We systematically searched the databases (Cochrane Library, PubMed, Web of Science, and MEDLINE) for randomized controlled trials (RCTs) using the following search terms: (aquatic exercise OR water-based exercise OR water exercise OR head-out water exercise) AND (functional fitness OR physical fitness OR physical capacity OR agility OR flexibility OR cardiorespiratory fitness OR aerobic capacity OR strength endurance OR strength OR quality of life) AND (postmenopausal women OR old women OR older women). All search terms were required to appear in the title or abstract. We also reviewed the reference list of the included literature. The Search was limited to database inception until July 2022. Two researchers (WSZ and SJM) independently completed the databases searching.

(i) RCTs; (ii) with exercise intervention as aquatic exercise or head-out water-based exercise; the control group did not participate in exercise. (iii) with participants being physiological postmenopausal women or women aged > 55 years (41–44); and (iv) with outcomes including physical fitness indicators and the overall QoL.

Two researchers (W-SZ and S-JM) independently conducted the selection of titles and abstracts from each database. The full text was obtained if the researchers deemed one study eligible. Two researchers (W-SZ and S-JM) independently extracted the study characteristics from eligible articles, including authors, publication year, age, sample size, exercise interventions, and primary outcomes. The corresponding authors of these studies were contacted in case of missing data. We deleted articles whose authors could not be reached or could not provide the data. The GetData software was used for extracting data if the results were presented as figures in the included articles (45, 46). Two researchers (W-SZ and S-JM) independently completed the data extraction and review. A third researcher (S-KZ) was invited and a consensus was reached at in case of discrepancies.

The Physiotherapy Evidence Database (PEDro) scale was used to assess the methodological quality of the included articles. The PEDro scale is based on 11 items, including eligibility criteria (not contribute to the total score), random allocation, concealed allocation, similarity baseline, subject blinding, therapist blinding, assessor blinding, >85% retention, intention-to-treat, between-group comparisons, and point and variability measures. Each study was assessed as “yes” (1 point) or “no” (0 points), with a maximum total score of 10. A study is considered to be of very good quality if it has a score of 9 or 10, while a score of 6 to 8 indicates good quality, a score of 4 or 5 indicates moderate quality, and a score of 0 to 3 indicates poor quality (47, 48). Two researchers (S-KZ and HX) independently performed the quality assessment, and a third researcher (W-SZ) was invited and a consensus was reached at if there was any discrepancy.

Data analysis was performed using the Cochrane Collaboration Review Manager (RevMan, version 5.4, Copenhagen, Denmark) software. Standardized mean difference (SMD) was employed if there were different outcome measures (49). I² index was used to test statistic heterogeneity. An I² >50% indicated high heterogeneity, and a random-effect model was applied (50). Sensitivity analysis was done by deleting studies one after the other (51). Egger's regression test was used to assess publication bias (52). The statistical significance level was set at p < 0.05.

Using the search strategy, 1,469 studies were retrieved, of which 1,421 studies were deleted because they were duplicates, animal studies, non-RCTs, or included non-postmenopausal women, and so on. Because the participants of 29 studies were on hormonotherapy, nutritional care, or the studies had no control group or failed to extract data, these studies were deleted. Sixteen RCTs (19 comparison groups) were finally included in the present study (Figure 1).

The 16 RCTs were published between 2006 and 2022 and involved 594 participants (320 received aquatic exercise interventions). The participants were aged from 54 to 74.9 years. Exercise duration was from 8 to 24 weeks, exercise frequency was from 2 to 5 times per week, and exercise time was from 30 to 60 min. Aquatic aerobic exercise (1, 28, 31, 32, 34, 35, 39), aquatic resistance exercise (29, 30, 53–55), and multicomponent exercise (aquatic aerobic and resistance exercises) (27, 31, 33, 34, 39, 56, 57) were the main types of exercises involved. The studies of Hafele, Alberton, Hafele, et al. (2022), Hafele, Alberton, Schaun, et al. (2022), and Silva et al. (2018) included both aquatic aerobic and multicomponent exercise (31, 34, 39). The participants compliance ranges from 73.6 to 100%. Each exercise session was supervised by the researchers. The characteristics of the included studies are summarized in Table 1.

The ranges of the quality assessment scores of the included studies was from 3–6. Two studies received scores of 6 (good quality) (31, 39), 13 studies received scores of 4–5 (median quality) (1, 27–30, 32–35, 54–57), and 1 study received scores of 3 (poor quality) (53). The mean score was 4.8 (Table 2).

