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Effects of tidal volume and respiratory rate on diaphragmatic motion velocities and expiratory muscle recruitment in healthy volunteers: A Tissue Doppler Imaging study

This study demonstrates that while increasing minute ventilation via either tidal volume or respiratory rate enhances diaphragmatic contraction and relaxation velocities in healthy individuals, elevating the respiratory rate imposes a significantly greater demand on diaphragmatic relaxation and expiratory muscle recruitment compared to increasing tidal volume.

Original authors: Eleni Soilemezi, Christos Nouris, Serafim-Chrysovalantis Kotoulas, Petros Morfesis, Dimitrios Matamis

Published 2026-09-20

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CC BY 4.0

Original authors: Eleni Soilemezi, Christos Nouris, Serafim-Chrysovalantis Kotoulas, Petros Morfesis, Dimitrios Matamis

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The human body is a master of balance, constantly adjusting how we breathe to match our needs. When we rest, our breaths are slow and deep; when we run, they become fast and shallow. This shift is not random but a precise calculation performed by the lungs and the brain to move enough air without exhausting the muscles. At the center of this effort is the diaphragm, a large, dome-shaped muscle that sits beneath the lungs and acts as the primary engine for breathing. When it contracts, it pulls down to draw air in; when it relaxes, it springs back up to push air out. For decades, scientists have understood that the speed and depth of breathing change how hard this muscle works, but a specific question remained unanswered: does the way we increase our breathing matter? If a person needs to move more air, does it make a difference whether they take deeper breaths or simply breathe more often?

A team of researchers at Papageorgiou General Hospital set out to answer this question by observing healthy volunteers in a controlled setting. They wanted to see if the pattern of breathing—whether driven by volume or by speed—changed the physical behavior of the diaphragm in ways that standard measurements might miss. To do this, they used a specialized ultrasound technique called tissue Doppler imaging. Think of this technology as a high-speed camera for muscle movement; instead of just seeing the muscle move, it measures exactly how fast the tissue is traveling and how quickly it speeds up or slows down. This allowed the researchers to look at the diaphragm not just as a pump, but as a dynamic engine with specific acceleration and deceleration characteristics. They also monitored a deep abdominal muscle called the transversus abdominis, which helps push air out, to see if it worked harder when breathing patterns changed.

The study involved twenty-one healthy adults who breathed in three different ways. First, they breathed naturally, as they would while sitting quietly. Next, they were asked to double the size of their breaths while keeping the speed the same. Finally, they were asked to double the speed of their breaths while keeping the size the same. In both experimental scenarios, the total amount of air moved in and out of the lungs increased significantly, reaching levels between sixteen and nineteen liters per minute, compared to about nine liters per minute during quiet breathing. The researchers recorded the muscle's performance over fifteen consecutive breaths for each condition, looking for subtle differences in how the diaphragm contracted to pull air in and how it relaxed to let air out.

The results revealed that while both strategies successfully increased the total air moved, they placed very different demands on the diaphragm. When the volunteers took deeper breaths, their diaphragm moved a greater distance, traveling about four centimeters compared to just under three centimeters during quiet breathing. The muscle also moved faster, reaching a peak speed of roughly 3.6 centimeters per second. However, when the volunteers breathed faster, the muscle moved even more quickly, reaching a peak speed of over 4 centimeters per second. The most striking difference appeared in how the muscle slowed down. During rapid breathing, the rate at which the diaphragm decelerated increased dramatically, jumping from a baseline of about 4.7 to over 20 units of acceleration. This suggests that breathing quickly forces the diaphragm to stop and reverse direction with much greater urgency than taking a deep breath does.

The study also shed light on the role of the other muscles involved in breathing. The researchers found that the deep abdominal muscle, the transversus abdominis, became significantly more active as the breathing rate increased. During rapid breathing, this muscle showed the highest level of electrical activity, suggesting it was working hard to help push the diaphragm back up and clear the lungs quickly. This finding supports the idea that when we breathe fast, we cannot rely on the lungs simply springing back on their own; we must actively engage our abdominal muscles to shorten the time available for exhalation. The data showed a strong link between how short the time for breathing out became and how hard the diaphragm had to work to relax. As the time to breathe out was cut in half, the muscle's need to accelerate its relaxation increased sharply.

These findings suggest that the way we breathe is not just a matter of volume or speed in isolation, but a complex mechanical choice that changes how our muscles function. Increasing the speed of breathing imposes a unique and intense demand on the diaphragm's ability to relax quickly, a task that requires significant help from the abdominal muscles. This is distinct from the demands of taking deeper breaths, which primarily require the muscle to stretch further and contract with more force. For doctors and researchers who use ultrasound to assess breathing muscle health, these results serve as a reminder that the context of the measurement matters. A muscle moving quickly might not be working harder in a general sense, but it might be under a specific type of stress related to time and speed that differs from the stress of a deep breath. The study confirms that the body adapts its breathing mechanics with precision, but it also highlights that rapid breathing places a distinct and heavy load on the speed of muscle recovery.

Technical Summary: Effects of Tidal Volume and Respiratory Rate on Diaphragmatic Motion Velocities and Expiratory Muscle Recruitment

Problem Statement
Minute ventilation (MV) is regulated by altering tidal volume ( $V_T$ ) and respiratory rate (RR) to meet metabolic demands. While it is established that breathing patterns reflect a balance between elastic and resistive work, the specific mechanical consequences of increasing MV via $V_T$ versus RR on diaphragmatic dynamics remain incompletely characterized. Specifically, it is unclear whether diaphragmatic contraction and relaxation velocities are driven predominantly by the volume of air moved, the frequency of breathing, or the overall level of ventilation. Furthermore, the role of expiratory muscle recruitment (specifically the transversus abdominis) in facilitating diaphragmatic relaxation under different breathing patterns requires further investigation.

