If I’m honest, I never really liked running, but every new year when I was in high school, I made the resolution to try out for the track team. We started training before snow was off the streets of Detroit by “running the stairs” – run down the hall, up three flights of stairs, back down the hall, down three flights.

They worked us hard and within a couple of weeks every one of us was hobbling around with shin splints.

We all recovered, but we clearly had an effective recipe for injury. What we didn’t know was that our shin splints were actually the early stages of stress fractures.

Most research into exercise-induced injuries involve military recruits who undergo intense exercise in their basic training. Injury rates in this group can be over 35%, but they’re highly variable and dependent on the training regime, as well as the level and type of previous exercise.

It’s not necessarily the intensity of exercise that seems to produce injuries. Rather injuries are more likely with a change in intensity or loading.

First peak for injuries

It might be expected that the frequency of injury would decline with time after the load increased, but this in not necessarily the case. Rather, there are two peak times for injury – one in the first two weeks and one in the period between eight and 11 weeks.

The first peak includes a high incidence of stress fractures, just like in my high school story. Bones get stronger and re-model in response to increased exercise, but the process takes time. Increased load produces stress on the bone and stimulates remodelling to build up bone strength.

The process of remodelling begins with re-absorption of old bone and results in the temporary weakening of the total bone structure. If the same high level of loading is repeated, then the weakened bone can be stressed and possibly even damaged. And this leads to a downward spiral that can culminate in a stress fracture.

Similar processes occur for other systems of the body, where the very loading that stimulates the capacity of a tissue to increase, frequently also results in a transient reduction in tissue strength.

Second peak

It’s the second peak of injury – at two to three months – that’s more reminiscent of many of the injuries we see in clinical practice. To understand this peak, we need to consider how the body responds to exercise. Allowing for the effects of age, disease or accident, generally if we keep doing what we’re doing we will maintain our capacity.

If we reduce the challenges in a particular area, its capacity reduces; increase the challenges, and capacity increases. So, for example, if your goal is more strength and muscle bulk, then high muscle load is necessary. If your goal is endurance, duration of exercise is important. For improving bone density, the activity needs to load the bones. To increase cardiovascular capacity, the cardiovascular system needs to be challenged.

Exercise is often thought of as having an effect on muscle strength and cardiovascular capability, so we build up our activity levels as our strength and cardiovascular fitness improves. Effective strength can increase within minutes or days due to neurological adaptations while actual muscle strength starts to increase within two weeks. Similarly, cardiovascular fitness starts to increase within three weeks.

Connective tissues such as tendons and ligaments also strengthen in response to load. Although their response is more rapid than bones, connective tissues respond much more slowly than muscles with changes only starting to occur over a period of months.

When we use our newfound muscular and cardiovascular capacity to ramp up the exercise load, the load is also increased on the other structures that haven’t had time to remodel. It’s typically these connective tissues, such as tendons and ligaments, where injuries start to occur after six or eight weeks of exercise.

A recipe for injury

Say you want to produce an injury. Start with a new type of exercise with plenty of speed, impact, and fatigue. Increase hard and fast, and add in some awkward positions and near maximal loads. Alternatively, be pushed by a friend or personal trainer who you trust against your better judgement.

Continue for a month or so. As you’re starting to have a better cardiovascular response and feel stronger, ramp up the intensity to tolerance.

The tendons and ligaments that haven’t built up yet can then be expected to start to break down. The tissues are subjected to the downward spiral of structural stress, reduced integrity prior to remodelling, more stress on weakened structures – your injury is ready.

Just like clothing or food, exercise goes through fashions. In the 1980s, it was aerobics. When the injury rates as high as 58% to 66% were recognised, low impact aerobics were introduced and the injury rates dropped somewhat.

Today, there are a wide variety of fashionable exercises some of which follow the recipe for injury even more closely than aerobics. Look for these elements: an unfamiliar type of exercise, repeated high levels of impact or high levels of effort, particularly those performed at high speed in unusual or awkward positions, continue when fatigued. Repeat frequently, don’t allow time for recovery and keep increasing as tolerated.

A cautionary tale

A man in his late 60s came to see me recently. He and his wife had started boot camp-type exercises twice a week about six weeks previously. The exercise program was such that they would just be recovering from the pain after their previous workout when it was time for the next one.

In the previous week, he had developed three separate problems – tennis elbow, a rotator cuff problem and knee pain. He asked me if I thought he was doing too much. Once he said the question out loud, I didn’t need to answer him and he didn’t end up needing any specific treatment. He just needed to back off and apply a common sense approach rather than his following his previous recipe for injury.

So, is “no pain, no gain” a recipe for injury? Not on its own and probably not for highly trained individuals who have built up to their level of exercise. But for the rest of us, more is not necessarily better.

Returning to that new year’s resolution. Perhaps the pain is not essential and, for most of us, a motto of “no brain, no gain” might be more appropriate.