I recently trained a client wearing a heart rate monitor on both a treadmill and an exercise bike. The training was intervals and her heart rate did the following: Walking at 6km/hr, her heart rate went up to 140 to 150 bpm running at 10km/hr (8-9/10 RPE). Her heart rate then decreased to around 120 bpm, we returned to walking and her heart rate went back up to 150 to 160 bpm and did not decrease markedly in two minutes of walking. However, when she ran again, her heart rate decreased to around the 120 bpm. On the exercise bike, her heart rate went up with the “hard” efforts and then returned in recovery periods. Can you please explain the low heart rate occurring on the treadmill “hard” efforts?
Whenever using heart rate, I urge that rather than using "whole" or absolute heart rate numbers like 140 bpm or 152 bpm, instead you should use relative heart rate numbers because it puts the data into perspective. Relative heart rate numbers are a percentage not a whole number. Relative heart rate is a percentage of an anchor point number like threshold heart rate, maximum heart rate, RPE, lactate (using blood samples) threshold or maximum oxygen consumption (% of VO2 max). For our purposes in your question, let’s use maximum heart rate because the math is easier. Let's say your client has a maximum heart rate of 200 bpm in running and 190 bpm in cycling. Then, using your example, her relative numbers during the workout in your question are as follows:
- Maximum heart rate: 200 bpm
- Interval exercise heart rate: 150 bpm
- Relative heart rate number: 75% of maximum heart rate
- Recovery heart rate number: 120 bpm
- Relative heart rate number: 60% of maximum heart rate
The hard effort (stress interval) was appropriate for a fitness enthusiast reaching 75 percent of maximum heart rate. The recovery interval was also appropriate taking her down to 60 percent level. I am reading that of the two types of recovery - active and total - that you had her stop completely (total) and allowed her heart rate to drop. Why it would return up to the interval exercise level is peculiar and not typical. You are right.
My suggestions: See if the activity is an "anomaly" or a one time occurrence. Or does it happen every time with every interval? My suspicion is that it was a one time occurrence since human physiology doesn't respond in this fashion normally.
Keep in mind, heart rate is affected by medications, environmental and emotional stresses, so you might want to dig deeper into those areas to see the root cause of this unusual cardiac response!