Are World Cup players the fittest athletes?

Being a professional soccer player requires speed, endurance, strength and skill -- key components of all-around fitness -- and World Cup players are at the top of their game. Could these be the world's fittest athletes?

Experts say it depends how you define fitness.

Soccer is certainly one of the most demanding sports because of the diversity of fitness aspects needed to perform well. However, other sports such as cross-country skiing, running and cycling seem to edge out soccer in terms of the number of calories that athletes typically burn, as well as their Vo2 max levels -- the maximum rate of oxygen athletes consume during a particular sports activity.

Joe English, National Advisory Coach to the Team In Training program, who trains marathon runners and ultra-marathoners and edits Running-Advice.com, told CBS News how he'd rank different types of athletes in terms of endurance, speed and similar criteria:

Marathon runners

Super-fit factors: Endurance and speed.

World record speed would be just over two hours at under 5 minutes per mile.

Professional cyclists

Super-fit factors: Endurance, speed, balance, strength.

"Tour de France riders this month will consume 10,000 to 12,000 calories per day and will ride for up to 6 hours per day for almost three weeks," English said.

Ironman triathletes

Super-fit factors: Endurance, strength and skill in three sports -- swimming, cycling, running.

Professionals cover the 140.6 mile course in under eight hours.

Cross-country skiers

Super-fit factors: Endurance, speed, strength, and balance.

"They also conduct their sport at high altitudes, meaning their have less oxygen at their disposal," English said. "Cross-country athletes burn at top speed burn about 1,500 calories per hour."

Soccer players

Super-fit factors: speed, balance, flexibility, and strength.

"Soccer players need to be able to create intense but short bursts of speed, quickly change direction and conduct skillful actions while moving at high speeds," English said.

How many calories do athletes burn?

Here are some of the sports disciplines in which athletes burn the most calories per hour. The following numbers are based on a 175 pound man performing at highly competitive speeds, according to rough estimates provided by English:

Cross-country skiing -- 1.500 calories per hour

Running -- 1,400 calories per hour

Cycling -- 1,100 calories per hour

Competitive soccer -- 800-1,000 calories per hour

Swimming -- 800 calories per hour

Race walking -- 700 calories per hour

Who has the highest VO2 max scores?

Among the world records for best VO2 max scores for female and male athletes, cycling , cross-country skiing and distance running tend to get the highest scores, according Topend Sports:

VO2 max records for male athletes:

97.5 Oskar Svendsen -- Cycling

96.0 Espen Harald Bjerke -- Cross country skiing

96.0 Bjørn Dæhlie -- Cross country skiing

VO2 max records for female athletes:

78.6 Joan Benoit -- Distance running

76.6 Bente Skari -- Cross-country skiing

76.0 Flavia Oliveira -- Cycling

So how do these athletes get so fit? Part of it is genetics and the other part is training, experts say. "It's a little bit of both," said Dr. Susan Joy, Director of Cleveland Clinic's Community Sports Health Network Factors.

Factors such as VO2 max, the size of the heart and the lungs really matter, and people can't really increase their VO2 max, Joy said, even though they can maximize it through training.