Here’s a phrase I want you to ponder, “cardio-respiratory endurance is modal specific.” What does it mean? Let me give you an example. Lance Armstrong won seven Tour de France victories and was considered the best cyclist of all time. Prior to becoming the greatest cyclist, he was a reknowned triathlete as a teenager. After retiring in 2005, he decided to run the 2006 New York Marathon. Yet the best cyclist in the world, who had triathlon experience, who was coached by elite marathoners, and who was likely taking the best performance enhancing drugs available at the time, was only able to perform above average at the marathon. Many people speculated that since he had the best cardio endurance of any athlete alive at the time, he would be able to dominate in an endurance event like the marathon. Yet human physiology proved them wrong.
You take the best cyclist in the world and put her in a boat, in running shoes, on cross-country skis, in a gi, or in any other event other than cycling, and she will cease to be dominant. How many times have you heard people new to jiu-jitsu remark that “grappling is a different type of cardio” or some version of that statement. At the most basic level, cardio-respiratory endurance is the exchange of oxygen for carbon dioxide. As you train, the body will get more efficient at intaking oxygen and expelling carbon dioxide. That’s a good thing. However, efficiency at running does not translate perfectly to efficiency at jiu-jitsu. Jiu-jitsu uses many more muscles than running, it requires changes of tempo and direction, it requires both isometric holding of positions and dynamic, explosive movements. Jiu-jitsu utilizes the anaerobic as well as the aerobic energy pathways. It should not come as a surprise that 20 minutes of cardio on a treadmill doesn’t have much carry over to a 7-minute grappling session. This is why you see people come in to jiu-jitsu for the first time and they seem fit from the gym, but they gas quickly when on the mat.
This is not an excuse for you to stop doing cardio!
If you want to improve your cardio-respiratory endurance on the mat (and you should!), then you need to train in such a way that has carryover to BJJ. You need to do short fast intervals. You need to do longer workouts with high rep full-body movements, You need to train with multiple modalities in a single workout. You need to change your workouts often to avoid plateaus.