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Rethinking the Resonance Frequency (RF) - Part 3: Slow-Paced Contraction

Updated: Jan 22

Slow-paced contraction

Slow-paced contraction (SPC) provides an alternative and unproven method for determining the resonance frequency (RF) in one or two sessions. Wrist-core-ankle contraction with crossed legs produces larger HR oscillations than wrist-ankle contraction with uncrossed legs. SPC enjoys five advantages over slow-paced breathing (SPB) protocols. First, clients can perform SPC correctly with minimal instruction. They don't have to overcome a lifetime of dysfunctional breathing habits. Second, SPC is more comfortable for chronic pain patients who often breathe faster than 20 bpm. Third, clinicians can more easily confirm compliance visually. Fourth, SPC is safer for clients whose rapid breathing compensates for an abnormal acid-base balance. SPB might endanger clients diagnosed with kidney disease. Fifth, many clients will find SPC at ~ 2 cpm easier to perform than SPB ~ 2 bpm to stimulate the vasomotor tone (VT) baroreflex.

We will discuss the equipment and displays used in SPC RF assessment, and outline RF protocols for stimulating the heart rate (HR) baroreflex and VT baroreflex.

A Disclaimer

SPC is a validated HRV biofeedback training method (Vaschillo et al., 2002). However, SPC is an experimental RF assessment procedure that requires extensive research.

Truman State University's Center for Applied Psychophysiology was among the first laboratories to use SPC to measure the RF. We have not seen published reports comparing the SPB and SPC methods to calculate the HR baroreflex RF. We have not found reports of the 2-week test-retest reliability of SPC RF measurements. Finally, we have not discovered reports SPC training using a SPC-determined RF.

The BioSource faculty wrote this post to show clinicians and researchers how to use SPC to measure the HR baroreflex and VT baroreflex RFs. We have described the "nuts and bolts" of SPC RF assessment. We hope this post will stimulate clinic and laboratory research to evaluate this complementary RF assessment method.

We strongly caution that the SPC RF assessment method is experimental, not validated, and not an accepted part of HRV biofeedback training.

Equipment for Resonance Frequency Assessment Using Slow-Paced Contraction

You will need an electrocardiogram (ECG) for publishable research and an ECG or PPG sensor for clinical work. Your data acquisition software may require a respirometer to measure mean HR Max - HR Min, which is the difference between the fastest and slowest HR across each breathing cycle.

ECG Sensor

Place ECG sensors on the torso to avoid movement artifacts from wrist contraction. The Polar H10 is a gold standard for ambulatory ECG monitoring. The H10 is compatible with the Optimal HRV application described in this post. Graphic © Polar Electro.

Below is a three-lead Thought Technology Ltd. ECG sensor. Clients may require assistance with sensor placement. Graphic © BioSource Software.

ECG chest placement

PPG Sensor

Since wrist contraction can produce movement artifacts, you must place PPG sensors on the earlobe. The HeartMath wireless PPG sensor is an excellent option. Graphic © Institute of HeartMath.


Graphic © BioSource Software LLC.


Pacing Displays

Using Smartphone Apps

You can repurpose the Optimal HRV application for SPC RF assessment. Their 14-minute RF procedure guides clients through seven 2-minute trials from 7.0-4.0 bpm in 0.5-bpm steps. The exhalations are longer than the inhalations:

Instead of paced breathing, contract the wrists-core-ankles with legs crossed for 3 s, starting 1.5-s before the peak of each cycle.

The application calculates the RF using weighted criteria, including LF power, RMSSD, and HR MinMax.

The bar chart below shows a logarithmic conversion of LF power, RMSSD, and HR MinMax values.


The table below shows the same data in their original units. Since Optimal HRV's algorithm assigns the greatest weight to LF power (gold), it selected 5.5 bpm as the RF. The highest RMSSD was also observed at 5.5 bpm. MinMax was highest at 7.0

Optimal HRV SPC chart

Using a Data Acquisition System

Data acquisition systems like the Thought Technology ProComp Infiniti and Mind Media NeXus allow you to create or purchase RF assessment suites.

ProComp Infiniti

NeXus 10

Below is a 6-cpm Truman Center BioGraph Infiniti RF assessment screen. A pacing display is center top. A spectral display showing HRV power distribution is bottom left. Red instantaneous HR and purple respirometer tracings are bottom right.

Participants contract the wrists-core-ankles with legs crossed for 3 s, starting 1.5-s before the peak of each cycle. Unlike SPB, the instantaneous HR and respirometer waveforms will not achieve synchrony with the alignment of peaks and valleys.

Slow-Paced Contraction Resonance Frequency Protocols for the Heart Rate and Vasomotor Tone Baroreflexes

The HR and VT baroreflexes have their own resonance frequencies, ~ 6 cpm and ~2 cpm, respectively. You may choose separate RF measurement protocols or combine them.

A single assessment session that combines two ranges can determine whether stimulating one baroreflex is best for your client and can pinpoint the contraction rate.

You can progress from 4.5 to 7.5 cpm in 0.5-cpm steps to find the HR baroreflex RF and/or 1.0 to 3.5 cpm in 0.5-cpm steps for the VT baroreflex RF.

Heart Rate Baroreflex Protocol

While reclining with feet supported by a chair, your clients can rhythmically contract their hands, core, and feet for 3 seconds. Record HRV at each contraction rate for 3 minutes with 2-minute buffer periods. Instruct your clients to follow the pacing display, starting contractions 1.5 s before each peak. Visually confirm compliance. Progress from 4.5 to 7.5 cpm in 0.5-cpm steps. Manually artifact each 3-minute data record and record criterion HRV values in a table. Use these metrics to select the RF.

The table below shows the contraction length and interval time (contraction + relaxation) for 4.5-7.5 cpm to stimulate the heart rate (HR) baroreflex.

Vasomotor Tone Baroreflex Protocol

For a quick review of the vasomotor tone baroreflex, see our post Rethinking the Resonance Frequency (RF) - Part 1: Understanding Resonance.

Here is a brief refresher. Vaschillo et al. (2002) described the HR and VT baroreflexes as closed loops and proposed that stimulating one closed loop activates its counterpart. Graphic adapted from Vaschillo et al. (2002).