In the last few decades, there has been a significant increase in the prevalence of obesity. In 2016, more than 1.9 billion adults were overweight and 650 million were obese worldwide. The global prevalence of obesity nearly tripled between 1975 and 2016.¹ Obesity increases the risk of type 2 diabetes mellitus, metabolic syndrome, cardiovascular disease, and mortality.²

South Korea experienced rapid socioeconomic growth during the 20th century, and its national income has been increasing until the present. Subsequent changes in eating habits and decreased physical activity in Koreans have led to overall body weight gain.^3,4 The prevalence of obesity in Korea increased dramatically from 26.0% in 1998 to 32.5% in 2013 within the last 20 years.⁵ Obesity increases health-related medical costs. Although socioeconomic costs in excess of $1,787 million have been incurred to address adult obesity in Korea, the prevalence of obesity continues to increase.⁶

To treat obesity and reduce weight, various methods have been used such as medication, behavior therapy, dietary restriction, increased physical activity, and exercise.⁷ However, it is not easy for individuals to lose weight since socioeconomic status and psychological background are involved. Social support plays a crucial role in boosting self-esteem and fostering body acceptance among women to a greater extent than among men.⁸ Hence, there is a need for a weight loss strategy tailored specifically to the female population.

According to the United States surgeon general, approximately 25% of American adults are completely sedentary, and more than 60% are not regularly active at the recommended level of 30 minutes per day.⁵ Therefore, physicians are attempting to develop various exercise methods that can effectively result in weight loss. One clinical trial found that the combination of aerobic exercise and resistance training resulted in greater weight loss and better glucose control than aerobic training alone.^9,10 Another clinical trial reported that combined exercise resulted in similar weight loss with only aerobic or only resistance exercise, and increased functional status of the subjects.¹¹ Nevertheless, further evidence collection is essential to substantiate the application of exercise prescriptions to patients.

Despite the acknowledged significance and necessity of integrated exercise, the design and practical implementation of exercise programs remain challenging in the field.^12,13 Meanwhile, Curves is an established chain of fitness center franchises providing a 30-minute workout program exclusively for women in a supportive environment. The Curves exercise circuit program includes the use of hydraulic resistance machines, and women move from one machine to the next with aerobic exercise in between, such as jogging in place or using a mini trampoline. The program provides the opportunity for exercise beginners and for women with tight schedules to get a workout. In addition, the supportive setting helps promote participation. The purpose of this study was to determine the effectiveness of the combination program of aerobic and resistance exercises by analyzing the effect of the Curves program on weight loss, and to measure whether it has practical effects on women’s exercise prescriptions.

Subjects

Women who registered between January 2015 to December 2015 in a supervised exercise program known as the Curves program and participated for more than a year were enrolled in this study. Data on metabolic or cardiovascular disorders, such as diabetes and hypertension, were not collected. Among the 12,643 women, we excluded women aged older than 60 years or younger than 20 years, and those whose body measurements were missing. Ultimately, 9,128 women aged between 20 and 60 years were included in our analysis. We limited the study population to women because we wanted to determine whether this particular exercise program aimed at women and combining aerobic and anaerobic exercise in a supportive setting would work in a pragmatic real-world environment. Our study received an Institutional Review Board exemption from the International St. Mary’s Hospital, Catholic Kwandong University, on January 4, 2018, under project number IS17OISI0077.

Exercise protocol & intensity

Each workout consisted of 12 pneumatic or hydraulic resistance exercises in a concentric circle. Participants were instructed to complete as many repetitions as possible in 30 seconds for each piece of equipment. In an interval fashion, they performed floor-based aerobic exercises for 30 seconds after each resistance exercise in an effort to maintain an exercise heart rate that corresponded to 60% to 80% of their maximum heart rate. Heart rate was measured via palpation of the radial artery or carotid artery during exercise. Participants were required to complete two circuits and stretch every part of the body, which took approximately 30 minutes. All workouts were supervised by trained fitness instructors, who provided information about the proper use of all equipment and assisted in maintaining the exact exercise technique. Attendance was recorded for each workout to monitor compliance with the exercise program.

According to related research,¹⁴ the Curves program falls under the category of moderate exercise intensity. Engaging in exercise at this intensity and duration aligns with the recommended guideline of 30 minutes of moderate physical activity. Notably, the energy expenditure derived from the Curves circuit program has the potential to contribute to more than 50% of the American College of Sports Medicine recommendations.¹⁵ These recommendations aim for a 300 kcal/day caloric deficit, facilitating the promotion and maintenance of weight loss specifically in sedentary and lower level aerobically fit females.¹⁴

Anthropometrics

Weights were measured with the subjects wearing light clothing. Body mass index (BMI) was calculated as weight in kilograms divided by height in meters squared. Underweight, normal weight, preobesity (overweight), obesity I, and obesity II or obesity III were defined as BMI <18.5, 18.5–22.9, 23.0–24.9, 25.0–29.9, and ≥30.0 kg/m², respectively.¹ Based on a study conducted on Korean citizens registered in the National Health Insurance Service Database, the aforementioned criteria were considered cutoff values. This decision was based on the alignment of valid criteria with the calculated cutoff values, which were derived from an increase in the hazard ratio for three comorbid diseases (type 2 diabetes mellitus, hypertension, and dyslipidemia).¹⁶ Waist circumference (WC) was measured to the closest 0.1 cm in an upright posture at the midpoint between the lower margin of the last palpable rib and the top of the iliac crest. Abdominal obesity was defined as a WC greater than 85 cm in women.¹ The average value of more than two measurements was used for all items recorded at baseline and follow-up.

