Static stretching will help. You need to stretch both the “Upper” Quadriceps (Rectus Femoris, Sartorius, TFL) and true Hip Flexors (Psoas and Iliacus) though, so you need both bent knee “Hip Flexor” stretches and straight(re at least) hip flexor stretches. Stretching your lower back (Erector Spinae, Quadratus Lumborum, Latissumus Doris, and External Obliques) and most likely chest (Pec Major and Pec Minor) should also help as those tissues are also likely chronically tight/shortened).
Strengthening your Rectus Abdominus, Internal Obliques, Transverse Abdominus, Gluteus Maximus, Hamstrings (especially Biceps Femoris), Middle and Lower Trapezius, and Rhomboids should also help. But, if the aforementioned (those in the first paragraph) muscle groups are extremely tight they may be inhibiting activation of these muscles and thus, IME you need to focus on lengthening them first before you start really trying to focus on “active flexibility”.
If your APT is really bad you can also try the following Hip Fkexor “release” technique. Place a foam roller on the floor and brace one end against something (wall works well), then place a pillow over the end and possibly another pillow on top of the for your neck. Next lay down, parallel to the roller, with your legs flat on the floor and the end of the stopping roughly between your shoulder blades. The second picture shows “hyperactivity”/tightness in the hip Flexors/lower back, and the first picture shows proper mobility in the hip Flexors/lower back. I’d suggest spending about 1/2 an hour in this position daily until your back can relax down to the floor (don’t “push” your back to the floor, let it relax down to the floor). It took me about 1 1/2 weeks of doing this consistently till I progressed from looking like picture 2 to being able to easily relax into looking like picture 1.
I wont get as technical as Sento
but I will touch on a few things-
rectus femoris - and piriformis
are two muscle groups that when tight
really impact your mobility and posture.
My hips always seemed tight- and not being a particularly large guy- whether road work when younder - weight room crap
or just life- my hips where always tight.
I am a big fan of smashing-
literally- LX ball or PVC on your quad- recutus femoris
and then some static stretching will do allot.
My quads particular my recuts femoris fires and grows much faster and caused some imbalances.
I often will wrap my knee with a voodoo band and then roll my quad- or sit on a LX ball with wrapped knee and do leg extensions- you can feel the junk in the quad loosening up
smashing- my hip flexor with a plate- literally- using a 45 and roling my hips, and then sitting on a LX ball legs crossed
and finding the piriformis - is a must.
pair this with some static and dynamic stretches.
very basic piriformis
some basic hip stretches
PNF I use this all the time there are better vids out there
big fan of band work for stretching
smashwerx this guy is great.
and here is his top ten.
this is a rabbit hole you can go down for ever.
Sentoguy laid out some really good stuff.
my 2 free cents-
find what is tight-
the easy solution- smash with LX ball pvc 45 lb plate etc.
to get it to release
stretch prep and more stretch as part of you r every day life to
get shit loose
