I've got a wide variety of clients -- high school athletes to weekend warriors to people going strong in their sixties.
One thing they all have in common -- pain. Inevitably, whether that's unstable hips causing low back discomfort in a high school basketball player, a new client who's never felt "the burn", or a client that has an old shoulder injury that prevents them from bench pressing, there is some sort of discomfort or even pain that arises.
Pain is interesting: it is at once simple (avoid doing the thing that causes pain) and at the same time complicated (your shoulder issues may be a hip imbalance!!).
Pain is subjective: some people are very pain tolerant, while others crumble with the slightest discomfort.
Pain is relative -- any pain does not mean a full stop of activities.
Here are three categories of pain that I see in my clients:
Muscle Burn -- this one is simple, you do 30 reps of bicep curls, your arms are going to be LIT. I wouldn't designate this as pain, but discomfort. Discomfort can get pretty high on it's own scale, but even a 10 on discomfort isn't a "PAIN," meaning there is not actual damage, its just the muscle is calling for oxygen, rest, energy, MERCY.
Joint/Muscle Pain -- this one is a little more complex, but say you have a isolated joint pain on the patella tendon (bottom front of knee) or your forearm hurts doing pull ups. This pain can range from 1 to 10 on a scale.
I'd want to avoid anything that causes pain to go from 0 to 7. I see this a lot in different ranges of motion -- "I'm good squatting till I get.... HERE... then it hurts"
But if it is a 6 or below, I think it is safe to work in that range of motion. There are mitigating techniques you can do to avoid pain here, which I'll discuss below.
System Pain -- this is more complex pain where multiple joints are involved, your knee hurts AND your hip hurts AND your quads feel tight. Your elbow hurts AND your shoulder hurts. Movement is inhibited a big amount.
Pain in this category is likely a constant ache. Working out with these types of pain should be done under the watchful eye of a trainer who knows what they're doing.
MITIGATING STRATEGIES FOR PAIn Types
Muscle Burn -- Stop being soft. Embrace discomfort. Growth is on the other side.
Joint/Muscle Pain -- If your knee hurts during ass to grass squats, a short term solution would be to NOT do ass to grass squats. Do deadlifts, RDLs, step ups, leg curls, leg extension as far as your knee will allow, do split squats, do box squats.
A long term solution would be finding out why your knee hurts -- perhaps it's your feet, perhaps it's your hip flexor, perhaps it's your thoracic spine.
Dissecting that issue on your own can be tough. A really good personal trainer or strength coach can help, a good physical therapist can also help. But hiring a physical therapist for this might be overkill.
But, again, a pain that is a 3 or so can be tolerated. If the pain increases as intensity increases, then a good coach will scale back the intensity -- lower the weight, shorten the range of motion, work the opposing muscle group for a minute to provide joint balance, do a different move to achieve same objective.
System Pain -- for multi-joint, systemic pain, it'd likely take a combination of a very skilled strength coach AND a physical therapist. You can likely work with a strength coach to hit specific muscles OUTSIDE of the system pain, but to help address system pain, working with a very good physical therapist would be a good start. That therapist would get you moving to the point where the pain is tolerable for more complex workouts and then hand you off to the strength coach.
Conclusions and final thoughts
Pain is subjective, dynamic, but isn't a death knell for your healthy and active lifestyle
It can be overcome w/ grit, resolve, and good people in your corner.
A great personal trainer or strength coach can find ways to work around previous injuries, achy joints, or isolated muscle pain.
When looking for a personal trainer or strength coach, you should take these things into consideration and ask probing questions about how trainers handle client pain, what relationships they have with physical therapists in the area, what injuries they've had in the past, how they got those injuries, and how they've overcome them.