Fourteen healthy men (mean age, 22.7 years [standard deviation (SD), 5.0 years], mean height, 172.1 cm [4.2 cm], and mean weight, 73.3 kg [13.4 kg]) volunteered to participate in this comparative cross-sectional study. The participants were university students majoring in physical education. The study sample comprised four football players, two rugby players, two long-distance runners, a sprinter, a tennis player, a baseball player, a volleyball player, a handball player, and a soccer player. The participants had no neuromuscular, orthopedic, or respiratory abnormalities.

All procedures involving human participants were performed in accordance with the ethical standards of the Institutional and/or National Research Committee and the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Written informed consent was obtained from all participants. The protocol was approved by the Ethics Review Board of Tokai Gakuen University (# 24-19).


The participants performed four active breathing tasks: quiet breathing (Q-Bre), deep breathing (Deep-Bre), pursed-lips forced expiration (Forced-Expi), and exertional nasal inhalation with expanding the thorax not to abdominal wall expansion (Exertion-Inspi). Specifically, Q-Bre was normal nasal tidal breathing (as a control). Deep-Bre was a complete nasal inhalation with submaximal expansion of the abdominal wall and relaxed expiration through the mouth. Forced-Expi was a completely forced exhale with pursed lips until the abdominal wall was hollowed. Exertion-Inspi was a complete nasal inhalation, sufficient to expand the thorax while maintaining the abdominal muscles in isometric contraction, not to abdominal wall expansion. The respiratory rhythm was applied by examiner cueing. The participants practiced repeatedly during a lesson from breathing exercise instructors (T.K. and J.U.). The four breathing tasks were performed in a random order in the supine (as a control posture) and elbow-toe postures for 30 s. Regarding the elbow-toe posture, the participants performed a prone plank posture on the floor, maintaining a straight trunk and lower extremities, with only their toes and forearms touching the floor (Fig. 1). They rested for 3 min between the tasks to eliminate the influence of fatigue.

Figure 1
figure 1

A schematic diagram of the experimental setup (a) for the active breathing task in the supine (b) and elbow-toe plank (c) postures. Intra-abdominal pressure data are collected by a pressure transducer placed intra-rectally. Airflow data are measured by a pneumotachograph attached to a face mask covering the nose and mouth. Abdominal muscle activity is measured during the task using electromyography. All participants perform the active breathing exercise at least three times. All elbow-toe tasks are performed with a straight trunk and lower extremities for 30 s.

IAP, breathing, and muscle activity measurements

Respiratory and IAP measurements were performed as previously described19,20. Briefly, IAP was measured using a pressure transducer (MPC-500, Millar Instruments, Inc., Houston, TX, USA) placed intra-rectally19,20,21. Before the actual measurements were taken, the maximum IAP was obtained based on the maximum voluntary pressurizations produced during Valsalva maneuvers performed in a standing posture (maxIAP). The highest value among three trials was used as the maximum. The IAP during the task was normalized using maxIAP (%IAP). The breathing volume of all the tasks was measured with a pneumotachograph (FM-200, Arco System, Inc., Kashiwa, Japan) using a face mask covering the nose and mouth (7400, Hans Rudolph Inc., Wyandotte, MO, USA). Respiratory volume was calibrated using a syringe calibrated at 3 L (763722, Sensor Medics Corp., Yorba Linda, CA, USA).

Surface electromyogram (EMG) measurements were conducted as previously described22. Preparing for the EMG, the surface of the skin was cleaned with alcohol and rubbed with sandpaper. Surface bipolar electrodes (Ag–AgCl, 6-mm contact diameter, 1.5-cm inter-electrode space) were placed on each of the muscles using Kinesio tape. Before measurements and throughout the testing period, all EMG signals were monitored and checked using a real-time oscilloscope display. EMG signals from the right transverse abdominis-internal oblique muscle (TrA-IO) and right external oblique muscle (EO) were recorded during the tasks. Electrodes for the TrA-IO were placed 2 cm inferomedially to the anterior superior iliac spine23 at an angle following the inguinal ligament. The activity of the TrA-IO could be best assessed from this surface location24,25. Electrodes for the EO were placed at the junction of a line drawn from the umbilicus to the anterior axillary line at an oblique angle following the muscle fibers. A reference electrode was placed on the right iliac crest. The EMG signals were amplified differentially using an AC amplifier (input impedance 5 MΩ, gain 1000–2000×, common-mode rejection ratio > 60 dB). Moreover, band-pass filtering was set at both low (time constant of 0.03 s) and high (2 kHz) cut-off filters (AB-620G, Nihon Koden, Japan). Before performing the actual tasks, all participants completed three consecutive trials of maximum voluntary contraction (MVC) of the isometric muscles21. MVC was performed to sustain the posture against the external force applied in the supine (for TrA-IO) and side-lying (for EO). The participants kept the area beyond their iliac crest hanging over the examination table in the supine or side-lying posture, with their pelvis and lower leg held in place by two examiners. Thereafter, another examiner applied the maximal resistance to the participant’s upper trunk with the hands using the whole body weight. The participants horizontally kept their trunk position against resistance, trying to hold the maximum force for approximately 5 s. The highest root mean square of EMG amplitude of each muscle during the three MVC trials (or Valsalva maneuver tests as mentioned below) was adopted as the MVC value used for subsequent statistical analysis (100% MVC). The root mean square of EMG data of the MVC trial was calculated for 100 ms (50 ms before and after the peak value). Moreover, Valsalva maneuver test was performed in the standing position to develop the maximal IAP (100% IAP)19,20. Valsalva maneuver test comprised a complete nasal inhalation followed by holding breath with maximal voluntary pressurization to the abdominal compartment as forcefully as possible.

IAP and airflow and EMG data were simultaneously recorded on a computer (chart 5.3, AD Instruments, Sydney, Australia) using an analog-to-digital converter (Power-lab 8sp, AD Instruments, Sydney, Australia) at a sampling rate of 2 kHz. Representative data are shown in Fig. 2. For each parameter, values were averaged from a series of three trials for each set of phases (inspiratory and expiratory).

Figure 2
figure 2

Representative data of air flow, IAP, and electromyography during the active breathing task (ad), with the inspiratory (↓) and expiratory phase (↑). The vertical dotted line indicates the elbow-toe posture interval. I inspiratory phase (↓), E expiratory phase (↑), IAP intra-abdominal pressure, TrA-IO transverse abdominal and internal oblique muscle, EO external oblique muscle, Q-Bre quiet nasal breathing, Forced-Expi completely forced expiration with pursed-lips until the abdominal wall hollowed, Exertion-Inspi exertional nasal inhalation with abdominal muscle isometric contraction to prevent abdominal wall expansion.

Statistical analysis

All data are expressed as mean ± standard error of mean. A two-factor (breathing and posture) repeated-measures analysis of variance was performed to assess statistical significance of the effects of interaction. Based on the model of Mauchly’s test, if the assumption of sphericity was not met, the Greenhouse–Geisser correction was applied. Then, Bonferroni post hoc testing was used for pairwise comparisons. All statistical analyses were performed using SPSS software, version 22 (SPSS Inc., Chicago, IL, USA). A p-value < 0.05 was considered statistically significant. In addition, the magnitude of changes in the values from the reference value (Q-Bre in the supine posture) was expressed as the effect size. Each outcome was classified as a small (0.2), moderate (0.5), or large (≥ 0.8) effect26. Finally, the muscle activity ratio between the local and global muscles was expressed as a relative value (TrA-IO/EO).

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