Egger test revealed that there was a relatively higher level of publication bias in aerobic capacity (t = −2.67, p = 0.045). There was no obvious publication bias in LLS (t = −1.38, p = 0.201), ULS (t = −0.02, p = 0.984), agility (t = −0.24, p = 0.815), flexibility (t = 0.21, p = 0.838), and overall QoL (t = −1.76, p = 0.177) (Supplementary material).

A decline in muscle mass and balance ability in the older adults commonly accompanies the aging process. This leads to limited mobility and loss of independent living ability, which greatly reduces the quality of life of the older adults (58, 59). It is particularly important to maintain LLS in the older adults. Past studies have indicated that LLS is the basic fitness of balance ability and an important factor in fall prevention in the older adults (60). The present study demonstrated that aquatic exercise significantly increases LLS (SMD = 1.37, p = 0.001, n = 334) and ULS (SMD = 1.86, p = 0.005, n = 106). The present findings conform well to those of Saquetto et al. (36), who observed that aquatic exercise significantly increases the muscle strength of knee extension (SMD = 3.34, p = 0.004, n = 216), knee flexion (SMD = 2.51, p = 0.007, n = 82), and arm curl (SMD = 6.78, p = 0.0001, n = 80). In the present review, 11 studies analyzed LLS, and 3 studies analyzed ULS, and the sample size of the included studies was larger. Therefore, our findings further updated the results of the previous study. The results of aquatic exercise efficacy in improving upper and lower extremity muscle strength were confirmed. According to many studies, resistance training can significantly improve muscle strength (61, 62). Resistance exercise in water is a great way for increasing strength, especially for ULS. Most of the studies included showed that the depth of the water reaches the xiphoid, which is beneficial for applying ULS exercise and increasing ULS (63). In addition, walking or jumping in the water is challenged by the drag and resistance of the water, thereby improving lower body muscle strength. The physiological mechanism of aquatic exercise to improve muscle strength mainly due to the improvement of neuromuscular system function (64). The trend of muscle strength decline was reversed (64). Previous studies have suggested that degenerated skeletal muscle recruitment patterns and functions are the main cause of decreased muscle strength (65, 66). Resistance training increases muscle strength by improving neuromuscular recruitment and muscle contraction (66, 67). Moreover, it may also be related to increased muscle mass, which is the main cause of increased muscle strength (68). Previous research confirmed that aquatic exercise significantly increases skeletal muscle mass (69, 70).

It is worth noting that aquatic exercise significantly improves LLS in postmenopausal women < 65 years old (SMD = 3.33, p = 0.02), but no efficacy was found in the ≥ 65 years subgroup. One possible explanation was that higher age-related muscle weakness, poor joint mobility, and poor balance limited body movement in a water environment lead to insufficient exercise intensity (71). Regarding the exercise types, both aquatic aerobic exercise (SMD = 0.81, p = 0.04) and resistance exercise (SMD = 4.51, p < 0.00001) significantly improved LLS. Aquatic resistance exercises induced greater magnitudes of improvement.

Regarding the ULS index, the same two studies were included in the age < 65 years and aquatic resistance exercise subgroups (29, 30). ULS was significantly improved (SMD = 2.44, p < 0.00001), but no efficacy was found in the age ≥ 65 years and aerobic exercise subgroups (SMD = 0.58, p = 0.16). Data for the age ≥ 65 years and aerobic exercise subgroups are from the same study (32). Therefore, the results of these two subgroups should be interpreted and applied with caution, and more studies are needed to be included for a more comprehensive interpretation in the future.

