Ever try the same thing as someone else and get different results? A friend gives you a recipe and you follow it to the letter. But your bread, casserole or soup just doesn’t turn out the exact same way. Perhaps your blender doesn’t mix as fast, or your oven temperature isn’t quite accurate. It couldn’t be operator error, could it? This same can happen with exercise.

The most studied aspect of exercise is fitness. How much strength and/or endurance you have to complete a specific task. There’s a wide range of responses to exercise in terms of fitness. Some people can double their fitness, while others may improve by only a small amount. And as hard as it may be to believe, yet others don’t improve at all. Or worse, fitness can go down. These latter two groups of people are termed non-responders.

Responders and Non-responders

There’s a number of reasons that can explain this. First off, how much you improve depends on your starting point. The better you are at something, the more it takes to improve. Conversely, the people who have the most to gain, also improve the most. It would take a lot of training for Usain Bolt to increase his speed compared to someone who’s never run before.

Age and sex, along with body size differences can also affect how one responds to a training program. Research studies attempt to control for these differences by recruiting people who have similar characteristics. To account for possible differences in other factors, studies randomize people to different comparison groups (for example, exercise or no exercise, or different types of exercise programs). By using chance to assign people to different groups, it’s hoped that each group is roughly the same. And generally it works out that way.

Then there’s the fact whether people did the exercise or not, or even did it at the proper level of effort. We know there’s a clear dose-response relationship between the amount of exercise you do and change in fitness. Sometimes people change their activity levels throughout the day to compensate for having exercised. But again, research takes this into account by having participants exercise in a lab under close supervision and measure activity levels outside of the lab.

Is it all in the genes?

But even when you take all these factors into account, we still see a range of responses. Could it be that some people’s fitness refuses to change? Is there something inherently different in these people? Perhaps. And it could be due to genetics.

To identify the role of genetics, researchers look to studies in families. In particular, how people in the same family respond to exercise compared to people in different families. From these studies, it’s estimated that genetics accounts for about 50% of the response to exercise.

Studies of twins go even further. There are two types of twins, monozygotic, who are genetically identical, and dizygotic who aren’t. Following three months of exercise, the greatest difference in fitness increases occurred between identical twin pairs. With very little difference among people of the same twin pair suggesting genetics is definitely involved.

Which exercise is right for you?

Just because you’re a non-responder to one type of exercise doesn’t mean you’re a non-responder to all types of exercise. This was tested in a group of identical and non-identical twins who underwent three months of aerobic training, followed by three months of strength training. There were non-responders following both types of exercise- people’s whose fitness or strength didn’t change.

What was encouraging, is that those people who didn’t respond to the aerobic exercise responded to the strength training, and vice versa. Only a small number of people (4%) didn’t respond to both types of exercise. This indicates each of us may have different types of exercise that works for us. Since only two types of exercise were tested, it’s likely those 4% who didn’t respond could benefit from another type of exercise.

So when it comes to your exercise routine, you may find some exercises work better for you than others. Keep in mind, fitness is just one of the many benefits of regular exercise. In one study, those people whose fitness didn’t improve still benefited from reductions in body fat and risk factors. From mental well-being to reducing your chances for disease, there are a great many reasons to exercise and you’re bound to experience a number of them.

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