Low versus high heart rate conditioning for fitness. Which is best?

You want to improve your cardiovascular fitness.  What approach will you take?  Aerobics classes, such as spin and pump, are one option.  These classes combine low intensity movement with high heart rate.  Another option is cross conditioning, e.g. CrossFit or F45, which combines high intensity movement with high heart rate.  A final option might be to go for a run or to cycle.  Observe anyone running or cycling and you're very likely to also observe someone working hard; exercising with a high heart rate. 

There may be more than one way to get fit but the common denominator appears to be that improvements in fitness are associated with exercising with a high heart rate.  If I want to get fitter, I need to work hard.  The harder I work, the greater the outcome.  As one goes up, so does the other.  Is this correct?  No, it's not.

The human body has two energy systems for exercise or activity: an aerobic or low power system, and an anaerobic or high power system.  The aerobic system powers us more or less all of the time.  The high power system does not kick in until muscle work approaches a peak.  It's only good for a few seconds up to about one minute.  The body draws on the aerobic system for work below maximum and when muscles are working at peak.  The aerobic system is used to replace the fuel used during anaerobic and aerobic work.  We experience this as high breathing rates during and after exercise.  Where fitness is concerned, regardless of context, the aerobic or low power system is paramount.

Returning to our question: which training approach is best, the question we need to ask and answer is this: what is the strongest signal to expand the aerobic system?  The aerobic system works at every intensity level but are they all equal where outcomes are concerned?  The answer to this question is that there is a heart rate band or zone that produces the strongest signal for the aerobic system to expand and it may surprise to know that it's not very high: 60-70% of max heart rate.  In this heart rate zone, your breathing rate is often not elevated much above normal and it is possible to chat with someone.  That's why the advice given to endurance athletes is to work at levels that would allow you to talk.  

Low heart rate training is not the only level of work endurance athletes need to complete but it should be most of it - about 80% of total training load or volume.  Aerobic fitness is also not the only kind of capacity for work needed for exercise.  Strengthening muscles, for example, is also useful!  However, if fitness or endurance are your primary goals then you need to know how to effectively perform and distribute work to gain the best outcome.  The most common mistake in endurance training and exercise for fitness is believing that the harder I work, the greater the outcomes.