Slow-Paced Contraction Training Using a Multichannel Biofeedback System

Perspective

Clinicians can deliver effective, slow-paced breathing (SPB) training using smartphone apps and multichannel data-acquisition systems.

Smartphone apps have been shown to increase HRV in users, mirroring results from ECG- and PPG-based multichannel biofeedback (Herhaus et al., 2022; Li et al., 2023; Limmer et al., 2022; Vagedes et al., 2025; Vann-Adibe et al., 2025; Vondrasek et al., 2023). Multichannel clinical systems mainly offer more complete physiology, higher data quality, and better protocol control than smartphone apps. Multichannel systems can confirm client breathing rates, which is critical for resonance frequency assessment, and 6-bpm and resonance frequency slow-paced breathing protocols.

Multichannel systems give a richer, mechanism-level picture. Are we increasing vagal activity in a way that is physiologically healthy (good HRV, normal CO₂, relaxed muscles, warm hands), or are we trading one problem (e.g., low HRV) for another (cold hands and reduced end-tidal CO2)?

Slow-paced contraction (SPC) training to increase HRV can also benefit from multichannel systems. For wrist-core-ankles resonance frequency assessment and training, a forearm EMG sensor can confirm that the client contracted at the prescribed rate.

Based on the groundbreaking work by the late Evgeny Vaschillo and colleagues (2011), SPC can be combined with SPB or can replace it when it is medically contraindicated (e.g., kidney disease).

Evgeny Vaschillo

For combined SPB and SPC training, a respirometer can assess your client's breathing rate and pattern. In this post, we will explain sensor placement, position, resonance frequency assessment, training protocols, success and failure indicators, and progress indicators.

Sensor Placement

Choose an ECG sensor, like Thought Technology Ltd.'s EKG-Flex/Pro. Use a chest or Erik Peper's lower torso placement, shown respectively. A lower torso placement preserves client modesty and reduces EMG contamination of the ECG signal.

Chest Placement

Lower Torso Placement

Place an EMG sensor like Thought Technology Ltd.'s Myoscan over forearm extensors and flexors to measure the rate of SPC.

For SPC + SPB, add Thought Technology Ltd.'s Respiration Sensor over the abdomen to measure respiration rate and pattern.

Resonance Frequency Assessment

Clinicians can use stepped or sliding protocols to measure client resonance frequencies.

Stepped Protocols

In a stepped protocol, instruct your client to contract the wrists, core, and ankles (or the wrists and ankles) for 2-minute intervals from 7.0 to 4.0 contractions per minute (cpm), decreasing in 0.5 cpm steps with 2-minute rest periods. Record physiological activity during SPC as separate 2-min epochs. Create a separate display for each RF trial and capture 2 minutes of EMG and HR waveforms on the same graph for each contraction rate (CR).

The top display with the moving yellow ball is designed to help clients contract at 6 bpm. The current contraction rate (5.58 bpm) appears on the right. The graph immediately below shows instantaneous heart rate (pink) and respiration (purple). Ignore the respiration signal when using SPC only. A raw ECG waveform is displayed toward the bottom of the screen.

Consider the following modified directions when introducing each CR: “Now try contracting your wrists, core, and ankles (or wrists and ankles) at this frequency (following the pacer)” (Lehrer et al., 2013, p. 101).

After your client completes 2 minutes of SPC, check on their comfort and verify that they followed the pacer by confirming the average CR for that trial. Repeat trials if the clients were 0.25 cpm too fast or slow. Assessment without an electromyograph lacks this quality control. We cannot verify that clients have contracted their muscles at the target rates in these cases.

Check for artifactual interbeat intervals and repeat invalid epochs after the client has rested for 2 minutes. Examine the segment spectral display for the location of LF peaks. When a peak occurs at 4.0 or 7.0 bpm, extend assessment with trials 0.5 cpm above and below this inflection point until LF amplitude decreases.

