RMSSD and HRV: Variability Is Health

Series: Breathing, Body, Consciousness, and the Shifting of the Tensional Selves (Eus Tensionais)

Introduction — Brain Bee (first-person consciousness)

As I read these lines, I’m not just someone who understands concepts.
I am a breathing body.

I notice that, at some moments, my chest expands with ease. At other times, my breath shortens without me realizing why. My heart doesn’t beat the same way all the time. Sometimes it speeds up, sometimes it slows down, sometimes it seems to find a comfortable rhythm—other times it feels far too tight.

None of this is a mistake.
None of this is a disease.
It is my body varying in order to stay alive.

Before any theory, before any data, this is what matters to feel:
to live is to vary.

HRV and RMSSD: what they’re really measuring

HRV (Heart Rate Variability) does not measure “calm,” nor “emotional balance” in any moral sense.
HRV measures variability between heartbeats.

A healthy heart does not beat like a metronome.
It responds, moment by moment, to the demands of the body, breathing, posture, blood pressure, environment, and the history of that organism.

Within the set of HRV metrics, RMSSD (Root Mean Square of Successive Differences) holds a central place because it captures rapid, beat-to-beat changes. It is especially sensitive to breathing and vagal tone.

In simple terms:

• HRV shows the broader landscape of variability.

• RMSSD shows the immediate capacity to adjust.

It’s not about being always high or always low.
It’s about being able to rise and fall when needed.

Variability is not permanent relaxation

A common mistake is to associate health with a continuous state of calm.
That does not exist in living systems.

A healthy body:

• tenses to act,

• sustains tension to work,

• reduces tension to reorganize,

• and tensions again when necessary.

If the heart only slows down, the body doesn’t act.
If the heart only speeds up, the body becomes exhausted.

Health is not the absence of tension.
Health is the ability to alternate states.
That is what HRV and RMSSD are showing.

Tensional selves as normal physiological states

Throughout the day, we move through different tensional selves (eus tensionais):

• a self for action,

• a self for attention,

• a self for rest,

• a self for defense,

• a self for coexistence.

Each of these selves is supported by:

• a breathing pattern,

• an autonomic state,

• a muscular and visceral organization.

As long as exchange is possible, the system remains healthy.
When that exchange is lost—when the body gets stuck in a single pattern—variability drops.
Not because the body “failed,” but because it lost room to vary.

Breathing: the most direct pathway to variability

Breathing is the main voluntary and involuntary modulator of the autonomic system.

With each respiratory cycle:

• inhalation momentarily reduces the vagal “brake,”

• exhalation restores it,

• the heart responds to this movement.

This dance between breath and heart is called cardiorespiratory coupling.
RMSSD is particularly sensitive to this process because it captures these fast oscillations.

When breathing loses variation—becoming trapped, held, rigid, or excessively controlled—the heart loses variation along with it.

This is not psychological.
It is physiological.

Variability is a requirement of DNA

Living systems do not seek rigid stability.
They seek dynamic stability.

From DNA to organ systems, life persists because it:

• adjusts,

• compensates,

• oscillates,

• reorganizes.

HRV and RMSSD are measurable expressions of this biological intelligence.
When we talk about bodily freedom, we are not talking about ideology.
We are talking about the minimum conditions for metabolism to remain functional.

Normality before diagnosis

It’s essential to understand:

• Low HRV at a given moment is not a failure.

• Reduced RMSSD during intense action is not a problem.

The issue is not a single drop.
The issue is the inability to recover.

A healthy body enters tension and exits it.
A “seized” body remains trapped.

This distinction is the foundation of the entire series.

How to recognize this in your own body

No technique. No method. No correction.
Only observation:

• Can my body change its breathing rhythm?

• Can I sense subtle differences in my heartbeat?

• Can I move from action to rest without collapsing?

• Can I tense by choice and release without guilt?

These questions matter more than any isolated number.

Closing

HRV and RMSSD are not goals.
They are mirrors of the capacity to vary.

To vary is health.
To vary is life.
To vary is what will allow us, in the next texts, to understand how the body learns, tenses, gets stuck, frees itself, and reorganizes—individually and collectively.

This text is part of the series Breathing, Body, Consciousness, and the Shifting of the Tensional Selves (Eus Tensionais), where different aspects of the same living system are approached from complementary angles.

References (post-2020)

Shaffer, F., & Ginsberg, J. P. (2020). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health.
→ Presents the main HRV metrics, including RMSSD, and frames variability as an indicator of physiological flexibility—not a fixed emotional state.

Laborde, S., Mosley, E., & Thayer, J. F. (2022). Heart Rate Variability and Cardiac Vagal Tone in Psychophysiological Research. Biological Psychology.
→ Discusses RMSSD as a sensitive marker of vagal tone and its direct relation to autonomic adaptation in real contexts.

Lehrer, P. M., et al. (2020). Heart Rate Variability Biofeedback: How and Why Does It Work? Frontiers in Psychology.
→ Shows how breathing modulates HRV, supporting the idea that varying cardiorespiratory rhythm is a trainable physiological function.

Thayer, J. F., & Lane, R. D. (2021). A Model of Neurovisceral Integration in Emotion Regulation. Biological Psychology.
→ Grounds the integration between brain, heart, and autonomic system as the basis for behavioral and emotional flexibility.

Kim, H. G., et al. (2021). Respiration–Heart Rate Coupling and Autonomic Regulation. Frontiers in Neuroscience.
→ Highlights cardiorespiratory coupling as a central mechanism by which breathing organizes heart rate variability.

Forte, G., et al. (2022). Heart Rate Variability and Interoceptive Awareness. Neuroscience & Biobehavioral Reviews.
→ Relates HRV and RMSSD to interoception, supporting the idea that feeling the body is part of physiological regulation.

von Rosenberg, W., et al. (2020). Respiratory Influences on Heart Rate Variability. IEEE Reviews in Biomedical Engineering.
→ Analyzes how different breathing patterns directly affect HRV, reinforcing breathing as a pathway for autonomic modulation.

Shaffer, F., Meehan, Z. M., & Zerr, C. L. (2020). A Critical Review of HRV Norms and Interpretation. Frontiers in Neuroscience.
→ Discusses the limits of HRV “norms” and emphasizes context and variation, aligning with the idea of dynamic normality.