As ages increase, it becomes increasingly challenging for the elder to move quickly and change direction (72, 73). Decreased agility is a key factor in predicting risk for recurrent falls (74). Exercise training is an important way to maintain and improve agility (75). In the present study, agility was significantly improved in the aquatic exercise group compared with the no exercise group (SMD = −0.67, p = 0.002). Our study results were in accordance with the meta-analysis of Saquetto et al. (36), in which 165 participants were included, and agility was significantly increased (SMD = −2.13, p = 0.05). Compared with the study of Saquetto et al. (36), our study has a superiority in including more studies (16 RCTs) and a larger sample size. The effect of improved neuromuscular function on increased muscle strength of the upper and lower limbs may have been responsible for the positive results (76, 77). Agility is the comprehensive embodiment of strength, speed, balance, and coordination, moreover, strength is the foundation of agility (78), and is directly associated with neuromuscular function status (79). In the present study, the overall results showed a significant increase in LLS of 1.37 kg and ULS of 1.86 kg. The results of subgroup showed that agility was improved only in the subgroup with age < 65 years (SMD = −0.98, p = 0.0003). Moreover, only postmenopausal women aged < 65 years showed significant improvement in LLS (SMD = 3.33, p = 0.02) and ULS (SMD = 2.44, p < 0.00001), further emphasizing the importance of strength in improving agility (79, 80). This may also be related to the improvement of joint range of motion. Previous studies have pointed out that agility and flexibility have a significant positive correlation (81), namely the better the flexibility performance, the shorter the agility test time. The present study also indicated that aquatic exercises can significantly improve the flexibility of lower limbs, and thereunto, only the flexibility of the subgroup aged < 65 years was significantly improved (SMD = 1.38, p = 0.008). Therefore, it is believed that the improvement of flexibility may be one of the possible reasons for the improvement of agility. Among the subgroups, agility was found to be improved in the age < 65 years subgroup, emphasizing that changes in agility improved by aquatic exercise were associated with age, precisely a relatively young age can contribute to better effects. Furthermore, subgroup analysis indicated that only aquatic resistance exercises can improve agility, reminding us that the importance and particularity of aquatic resistance exercises should be significantly considered when designing aquatic exercise programs in the future.

6MWT, as an important index that assesses the aerobic capacity, was adopted in the included studies. 6MWT was a submaximal exercise ability test for the middle-aged and the old adults (82, 83). The present study demonstrated that aquatic exercises cannot significantly improve aerobic capacity in postmenopausal women. In addition, subgroup analysis showed that the aerobic capacity was not improved in the aged < 65 years (n = 38), aged ≥ 65 years (n = 155), aerobic exercise (n = 139), and multicomponent exercise subgroups (n = 54). Our findings are consistent with those of the study of Ha et al. (1), who included participants performing aquatic aerobic exercises 3 times a week for a 12-week duration (1). They found that 6MWT was not improved. No significant improvement of 6MWT was found in the study of Perkins et al. (2021) with 60 min of aerobic exercises 5 times a week for a 17-week duration (33). The controversial findings were possibly associated with the low impact when performing aquatic exercises, which was caused by buoyancy and reduced the muscle loads. Although the resistance to water caused the muscle to produce a contractile load, it may not be sufficient to induce a large cardiopulmonary response and therefore did not produce a better adaptive increase in cardiopulmonary function. Furthermore, it may also be related to the lower heart rate level during aquatic exercises. To our knowledge, the reduction in heart rate was mainly due to hydrostatic pressure. Hydrostatic pressure increases venous return and decreases peripheral blood volume. As a result, end-diastolic volume and stroke volume increase, thereby reducing the heart rate (84, 85), accompanied by increased vagal and parasympathetic activity and decreased sympathetic activity caused by atrial and arterial baroreflex mechanisms (86, 87). This results in a lower cardiopulmonary load without an adaptive increase in cardiopulmonary function. We mainly adopted 6MWT to assess the aerobic capacity, and there were also other studies using the VO_2max index. The study of Saquetto et al. (36) using VO_2max index included 4 articles and the results demonstrated that aquatic exercises could increase VO_2max by 4.12 ml/kg, which was inconsistent with the results of our study. Therefore, there was still some controversy about the results of aquatic exercise on improving the aerobic capacity level, and we should be especially cautious when interpreting and applying these results. Our study indicated that there was a relatively higher level of publication bias in aerobic capacity. It may be mainly related to the small number of the included articles. It is well known that participating in exercise for a period of time is helpful to improve aerobic capacity (88), but in the end, it shows negative results in the present study. It is also may be that the research articles with positive results have not been published or published in non-English journals.

