In this study, we presented novel findings showing an association between several combinations of breathing patterns and postures on abdominal muscle activities based on EMG and IAP, while observing respiratory volume. We found that TrA-IO and EO activity including IAP was only associated with breathing pattern and posture during the expiratory phase. Additionally, in the inspiratory phase, TrA-IO activity and IAP were remarkably affected by breathing pattern, while EO activity and IAP were affected by postural tasks. Thus, our results may fill the gap between basic and clinical practice regarding the importance of combining posture exercises with breathing tasks.
The greatest TrA-IO activity was observed in the Forced-Expi breathing pattern at the expiratory phase in the supine posture (47.6% of MVC). Even in the supine posture, the TrA-IO activity level in Forced-Expi breathing was still relatively higher than that in some of the plank exercises without breathing tasks reported in previous studies [24, 25]. TrA-IO activity was primarily affected by Forced-Expi, as evidenced by the greatest IAP development, which induced the greatest respiratory volume approximately 4–5 times, compared with that in Q-Bre (Table 1). In the qualitative observation of IAP dynamics and TrA-IO activity, Forced-Expi induced primary positive pressuring at the expiratory phase, corresponding with the phasic activity of the TrA-IO after negative pressuring at the inspiratory phase (FIGURE 2-c). According to observations of ultrasonographic visual change, the mean thickness of the IO muscle significantly increases at end-expiration phases [26]. Furthermore, TrA/IO thickness increases by approximately 1 mm—a considerable increase—after the breathing exercise of maximum expiration with the maximal abdominal contraction maneuver [1]. Forced expiration corresponding with the positive pressuring would promote concentric activity of the TrA-IO; however, we must consider the risk-benefit ratio between cardiorespiratory burden and exercise intensity. The greatest IAP value in the Forced-Expi (24.9% of IAP) was moderate and comparable with the result of 45% maximal lifting effort during isometric lifting found in a previous study [16]. If a few breathing trials did not have any issues, but were repeated several times, it was still necessary to consider the cardiorespiratory burden.
The second greatest TrA-IO activity value was observed in the Exertion-Inspi pattern throughout the inspiratory and expiratory phases in the elbow-toe posture (about 30–40% of MVC). Despite this, the respiratory volume was only about 1.2–2.0 times that of the Q-Bre (Table 1). The TrA-IO activity was triggered by the inspiratory phase rather than by the expiratory phase, contrary the pattern observed in Forced-Expi (FIGURE 2-d). The Exertion-Inspi method resembles the Pilates methods [15], which was a complete nasal inhalation while maintaining the abdominal muscles in isometric contraction. To maintain the isometric contraction, the TrA-IO activity needs to be balanced against the increasing pressure produced by the descent of the diaphragm in inhalation [27, 28]. Thus, endurance-related training is recommended for the sustenance of TrA/IO activity without being affected by the breathing event, as TrA/IO activity plays a role in respiration and the postural control effect [8–10]. Moreover, it would improve the risk-benefit ratio, because the IAP value in Exertion-Inspi was smaller (12.4% of IAP) than that in Forced-Expi. This low value allows us to sustain exercise without cardiorespiratory burden [16]. Regarding exercise instruction, because of the small respiratory volume in Exertion-Inspi, the instructor emphasized the need for isometric contraction of the TrA-IO.
Compared with the Q-Bre task, there was no increase in the activity of the TrA-IO in the Dia-Bre. TrA-IO activity in Dia-Bre in the elbow-toe posture was only about 20% of the MVC, which was the same as that of some of the plank exercises without breathing in previous studies [24, 25]. Even if we focus on the large inspiration pattern, which induced a 2.0–2.4-fold respiratory volume in Dia-Bre compared with that in Q-Bre, the relaxed-deep breathing (Dia-Bre) resulted in insufficient TrA-IO activity. In this breathing pattern, the abdominal muscles only expanded passively, contrary to what was observed in Exertion-Inspi. Although the IAP value in Dia-Bre was low, the elasticity of the passive expansion of abdominal muscles nullified the IAP produced by the descent of the diaphragm during inhalation [27, 28]. Thus, the Dia-Bre pattern did not affect TrA-IO activity and IAP dynamics, and only promoted the passive expansion of the abdominal muscles, even in the elbow-toe posture.
EO activity was affected by the posture tasks. The activity was 3–6 times larger in the elbow-toe posture than it was in the supine posture, regardless of the type of breathing pattern. This result is in accordance with the results of previous studies [24, 25], although it is impossible to compare them directly because of the different MVC trials and a lack of supine posture data in the previous studies [24, 25]. Imai et al. [24] and Okubo et al. [25] found that EO activity increased in several additional postural tasks in addition to the elbow-toe posture, rather than in the local muscles. These results in combination with ours suggest that EO activity contributes more to posture tasks than to breathing-related tasks.
We found a unique result regarding the relative values of TrA-IO and EO activity. The TrA-IO/EO ratio of < 1.0 in the elbow-toe posture indicated that the contribution of EO activity was higher than that of TrA-IO activity. Conversely, a ratio > 1.0 in Exertion-Inspi at the inspiratory-expiratory phase and in Forced-Expi at the expiratory phase indicated that the TrA-IO was also a contributor, together with the EO. Performing the dual task of posture and breathing may be useful in activating all the abdominal muscles. The dual task is also likely to have an effect on muscle strength and physical performance. Additional research is required to determine the training effect on physical performance and the synergistic effect of muscle function.
There are several limitations to this study. First, the participants were university students majoring in physical education who were not accustomed to routine practice of the breathing methods used, such as the Pilates method. The abdominal muscles may be activated to a higher extent in people practicing controlled breathing. Second, we used surface EMG rather than fine-wire EMG. Nonetheless, the muscle activity of the TrA-IO and EO significantly differed among several breathing patterns and postural tasks. Thus, our goal was achieved. Lastly, we could not assess the activity of the diaphragm because of technical reasons, although its activity plays an important role in IAP development.