Running 04 – The 10% rule and old injuries: increasing load safely

The 16-week training plan is stuck to the fridge door, held by two magnets. The second column lists the kilometers for each week, each week a little more than the last, exactly the “add no more than 10% a week” that running groups pass around. You have just colored in the box for week five with a marker. Under the desk sits an unopened box of new shoes, saved for week nine, when the plan starts including trail runs (running on mountain and hill paths).

That sheet of paper has no box for last year’s rainy season. Back then your right heel hurt so much that you put your shoes away for almost two months, and the first few steps after getting up in the morning meant holding on to the wall. It doesn’t hurt now, so the old story isn’t written down anywhere.

Yet the research points right at that missing line. Increasing your distance gradually is the right thing to do. It’s just that the 10% figure does not reduce injuries the way people assume, while having had an injury in the past 12 months is the most consistent warning sign of a new injury that studies in recreational runners have found.

The column of numbers on the fridge door

The 10% rule sounds very reasonable: increase slowly, and muscles, tendons and bones have time to catch up. Buist and colleagues (2008) put it to the test in a randomized trial of 532 novice runners. One group trained for 13 weeks, with weekly distance rising by no more than 10%. The other followed a standard 8-week plan that increased faster. At the end, 20.8% of the slower group were injured, and 20.3% of the standard group. The two figures almost coincide, and the difference was not statistically significant.

Damsted and colleagues (2018) compared people increasing by an average of 10% a week with people increasing by 24% a week, and also found no difference in injuries between the two groups.

That result does not mean you can increase as much as you like. It shows that one fixed number, applied to everyone, is not enough to keep anyone from getting injured. Ten percent for someone running 20 km a week and for someone running 60 km are two very different loads, not to mention that those two people bring two completely different injury histories.

The 30% threshold and a p-value

After the 10% rule, running groups often pass on one more benchmark: increasing by more than 30% over two weeks puts you in the danger zone. This benchmark traces back to a study by Nielsen and colleagues (2014) in 874 novice runners.

Read closely, that study says much less. Overall, the number of people injured did not differ between the fast-increase and slow-increase groups. Only when “distance-related injuries” were separated out did the group increasing by more than 30% have a risk ratio of 1.59, meaning a risk about 1.6 times that of the slow-increase group. The confidence interval for that figure, that is, the range the true figure may fall within, runs from 0.96 to 2.66, which means it still touches 1, the “no difference” level, and the p-value is 0.07, short of the threshold for statistical significance. This analysis was also exploratory, done to raise questions for later research. A signal like this is worth watching, but it cannot yet be drawn as a red line on a training plan.

More recently there is also a way of calculating the ratio between this week’s load and the average load of the previous few weeks, called the acute:chronic workload ratio (ACWR). The data for this ratio come mainly from team sports and are of low quality, and the calculation itself has been criticized as statistically misleading (Impellizzeri and colleagues, 2020). In endurance runners, there are no studies at all yet. The clinic therefore does not use this ratio when advising runners.

The old bone that knows when rain is coming

Older people in the family often say that the spot where they broke a bone years ago “knows when the weather turns”: it aches before the rain even falls. We’ll leave rain forecasting out of this article. In runners, an old injury site forecasts something else, and for that there are data.

Saragiotto and colleagues (2014) pooled studies on risk factors in runners and found that the most consistent warning sign was an injury in the previous 12 months. The things runners like to scrutinize and blame predict far less well. Later studies (Peterson and colleagues, 2022; Malisoux and colleagues, 2022) show that running-form features, the impact force as the foot strikes the ground, the loading rate and step rate hardly help predict which recreational runners will get injured. A slow-motion video of your running form, viewed frame by frame, is fun to watch, but if you want to know who is likely to get hurt, a question about last year is more reliable.

The word “injury” in these studies is broader than runners usually think. Under the common definition researchers agreed on in 2015 (Yamato and colleagues), it means running-related musculoskeletal pain in the lower limbs that makes you reduce or stop running for at least 7 days, or for 3 consecutive training sessions, or that makes you see a doctor. The two-month bout of heel pain in the opening scene obviously counts. A bout of knee pain that made you skip three sessions in a row also counts, even if at the time you only called it “a bit tired.”

