During cognitive training, an athlete’s nervous system is constantly adjusting — even if you can’t see it from the outside.
Minute-on-minute heart rate variability (HRV) tracking gives you a live view of what’s happening internally. It reveals how the nervous system is reacting, adapting, or beginning to fatigue under pressure.
Two metrics give you that window:
rMSSD – tells you how well the nervous system is staying calm and adaptive
SDNN – shows how much total pressure the nervous system is carrying over time
Together, these two values help you coach smarter, spot fatigue before it affects performance, and tailor recovery with precision.
What is rMSSD?
rMSSD reflects the activity of the parasympathetic nervous system — the system responsible for recovery, control, and calm under stress. It’s a short-term measure that changes quickly, making it ideal for tracking real-time nervous system adaptability.
In the graph above, rMSSD starts high — a sign that the athlete’s nervous system is calm and fully recovered at baseline. But it drops sharply within the first minute, indicating an immediate response to cognitive or emotional stress.
From minutes 2 to 7, the nervous system attempts to recover, fluctuating between 50 and 70, but never regains the initial high state. In the final minute, rMSSD rebounds significantly — showing the system still has the ability to reset once the load decreases.
What this means:
The task created real mental strain. The nervous system initially struggled, but the final rebound confirms the athlete still had resilience in the tank — a sign of good overall capacity.
What is SDNN?
SDNN tracks the total variability in heart rate, reflecting the combined activity of the sympathetic (stress) and parasympathetic (recovery) systems. It gives you a big-picture view of how the entire nervous system is handling sustained load.
In this session, SDNN begins very high — a flexible, well-recovered starting point. From minute 1 to minute 9, it declines steadily, indicating a gradual accumulation of system-wide stress.
At minute 9, SDNN hits its lowest point — the nervous system is fully compressed and fatigued. But at minute 10, there’s a strong rebound, which shows the system can still recover quickly once the stress is lifted.
What this means:
Even though the athlete’s system was heavily taxed, it wasn’t broken. That rebound tells you recovery capacity is still intact — a good sign of long-term nervous system health.
How to read both together
Think of rMSSD and SDNN as two parts of the same system.
rMSSD tells you what’s happening right now — whether the nervous system is staying composed during the session.
SDNN shows the full load over time — and whether the system can bounce back after being pushed.
In this session, the nervous system showed:
An early stress response (rMSSD drop)
Sustained strain (gradual SDNN decline)
Late-stage fatigue (low points in both)
And resilience (clear rebounds at the end of both metrics)
This is exactly what you want to understand as a coach: not just when fatigue hits — but how the system fights back.
What you can do as a coach
This data lets you coach beyond appearances or guesswork.
If your athlete shows an early rMSSD drop and no recovery, the system is overloaded. If SDNN bottoms out but rebounds sharply, they still have adaptation capacity — they’re just temporarily drained.
You can use this insight to:
Adjust load on the fly during a session
Identify when to switch to recovery work
Catch signs of nervous system fatigue before performance drops
Validate whether your recovery strategies are actually working
Summary
rMSSD = short-term nervous system control
SDNN = total nervous system strain and recovery ability
Tracking both helps you understand not just what your athlete is doing — but how their nervous system is handling it.