During extended exercise, your heart rate is likely to rise even when the mechanical output (power or pace) stays the same. You may be running a flat course at a steady 5min/k and start the workout at 135bpm, but by the end of the workout your heart rate is 150bpm. This phenomenon is called ‘cardiac drift’.While it’s not fully understood, two factors seem to be at play.
First, as we become more dehydrated, our blood volume drops so the heart must pump with more frequency for the same cardiac output to the muscles.
Second, as our more efficient ‘slow-twitch’ muscle fibres tire, less efficient fibres are recruited that demand more oxygen for the same output.
As many of us use heart rate to prescribe training, this can be confusing. E.g. a 2hr run at 130bpm beats –am I referring to the starting heart rate, knowing that it will build over the workout? Or should I adjust the pace down over the course of the workout to hold that pace? I believe heart rate is a great indicator of overall systemic stress. So, in practice…
Give yourself an acceptable ‘zone’ rather than a fixed heart rate. I’d suggest 5-7%.
Start your workouts at the low end of this zone knowing that it will build.
Stick to this zone! If it’s a hot day, don’t be afraid to slow the pace.