Sliding Protocols

Fisher and Lehrer (2022) described a 15-min automated sliding protocol in which participants breathed between 4.25 and 6.5 bpm, with a constant 67.04-ms rate of change. Since peak-trough amplitude proportionally increases as clients approach the RF, the sliding protocol measures this metric at 78 frequencies. In contrast to the subjectively weighted multiple criteria of the stepped approach, the sliding protocol identifies the frequency with the largest peak-trough amplitude.

The mean absolute difference in RF between the two methods was 0.22 ± 0.169 bpm. The authors raised the issue of whether this slight difference between RF estimates is too small to affect physiological or psychological outcomes. Although the sliding protocol was designed for SPB, it could be readily adapted to SPC.

Selecting the Resonance Frequency

HR Max-HR Min is a Good RF Criterion

While smartphone apps can extract breathing information from the interbeat intervals (see Optimal HRV), multichannel biofeedback systems employ respirometers to determine when each breathing cycle starts and ends. Either method allows us to calculate a new RF criterion: HR Max-HR Min. Fisher and Lehrer (2022) used peak-trough (RSA) amplitude to determine the RF. HR Max-HR Min measures RSA amplitude. It is a workable RF criterion because RSA amplitude the most important HRV source, with the heart rate baroreflex and vascular tone baroreflex next in importance. Training to increase HR Max-HR Min can raise resting HF power (a proxy for vagal tone), depending on the protocol and population (Bourdillon et al., 2025).

HRV Training to Increase HR Max-HR Min Resembles Resistance Training

In resistance training, you expose muscle fibers to repeated, structured loading, which triggers both acute recruitment and long‑term adaptations (hypertrophy, improved neural recruitment). HRV biofeedback applies a comparable logic to baroreflex and vagal control.

Each session of resonance‑frequency breathing produces large, regular oscillations in blood pressure and RR intervals. During practice, baroreflex gain and low‑frequency HRV (reflecting the 0.1 Hz oscillation) increase markedly, demonstrating that the reflex arcs are being intensely recruited in real time (Lalanza et al., 2023; Lehrer et al., 2003).

Over weeks or months of daily practice, several studies report that even outside the training periods, individuals show higher resting baroreflex sensitivity and more robust vagally mediated HRV (Deschodt‑Arsac et al., 2018; Lehrer et al., 2003). The same pattern—dose–response improvements in HRV and baroreflex sensitivity with repeated cardiovascular training—has also been seen with exercise training, which further supports an “exercise‑like” model of autonomic plasticity (Lellamo et al., 2013).

In other words, the reflex arcs that couple blood pressure to heart rate and vascular tone appear capable of both short‑term recruitment (large oscillations during a session) and long‑term adaptation (higher gain and HRV at rest) when regularly “loaded” by slow, coherent breathing and biofeedback. That is precisely the pattern we expect from an exercised system.

HR Max-HR Min Doesn't Index Vagal Tone

Although HR Max-HR Min is a reasonable RF criterion, readers should not conflate it with vagal tone. HR Max–HR Min is “especially sensitive to the effects of respiration rate, independent of vagus nerve traffic” and therefore reflects RSA amplitude rather than vagal tone per se (Shaffer & Ginsberg, 2017, p. 12). In fact, Marmerstein et al. (2021) demonstrated in an animal study that overall vagal activity, when directly measured, was not associated with RSA magnitude.

Grossman (2024) reminds us that RSA amplitude is influenced by more factors than cardiac vagal tone: respiration rate and tidal volume, sympathetic activity, age, gender, non-parasympathetic mechanical factors, behavioral and psychological states, cardiac aliasing, intrinsic cardiac nervous system interactions, and hypercapnia (CO2 elevations).

Why Not Prioritize LF Power?

LF power is a widely-used RF selection criterion and successful HRV biofeedback training protocols target LF power. For example, the Institute of HeartMath's coherence metric is maximized when there is a narrow peak between 0.09 and 0.14 Hz in the LF range.

The rationale for targeting HR Max-HR Min is that RSA exercises the HR baroreflex, while LF power confirms pacing precision.

