Training Tip: Are you “Cardiac Drifting”?

Mark Baines, Vice President of Program Development
Cardiac drift is the tendency for the heart rate to rise gradually throughout a workout due to dehydration and/or rising temperature within the muscles despite a fairly consistent level of effort or steady work rate. In 1990, a study was performed in Italy with 34 male, recreationally athletic subjects to assess changes in heart rate while training at the lactate threshold level. During a test requiring 60 minutes of work, the study found that heart rates increased significantly after only 20 minutes of exercise at the lactate threshold.

The authors of the study suggested that these heart rate changes, despite consistent work rate, may be due to a gradual increase in body temperature over the first 20 minutes. Depending upon the individual’s current conditioning, and other external factors, it was not possible to maintain the same heart rate and a consistent level of effort in almost all cases.

Cardiac drifting is very normal and typical in well trained individuals (an untrained or deconditioned individual would not see much consistency at almost any consistent level of effort). It should be expected that the heart rate will vary within 5-10 percent or so at a consistent level of effort after as little as only a few minutes of training. The implication for training is that it is not always possible to maintain a consistent level of effort if the heart rate continues to drift upward at a given intensity (particularly when the heart rate rises above the individual’s lactate threshold). At some point in time, with prolonged effort, the individual may be forced to decrease effort dramatically or reach the point of complete exhaustion.

Several years ago, the great endurance athlete, Ingrid Kristiansen, set out to run a 10k under 33 minutes. However, at one point in the race, her heart rate monitor let her know that her heart rate was drifting upward despite the consistent level of effort which she had grown accustomed to giving. It was quite hot that day. Fortunately, she was wise enough to decrease her pace and allow her heart rate to return to an expected level to avoid “blowing up” completely and she was able to finish the race, although not at a record pace. If she had continued to pursue the same pace, she might not have finished the race at all. While power output and pacing are obviously important variables to track and pursue, we cannot ignore the body’s internal responses for the sake of power output or pace, or we will experience lackluster or traumatic results.

Pay attention to heart rate AND perceived exertion. You must know both, you must understand booth, and you must constantly monitor both if you expect to provide the right training intensity for your clients and athletes. Whether or not your client is breathing hard is far from the ideal way to determine intensity in cardiovascular exercise or more intense training circuits.

The above information was excerpted from the NESTA Heart Rate Performance Specialist (HRP) manual.

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