High-intensity interval swimming improves cardiovascular endurance, while aquatic resistance training enhances muscular strength in older adults
This study followed the recommendations for reporting experimental randomized studies as described in the updated version of the Consolidated Standards of Reporting Trials (CONSORT)30.
Study design
The study design was a randomized controlled trial involving sedentary older adults assigned to one of four groups: three experimental interventions (FS, ART, or FS&ART) and one control group. The randomization aimed for a 1:1 allocation ratio, ensuring equal chances for all participants to be assigned to any of the four groups. The evaluations were conducted twice: once before the intervention period and again after the 16-week intervention period.
Participants
The study protocol received approval from the Chengdu Institute of Physical Education ethical committee (ID 100/2024). Participants were fully informed about the study design and protocol and provided their consent by signing a voluntary informed consent form. The study adhered to ethical guidelines for research involving human subjects as defined in the Declaration of Helsinki.
The inclusion criteria were established before recruiting participants. These criteria were: (i) aged between 60 and 70 years old; (ii) not having engaged in regular physical exercise or a program for at least three years prior to the study; (iii) lacking significant physical limitations that could impair mobility or the ability to exercise; (iv) not having chronic diseases for which physical exercise might be risky; (v) having familiarity with swimming, including basic water comfort, proficiency in basic swimming skills, and breath control; and (vi) attending all evaluation appointments and maintaining at least 80% adherence to the total number of intervention sessions.
The recruitment process included informing community centers and elder care homes in the region to ensure they received pertinent information identifying key aspects of the program. Additionally, announcements were made in community spaces such as local supermarkets and other public places to reach as many potential volunteers as possible. This communication effort extended over 4 weeks with the goal of maximizing volunteer participation.
Among the 114 participants who initially expressed their willingness to volunteer for the current study, 34 were excluded because they failed to meet at least one of the inclusion criteria. Specifically, 11 were over 71 years old, 15 did not possess the basic swimming skills required for participation in the classes, and 8 had severe health conditions such as uncontrolled hypertension, severe respiratory issues, or severe neurological disorders. Therefore, 80 participants were deemed eligible to be assigned to the groups.
Interventions
The interventions were conducted twice a week for 16 consecutive weeks (Fig. 1). The sessions took place in conditioned indoor swimming pools. Both the FS and ART groups participated in classes with no more than 8 participants each, supervised by two coaches per group to ensure the interventions’ effectiveness. Schedules were adjusted according to participants’ availability, ensuring they maintained two weekly sessions with at least a 24-hour rest period between them. In the case of the FS&ART group, they had one weekly session of FS and one weekly session of ART.
Study design. FS: free swimming; ART: aquatic resistance training.
Both the FS and ART classes began similarly with a standardized dry-land warm-up involving a general mobilization of the upper and lower limbs. This warm-up consisted of 5 min of slow, dynamic movements for the upper limbs and trunk, followed by 5 min of slow movements for the lower limbs.
The FS group engaged in high-intensity interval training, performing free-swimming exercises as follows: from weeks 1 to 5, they completed 6 sets of 30-second all-out front crawl intervals, with 3 min of passive recovery between sets. Between weeks 6 and 10, the group increased to 8 sets of the same intervals and recovery duration. Finally, from weeks 11 to 16, the number of sets rose to 10, while maintaining the other parameters. Throughout all sessions, two coaches monitored the participants, providing verbal encouragement during activities and assessing their effort and commitment.