In practice, we want to maximize both metrics, since HR Max-HR Min will be greatest when pacing at your clients' RF within the LF range. To date, we have not found head-to-head comparisons of HR Max-HR Min and LF training.

Slow-Paced Contraction Training Protocols

We will describe sitting position, muscle recruitment, SPC and SPC+SPB protocols, success and difficulty indicators, and when you should use each protocol, in this section.

Position

Clients should recline with their ankles crossed and feet supported by a footrest or separate chair. Although the original Vaschillo protocol only contracted wrists and ankles with legs uncrossed, we have observed greater RSA using wrist, core, and crossed-ankle contraction. RSA represents breathing-mediated changes in peak-to-trough heart rate differences.

Muscle Groups

Instruct your clients to gently simultaneously contract their wrists, core, and ankles for SPC or wrists and ankles for SPC+SPB.

Borrowing from Dr. Erik Peper's approach, encourage effortless contraction to ensure a smooth rhythm and minimize fatigue.

Erik Peper

Your clients should use about 25% of their maximum effort. They should feel as if their wrists-core-ankles are contracting themselves, rather than forcing the movement.

This gentle approach promotes sustainable practice and prevents the activation of stress responses like vagal withdrawal that could counteract the training benefits.

They should hold each contraction for 3 seconds.

Below is an BioGraph Infiniti display we have repurposed for SPC training. Clients should start 1.5 seconds before each cycle's peak. End 1.5 seconds after each cycle's peak.

Slow-Paced Contraction With or Without Slow-Paced Breathing

You can combine SPC with SPB or use it alone to increase HRV. When combining these techniques, limit contraction to the wrists and ankles to permit the abdomen to expand and the diaphragm to descend. Instruct your clients to start inhaling when they start contracting and start exhaling when they relax their muscles. Below is an video of slow-paced contraction with slow-paced breathing.

Next, is a video of slow-paced contraction alone.

Training Sessions

Session Activities

Success Indicators

Difficulty Indicators

Training Trial Review

Fit an entire 3-minute segment on the screen to review it together. When your client succeeded, you might point out where they succeeded.

Promote Mindfulness

Before starting the next segment, you might ask: "What were you doing when the display became wavelike and regular? What happened when the display became more jagged and irregular?" If accessory SEMG exceeded 2 microvolts, point this out on the display, ask them if they felt the heightened breathing effort, and encourage them to "let their shoulders relax and allow themself to breathe."

Reassure them that it's normal for the tracings to be choppy when people start training and that they will gradually become more wavelike as their breathing becomes more rhythmic and regular.

Instead of overwhelming them with corrections, ask them to experiment with one or two changes at a time. For example, "Effortless breathing is rhythmic like ocean waves. Allow your stomach to gently expand and contract as you follow the pacing display."

Session Review

After your client has completed six 3-minute training segments, take a 3-minute post-baseline without feedback. After the post-baseline, ask your client how they felt and what they learned during the training session. Display the entire session on one screen and highlight where they succeeded and where they need more work.

Comparing Pre- and Post-Baseline Values

Without pacing or feedback, your client should breathe at typical rates during the pre- and post- baselines. HF power, RMSSD, and hand temperature may increase, while the skin conductance level may decrease.

The graphic below shows HF power in blue during a pre-training baseline, HRVB training, and a post-training baseline.

The y-axis shows power in each band. HF power increases from ~100 μV during the pre-training baseline to ~300 μV during the post-training baseline. This change is important because increased HF power can signal greater vagal tone.

Also, note the greater LF power concentration post-training compared with pre-training during which the client breathed at typical rates. Dr. Inna Khazan generously provided the spectral plots.

Important HRV Biofeedback Training Elements

The clinician's relationship with their client is foundational. HRV biofeedback apps like Optimal HRV allow daily monitoring and practice.

When Should You Combine SPC with SPB?

Dr. Inna Khazan combines SPC with SPB when her clients struggle to learn SPB. SPC can serve as a powerful pacing cue for their breathing rhythm, and model of the effortlessness we encourage.

