In this write up, I’ll cover:
What is Cardiac Drift and what causes it?
What is Submax Fatigue and how to recognize it?
Review my Sunday long run data and show how both are present
Let’s dive in:
What Is Cardiac Drift?
A gradual rise in heart rate during endurance exercise when intensity and pace remain constant.
The body starts working harder to maintain the same level of output.
Here’s a good visual from Phil Maffetone:
3 Factors That Cause Cardiac Drift
#1: Dehydration
When we lose water through sweat, blood volume decreases.
This makes the heart work harder to circulate blood throughout the body.
#2: Increased Core Body Temperature
Muscle contractions (plus many other exercise factors) create heat.
To dissipate excess heat, the heart pumps more blood to the skin so we can sweat and cool down.
#3: Outdoor Conditions: hills, humidity, temperature, altitude, etc.
These can be additional physiological stress that increase demand on the heart without a noticeable change in intensity/output.
Submax Fatigue
While the factors above can lead to cardiac drift, there’s one thing to always consider:
We are carrying fatigue.
We typically associate fatigue with hard workouts that leave us gasping for air and noticeably sore afterwards.
But fatigue accumulates during all movement and exercise.
Moderate levels of fatigue can actually produce a more significant physiological response than more extreme fatigue.
The chart below shows the physiological impact of cardiac drift on pace when HR is capped.
Cardiac Drift In My Sunday Long Run
Yesterday, I ran 10 miles at a steady, endurance effort.
Here’s a look at my charts with first and second half Pace and HR data summarized below:
First Half Data:
Avg. Pace: 9:30 min/mile
Avg. HR: 124 BPM
Second Half Data:
Avg. Pace: 9:15 min/mile
Avg. HR: 136 BPM
Since I wasn’t strict on capping HR, my pace stayed relatively constant while my HR rose significantly.
Conditions Matter, But Is It That Complicated?
I started my run at 7:30 am and the temperature warmed up as the sun continued to rise.
I wore 2 layers, gloves and a neck gaiter. I took the neck gaiter off at my 32 min bathroom break.
I was a bit warm and there were some slight inclines on my route, but here’s what’s more important:
It was at the end of a 10 hour training week
I had a 17 hour training week the week prior which included a 100 mile ride with 5k feet of gain
I spent the day before the run at a birthing class from 9-5 in a room without windows and under fluorescent light (this exhausted me)
I was carrying fatigue.
The Process
Success in endurance sports (or anything really) doesn’t come from the sessions that go perfectly and have super sexy data.
Success comes from the middle of the road, mediocre sessions that stack up over months and months of time.
Success comes from staying committed to The Process.
The win from Sunday was that I woke up early and ran 10 miles (before another half day of birthing class).
I got some good data that suggests I’m fatigued and could benefit from rest.
And right on cue, that’s what’s on the plan today.
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