The ART group sessions were conducted in a pool heated to 29 to 32ºC, with the water level set approximately at xiphoid depth. The pool depth ranged from 1.2 to 1.5 m, and participants were individually positioned in the pool according to their height to ensure the water reached the xiphoid depth. The participants were kept distanced from the wall of the swimming. Previous study has shown that this level enhances muscular engagement31. The ART group utilized resistance aquatic hand paddles (Jinhua Rundoer Sports Goods Co., Ltd., China) for upper limb exercises and hydro-tone hydro boots for lower limb exercises. Each session consisted of two consistent exercises for the upper limbs and trunk: Chest Press/Upper Back and Water Arm Lifts. Lower limb and trunk exercises included Leg Lifts, Squats, and Jumps. All exercises were performed with maximal effort and full range of motion to ensure maximum intensity. Similar to the FS group, during weeks 1 to 5, participants completed 6 sets (a set per exercise) of 30-second all-out ART intervals with 3 min of passive recovery between sets. From weeks 6 to 10, the group increased to 8 sets of the same intervals and recovery duration. In this case, all the exercises were performed in one set, except for the chest press and squats, which were increased to two sets each. Finally, from weeks 11 to 16, they performed 10 sets (two sets each of chest press, upper back exercises, squats, and jumps, and one set each of water arm lifts and leg lifts) while maintaining the same parameters.
Throughout all sessions, two coaches monitored the participants, offering feedback to enhance exercise performance, providing verbal encouragement during activities, and evaluating their effort and dedication. In both experimental groups, participants were also familiarized with exertion scales. They were instructed to perform the exercises at maximum effort, aiming for a perception of “very, very hard” (19–20 arbitrary units) on the 6–20 Borg scale. Because maximum effort is highly individualized, the number of repetitions per set for ART and the distance swum for FS were also dependent on each participant’s level and therefore varied.
The control group continued with their regular daily routines and were not enrolled in an experimental group. Throughout the intervention period, they were contacted twice to monitor their activities and ascertain if they had voluntarily begun any physical exercise training programs.
Outcomes
In a controlled indoor facility with a standardized temperature of 22ºC and relative humidity of 50%, assessments were conducted. Evaluations took place in the morning. Each participant completed all assessments during a single morning session on the same day. Participants followed a consistent evaluation procedure throughout. Initially, they provided demographic information, and completed the International Physical Activity Questionnaire32. Subsequently, they underwent blood pressure measurements and anthropometric assessments, including height, body mass, hip and waist circumference, and fat mass estimation using a body composition analyzer.
Afterwards, participants participated in a standardized warm-up session comprising 5 min of walking, 5 min of upper limb and trunk mobility exercises, 5 min of lower limb mobility exercises, and 5 min of balance exercises. Following the warm-up, participants underwent assessments for senior fitness test, starting by the chair stand test, arm curl test, chair sit and reach test, back scratch test, 8-foot up and go test, and concluding with the six-minute walk test. The sequence of assessments was the same for all participants, with a 3-minute passive rest period provided between each test.
Anthropometry
Participants wore either sports shorts or sport pants along with a t-shirt during the assessments. Height (cm) was measured using a stadiometer (model 213, SECA) with a precision of 1 mm. Participants stood barefoot, with heels together, back straight against the vertical measuring rod, and head positioned to ensure accurate marking of the top of the head by the sliding horizontal bar.
Body mass (kg) and fat percentage (%) were assessed using a multifrequency segmental body composition scale (DC430-PMA, Tanita). Hip circumference (cm) was measured at the widest part of the buttocks, while waist circumference (cm) was measured at the narrowest part of the torso using a flexible tape measure for precise measurement.
These evaluations were conducted by two experienced assessors, each with more than three years of experience in performing these specific assessments.
Blood pressure
Blood pressure assessments utilized an automated sphygmomanometer (Omron M2 Compact, HEM 7102-E), following standardized procedures33. Participants were seated comfortably with their arm supported at heart level. After a 5-minute rest period, two consecutive readings were taken at 1-minute intervals. The average of these readings was then recorded as the participant’s blood pressure measurement, encompassing both systolic (mmHg) and diastolic (mmHg) values for further analysis.
Chair stand test
Following standardized protocols34, the Chair Stand Test, adapted from the Senior Fitness Test, was administered. Participants were instructed to rise from a 44 cm high chair without armrests using only their legs. A dedicated observer monitored each participant individually, recording the number of chair stands completed within a 30-second period using a counter. The final count (n) was documented for subsequent data analysis.