Methodology
The study utilized a prospective design involving 21 healthy volunteers (12 men, 9 women; mean age 28 ± 4 years). Participants underwent three distinct breathing conditions:

Quiet Breathing: Spontaneous, comfortable breathing.
High- $V_T$ Breathing: Tidal volume was approximately doubled while maintaining the baseline respiratory rate.
High-RR Breathing: Respiratory rate was approximately doubled while maintaining the baseline tidal volume.

Data were collected over 15 consecutive breaths for each condition. The study employed three primary measurement modalities:

Respiratory Mechanics: Airflow was measured via pneumotachography to derive $V_T$ , RR, inspiratory time ( $T_i$ ), expiratory time ( $T_e$ ), and MV.
Diaphragmatic Tissue Doppler Imaging (TDI): Performed on the right hemidiaphragm using pulsed-wave TDI to measure Peak Contraction Velocity (PCV), Peak Relaxation Velocity (PRV), and the Diaphragmatic Relaxation Slope (the steepest rate of velocity change during expiration).
Electromyography (EMG): Surface EMG was recorded from the transversus abdominis to assess expiratory muscle recruitment.

Statistical analysis involved the Friedman test for overall differences and Wilcoxon signed-rank tests with Holm adjustment for pairwise comparisons. Repeated-measures correlations were used to assess within-subject associations between ventilatory and TDI variables.

Key Results
All respiratory and diaphragmatic variables differed significantly across the three conditions ( $p < 0.001$ ).

Ventilatory Changes: MV increased from 8.93 L/min (quiet) to 16.56 L/min (high- $V_T$ ) and 19.23 L/min (high-RR). High- $V_T$ breathing increased $V_T$ by ~86% with no change in RR, while high-RR breathing increased RR by ~98% with minimal change in $V_T$ .
Diaphragmatic Velocities: Both PCV and PRV increased significantly in both experimental conditions compared to quiet breathing. However, the magnitude of increase was generally greater during high-RR breathing.
- PCV: 2.59 cm/s (quiet) → 3.62 cm/s (high- $V_T$ ) → 4.22 cm/s (high-RR).
- PRV: 2.53 cm/s (quiet) → 3.57 cm/s (high- $V_T$ ) → 4.63 cm/s (high-RR).
Relaxation Dynamics: The most distinct finding was observed in the relaxation slope. It increased from 4.74 cm/s² (quiet) to 7.65 cm/s² (high- $V_T$ ) and dramatically to 20.29 cm/s² (high-RR).
Expiratory Muscle Activity: Transversus abdominis EMG activity increased progressively across conditions (0.012 mV → 0.022 mV → 0.032 mV). Notably, measurable expiratory EMG activity was detected in only 12 of 21 participants, though palpation confirmed abdominal muscle activation in all.
Correlations: Strong positive associations were found between MV and PCV ( $r=0.87$ ), PRV ( $r=0.85$ ), and relaxation slope ( $r=0.78$ ). A strong inverse association existed between relaxation slope and expiratory time ( $T_e$ ) ( $r=-0.85$ ).

Key Contributions
The study demonstrates that the method used to increase minute ventilation significantly alters diaphragmatic mechanics:

Pattern Dependence: Increasing MV via respiratory rate imposes greater temporal and mechanical demands on diaphragmatic relaxation than increasing MV via tidal volume, even when the resulting MV is comparable.
Relaxation Slope Sensitivity: The diaphragmatic relaxation slope is highly sensitive to respiratory frequency and available expiratory time, showing a markedly steeper increase during rapid breathing compared to deep breathing.
Expiratory Recruitment: Rapid breathing necessitates increased recruitment of the transversus abdominis to facilitate rapid diaphragmatic relaxation and restoration of length, a mechanism less pronounced during high-tidal-volume breathing.

Significance and Claims
The authors conclude that the physiological context (specifically the breathing pattern) is a critical determinant of diaphragmatic TDI parameters. The findings suggest that:

Interpretation of TDI: Clinicians and researchers must account for respiratory rate, tidal volume, and expiratory time when interpreting diaphragmatic TDI-derived velocities. Comparing measurements obtained under different physiological conditions without adjusting for these factors may lead to misinterpretation.
Relaxation Slope as a Marker: The relaxation slope may serve as a useful TDI-derived marker for the temporal demands imposed on the diaphragm, particularly in scenarios involving high-frequency breathing.
Physiological Mechanism: The study supports the hypothesis that expiratory muscle recruitment becomes increasingly important for rapid diaphragmatic relaxation when expiratory time is shortened, helping to restore the diaphragm to its optimal operating length before the next inspiration.

The authors explicitly note limitations, stating that findings in healthy volunteers cannot be directly extrapolated to patients with obstructive or restrictive lung diseases where mechanics (e.g., dynamic hyperinflation) differ. Additionally, the study assessed electrical activity and motion velocities rather than direct measurements of muscle work or metabolic cost.

Technical Summary: Effects of Tidal Volume and Respiratory Rate on Diaphragmatic Motion Velocities and Expiratory Muscle Recruitment

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