flat back below 90 degrees for any standing split/pike work?

In an instructional video in the middle split series, Wes explains that an athlete doing a standing pancake should be able to get the upper body slightly below horizontal (i.e. below 90 degrees) with a flat back before any rounding of the back.  

I'm very inflexible and prone to lower back discomfort, so I have 2 questions:

1) Would this recommendation also apply to the standing pike in front split series?  I've been assuming "yes" for several weeks but wanted confirmation.

2) Would this recommendation also apply to Jefferson curls?  

Thanks for any guidance you can give,

Craig

I m also interested in that. In Jefferson Curls I already know the answer, definetely not, you must round your back. But in Front Split Series when doing pike with straight back i might have 120 grades between my tronk and my legs or even more! So sometimes it s very difficul to execute enough tension on my hamstrings what I solved a little bit by placing my hands on a table or similar. I also took this recommendation from MS and thought it was also said in FS videos.

Jefferson Curl must be performed with posterior pelvic tilt, the goal of the exercise it primarily the spine extension strength work and not the flexibility itself.

On the pike variation of the front split the more you can arch you back the better it is.

Thank you for the responses. 

So a general principle or rule is that once I've lost the arch, then I'm probably at the limit of safe ROM and should go no farther?

19 hours ago, Craig Copeland said:

Thank you for the responses. 

So a general principle or rule is that once I've lost the arch, then I'm probably at the limit of safe ROM and should go no farther?

@Wesley Tan seems to say that most FSs and MSs work barring vertical shin work and supine middle split work from those respective series require APT and that we should focus on that/not go further than that. Still refining this myself.

Not yet, rounding the back depends on different factors non directly involved on rom like for example the muscles mass of upper-lower limb during a pike compression. I prefer to spend some time on the arched back position because this is extremely important to mobilize the upper par of hamstring.

Craig, to your original questions:

1) Yes. My understanding is we maximize APT for straddles and pike work, especially if you know your hamstrings have limited rom. By anteriorly rotating your pelvis you're moving your pelvis away from your femurs thus effectively lengthening those tissues first.

2) No. Hold PPT for as long as possible (while going down). This anchors the pelvis so we can focus on the spine articulation.

In order to really open the hamstrings and adductors for both pike and pancake, it is crucial that one develop strength in APT. I have found that students with very limited hamstring mobility and thus restricted APT in the above stretches, do well when focusing on building strength in APT, this means hip flexor and lower back strength. Once these muscles are really engaged, the nervous system can inhibit contraction of the hamstrings through a process known as reciprocal inhibition.

One way of doing this is in attempting to full squat with a definite ATP.  Even some slight resistance can help, like the beginning of a good form deadlift with a low weight such as 10-20kg, the weight can be used to anchor the arms and body whilst activating a maximum APT. This is could also be done using the lower rungs of stall bars in an isometric fashion. In this example the effort would be on the APT and not on actually lifting the weight. Done correctly you should feel a strong contraction in both hip flexors and lower back. 

Thank you for the responses. Very helpful.