Most small-business owners understand the concept of diminishing returns: The idea that, when other variables stay constant, at some point putting in additional time and effort results in increasingly smaller results. 

Since resources are always limited, figuring out where to spend your entrepreneurial time so it delivers the best bang for the, um, hour is critical.

That same premise extends to health and fitness. If you're like many entrepreneurs, you try to stay reasonably fit. Not just because it's good for you, but because exercise helps you perform better under stress. Can elevate your mood for up to 12 hours. Can even make you a little smarter.

Still: How "fit" is fit enough? 

If you want to run a marathon, your definition of "fit" will differ from most. But if you want to compare yourself with other people and see where you currently stand -- and, more important, get a sense of where you would like to stand -- here are three simple tests you can do at home.

If you fall in the "average" range, that's good. If you fall closer to the "excellent" range, that's great.

And may be a sign that doing more in an attempt to increase your score might push you into the land of diminishing returns.

Here we go:

Upper Body Strength

The American College of Sports Medicine recommends using a pushup test to assess upper body strength and endurance.

To do pushups their way, start at the top, go down to the 90-degree mark, and push back up without locking out at the top. Women can do plank-version pushups or modified (from the knees) pushups.

Then just count how many you can do in one set. (Quick rest breaks at the top are OK.)

Here's a graph so you can see where you stand. (All images are courtesy of research scientist Schalk Cloete; for more, check out his deep dive into the subject.)

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Comparing yourself with others provides a reasonable sense-check.

But also keep this in mind: Research shows that men (not sure why they didn't include women) who could do 40 or more pushups were 96 percent less likely to experience a cardiovascular event than those who could only do 10 or fewer. In fact, pushup capacity was more strongly associated with reduced cardiovascular disease risk than aerobic capacity.

So if you want to increase the number of pushups you can do, here's a simple process you can follow (scroll down to "How many pushups do you want to do?"). Do it three times a week for 10 minutes, and after three weeks you'll definitely be stronger.

Lower Body Strength

Time for some squats, no weight required. (Think "air squats.")

To conduct this test, find a chair that, when you sit on it, puts your thighs at a 90-degree angle to your lower legs. Then put your hands on your hips, lower yourself until your butt grazes the chair, and then straighten back up. (But don't lock your knees at the top.)

Do as many as you can until you run out of (leg) gas.

Here's the graph:

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Want to be able to do more? Like many things, increasing the number of squats you can do is just a matter of time and effort: Do four or five sets of squats to failure three times a week, and in three weeks you'll definitely be stronger. 

And with a great outcome: Squats can strengthen your lower body and core, improve your flexibility, and reduce your risk of injury.

And in my case, help keep me from sounding like an old man when I get up after sitting too long.

Cardiovascular Fitness

This one's a little trickier, since there are a variety of ways to evaluate cardiovascular fitness. Stress tests. Exertion/heart rate tests. Shoot, whether you can run a mile, and how fast you run it, qualifies.

Another is VO2 max, the maximal volume of oxygen that can be inhaled and absorbed by a body. Generally speaking, the higher your VO2 max, the better your cardiovascular fitness (within genetic reason, of course.)

One way to estimate your VO2 max is to use a fitness calculator like this. Answer a few questions and you'll learn your "expected" VO2 max (based largely on things like age) and your estimated VO2 max (based on activity levels, resting hear rate, and waist size.)

Then see how you stack up:

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There are a number of ways to improve your cardiovascular fitness. Walking (briskly) is a great start. So is jogging. So is cycling, rowing, elliptical training... or if you want to double-dip and get some strength gains at the same time, consider doing HIIT workouts. Research shows that 11 (intense) minutes a day can make a meaningful difference.

Which is where diminishing returns come into play. If you want to enjoy the benefits of reasonable -- not extreme, just reasonable -- fitness, you don't have to spend hours on a treadmill. You don't have to spend hours at the gym.

You just need to do a few key things that make a big impact.

And then do them consistently.

Which, I'm guessing, is the same approach you take to running your business.

Quick update: Since a number of you have asked, here are my results. (But never forget that it's not a competition. The only comparison that matters is when you compare who you are today to who you wish to someday become.)

  • Pushups: 52
  • Squats: 61 (I did them without locking out or taking a pause at the top, which definitely upped the burn factor and made it as much a test of pain tolerance as endurance)
  • VO2 max: 56