Inna Khazan

Matt Bennett, Optimal HRV co-founder, recommends it to increase HRV for clients who have mastered SPB. He finds that their HR Max - HR Min and low-frequency values are greater when they combine SPC with SPB. They can see improved results after training trials using the Optimal HRV application

Matt Bennett

Several clinicians who treat Postural Orthostatic Tachycardia Syndrome (POTS), which is characterized by an excessive increase in heart rate upon standing, report that they have minimal HRV and require the combined approach to achieve clinical gains.

Data from the Truman Center for Applied Psychophysiology

The data were obtained from a healthy Truman undergraduate who had mastered slow-paced contraction and slow-paced breathing.

Time-Domain Measurements

The first Kubios table shows 6-cpm SPC. The RMSSD was 55 ms.

The second table shows 6-cpm SPC with 6-bpm SPB. The RMSSD was 88 ms.

Frequency-Domain Measurements

The first table shows 6-cpm SPC. Low-frequency power (ms2/Hz) using the FFT method was 5046. The participant breathed normally at 7.8 bpm.

The second table shows 6-cpm SPC with 6-bpm SPB. Low-frequency power (ms2/Hz) using the FFT method was 12277. The participant breathed at the target of 6 bpm.

Pilot Summary

These data suggest that two oscillators, 6-cpm SPC and 6-bpm SPB, can synergistically increase RSA and HRV time- and frequency-domain measurements. Confirmation awaits a planned Truman Center randomized controlled trial.

When Should You Use SPC Alone?

Prioritize SPC when SPB is not advisable due to dysfunctional breathing patterns or medical contraindications that make respiratory interventions unsafe or ineffective. SPC offers a means of engaging the autonomic nervous system through rhythmic, low-intensity skeletal muscle contractions that mimic the oscillatory effects of breathing on HRV and vagal modulation without directly altering respiration. This approach supports parasympathetic activation and enhances baroreflex engagement while maintaining safety for individuals with conditions such as chronic obstructive pulmonary disease (COPD), panic-prone respiratory sensitivity, or unstable cardiovascular responses. In such cases, SPC serves as a functional analog to SPB, providing a gentle, body-based entrainment method that stabilizes autonomic tone without invoking potentially disruptive ventilatory dynamics.

Dysfunctional Breathing

Slow-paced breathing can be challenging for clients who breathe dysfunctionally, particularly those who exhibit overbreathing or hyperventilation patterns. These individuals may struggle with the precise respiratory control required for effective SPB training.

SPC provides an alternative pathway to cardiovascular resonance that bypasses problematic breathing patterns, allowing these clients to access the benefits of HRV biofeedback training without respiratory-based interventions.

Medical Conditions

Elevated Respiratory Rates in Clinical Populations

Disorders that affect respiration may elevate breathing rates to 18-28 breaths per minute, according to research by Fried (1987) and Fried and Grimaldi (1993). These elevated rates make it extremely difficult for clients to slow their breathing to the 4.0-7.0 bpm range required for effective SPB training.

Conditions such as anxiety disorders and panic disorder can chronically elevate respiratory rates, making SPC a more accessible alternative intervention approach.

Pain-Related Respiratory Changes

Sustained pain increased the respiration rate from 13.2 to 17.7 bpm in research conducted by Kato, Kowalski, and Stohler (2001). Patients experiencing chronic pain may be unable to slow their breathing to the 4.0-7.0 bpm range necessary for effective SPB training.

Pain-related respiratory changes reflect the body's natural stress response and autonomic nervous system activation, making breathing-based interventions more challenging to implement effectively in these populations.

Metabolic Acidosis

SPB may be medically contraindicated when altered breathing patterns could be hazardous for clients suffering from diabetes (Kitabchi et al., 2009) or kidney disease (Kim, 2021) that produce metabolic acidosis—excess acid in the body fluid.

In these conditions, the body relies on specific respiratory patterns to maintain acid-base balance, making interventions that alter breathing potentially dangerous to the client's physiological homeostasis.