Arm curl test
The Arm Curl Test, adapted from the Senior Fitness Test, adhered to established protocols34. Participants seated on a chair held a weight in one hand with a suitcase grip (palm facing towards the body), positioning their arm vertically beside the chair. They were instructed to stabilize the upper arm against the body while performing bicep curls through a full range of motion. Women used a 5-pound dumbbell, while men used an 8-pound dumbbell, aiming to complete as many curls as possible within a 30-second timeframe. An observer dedicated to each participant recorded the number of curls completed using a counter. The final count (n) was documented for further analysis of arm strength.
Chair sit and reach test
Following standard procedures34, the Chair Sit and Reach Test, part of the Senior Fitness Test, was administered. Participants were seated on a 44 cm high chair, with one leg extended straight and the other foot flat on the floor. They reached forward with both hands along a measuring tape positioned over the extended leg, striving to achieve maximum stretch while maintaining straight knees. Participants were instructed to hold the reach for 2 s. An observer focused solely on one participant at a time recorded the distance reached between the fingertips and the toes in centimeters. This measurement (n) was utilized to assess participant flexibility further.
Back scratch test
Conducted according to established protocols34, the Back Scratch Test, part of the Senior Fitness Test, was administered. Participants were instructed to stand comfortably with one arm reaching over the shoulder and down the middle of the back, and the other arm reaching up the middle of the back and over the shoulder. The distance between the middle fingers of both hands was measured using a ruler, where positive numbers indicated overlap and negative numbers indicated a gap between the fingers. Prior to the recorded evaluation, participants underwent two familiarization trials. The final score (cm) was based on the most accurate measurement rounded to the nearest centimeter.
The 8-foot up and go test
Following standardized procedures34, the 8-Foot Up and Go Test was administered. Participants began seated in a chair with their back against the chair’s backrest. At the signal “Go”, they rose from the chair, walked briskly to a cone positioned 8 feet away, turned around, returned to the chair, and sat down. Timing commenced with the initial movement and ceased upon the participant’s return to a fully seated position. Each participant completed a practice trial for familiarization, followed by a recorded trial for evaluation. The best recorded time in seconds was utilized for subsequent analysis.
Six-minute walk test
Conducted according to standardized procedures34, the six-minute walk test was administered. Participants were instructed to walk along a rectangular course spanning 45.72 m, with cones marking intervals along the path. Participants were encouraged to walk at their own pace and could take breaks if necessary. An observer timed the duration and recorded the distance covered to the nearest meter. The total distance (in meters) walked within the six-minute period was subsequently recorded for further analysis.
Sample size
Based on previous studies with an effect size of 0.18035, and using an effect size f of 0.47, a power of 0.95 was calculated for four groups, two measurements, and a significance level of 0.05. Using G*Power (version 3.1.9.6.), a recommended total sample size of 24 was retrieved for repeated measures, specifically for within-between interactions.
Randomization
The simple randomization process was conducted manually: an equal number of opaque envelopes, each labeled with a group name, were prepared. A researcher then randomly picked an opaque envelope to assign participants to groups. Allocation concealment was maintained by conducting the randomization and group assignments before the first evaluation, keeping these assignments consistent throughout the study.
Blinding
In this study, only the evaluators were blinded to the participants’ assignments. They were unaware of which participants belonged to each group as they were solely involved in the evaluation process.
Statistical methods
An investigation into the comparative performance of the FS, ART, FS&ART and control groups at two distinct time points employed a mixed ANOVA, aiming to uncover potential interactions between group dynamics and assessment intervals. Prior to analysis, the assumptions of normal distribution and variance homogeneity were validated using the Kolmogorov-Smirnov test (p > 0.05) and Levene’s test (p > 0.05), respectively. Post hoc examinations utilized the Bonferroni test to identify specific group differentials, while effect size was assessed via partial eta squared. Additionally, Cohen’s d was used to determine the effect size in pairwise comparisons, with the following interpretation based on the magnitude of the differences36: <0.2, trivial; 0.2–0.6, small; 0.6–1.2, moderate; 1.2–2.0, large; 2.0–4.0, very large; and > 4.0, nearly perfect. All statistical computations were conducted using IBM SPSS Statistics (Version 27.0, IBM Corp., Armonk, NY), with a significance level of p < 0.05 set for all analyses.
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