Decreased flexibility is associated with the development of musculoskeletal disorders, progressive disability (89), and an increased risk of falls in middle-aged and old adults (90). Exercise is a favorable way to maintain and improve flexibility (91). The present study demonstrated that aquatic exercises can improve lower limb flexibility in postmenopausal women (SMD = 0.91, p = 0.002). Our study results were consistent with the study of Saquetto et al. (36), including only 3 RCTs vs. 12 RCTs in our study, thereby confirming the effectiveness of aquatic exercises on improving lower limb flexibility. Previous studies have shown that exercises are efficacious in improving flexibility. Aquatic exercises, as an intervention method in our study, can make people more relaxed and the action more stretched. Moreover, the buoyancy of water can reduce the fear of falling in middle-aged and old postmenopausal women, and higher water temperature had the effect of hot compress and massage, which further reduced the stiffness of tissues and muscles around joints (92), thus bringing better advantages to the improvement of joint mobility. In addition, our study has confirmed the effectiveness of aquatic exercise on improving muscle strength, especially the increased muscle strength of the lower limbs leading to more stable joints. Under the hydrostatic pressure of water, the blood circulation is better, the blood flow around the joint is more, and the metabolic wastes are recovered and disposed of, thus improving the range of motion of the joint. The subgroup analysis results of the study showed that the flexibility of the aged < 65 years (SMD = 1.38, p = 0.008) and the resistance exercises subgroups (SMD = 2.49, p = 0.04) was significantly increased, while the aged ≥ 65 years, aerobic exercises, and combination exercises subgroups were not significantly improved. It followed then that aquatic exercise can improve the flexibility of postmenopausal women aged < 65 years, and resistance exercise had a better effect on improving flexibility. Due to the importance of flexibility to the ability of voluntary physical activity and fall prevention, older adults should maintain a certain range of joint motion (93).

The decline of physical function associated with aging influenced the ability of daily independent living of older adults. Meanwhile, they had various chronic diseases and emotional burdens, which affect the overall QoL (16, 94, 95). The present study demonstrated that aquatic exercises can significantly improve the overall QoL in postmenopausal women (SMD = 1.04, p = 0.04), moreover, subgroup analysis indicated that aquatic exercises were significantly associated with the effectiveness in the subgroup aged < 65 years and ≥ 65 years. Since only one article was included in the subgroup of age < 65 years (30), caution should be used in the interpretation and application of this finding. Most noteworthy, although no significant improvement was observed in ULS, LLS, agility, and flexibility in the subgroup aged ≥ 65years, aquatic exercise had positive effects on postmenopausal women of this age. It was concluded that aquatic exercise may be a better exercise method to improve the overall QoL of old postmenopausal women. Our findings are consistent with those of the study of Hafele et al. (39), which used aquatic aerobic exercise or a combination of aquatic aerobic and resistance exercise for 60 min three times a week for 16 weeks and showed significant improvements in overall QoL in both exercise intervention groups. However, there was no improvement in the control group (39). Similar to our study, Silva et al. (34) showed a significant 17% improvement in overall QoL in the aquatic aerobic exercise group adopting 12-week aquatic aerobic exercise or a combined aquatic aerobic and resistance exercise intervention with exercise twice a week. Although no beneficial effect of combined aquatic aerobic and resistance exercise was found (34), there was no denying of the relevance of aquatic exercises via improving postmenopausal women population in the area of the body (relating to the pain or discomfort, energy or fatigue, sleep, rest, mobility, daily activities, drug dependence and performance), psychology (including emotion, learning, memory and attention, self-esteem, appearance, spiritual, social, religious, and positive or negative thinking), social identity (personal relationship, social support and sex) and environment (including physical safety, home environment, financial security, information evaluating opportunities, participate in social or cultural activities and leisure time activities). This can be explained by previous studies, which showed that social and psychological problems such as depression, anxiety, or social isolation were significantly associated with chronic diseases (30, 96). In addition, subgroup analysis results showed that the resistance exercise and combination exercise subgroups had a significant improvement in overall QoL, while the aerobic exercise subgroup had no significant effect. The importance of resistance training for postmenopausal women was further emphasized here.

There are some limitations in the present study. First, few high-quality studies were included, and most were of moderate quality, especially in terms of blinding subjects, coaches, and measurements. Second, although more articles were included than the study of Saquetto et al. (36), there may still be the risk of insufficient literatures in the analysis of some indexes, especially in the subgroup analysis, so the results of the present study should be interpreted with caution when practicing.

In the present systematic review and meta-analysis, we evaluated and updated the effects of aquatic exercise on physical fitness and quality of life (QoL) in postmenopausal women. The present findings indicate that, aquatic exercise significantly improved ULS, LLS, agility, flexibility, and overall QoL in postmenopausal women. Aquatic resistance exercise is recommended as the best option for postmenopausal women to improve physical fitness and QoL.