Years of running also play a part. In recreational runners, there are about 7.7 injuries for every 1,000 hours of running; in novice runners it is 17.8, more than double (Videbæk and colleagues, 2015). Someone who has just recovered from an injury and is only starting to train again should be the slowest in the group to increase load.

The figures above tell you who needs to be more careful, but they cannot yet show what needs fixing in that person.

The box of new shoes under the desk

On the training plan, load has only one unit: kilometers. For tendons and bones, load lies in many other places too. Joyce and Lewindon (2016) point out that changing the running surface, your shoes or your running form all add load, even when the kilometers stay the same, and they advise avoiding sudden increases, especially after a break or after an injury.

Read the training plan again that way, and week nine packs three changes in at once: shoes you are not used to, trail surfaces instead of asphalt, and hills appearing for the first time. That week’s number is only a little higher than the week before, while your legs have to carry all three at the same time.

Working from that principle, the clinic advises separating the changes, one thing at a time. Choose shoes by how well they fit and how comfortable they feel (Nigg and colleagues, 2015), then wear them gradually over a few short runs before bringing them into a long run. Switching quickly to minimalist shoes, with thin soles and almost no cushioning, raises the risk of injury. As for trail, running downhill forces the thigh muscles to contract while being stretched, which often causes muscle soreness the next day. One downhill run beforehand makes later ones less painful (Tallis and colleagues, 2024), so practice downhill running on short stretches first and then gradually longer ones.

The same goes after a break. Going straight back to the distance of your last week before the break is a kind of sudden increase, even though on paper it looks like “just running as before.”

Two hard sessions side by side

Try laying out a week. If the long run falls on Sunday, put the fast run on Tuesday or Wednesday, with an easy run or rest on the day in between. A hill session also counts as a hard session, so don’t place it right next to the other two. That gap is the time a tendon needs to recover after a high-load session, about 2–3 days according to Joyce and Lewindon (2016).

Sahrmann uses a 48-hour benchmark when letting people with shin pain return to running, and the clinic borrows this benchmark for the load-building phase before a race: if the old injury site is still sore or swollen on the third day after a hard session, don’t increase next week; step back one level. Pain concentrated at one point on a bone that gets worse with each session, or groin pain when you put your weight on one leg, is a different matter. In that case, stop running and see a doctor.

The textbook on the basic theory of Chinese medicine has a four-character phrase, “tire yourself, but not to exhaustion” (劳而不极 / 勞而不極, láo ér bù jí). A hard session is meant to tire you. Exhaustion usually comes when two hard sessions sit right next to each other, on top of a new pair of shoes and the first hill.

Supervised hip and core exercises

Among the ways to prevent injury, strength training has one noteworthy trial. Leppänen and colleagues (2024) followed 325 novice runners for 24 weeks. The group doing hip and trunk strength exercises (hip and core) had fewer injuries in the legs (lower limbs), with a risk ratio of 0.66, meaning the risk was only about two-thirds of that in the comparison group. In the same trial, ankle and foot exercises did not reduce injuries, and acute injuries were actually more frequent. So not every exercise program gives the same result.

Pooling 9 trials in 1,904 endurance runners, Wu and colleagues (2024) did not find a clear benefit from strength training in general. The benefit in the “supervised” group appeared only in an additional analysis done afterward, so it still awaits confirmation. What can be said for now: supervised hip and core training may help, while training on your own has not been proven.

Runners with a past injury: when to get checked before increasing load

Not everyone who has ever been in pain needs to see a doctor before every new training plan. Get checked before starting a phase of increasing distance if you:

• Still feel soreness at the old injury site when running, or still have pain in the first few steps in the morning;
• Get pain again in exactly the same spot every time you extend your distance, even if earlier scans were normal;
• Have had a stress fracture (a crack in the bone from repeated overload) in the femoral neck (near the hip joint), or two or more stress fractures within 2 years;
• Have an ankle you once sprained that still often feels loose and rolls easily on uneven ground;
• Have just taken several weeks off running because of an injury and plan to go straight into a race training plan;
• Are a woman who runs a lot and whose periods are becoming less frequent during the training season.

As for the ankle, up to 18% of people still have symptoms many years after a single sprain (Jull and colleagues, 2015). To prevent another sprain, the clinical practice guideline published in the journal JOSPT (Martin and colleagues, 2021) gives its strongest recommendation to an ankle brace combined with proprioceptive training, that is, training to sense joint position and keep your balance.

