Jürgen Klinsmann understands what it takes to compete in a World Cup. With eleven goals for the German national team across the 1990, 1994 and 1998 tournaments, he is still the sixth leading goalscorer in World Cup history.

As he prepares the U.S. men’s national team for this year’s trip to Brazil, his message of preparation begins with world-class fitness. Now, a new research review from three sports scientists confirms Klinsmann’s obsession with being in top condition.

“The level in the World Cup is two or three levels higher, and the reality is that the last two years of World Cup qualifying and the Gold Cup don't give you the real picture,” Klinsmann, the U.S. head coach, told U.S. Soccer. “The global picture is facing the strongest nations in the World Cup, and you need to be prepared. It’s not easy to put a number on it, but it requires at least 30 to 40 percent more than what we have needed so far."

At the team’s January camp in southern California, players went through extensive testing for VO2 max, speed-shuttle runs, vertical-leap measurements as well as other measures of fitness. “It's crucial for us to benchmark them throughout the year and always have this data,” Klinsmann said. “You need to be able to understand what players are going through, whether it's injuries, losing form, or sometimes losing focus. We can tell them what they are lacking and where they can improve.”

Sports scientists from Australia, Norway and Spain couldn’t agree more, as long as the training and monitoring program is well defined. In an upcoming World Cup special issue of the International Journal of Sports Physiology and Performance, David Pyne, Matt Spencer and Iñigo Mujika describe how traditional fitness metrics need to be combined with new monitoring technology for a complete training program.

They argue that the obvious collection of stats, such as speed, strength, power, endurance and agility need to be supplemented with more subjective reports of overall athlete health. “The major development in the last decade is the use of daily health and well-being measures,” wrote the authors. “A range of measures including fatigue, muscle soreness, ratings of perceived exertion, presence of illness, mood state, and sleep quality and duration, are commonly recorded.”

Some coaches argue that tracking on the field performance results is a more direct measure of player fitness. A dip in a player’s usual form is a sign they are either fatigued or overtrained. While this observation is a symptom, it doesn’t get to the cause of the problem.

Pyne et al recommend in-season physiological testing to narrow down the possible issues: “Physiological testing can provide valuable insights on the factors that contribute to and regulate exercise performance, and is thus complementary rather than competing with performance testing. For example, submaximal heart rate testing using standardized protocols can be utilized for frequent, time-efficient and non-exhaustive testing of intermittent exercise capacity of high-level football players.”

Of course, the collection of gigabytes of data won’t help win games on the field if there is not proper interpretation and communication of results back to players and coaches. “Timing is everything - where possible it is always best to give verbal feedback of results and their brief interpretation at the time of testing,” wrote Pyne. “Players’ interest will wane quickly if they are tested or monitored but don’t hear back on the results.”

The best way, according to the authors, to present training information is in electronic form that allows access from multiple online platforms, including mobile: “Digital management of results and feedback is essential in contemporary football. Programs that don’t employ a fully featured web-based system that generates group and individual reports, with overlay charts, reference ranges, an accessible interface or dashboard, and automated alerts via e-mail, SMS text, or social media, are probably off the pace.”

The American team has a demanding task ahead of them in Rio, but Klinsmann is confident that the last two years of physical and mental preparation will serve them well, "The stronger their foundation is in terms of fitness and overall shape, the easier it is for the player to express himself and have confidence in what he's doing.”