your waist (Prasaritta Padottanasana B) or you have your hands clasped behind your back (Prasaritta Padottanasana C Shoulder Stretch) then you can use your hip flexors, as well as gravity, to drive the stretch.
The ebook (and optional video) Muscle Control For Better Flexibility teaches you how to activate these muscles in the context of a yoga routine.
This routine includes exercises specifically to help you learn to feel and activate these muscles.
If all you want is some simple exercises to teach you how to activate the above muscles, the video Basic Muscle Control: Quads, Hip Flexors, Hamstrings, Gluteus Maximus has exactly that. Basic exercises for activating those muscles, plus the no-fail exercises that I use as a back up for the students who have difficulty.