Respiratory Acidosis

Common respiratory acidosis causes include chronic obstructive pulmonary disease (COPD), asthma, pneumonia, and neuromuscular disorders that affect breathing muscles.

These conditions impair the lungs' ability to eliminate carbon dioxide effectively, leading to respiratory acidosis. Interventions that further alter breathing patterns could exacerbate these underlying physiological imbalances.

Patients may breathe rapidly to protect acid-base balance in medical disorders that cause a decrease in blood pH, leading to acidosis. Rapid breathing helps to expel carbon dioxide (CO2) from the body, which in turn can increase the pH and counteract the acidosis.

This compensatory hyperventilation represents a crucial physiological adaptation that maintains homeostasis. Interventions that interfere with this compensatory mechanism could compromise the patient's health and safety. SPC provides a safe alternative that can improve heart rate variability and autonomic balance without disrupting these essential respiratory compensatory mechanisms.

Key Takeaways

1. SPC is a core HRV exercise pioneered by Evgeny Vaschillo’s group; it can be paired with SPB or used instead when SPB is contraindicated (e.g., metabolic/kidney conditions).

2. Teach with a three-lead ECG sensor lower-torso placement when using a data acquisition system like the ProComp Infiniti.

3. Position clients reclined, ankles crossed and supported; cue gentle, effortless wrist-core-ankle contractions held ~3 s. Encourage 25% effort to avoid vagal withdrawal.

4. Time each contraction to the pacer peak (start ~1.5 s before, end ~1.5 s after) to synchronize with cardiovascular rhythms and amplify resonance effects.

5. Combine SPC+SPB when appropriate and prioritize SPC alone for dysfunctional breathing or when slowing respiration would be unsafe.

Appreciation

The Truman Center for Applied Psychophysiology research staff made this post possible. A special thanks to my amazing Lab Managers, Isaac Compton and Emma Suchsland, who teach and supervise this dedicated team of 33 undergraduates. Isaac Compton modeled our wrists-core-ankles SPC technique.

Glossary acid–base balance: the regulation of the body’s acidity and alkalinity (pH), primarily through respiratory control of carbon dioxide and renal handling of acids and bicarbonate, to keep blood chemistry within a narrow healthy range.

acidosis: a state in which body fluids become too acidic (low pH), which can result from metabolic processes (metabolic acidosis) or inadequate removal of carbon dioxide by the lungs (respiratory acidosis). 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. baroreflex gain: a measure of how strongly the baroreflex changes heart rate for a given change in blood pressure, often expressed as beats per minute per millimeter of mercury; higher gain indicates a more responsive baroreflex. chronic obstructive pulmonary disease (COPD): a group of lung diseases (such as chronic bronchitis and emphysema) that limit airflow and reduce the lungs’ ability to eliminate carbon dioxide, predisposing patients to respiratory acidosis. coherence: a narrow peak in the BVP and ECG power spectrum between 0.09 and 0.14 Hz. effortless contraction: contraction force around 25% of maximum effort, analogous to Erik Peper's concept of effortless breathing. electrocardiogram (ECG): a recording of the heart’s electrical activity from the body surface, providing precise R‑wave timing for heart rate and HRV analysis. high-frequency power (HF power): a frequency‑domain HRV index representing variability in the high-frequency band (typically 0.15–0.40 Hz), often used as a marker of respiratory sinus arrhythmia and cardiac vagal modulation when breathing is in that range. HR Max – HR Min: an HRV index that calculates the average difference between the highest and lowest HRs during each respiratory cycle. hypercapnia: the elevation of carbon dioxide levels in the blood or end‑tidal air, which can occur when ventilation is insufficient relative to metabolic production of CO₂. interbeat interval (IBI): the time between successive heartbeats, typically measured from R‑wave to R‑wave on the ECG; the sequence of IBIs is the raw data used to compute HRV. low-frequency (LF) band: a 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 4.0-7.0 times a minute. metabolic acidosis: a condition in which excess acid accumulates or bicarbonate is lost from body fluids, causing blood pH to fall below normal due to non-respiratory causes such as kidney dysfunction or diabetes. multichannel biofeedback system: a clinical data‑acquisition platform that records several physiological signals at once (e.g., ECG, respiration, EMG, skin conductance, temperature), allowing comprehensive assessment and training. photoplethysmography (PPG): an optical technique that uses changes in light reflected or transmitted through tissue to track blood volume pulsations, providing a pulse signal from which a “pulse rate variability” surrogate for HRV can be derived. postural orthostatic tachycardia syndrome (POTS): an autonomic disorder characterized by an excessive increase in heart rate on standing, often accompanied by low HRV and intolerance to orthostatic stress. resistance training: an exercise paradigm in which muscles repeatedly work against a load, progressively increasing strength and endurance; used here as an analogy for repeatedly “loading” baroreflex and vagal pathways in HRV training. resonance frequency (RF): an individual's breathing or contraction rate at which the cardiovascular system exhibits maximal, coherent oscillations (often near 0.1 Hz), driven by baroreflex dynamics.

