The Trunk Rotation May Reduce Lung Volume and Respiratory Muscle Strength Prominently in Female Than Male

It is known that gender affect pulmonary function, associated with anatomical differences between male and female. However, the effects of trunk rotation on respiratory variables and its differences between males and females remain unclear. We examined the effects of gender and physical characteristics on postural changes in healthy young people. In this study, 9 males and 11 females (22 ± 1 year old for both males and females) were enrolled. We measure the vital capacity (VC), inspiratory capacity (IC), tidal volume (VT), expiratory reserve volume (ERV), inspiratory reserve volume (IRV), and force vital capacity (FVC) and respiratory muscle (PImax and PEmax) with rest posture in the sitting position (rest posture) in sitting position and 30° trunk rotation both genders in the sitting position (rotational posture). The value of VC, IC, ERV, IRV, FVC, or FEV1.0 for males were signicantly higher than that for females in both postures. In both genders, the VC, ERV, FVC, FEV1.0, and PEmax values in the rotational posture were signicantly lower than that in the rest posture. Further, in females, there was a signicant decrease in PImax in the rotational posture compared with the resting posture. This study indicated that trunk rotation may limit pulmonary function prominently in female than in male. These nding may provide important insights on gender differences in respiration in daily living.


Introduction
It is known that there are signi cant differences in pulmonary function between male and females.
Females are characterized by a smaller-sized rib cage and airways relative to lung size 1, 2, 3, 4 and a greater contribution of inspiratory rib cage muscles than males 5 . These anatomical differences between males and females may affect performance in activities of daily living (ADL).
Posture also affects pulmonary function and the contribution of the rib cage and abdomen. Previous studies have investigated posture effect on chest wall kinematics (Craig 1960). Verschakelen et al. have showed the effect of gender on chest wall kinematics 7 . There are differences in the composition of thoracic dimensions and con guration between genders 8,9 . However, few studies have examined the effects of gender and physical characteristics on respiratory changes relative to posture.
In this study, we attempt to elucidate the effects of gender and physical characteristics on respiratory variables during rotational posture in healthy people.

Subjects
There were 20 healthy young people (22 ± 1 year old for both genders) were enrolled in this study. The inclusion criteria were nonsmoker and with no cardiac and pulmonary diseases.

Pulmonary function
Pulmonary function was assessed using a spirometer (H-801, CHEST, Japan), according to ATS/ERS statement on pulmonary function test 10 . The variables such as vital capacity (VC), inspiratory capacity (IC), tidal volume (VT), expiratory reserve volume (ERV), inspiratory reserve volume (IRV), force vital capacity (FVC), forces expiratory volume in one second (FEV1.0), or forces expiratory volume % in one second (FEV1.0%), were measured. Respiratory muscle strength was determined by measuring the maximal inspiratory pressure (PImax) and maximal expiratory pressure (PEmax) using a mouth pressure meter (IOP-01, Kobata, Japan), following ATS/ERS statement on respiratory muscle testing 11 . The maximum value of the three maneuvers varying by less than 20% was recorded. These variables were measured in the resting or 30° trunk rotation in the sitting position. Participants performed a test in two sitting postures with rest period.

Statistical analysis
Respiratory variables are expressed as mean ± standard deviation. The data were analyzed using the IBM SPSS Statistics software version 24.0 (IBM Corporation, Armonk, NY, USA). The differences between males and females, and the two sitting postures in each variable were analyzed using Student's t-test, with a signi cance level set at p < 0.05 for all statistical comparisons.

Ethical approval and consent to participate
This study's ethical approval was granted by the Ethics Review Board of Yamagata Prefectural University of Health Sciences, Yamagata, Japan (#1801-23). This study was carried out in accordance with the recommendations of the Ethics Review Board of Yamagata Prefectural University of Health Sciences. All participants gave written informed consent in accordance with the Declaration of Helsinki.

Results
Twenty healthy young people participated in this study. The characteristics of participants are shown in Table 1 Furthermore, the results indicated signi cant decrease in ERV and FEV1.0 during respiration in the rotational posture compared to that in the rest sitting posture. According to a previous study, obesity and external pressure on the rib cage reduced ERV 13 . The muscle strength variables for forced expiration were signi cantly decreased in the rotational posture. The agonist muscle for trunk rotation includes the external and internal oblique abdominal muscles, which are the most active during forced expiration.
Therefore, this result suggests that rotational posture limits the activity of the forced expiratory muscles.
PImax was signi cantly decreased in the rotational posture compared with that in the rest posture in females, although it did not change in males. It is known that chest wall kinematics is signi cantly in uenced by gender 8 , 9 . There are differences in the relative contribution of the rib cage and abdomen during respiration. Bellemare et al. reported that the differences in the composition of thoracic dimensions and con guration between males and females produce differences in the contribution of the rib cage and abdomen during ventilation 5 . Females have smaller airways relative to their lung size 1 . We deduced that the rotational posture, restricting the range of motion of the rib cage, affected the inspiratory muscle strength for female.
This study exhibited that respiratory variables were signi cantly decreased by rotational posture even in healthy young people with average muscle strength. Therefore, the elderly or chronically ill patients speculated cause further affected by posture.

Conclusion
The present study revealed that posture and gender differences affect the lung volume and respiratory muscle strength. These ndings may provide important insights on gender differences in respiration in daily living activities.