I have had severe problems with back mucles spasming that sound very similar to what you are describing - it is separate from the PMR and won't show up as raised ESR/CRP because it isn't inflammation.
I discussed this with an orthopaedic specialist in Germany/Austria who has done research on something called myofascial pain syndrome (MPS). Where PMR is caused by the action of substances called cytokines that are shed into the body as a whole in the early morning (hence the morning stiffness so many patients experience) in MPS these sytokines are found in small packages, to some extent all over muscles but also in specific trigger points on either side of the spine: one pair in the trapesius muscle in the shoulders between neck and upper arm, one pair in the low back about where a baby's dimples are and another pair about rib level above the waist. They can be felt as hard knots in the muscle and are extremely painful if a physio for example digs their fingers into them! They cause the muscle to be hard in spasm to protect you from pain - and that in itself can be very painful - and can also end up pinching nerves and causing referred pain into the neck and arms, around into the rib area and into the upper legs as well as the buttocks along the line of the sciatic nerve. Any wrong move makes the pain worse and you adopt a poor posture to avoid pain - and the muscles become progressively harder and weaker. For a long time they adapt, they are very flexible in that sense, but eventually they give up the unequal struggle.
The orthopaedic specialist told me he finds it often happens alongside PMR - the PMR affects how we walk and stand and may worsen or even cause the muscle spasming. At higher doses of pred the inflammation is relieved to some extent and we notice it less, often blaming a PMR flare, but as the daily doses reduces this pain starts to reappear - again, often thought mistakenly to be a flare. After a total collapse with excruciating pain over my sacroiliac joint I was referred to a pain specialist here in Italy where I live and she agreed with the scenario I just outlined. She treated it with local cortisone injections to relieve the more localised pain and used a technique called needling over the whole of my back - I have found references to it in the USA, I'm not sure about the UK, I've never come across it in the NHS although GPs in German-speaking countries use it regularly. She also did manual mobilisation of the trigger points - physiotherapists also use that technique.
All through the first 5 years of PMR, with no pred to help, I did at least weekly Pilates classes and also some yoga. My back muscles were certainly not weak but I had occasional bouts of this MPS. I had some physio appointments with a couple of physios who understood the problem and were able to give some relief. An osteopath also helped a bit but the biggest game-changer for me in the UK without doctors who would acknowledge anything was wrong was Bowen therapy.
Both on this forum and the other two PMR/GCA forums in the UK there are now quite a few people who have tried Bowen therapy and gained great relief from it. One friend of mine was stuck in bed for weeks, she could just get to the loo in the next room using a zimmer frame. After some weeks of nagging by me she gave in and booked a session with my former Bowen therapist. A friend got her down to the car and at the place she got across the pavement with her friend on one side and a walking stick on the other - no room in the car for the zimmer! After an hour session she walked back to the car unaided - and forgot her walking stick! She wasn't totally pain-free but wasn't far off after 3 sessions. She now returns every few months for a "top up".
In the meantime, the regional healthcare trust there is sponsoring a trial of Bowen therapy alongside more traditional techniques to see if it enhances pain relief in such cases. Google Hewitt bowen therapy Durham to find her site and information about Bowen - there are also many videos online which demonstrate it. It looks crazy that such a simple and non-aggressive treatment can achieve what it does and I realise many people will be very sceptical. A good therapist will tell you will will know within 3 sessions if it will work for you - it isn't an open-ended therapy that you will need for the rest of your life and which will cost thousands of pounds. You may need the odd follow-up session but not every week for ever.
By the way, I had a whiplash injury some years ago and the orthopod I saw at the time told me that the pain my lower back was arthritis-related and I would have to live with it. I couldn't walk more than a few hundred yards without excruciating pain across my sacroiliac joints. The physio and the pain specialist were both of the opinion it was entirely muscular in origin and it had developed so badly because I wasn't able to go to my Bowen lady and do Pilates (none available here at the time). They sorted my back out and that low back pain has disappeared altogether. I still get twinges - a cortisone shot and/or a session of Bowen seems to keep it at bay. I'm retired, and while there are a few Pilates classes here they are eye-wateringly expensive so haven't gone back to doing it. But the pain that you describe is gone and as a result I have got my pred dose down to 4mg - before 9mg was the stopping point, the other pain came back too badly and we thought it was a PMR flare.
One research group is hoping to look at whether dealing with this sort of thing as well as bursitis on a separate basis may help reduce the overall oral pred dose needed in PMR - cortisone injections target where the problem is and that doesn't affect the rest of the body as much and physical therapies used properly don't have side effects. Bowen doesn't anyway!
Do consider trying it - and ask your physio if she can find the hard knots in the muscles and work on them. It can be painful in the short term (for a day or two) but longer term it is worth it. I also go for massage where they work on them too.