It’s one of the most complex, contentious questions in sports: Who are the fittest athletes in the world?

Sports Illustrated accepted the challenge and ranked the best-conditioned male and female athletes in the world on the 2019 Fittest 50 list, consulting a panel of trainers, exercise physiologists and performance experts to evaluate athletes on the following criteria: performances over the last 12 months; demands and risks of their respective sports; durability; training regimens and other physical benchmarks including power, speed, strength, agility, endurance, flexibility and more.

While working to compile the Fittest 50 list, I spoke with Dr. Christopher Lundstrom—a member of the expert panel, an exercise physiologist and coach of elite distance runners at the University of Minnesota—about the measures we should use when comparing athletes of all shapes and sizes, from disparate sports.

Jamie Lisanti: Determining the best-conditioned athlete is a difficult task. What criteria should we use?

Christopher Lundstrom: I think you have to look at the ability of an individual to perform at a very high level in a range of activities or conditions. To be able to do a lot of different things to adapt to different demands. My background is in track, so I think of the 400- and 800-meter runners who are really fast, but can also have really good stamina and endurance as well. You could probably put them out on the soccer field and they’d do pretty well.

You have to look at the ability to produce power in a variety of planes of movement as well. You can think of that as agility and dynamic flexibility—the ability to be powerful in a variety of movements and movement patterns.

You can look at strength to weight ratio as a big measure. Anytime you look at a measure of the ability to produce strength or power, it really should be adjusted for body mass. Because there is no way you can expect a 100-pound gymnast to produce the same gross of force as 250-pound person. But what they are able to do with their body is amazing because they have an incredible strength to weight ratio.

Sports Illustrated's Fittest 50 2019: Ranking the Best-Conditioned Athletes

And you can see that across all the sports. If you think about Kipchoge in the  marathon, he’s not able to produce a ton of force but he is able to produce a lot for his body weight and for a long period of time.

A much under looked component is also the psychological fitness—people usually talk about this in terms of grit and mental toughness. Just the ability to keep going and push through when it becomes difficult, whatever the activity or sport may be.

What traits or characteristics do professional athletes have that make compiling this list difficult?

CL: It is really challenging because the more elite you become in a particular area or sport, the more specialized you become. And the less fit you become for the opposite types of activities. For example, you can look at the extremes: an elite marathoner who’s body fat percentage is so low and their ability to produce any power with their upper body is extremely low. You could put them in a cold environment and they are going to have a difficult time surviving because their body fat is so low. And then on the other end of the spectrum, you have someone like an NFL player who is super powerful with a lot of muscle mass and can produce tons of power. But if you put them out in the desert and asked them to keep doing something for 30 minutes, they are going to be toast and overheat.

Each professional sport has a special set of demands, but may not meet all of the physical criteria. What groups of athletes do you think check all or most of the boxes?

CL: On the exercise physiology side, we think about it in terms of energy systems. The ability to move in a variety of planes of movement. And perform a lot of complex, neuromuscular firing patterns to execute movements. The sports where you need all three energy systems seem to be pretty darn good—something like soccer, something with repeated sprints, but an endurance component too, and the ability to recover between high intensity bouts.

Hockey is another—it’s a power-oriented sport. You have to have an anaerobic ability to recover between the bouts, with limited recovery in between. In track, 400- and 800-metere runners have all energy systems firing. In football,

What about decathletes?

CL: There is an endurance component because they are doing the 10 events over ywo days. But they have a good amount of time between those events. And most events are very short duration. For the same person to be able to throw the shot put and do the the hurdles that’s just an incredible range, and then they make them runt he 1500m at the end. And that brings in the endurance component.

What about basketball players?

CL: Definitely, I think that’s a great example. You’ve got the repeated sprints. You have an endurance component. But obviously what makes a great player is their ability to produce a lot of power in a short period of time to jump, sprint and get open on the court. The thing that is tricky with a sport like basketball is that the skill component is a huge part of what separates a good from a great player. And skill is different from fitness for sure. But at the same time, I think the person that is performing well at the end of the game—it’s not because they are more skilled than someone else, it’s that they have the fitness to still be playing really well that the end of the game, so that gives them an edge.

How does age play impact fitness and play a role in conditioning as time goes on?

CL: The ability to stay healthy and continue to train year after year is indicative of a true underlying fitness. Whereas a college basketball player, for example, might have a really bright peak but then due to whatever underlying health issue or susceptibility to injury, they might not last. So I think that there is something to be said about that staying power.

I think for somebody who is later in their career, it becomes more of a case of: they are just training smarter and they know what they need. They have mastered the art of recovery and preparation. Are they more fit than they were before? Probably not. But they’ve got the intelligent approach to preparation that allows them to maintain their fitness over a long period of time, which is certainly impressive in its own way.

On the opposite end, what about younger athletes, in their teens or early 20s? Or rookies in their first year as a pro, like Saquon Barkley or swimmer Caeleb Dressel?

CL: This is for the fittest of this year, or the last 12 months. I think it would be hard to debate if they are performing at a high level. To be doing that kind of stuff you need to be at an incredible point of fitness, no matter the age. I would question it more if it were an Olympic sport athlete who has just one glorious season or Olympic Games, where they peak for one event. I think the reality of a sport like football is that the demands are such that you are going to be your most fit in your early to mid-20s. That’s kind of just the wear and tear of the game.

What makes this question so difficult to answer?

CL: David Epstein talks about how athletes have become so much more specialized the elite level. And that’s essentially at the foundation of the challenge of answering this question. The more elite you become in one particular sport, the less likely it is to be able to take that and crossover and perform at a similar level in a different sport. My thought would be to encourage anyone who is thinking about an athlete in this context to think: How would they do in these five other sports or events? And that gets at the root of the challenge. Could they even survive? Could LeBron complete an Ironman triathlon? Maybe he would drag himself to the finish line. But on the other side, what if you put Eliud Kipchoge out on the court? Could he survive for two minutes out there without getting crushed? It’s an incredible challenge but it’s a really cool mental exercise to think about how we measure fitness.

It’s kind of like asking: Who is the smartest? You’re not going to get the same answer from anyone who is specialized in his or her field. Everyone is going to have a different answer. But that's what makes it fun.