Do you think you've got your game on when it comes to physical activity? You may take long walks, cycle, and go to the gym and think that you're doing pretty swell, but a new study questions how well you actually evaluate your level of physical activity.

Do you, as the song says, like to "move it move it?" Do you think you're "physically fit?"

You may be far from couch potato status, but does that mean that you're really as active as you think you are on a daily basis?

Granted, staying active can often be challenging due to reasons outside of our control. Perhaps we're sick, our workplace isn't within walking—or cycling—distance, or we've broken a leg.

Regardless of such obstacles, many of us think that we do pretty well in terms of being active and staying fit. I, for one, know that I'm doing my part: I've started doing yoga, I walk more, I opt for the stairs rather than the elevator, and I'm getting pretty addicted to my standing desk at work.

I might not be a fitness hero, but I think I'm doing pretty well for myself—and, if someone were to ask me how physically active I considered myself to be, I'd say "moderately so."

That being said, a new study suggests that my self-evaluation might be more wishful thinking than objective assessment.

Scientists at institutions across Europe and the United States—including the University of Southern California, Los Angeles, University College London in the United Kingdom, and Tilburg University in the Netherlands—put together a project testing how accurately people rate their physical activity status.

Lead study author Arie Kapteyn and colleagues' findings—which have now been published in the Journal of Epidemiology and Community Health—aren't very flattering.

In short, we're all less active than we think we are. And, study respondents from the US tended to overestimate their activity levels the most.

Those in US gravitate toward extremes

Essentially, the researchers asked whether or not people from different countries, backgrounds, and ages accurately self-report their own physical performance.

In order to answer this question, they worked with 748 people from the Netherlands, 540 from the US, and 254 from the UK. All the participants were aged 18 and older, and about half of them were women.

To check how accurate people were in their self-evaluations, the scientists tried a two-pronged approach: they asked the participants to report how active they thought they were, as well as objectively measured the participants' activity levels using wrist-worn accelerometers.

The subjects were asked to rate their activity levels using a five-point scale—from "very inactive" to "very active"—and their performance was monitored by accelerometers over a 7-day period.

At the end of the study, Kapteyn and colleagues found that, generally, participants from all three countries had a tendency to overestimate how active they were on a daily basis, though the average estimates across the board were largely about the same.

However, Dutch and English participants were more likely to consistently declare that they led a "moderately active" lifestyle, whereas participants from the US leant toward the two extremes of the five-point scale, either indicating that they were "very inactive" or "very active."

The case-by-case accelerometer monitoring also revealed discrepancies about the realities of the matter: people in the US turned out to be much less physically active than participants from the other two countries.

And, strikingly, the percentage of US individuals that qualified as "inactive" was twice as large as that of inactive Dutch and English participants.

'Different age groups, different standards'

Another discrepancy turned up when the researchers analyzed the self-reports by age group: older people were generally likely to say that they were quite as active as their younger counterparts when, in fact, the opposite was true.

Kapteyn and team noted that, across the three countries, people became less physically active, on average, the older they got. This might not sound like much of a surprise to anyone. Still, older participants seemed to consistently overestimate their performance.

"Individuals in different age groups," explains Kapteyn, "simply have different standards of what it means to be physically active. They adjust their standards based on their circumstances, including their age."

The data captured by the worn devices indicated a rather disheartening reality: 60% of older participants in the US turned out to lead inactive lifestyles. Among mature Dutch people, 42% were inactive, and the same was true of 32% of the UK participants in this age group.

Considering what these results mean in the grand scheme of things, the researchers argue that the proven discrepancies between self-evaluations and objective measurements cannot be ignored.

"[P]eople in different countries or in different age groups can have vastly different interpretations of the same survey questions," says Kapteyn.

Many health and fitness studies rely on self-reported information, the researchers note, which may end up skewing the results because the data aren't as accurate as they might be. That's why, they add, scientists would do much better to turn to wearable devices instead.

"When you rely on self-reported data," notes Kapteyn, "you're not only relying on people to share a common understanding of survey terms, but to accurately remember the physical activity that they report."

"With the wide availability of low-cost activity tracking devices, we have the potential to make future studies more reliable."

–Arie Kapteyn

And what lesson is there in it for the rest of us? Consider investing in an accelerometer.