Joints and Cartilage: What Actually Helps and What Wastes Your Time

How cartilage really works, how much load it tolerates, which exercises and numbers help painful joints, and the signs that mean you should see a doctor.

Joints rarely hurt because you wore them out; far more often they hurt because the load changed faster than the tissue could adapt. Cartilage is slow tissue, but it is not dead tissue — it responds to pressure, just differently than muscle does. Here is what is actually known about it and what you can do over the next 12 weeks.

How cartilage actually lives

Articular cartilage in the knee is roughly 2 to 4 millimetres thick and is 65-80% water. It has no blood vessels, no nerves and no lymphatics. That is why it repairs slowly: muscle shows clear remodelling 24-48 hours after a hard session, while changes in cartilage and connective tissue are measured in months.

The second consequence matters more: cartilage itself cannot hurt. Pain comes from the bone underneath it, the synovium, the capsule, tendons and muscle. That is why imaging and symptoms often disagree — in people over 40 with no complaints at all, MRI shows cartilage changes in the knee in roughly 30-40% of cases. A report that says you have damage does not automatically mean that is the source of your pain.

Cartilage feeds by diffusion from synovial fluid, and the pump is cyclic loading and unloading. In plain terms: movement feeds the joint. A few weeks of full immobilisation measurably thins the cartilage layer, while regular moderate loading is associated with a thicker, more resilient layer.

Load is a stimulus, not damage

The common belief is that running and squatting wear out knees. The data do not support it: recreational runners show hip and knee osteoarthritis prevalence around 3-4%, sedentary people around 10%, and elite athletes with extreme mileage around 13%. The problem is not load, it is the jump in load.

Increase weekly volume (mileage, total sets, tonnage) by no more than 10% per week.
Do not change three variables at once — volume, intensity and exercise selection. Change one per 3-4 week block.
After a break longer than 3 weeks, restart at 50-60% of your previous volume and rebuild over 4-6 weeks.

A plan for a joint that already hurts

The working rule used in rehab: pain up to 4/10 during the exercise is acceptable, and by the next morning it must return to the level it was before the session. If it is worse the next morning and stays worse for 24 hours, the previous session was 20-30% too much.

Isometrics to calm it down. 5 sets of 45 seconds at about 60-70% of maximum effort, 2 minutes rest between sets, once or twice daily. For the knee: wall sit or Spanish squat. For the shoulder: pressing into a wall in external rotation.
Strength, 12 weeks minimum. 3 sets of 8-12 reps, 2-3 times per week, with a 3-second lowering and 1-second lifting tempo. For knee pain target quadriceps and glutes; for the shoulder, external rotators and lower traps at 3 sets of 12-15.
Range of motion without pain. If a full squat hurts, use a leg press through 0-60 degrees and add 10-15 degrees every 2 weeks.
Check symmetry. A strength difference of more than 10-15% between left and right leg is a real risk factor and a sensible target for the next 2-3 months.

If 12 weeks of consistent work moves nothing — not strength, not pain, not function — the answer is not more patience. It is a different plan or a proper diagnosis.

Kilograms and steps

During ordinary walking the knee takes forces 3-4 times body weight, and up to 6 times when descending stairs. Every kilogram of body mass therefore costs the knee 3-6 kilograms of force per step. Losing 5% of body mass produces a measurable drop in pain; around 10% produces a clear difference in function. A realistic pace is a 300-500 kcal daily deficit, roughly 0.3-0.7 kg per week, with strength training kept in place.

For daily activity in people with knee osteoarthritis, six to eight thousand steps a day is a reasonable target; below four thousand, function tends to decline.

Nutrition that makes sense

Protein at 1.6-2.2 g per kilogram of body weight daily, spread across 3-4 meals of 25-40 g. This is the base for the muscle that protects the joint.
Collagen or gelatin, 10-15 g with 50 mg of vitamin C, taken 30-60 minutes before loading. Evidence is limited and the effect is small, but the risk is negligible.
Vitamin D only based on a blood test; deficiency is usually treated with 1000-2000 IU daily.
Omega-3 at 2-3 g of EPA plus DHA daily — a modest reduction in morning stiffness in inflammatory conditions.
Glucosamine and chondroitin have unconvincing results. If you try them, give them 8-12 weeks and then judge honestly; do not build a plan around them.

What does not help

Complete rest longer than 2-3 days — it reduces pain short term while weakening muscle and cartilage long term.
Joints cracking without pain or swelling. That is not damage and needs no action.
Braces and tape as a permanent substitute for getting stronger.
Daily anti-inflammatories so you can train through pain — that just removes the only signal you have.
Avoiding squats, stairs or running for life because of one scan.

When to see a doctor

Swelling that appears within 1-2 hours of an injury, or a joint that locks and will not straighten.
You cannot take four steps with weight on the leg.
Redness, a hot joint and a temperature above 38 °C — urgent, same day.
Morning stiffness lasting more than 45-60 minutes, in several joints, going on for weeks.
Pain that wakes you at night, swelling with no injury, or unexplained weight loss.
Pain that has not improved after 6-8 weeks of consistent strength work.

The short version

Cartilage has no nerves and cannot hurt on its own; damage on a scan is common in people over 40 who have no symptoms at all.
Movement feeds the joint — immobility thins it, and weekly jumps in load above 10% overload it.
Strength is the base: 3 sets of 8-12 reps, 2-3 times a week, for at least 12 weeks, with pain up to 4/10 that is back to normal by morning.
Every kilogram lost removes 3-6 kilograms of force from the knee per step; a 5-10% drop in body mass is noticeable.
Swelling within two hours, a locked joint, a temperature above 38 °C or an inability to walk belong with a doctor, not in a training plan.