Martial arts are exhausting, as anyone who's traded a few punches, kicks, or throws can attest. But where exactly does the energy come from? Every form of exercise uses a different combination of the body's metabolic systems for energy. Cyclical sports such as running and cycling are relatively easy to replicate with exercise machines in a laboratory, but that's harder to do with more unpredictable sports such as martial arts. So a team of Brazilian researchers have taken the lab into the dojo to study the energy requirements of the Japanese art of judo.
Three different systems convert food to energy. During long periods of moderate exercise, aerobic metabolism does most of the work, using oxygen to turn sugar into energy, water, and CO2. Running a marathon or cycling for 100 miles, therefore, is almost entirely aerobic. For shorter, more intense exertion, or when the oxygen runs out, muscles can break down sugar anaerobically, although that system is far less efficient and produces muscle-burning lactic acid as a byproduct. Lastly, for very short bursts of energy, such as a 10-second sprint, muscles can rely on another type of anaerobic system: they use up energy-storing compounds, called phosphagens, in muscular tissues.
The usual method for measuring metabolism requires athletes to stay in place, on a treadmill or standing bicycle, while machines track the gas composition in the athlete's breath. Blood tests measure the amount of sugar and other metabolites. To get the metabolic profile, those numbers are plugged into a mathematical model of human metabolism.
But a martial art like judo or a team-based sport such as soccer involves "more complex actions," says Emerson Franchini, a physiologist at the University of São Paulo, Brazil. Rather than repetitive motion, these sports require constantly changing muscle groups, as well as adaptation to a partner or team member. Assessing metabolism during these activities is tricky: Judo can't be performed while standing in one place. And throwing people across the lab can make a mess.
So Franchini's team outfitted judo practitioners with a portable physiology lab: a mask attached to a device worn on the torso that analyzes gases in the martial artist's breath and measures the pulse. Once the athletes were hooked up, combat began. In one version of the experiment, they performed three different kinds of throws, one every 15 seconds for 5 minutes (see video). In another, the athletes sparred at high intensity, throwing each other as fast as possible.
Metabolically, judo turned out to be a mix of aerobic sports like running and anaerobic sports like weightlifting, the researchers report online this month in the Journal of Visualized Experiments. "Normally, judo is thought to be highly lactic" because it requires intense bursts of energy, Franchini says. But the data revealed that phosphagen metabolism was crucial for throwing people, and aerobic metabolism was also higher than expected.
Franchini says the findings should help judo teams train. By knowing their energy expenditures, for example, martial artists can better customize their diet.
"This is the best method we have for measuring physiology for a sport like this," says Carl Foster, a physiologist at the University of Wisconsin, La Crosse, who was unconnected with the study. But the method also suffers from "a kind of Heisenberg uncertainty problem," he adds. "How can we be sure that wearing that equipment isn't interfering with the sport? ... They're measuring something, but is it the same as competitive judo?"