Primary and secondary outcomes

The primary outcomes assessed included achieving weight loss thresholds of more than 5% and more than 10%. Additional analyses were conducted on various outcomes such as weight (in kilograms), WC (in centimeters), and BMI. Furthermore, changes in obesity class were extracted during the follow-up period to evaluate the effectiveness of weight maintenance.

Statistical analysis

The data are presented as mean±standard deviation for continuous variables and as percentage and 95% confidence interval for categorical variables. Women were divided into the following four groups according to exercise frequency: <1, 1, 2, and ≥3 days/week.

We performed one-way analysis of variance for continuous variables and the chi-squared test for categorical variables. Logistic regression analysis was used to calculate the odds ratio of reaching more than 5% or more than 10% weight loss among the different exercise groups. P-values less than 0.05 were considered to indicate significance, and all the statistical analyses were performed using SPSS version 15.0 (SPSS Inc.).

The baseline characteristics of the participants according to exercise frequency are shown in Table 1. Of the 9,128 participants, 644, 2,618, 3,199, and 2,667 women exercised <1, 1, 2, and ≥3 days/week, respectively. The women who exercised most frequently were older, homemakers, and married. Height, weight, WC, BMI, and obesity class were significantly different among the groups. The average follow-up duration was 625.3±151.2 days.

When comparing the groups by exercise frequency, a dose-response relationship was found between the exercise frequency and the reduction in weight, WC, and BMI. A more than 5% weight reduction was achieved in 12.7% of the <1 day/week group, 14.8% of the 1 day/week group, 15.0% of the 2 days/week group, and 17.2% of the ≥3 days/week group. The WC decreased by –0.66±4.50, –0.91±5.28, –1.43±4.78, and –1.65±4.98 for the <1, 1, 2, and ≥3 days/week groups, respectively (Table 2).

The odds ratios of more than 5% and more than 10% weight reduction between the exercise groups are shown in Table 3. After adjustment for age, BMI, marital status, profession, area, and duration of exercise, the odds ratios for weight reduction increased among the increased frequency exercise groups.

Fig. 1 presents the changes in the obesity class during the follow-up period. The proportion of women with a normal weight who maintained their weight was 95%, whereas 2.6% of women with a normal weight became pre-obese. The proportion of pre-obese women who achieved a normal weight was 32%, whereas 63.8% of the pre-obese women maintained their weight and 3.6% of the pre-obese women changed to obesity I. The proportion of women with obesity I who achieved a normal weight was 4.5%, whereas 23.5% of women with obesity I became pre-obese, 70.5% of women with obesity I maintained their weight, and 1.3% of women with obesity I changed to obesity II or obesity III. The proportion of women with obesity II or obesity III who achieved a normal weight was 0.8%, whereas 1.9% became pre-obese, 30.6% had obesity I, and 66.7% maintained their weight.

A significant dose-response relationship was found between the amount of exercise and weight loss. In the clinical field, the weight reduction goal is defined as losing more than 5% of the initial weight in the short term of within 6 months, or more than 10% of the initial weight in the period of 1 year or more.^2,3 Our results showed that more than 20% of the individuals in the ≥3 days/week group achieved a weight reduction greater than 5%. Moreover, an increase in exercise frequency was associated with a statistically significant reduction in WC. The majority of these weight loss effects persisted throughout the follow-up period. Additionally, each parameter exhibited a dose-response relationship with exercise frequency.

The combined aerobic and resistance training program was effective for long-term weight loss and maintenance.¹⁷ This finding supports the recommendation that frequent exercise should be included in the management of obesity.

Our findings in women are in agreement with previous studies demonstrating that combined exercise was effective for weight loss and for >1-year of long-term weight maintenance.^2,3

The primary characteristics of the Curves program include the following: (1) enhanced accessibility and convenience of exercise; (2) integrated circuit training that combines both aerobic and resistance exercises; and (3) tailored specifically to women’s fitness.

The distinctive feature of the Curves program lies in its provision of a convenient, appointment-free environment exclusively for women, enabling short-duration, pressure-free exercise sessions at nearby centers. With fixed exercise times and intensity levels, the program encourages frequent visits to the center, increasing participation frequency.

