Effects of Water Immersion in Different Water Depths on Respiratory Function and Respiratory Muscle Strength among Elderly People: An Observational Study

The aim was to investigate the effect of three water levels (umbilical, 4 th rib, and clavicular) on the respiratory function and respiratory muscle strength among elderly. Spirometry and respiratory strength were measured on land as baseline data. Next, water depth conditions were determined randomly, and spirometry and respiratory muscle strength were measured at each water level. The Vital Capacity and Expiratory reserve volume in the clavicular level were significantly lower than those in the land and umbilical trials. No significant difference was observed in other respiratory functions. Chest circumference and respiratory muscle strength were not significantly different among all conditions.


Introduction
Aquatic exercise has recently been introduced as a part of health enhancement plans in rehabilitation and sports clubs [1] [2]. Aquatic exercise can reduce the self-weight load, thus allowing persons with obesity, joint diseases, or lumbago to perform exercise safely [3] [4]. In addition, water viscosity and pressure can be used as exercise loads to enhance extremity muscle strength [5] [6].
Breathing underwater requires great effort mainly for the following two reasons: first, the blood volume shifts into the chest cavity due to increased venous return from the lower extremities; and second, the chest wall and diaphragm inflexibility shifts toward the cranial side due to the hydrostatic pressure, resulting in restricted pulmonary compliance [7] [8]. Regarding the effects of water depth on respiratory function, the pulmonary vital capacity (VC), forced expiratory volume during the first second (FEV 1.0 ), and functional residual capacity (FRC) decrease during water immersion at the clavicular or cervical level [9] [10]. Furthermore, de Andrade et al. reported that decreased maximum inspiratory muscle strength during water immersion was greater when the water level was at the clavicle than at the xiphoid process due to the higher water pressure at the clavicular level (CL) [11]. We also suggested that forced respiration during upright water immersion up to the CL resulted in greater inspiratory muscle fatigue than at shallower depths in healthy young men [12].
As mentioned above, underwater environment exercises have been incorporated in various rehabilitation programs and are widely adopted in respiratory, heart, and other diseases. However, although several studies have investigated the effect of different water depths on respiratory function in young people, studies investigating this effect in elderly people are limited and with unclear findings.
Therefore, this study aimed to investigate the effect of three water levels (umbilical, 4 th rib, and clavicular) on the respiratory function and respiratory muscle strength among elderly people.

Participants
A total of 14 elderly people were included in the study. Participants with history of respiratory or cardiovascular disease, hypertension (resting systolic blood pressure [BP] of ≥140 mmHg and/or diastolic BP of ≥90 mmHg), diabetes, or obesity (body mass index [BMI] of ≥30 kg/m 2 ), or smoking habit were excluded. In addition, those with kyphosis were excluded to eliminate the influence of spinal deformity. Eligible participants who met the inclusion criteria were included in the study after familiarizing themselves with the experimental protocol, such as the measurement method using spirometry described below. Of the 14 registrants, two had a history of smoking and two had kyphosis and hence, they were excluded from the study. Therefore, statistical analyses were carried out using data from 10 participants. The participant characteristics are summarized in Table 1.
This study was approved by Aino University Research Ethics Committee and conformed to the standard set by the Declaration of Helsinki. Written informed consent was obtained from all participants prior to the experiment.

Material and Methods
An underwater treadmill (HOKKODENKI treadmill KRT-2500P) was used as Data are means ± SD the water tank, and the water level was set to three settings: umbilical level (UL), was measured at the xiphoid process level.
Measurements were performed according to the protocol below. At the laboratory, participants' weight and height were measured before the first experiment.
BMI (kg/m 2 ) was calculated as body weight (kg) divided by height (m) squared.
Then, participants practiced spirometry and respiratory muscle strength measurement methods, and all of them were trained. Then, 30 minutes after completing the practice, the experiment was started. Spirometry, respiratory strength, and chest circumference were measured 10 minutes after sitting on land and recorded as baseline data. Next, water depth conditions were determined randomly, and spirometry and respiratory muscle strength were measured 10 minutes after assuming a sitting position at each water level.

Statistical Analysis
All statistical analyses were performed using the StatView statistical software package (Ver5.0, SAS, Cary, NC, USA). Data were expressed as mean ± standard deviation (SD) unless otherwise indicated. One-way analysis of variance with repeated measurements and subsequent multiple pairwise comparisons (Scheffe method) was performed to evaluate the effect of upright water immersion at different water depths on the respiratory function and respiratory muscle strength. Statistical significance was defined by a P value of <0.05.

Discussion
This study demonstrated that VC and ERV in the CL were significantly lower than those in the land and UL trials. Agostoni et al. stated that respiratory functions, such as VC and forced expiratory volume in 1 s of water immersion, were reduced in proportion to the water depth and that the reduction become remarkable when the depth reached the CL [9]. Dahlback and Buono et al. also reported that VC was significantly decreased by water immersion at the CL [13] [14]. Our results are consistent with those of previous studies. Kurabayashi et al.
reported that CC was reduced by 0.5 cm underwater at the CL depth than that before submergence [15]; however, our results did not show a significant difference regardless of water depth. Chest compliance has been reportedly reduced in elderly people [16]. Therefore, they are less likely susceptible to CC changes due to hydraulic pressure on the chest wall. The abdominal and chest walls are speculated to be compressed by hydrostatic pressure during water immersion, resulting in a cranial shift of the diaphragm, which decreased VC. This cranial shift also minimizes the alveolar size at the end-expiratory phase, which may decrease ERV. In this study, neither PImax nor PEmax was affected just by water immersion at any depth. In terms of PImax, our results were consistent with that of Schoenhofer et al.'s, which showed that PImax tended to decrease from the baseline after water immersion at the CL depth. PImax is generally recommended to be measured at the maximum expiratory phase based on the length-tension relationship theory [17]. Briefly, the maximum expiratory phase is determined by the balance between expiratory muscle contraction and opposing elastic dilatation pressure of the lungs and thorax. At this phase, the inspiratory muscles and PEmax during water immersion was not different at the iliac crest, xiphoid process, or CL [11]. Primarily, the decreased VC that is accompanied by submergence may be unfavorable to measure PEmax from the above length-tension relationship perspective. However, hydrostatic pressure against the entire thorax at the CL depth is speculated to have assisted expiration, resulting in the minimum influence on PEmax.
There are some limitations in the present study. The results of the present study with the small number of subjects suggest that VC and ERV decreased when water immersion was at the CL. We will increase the number of subjects more from now on, and have to consider whether it'll be a similar result. In addition, water resistance changes depending on the movement speed, moving with low resistance may be possible by changing the speed even if the movement is the same. Therefore, when performing respiratory exercises underwater, the exercise load may be reduced if the inspiratory speed is slow. Further detailed exercise methods that affect the respiratory function should be investigated in future studies.

Conclusion
In conclusion, we demonstrated that VC and ERV decreased when water immersion was at the CL in the elderly, but did not affect the respiratory muscle strength. Further studies are needed to develop an exercise regimen that can be utilized in the field of health promotion for elderly that makes the most of the characteristics of submersion and aquatic exercise.