Co-activation
Also called coactivation, co-inhibition, autonomic space
Both branches of the autonomic nervous system rising, or falling, at the same time. They are not a seesaw, so one HRV number cannot place you on a single scale.
The usual picture of the nervous system is a seesaw: more “ fight or flight Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system ” means less “ rest and digest Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system ”, and the other way round. Berntson, Cacioppo and Quigley (1991) showed that the picture is too simple. The ways the two branches can act do not lie on one line from parasympathetic to sympathetic; they spread over a two-dimensional “autonomic space”. Each branch can go up or down on its own, so four patterns are possible: one up and the other down (the seesaw), both up (co-activation), both down (co-inhibition), or one changing while the other holds.
Why it matters for HRV.
- Balance labels mislead. A “balance” or “stress” score treats your state as a point on one line. If both branches rise together, heart rate may barely change while the brake and accelerator are both working harder.
- Speeds differ. The brake acts in under a second; the accelerator starts after a delay of up to 5 seconds and takes 20 to 30 seconds to reach full effect (Shaffer, McCraty and Zerr, 2014). When both are pressed at once, the brake shows first and the accelerator takes over later, as the “both at once” button below shows.
- One number cannot tell them apart. A low HRV can come from a weak brake or a strong accelerator, which is why HRV is read as a trend next to resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate and how you feel.
How fast each nerve acts
A teaching model, not a measurement
- Delay before it moves
- 0.3 s
- Time to 90% of the change
- 1.5 s
- Rate range
- 54 to 70 bpm
Tap the vagal brake and the very next beats come later. The slowing starts within a fraction of a second and is complete within about two seconds. Let go and the rate is back just as fast.
The buttons work when JavaScript is on.
A line chart of heart rate over 50 seconds, resting at 70 beats a minute, with the press shaded from 5 to 30 seconds and a tick under the chart for every heartbeat. Tapping the brake drops the rate within about 1.5 seconds and it recovers as fast once released. Pressing the accelerator does nothing for 3.0 seconds, then the rate climbs and settles after about 21.4 seconds, and drifts back slowly after release. Pressing both slows the heart first and speeds it up later. Three buttons choose which to press, and readouts give the time to the first change and the time to settle.
The lesson The nervous system behind HRV covers both branches.