Tendons and bones also wear with age. Middle-aged people who often have aching back and knees, whose legs feel weak on long runs and hurt more the harder they work, are classed in Eastern medicine as having Liver and Kidney deficiency (肝肾虚 / 肝腎虛, gān shèn xū), meaning the Liver and Kidney organs as understood in Eastern medicine are both weakened. After an examination, the doctor may prescribe a decoction in ready-to-drink 50 ml pouches aimed at tonifying the Liver and Kidney and strengthening the sinews and bones. This approach rests on the classics, has not been tested in runners, and the pouches cannot replace increasing load gradually. If you run races with doping control, tell the doctor before anything is prescribed.

Last year’s rainy season is still in your foot

Just because the heel no longer hurts doesn’t mean the nervous system has forgotten it. When they stimulated the receptors in the ankle ligaments (tiny nerve endings that sense the position and tension of the joint), Freeman and Wyke found that the activity of the calf muscles and the tibialis anterior (the muscle running down the front of the shin) changed accordingly. In people whose joint capsule injury has healed, the postural reflexes of the muscles crossing that joint may still be off. Applied Kinesiology (AK), a way of testing muscle strength by hand to read the state of the body, interprets this as joint receptors firing incorrectly, so muscles are switched on or held back in ways that do not match what the body needs (Walther, Synopsis, p. 60).

That imbalance lingers because of the cerebellum, as Schmitt explains it. Throughout the time a wound is healing, pain receptors fire signals continuously, and the cerebellum has to readjust the reflexes that contract and relax the muscles, like someone at sea for a long time gradually getting used to the waves. If the injury is serious enough, that readjustment is kept as a “new normal,” bringing with it a lopsided posture, a changed order of muscle contraction and a changed gait (Synopsis, p. 186).

While the imprint of an old injury is still active, the patient still responds to treatment, but the results may not last (p. 184). If you want to increase load, it is worth finding it first, and the Injury Recall Technique (IRT), developed by Schmitt, does exactly that. You place your hand on the spot on your heel that used to hurt. The doctor makes a very gentle movement at the ankle, with no jerking, following Schmitt’s description of IRT, and then tests a muscle that is currently strong. If the rainy-season bout of pain is still “stored,” that muscle weakens, whereas just placing the hand or just making the movement does not weaken it (p. 184–185). During treatment, your hand stays on the old spot, along with a similar gentle movement. Afterward, the calf muscles, tibialis anterior and other muscles around the heel are retested to see whether they have stopped working below threshold (weakening when tested).

On the same page, Walther records an observation from podiatry: almost every significant injury leaves an imprint on the foot, usually on the same side (p. 184). So the doctor also asks about past ankle sprains, falls from a motorbike and old surgical scars. If the heel is still swollen or the pain keeps getting worse, the first step is still an examination and imaging when needed, with IRT left for later.

The extra line in the corner of the page

Take a pen and write the missing line in the corner of the training plan on the fridge door: right heel, last year’s rainy season, two months off. Then open the shoe box earlier and wear the shoes on the short runs of weeks six and seven, so that by week nine the only change left is the trail surface. Keep the long run and the hill session at least two days apart. The kilometer column can keep rising as before; you just need to read it together with the line in the corner.

If your heel is still sore in the first steps of the morning, or the pain comes back every time your long run grows by a few kilometers, bring your training plan and the shoes you are running in to an appointment before increasing further. Dr. Dinh Ngoc Duc (MD, MSc in Traditional Medicine) will look at the old injury site, examine you while you move, and then work with you to adjust the pace of your load increases for the remaining weeks. Book a consultation.

Worth keeping

• Write down every injury in the past 12 months, including any time you only had to skip 3 runs in a row;
• Increase your distance gradually, but don’t treat 10% or 30% as a safe line;
• Count new shoes, a new running surface and added hills as part of the load increase, and change only one thing at a time;
• Leave at least 2 days between two hard runs;
• If pain or swelling at the old site is still there after 48 hours, step back one level the following week;
• If you want to do hip and core strength training, find someone to guide you;
• If the old injury has not fully cleared or hurts again when you extend your distance, get checked before increasing further.

References

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