Before CrossFit burst on the fitness scene over two decades ago, the conventional wisdom in exercise science was that training to improve aerobic fitness — endurance — must be separated from training to improve strength due to the “interference effect.” The consensus was that if endurance and strength training were conducted in the same workout, the endurance training would interfere with or reduce the strength gains and prevent one from achieving optimal fitness. This line of thinking spurred split training templates where strength work was conducted on Mondays, Wednesdays, and Fridays, and cardio was performed on Tuesdays, Thursdays, and Saturdays.
In his CrossFit Journal article “Removing Interference,” Lon Kilgore explains why combining traditional long, slow distance training with strength training in the same workout hampers strength development when compared to strength training alone. Kilgore cites the “chronic hypothesis,” which proposes that working muscles cannot adapt simultaneously to two competing training stresses, especially when the adaptations each modality induces diverge widely, as in the case of endurance and strength training. When presented with two diverse challenges to adapt to simultaneously, for example, long, slow distance running and weight training in the same workout, the body will bias adaptation toward the least permanent fitness quality. In this case, cardiovascular endurance degrades much faster than strength, so it receives more physiological attention, thereby blunting strength adaptation.
CrossFit revolutionizes the traditional approach to fitness. It defies the conventional wisdom that strength and endurance training should be separate and instead combines them in the same workout. This unique approach, characterized by variance and intensity, yields remarkable results.
In CrossFit, we have single-modality monostructural days consisting of a long run, row, or bike that almost exclusively trains cardiovascular endurance without interference. We also have heavy days of lifting near-maximal weights in squats, deadlifts, presses, or Olympic lifts. These are pure strength days without interference. When we combine training modalities, we program unlimited combinations of running, biking, or rowing intervals, calisthenics, and/or strength movements into fast-paced, short-duration, high-intensity glycolytic (anaerobic) workouts. These met-cons bridge the gap between aerobic workouts and strength days, allowing for tremendous aerobic conditioning and improvements in strength.
While some interference still exists with our mixed modality methods, it is significantly less than the interference that occurs when aerobic and strength training are paired together. The truly maximal gains of the specialist — the 700-lb deadlift or the 2:06 marathon — are not possible when athletes are subjected to stressors outside their area of specialty. However, how we organize our programming is the best approach for developing the strength and cardiovascular conditioning characteristic of the broad, general, and inclusive fitness we seek to fuel our physically demanding professions, sports, or hobbies.
As a final note, you may be asking yourself why the routine of lifting weights on M/W/F and doing cardio on T/R/Sat isn’t the way to go. Doesn’t this avoid interference? This training schedule may avoid interference initially, but as results begin to plateau, athletes will have to row, run, swim, or bike longer and longer distances just to maintain their aerobic fitness. These longer workouts will begin to steal from the strength work. On the strength side, heavier weights or more sets and reps will be required to elicit gains. This will be very difficult to accomplish with the increased volume of aerobic work. Requiring more and more work to keep adapting is a losing proposition; we always run out of time or get injured. Furthermore, such a routine is inefficient — the fitness return for the time invested is low — and it omits training for many fitness qualities, such as the intense glycolytic work we value in CrossFit.
Most importantly, for the type of fitness we are looking for in CrossFit, we want to “…strive to blur distinctions between ‘cardio’ and strength training. Nature has no regard for this distinction or any other…” — meaning if we follow a segmented routine of strength work on one day and cardio on another, we’ll end up with segmented capacity. Inevitably, we’ll fail miserably when strength and cardio requirements are blended as they are in combat, sports, and life. CrossFit’s method of training gives us the best shot at developing excellent capacity in all the fitness qualities we need to prepare for the unknown and unknowable.
About the Author
Stephane Rochet is a Senior Content Writer for CrossFit. He has worked as a Flowmaster on the CrossFit Seminar Staff and has over 15 years of experience as a collegiate/tactical strength and conditioning coach. He is a Certified CrossFit Trainer (CF-L3) and enjoys training athletes in his garage gym.
CrossFit and the Interference Effect