resonance frequency assessment: a procedure in which clients practice at several breathing or contraction rates while clinicians examine HRV, baroreflex-related indices, and waveform shape to identify the rate that produces the strongest resonance pattern. respiratory acidosis: a condition in which the lungs fail to eliminate sufficient carbon dioxide (CO₂), leading to its buildup in the blood and a corresponding decrease in pH from impaired ventilation or gas exchange. 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 device that measures breathing rate and pattern, often via chest or abdominal movement, enabling precise identification of inhalation and exhalation cycles during HRV training. RMSSD: the square root of the mean squared difference of adjacent NN intervals in milliseconds. SDNN: the standard deviation of the normal (NN) sinus-initiated IBI measured in milliseconds. skin conductance level (SCL): a measure of tonic electrical conductance of the skin, influenced by sweat gland activity and sympathetic arousal; decreases in SCL can indicate reduced sympathetic activation during successful HRV training. sliding protocol: a resonance assessment method in which the pacing rate continuously and smoothly changes across a target range (for example, from ~4.25 to 6.5 breaths per minute), allowing estimation of resonance frequency from the point with the largest peak–trough amplitude. slow-paced breathing (SPB): breathing in the adult 4.0-7.0 breaths per minute range to stimulate cardiovascular resonance and increase heart rate variability. slow-paced contraction (SPC): wrist-ankle or wrist-core-ankle contraction in the adult 4.5-6.5 contractions per minute range as an alternative to breathing-based HRV training. stepped protocol: a resonance assessment method that presents discrete breathing or contraction rates in fixed steps (for example, from 7.0 to 4.0 cpm with rest periods), so that physiological responses can be compared across separate 2‑minute epochs. vagal tone: functional level of parasympathetic (vagal) influence on the heart, often inferred from HRV indices such as RSA or HF power under controlled breathing conditions, though not directly equivalent to any single HRV metric. vagal withdrawal: the reduction or inhibition of parasympathetic (vagal) influence on the heart, typically during stress, exercise, or cognitive demand. When the vagal input decreases—i.e., withdrawal occurs—heart rate increases, reflecting a shift from parasympathetic to sympathetic dominance. wrist–core–ankles protocol: a specific slow-paced contraction setup in which clients rhythmically contract muscles in the wrists, core, and ankles at a prescribed rate to drive cardiovascular resonance and increase HRV.

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About the Author

Fred Shaffer earned his PhD in Psychology from Oklahoma State University. He earned BCIA certifications in Biofeedback and HRV Biofeedback. Fred is an Allen Fellow and Professor of Psychology at Truman State University, where has has taught for 50 years. He is a Biological Psychologist who consults and lectures in heart rate variability biofeedback, Physiological Psychology, and Psychopharmacology. Fred helped to edit Evidence-Based Practice in Biofeedback and Neurofeedback (3rd and 4th eds.) and helped to maintain BCIA's certification programs.

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Slow-Paced Contraction Training Using a Multichannel Biofeedback System.pdf

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