Monitoring intensity of cardiovascular exercise is all about determining the proportion of fat and carbohydrates being used to power the activity. The gold standard is gas exchange analysis, which involves collecting exhaled air and determining the amount of oxygen used and carbon dioxide produced to directly calculate the fat vs carbohydrate ratio. The heart rate monitor, although accurate in measuring actual heart rate, relies on formulas to estimate this fuel ratio. These formulas are not individualized and fail to account for changes in fitness, health status, or a myriad of other factors that can affect physiology that would degrade the accuracy of the estimations.
The breathing rate and intensity are directly affected by the fuel ratio being used at the time. Self-monitoring of breathing rate may seem like a step backwards in technology, but it’s really a step closer to source of the desired “data.” There are two ventilatory thresholds that are of concern to the aerobically training athlete, dubbed “VT1” and “VT2.” When breathing can be done through the nose, is slow and controlled and conversation isn’t a struggle, at least 50% of the energy being produced is derived from fat. Theoretically the limitation on the duration of this activity is the fuel stored, which for fat, is almost limitless even in the thinnest of rock jocks. The upper limit of this predominantly fat driven zone is VT1. Once you cross VT1, exercise intensity is too high to be carried mostly by fat metabolism, and carbohydrate metabolism kicks in at a higher rate as does its byproduct, lactate.
At moderate exercise intensities, lactate can be removed at a rate that is tolerable; breathing rate increases and mouth breathing becomes necessary. As exercise intensity continues to rise, carbohydrate use escalates and lactate production starts to outrun lactate removal. Carbon dioxide is a byproduct of cellular metabolism, and its concentration grows in the blood as well, triggering the brain and body to cause the depth of breathing to suddenly increase. The athlete will take that first deep breath, the breathing rate also sharply rising. This threshold is VT2. Conversation is difficult (only a few words at a time) and maintaining the intensity of exercise at this level is limited to minutes (duration sustainable is inversely proportional to intensity above VT2).
Although rock climbing performance is not limited by general aerobic or anaerobic capacity, aerobic training can improve health and increase metabolism, which both beneficial to rock climbing performance (much of this through weight management.) Improving the efficiency of fat metabolism is the biggest benefit of cardiovascular exercise for the aspiring rock climber, thus the majority of aerobic training should be done below VT1. Nose breathing it is!
After a career of literally being a lab rat, constantly staring at digital outputs of my training efforts, I can look back and see how this one-sided mentality was detrimental to my career and health. More importantly, it stole much of the original reason I chose to be an athlete, the enjoyment. Your body will not lie to you so heed all its signals and warnings. Strive to become attuned to the host of signals it can give you while training. Monitoring breathing is a first great step in this direction, it is directly related to the processes you are attempting to improve, and not being distracted by a heart rate monitor helps maintain the joy that the training can provide.