Do not stretch hams or hip flexors before lifting. Possibly, not ever.
I agree that stretching hams before exercise is generally a bad idea, and that stretching any muscle before using it, will only reduce its ability to generate force.
However, if you have anterior pelvic tilt, causing hamstring and abdominal inhibition, then stretching psoas before exercise is a good idea (as long as you don’t need much force from the psoas which you won’t in deadlifting or squatting).
Whaen you reach the top of the deadlift, and your overtight psoas are tilting your hips antreiorly, causing a hyperlordosis in the lumbar spine and you have a large amouint of weight in your hands, what do you think will happen to the facet joints? That’s right, they will likely bear a lot more load than they should, irritating them and causing pain.
Tight psoas may be a problem. But classic psoas stretches do not work, at least for me. They ALWAYS cause me pain.
In general I am not a fan of isolated static stretches of problem muscles as a means to fix pain arising from dynamic movements. I did a ton of static stretching with no success. Instead, I find a combination of soft tissue work and more complex movements, like those in McGill’s book and Egoscue’s book, fix the overly tight psoas and anterior pelvic tilt.
The pattern of muscles firing during a movement like a deadlift is very complex. I do not believe that anyone knows, especially for someone in pain, that the psoas is causing the problem during the actual deadlift movement. And even if it is, the static stretch may not even target the same muscle fibers that are causing the problem during the movement. (My perspective here comes from being a cognitive neuroscientist specializing in motor learning and motor control.)
There is a long history of recommending static stretches to solve movement-related pain, with limited success – in fact, perhaps just as many failures as successes, which suggests that the approach is no better than a shot in the dark.
McGill actually observed an experienced powerlifter rupture a disc while deadlifting in his lab while hooked up to all kinds of measuring devices (EMG, etc.). If I remember correctly, the injury was caused by a little multifidus that failed to fire. The movement was fine except for that one little muscle that failed to support that one disc for that one little fraction of a second but instead let it get hammered by the weight.
Whatever the reasons may be, classic psoas stretches did NOT fix my anterior pelvic tilt, did NOT help my form in any lift, and virtually always triggered pain. On the other hand, spinal stability movements, hip mobility movements, glute activation movements including one-legged stuff, and soft tissue work DID fix those problems. (And interestingly, it was soft tissue work on the glutes that was so helpful, not on the psoas.) So my reasoning about static stretching is, when in doubt, leave it out. And over the internet there is always doubt.
(BTW I enjoy the information you give on this and other threads, and I’m so glad to see more people studying issues related to injury treatment and prevention for lifters. There was nothing much out there years ago when I started lifting, and what I didn’t know DID hurt me.)