Updated: Mar 24
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.
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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.
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.
Graphic © BioSource Software LLC.
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
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.
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).
Each baroreflex is a potential target for HRV biofeedback training. SPB and SPC at ~ 6 bpm/cpm can stimulate the HR baroreflex, separately or synergistically. SPC at ~ 1 cpm can activate the VT baroreflex.
Note. Bottom left: the lung and muscle icons indicate that SPB and SPC, alone or together, can stimulate the HR baroreflex at ~ 6 bpm/cpm. Bottom right: the muscle icon signals that SPC can activate the VT baroreflex at ~ 2 cpm.
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 1.0 to 3.5 cpm in 0.5-cpm steps. Manually artifact each 3-minute data record and record criterion HRV values in a table.
The next table shows the contraction length and interval time (contraction + relaxation) for 1.0-3.5 cpm to stimulate the vasomotor tone (VT) baroreflex.
How to Perform Slow-Paced Contraction
The contraction force should be moderate, not maximal, to ensure a smooth rhythm and minimize fatigue.
Fine-Tuning the Resonance Frequency
Consider rechecking the RF for any assessment protocol you use. We recommend this step since SPC is a novel task, and consistently following a visual pacing display can be challenging. Also, since most HRV apps do not artifact trial data, replication provides a minimal check on measurement error.
Manual artifacting is a best practice since even a single false heartbeat can significantly distort HRV metrics (Berntson et al., 1997) and RF selection.
Following a resting baseline at the start of the first training session, clinicians can instruct clients to perform SPC 0.5 cpm slower than the RF, at the RF, and 0.5 cpm faster than the RF. This step helps to confirm the RF through replication. Finally, after artifacting, clinicians should evaluate the three trials: RF - 0.5 cpm, RF, and RF + 0.5 cpm.
SPC RF assessment is experimental, not validated, and not an accepted part of HRV biofeedback training. 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. You can repurpose RF breathing pacing displays for SPC. We have provided examples using smartphones and data acquisition systems. The Optimal HRV application provides automated assessment from 7.0-4.0 cpm. Data acquisition systems offer greater flexibility in selecting pacing frequencies. Instruct clients to start wrist-core-ankle contractions 1.5 s before the display peak. Assessment can progress from 4.5-7.5 cpm for the HR baroreflex RF and 1-3.5 cpm for the VT baroreflex RF. Clinicians must artifact trial data before calculating the RF. Finally, we recommend confirming the RF at the start of the first training session.
baroreflex: baroreceptor reflex that provides negative feedback control of BP. Elevated BP activates the baroreflex to lower BP, and low BP suppresses the baroreflex to raise BP.
electrocardiogram (ECG) sensors: electrodes that record the heart's electrical signal.
heart rate (HR) baroreflex: the closed-loop encompassing the cardiovascular control center, heart rate control system, and blood pressure control system.
heart rate variability (HRV): the beat-to-beat changes in HR involving changes in the RR intervals between consecutive heartbeats.
HR Max – HR Min: an HRV index that calculates the average difference between the highest and lowest HRs during each respiratory cycle.
HRV frequency-domain measurements: metrics that quantify absolute or relative power distribution into four frequency bands, revealing the sources of HRV.
HRV time-domain measurements: metrics that quantify the total amount of HRV.
low-frequency (LF) band: an HRV frequency range of 0.04-0.15 Hz that may represent the influence of PNS and baroreflex activity when breathing or contracting muscles between 4.5-6.5 times a minute.
peak frequency: the HRV frequency with the greatest power.
photoplethysmographic (PPG) sensor: a photoelectric transducer that transmits and detects infrared light that passes through or is reflected off tissue to measure brief changes in blood volume and detect the pulse wave.
resonance: an amplification process in which an external force causes a closed-loop (negative feedback) system to oscillate with greater amplitude at its inherent resonance frequency (RF).
resonance frequency (RF): the frequency at which a negative-feedback system with a fixed delay, like the cardiovascular system, can be activated or stimulated.
resonance frequency (RF) assessment: identifying the stimulation frequency producing the greatest amplitude heart rate oscillations in one or two sessions using slow-paced breathing or contraction methods.
respiratory sinus arrhythmia (RSA): the respiration-driven heart rhythm that contributes to the high frequency (HF) component of heart rate variability. Inhalation inhibits vagal nerve slowing of the heart (increasing HR), while exhalation restores vagal slowing (decreasing HR).
respirometer: a sensor that changes resistance to a current as it expands and contracts during the respiratory cycle.
RMSSD: the square root of the mean squared difference of adjacent NN intervals in milliseconds.
slow-paced breathing (SPB): low-and-slow breathing at ~ 6 bpm for adults.
slow-paced contraction (SPC): wrist-core-ankle contraction with legs supported and crossed at rates of ~ 2 or 6 cpm.
time-domain measurements: quantify the total amount of heart rate variability.
vagal tone: parasympathetic activity measured by the natural log of HF power when breathing at normal rates without feedback.
vascular tone (VT) baroreflex: the closed-loop encompassing the cardiovascular control center, vascular tone control system, and blood pressure control system.
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Meehan, Z. M., & Shaffer, F. (in press). Adding core muscle contraction to wrist-ankle rhythmical skeletal muscle tension increases respiratory sinus arrhythmia and low-frequency power. Applied Psychophysiology and Biofeedback.
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