In our study, exercise frequency was categorized into four groups, with three or more sessions down to fewer than one session per week. This categorization stems from the recognition that patients’ actual exercise engagement in clinical settings often falls significantly below the World Health Organization guidelines, which recommend a minimum of 150 minutes of medium-intensity physical activity or 75 minutes of high-intensity physical activity weekly.¹⁸ While self-reported satisfaction with these recommendations was noted by 65% of the National Health and Nutrition Examination Survey population, objective accelerometry measurements indicated that only 5% met the recommended activity levels.¹⁹ Given the challenge of meeting these guidelines in clinical settings, our study design designated the group engaging in medium-intensity exercise for more than 90 minutes per week as the most active group, aligning with practical clinical compliance. Notably, our findings demonstrated a dose-response relationship between weight loss and exercise frequency below guideline recommendations, signifying significant results.

Individuals visiting clinics often find it challenging to achieve the recommended physical activity levels in their daily lives, prompting questions about the potential benefits of brief, infrequent exercise compared to no exercise at all. Our study revealed a statistically significant dose-response relationship with weight loss, even at below guideline-recommended activity levels, providing valuable evidence for addressing such inquiries in real clinical settings.

Furthermore, our study indicated that the weight loss effects were sustained (Fig. 1). Given the high dropout rate of approximately 50% among adults who initiate exercise within the first 6 to 12 months,²⁰ our findings suggest that exercise contributes to enhancing adherence to maintain weight loss.

The combination of both aerobic and resistance exercise in the circuit program not only enhances patient compliance but also contributes to tangible improvements in weight loss and physical function. Individuals often face a dilemma when initiating exercise, deliberating between choosing aerobic or resistance exercises. The ideal choice is to implement both types of exercise, but this is difficult without any guidance. The circuit program’s incorporation of combined exercises streamlines this decision-making process at the outset, offering psychological benefits by allowing participants to enjoy the effects of both exercise modalities.

Theoretically, the combination of aerobic and resistance exercises may potentially interfere with each other’s physiological benefits. Aerobic exercise enhances cardiovascular adaptation by increasing peak oxygen consumption without significantly impacting strength, whereas resistance exercises increase strength without greatly increasing maximum oxygen consumption.²¹ However, according to the results of the 2017 randomized controlled trial conducted in elderly individuals,¹¹ a combination of both exercise modalities had a more pronounced effect on weight loss and functional improvement than did individual exercise modalities. This suggests that the combined program not only surpasses the efficacy of aerobic-only exercise for weight loss but also further enhances overall physical function.

Gender differences play a role in the effects of exercise and the factors influencing weight loss. Existing studies indicate that a supportive environment is crucial for women to maintain a low BMI,⁸ and there is evidence suggesting that sustaining weight loss through continuous exercise is more challenging for women than for men.²² Considering these points, there is a need for quantitative research specifically focusing on the effectiveness of exercise programs tailored to women.

This study has several strengths. First, our data were from a large number of participants who exercised longer than 1 year. Second, the women were supervised by trained fitness instructors. Third, we also assessed weight loss maintenance. A period of more than 1 year was appropriate to determine the weight maintenance effect of exercise.^4,5 Fourth, by analyzing real-world data and producing results, we have contributed to the creation of evidence that can be applied directly to real-world clinical practice.

Our study has several limitations. First, our study was not a randomized clinical trial, and baseline characteristics were not identical among the different exercise groups. Women in the ≥3 days/week group were older than those in the <1 day/week group. Despite being older, those in the ≥3 days/week group experienced more weight loss. However, using real-world data, we found complex results in an actual clinical setting. Second, dietary composition among the different groups was not examined. Dietary modifications, such as caloric restriction or a high-calorie diet, influence weight loss. Third, body composition and functional state, including energy expenditure and maximal oxygen consumption (VO₂max), were not measured because lean body mass, fat mass, and peak oxygen uptake are useful for estimating the effectiveness of an exercise. Fourth, most of our study participants were normal in weight. However, detailed analysis revealed that a limited number of individuals within the normal BMI range transitioned during the follow-up results to the underweight category (Fig. 1). This mitigated concerns about the potential for excessive weight loss and increased morbidity among subjects within the normal weight range. Finally, adverse effects of exercise, such as injury, fatigue, or musculoskeletal disease, were not recorded.

While the benefits of exercise are widely acknowledged, it is imperative to develop an exercise program that not only captures but also maintains frequent engagement in these activities. Additionally, it is equally crucial to conduct research on the effectiveness of such programs. Our study revealed that a woman-centric, short-session, combined exercise program proved effective in promoting weight loss within a real-world setting. Moving forward, further clinical trials will likely be needed, taking into account factors such as calorie restriction, intermittent fasting, and modified diets, such as low-fat or low-carb diets.

The study results verified that there is a statistically significant link between the Curves program—a women-targeted exercise regimen characterized by high accessibility and combined aerobic and resistance exercise training—and effectiveness in weight loss and the maintenance of weight loss.

The authors declare no conflict of interest.

We are grateful to the study participants for their contributions.

Study concept and design: BO; acquisition of data: HJO; analysis and interpretation of data: HJK, HJO, and BO; drafting of the manuscript: JSL and HJH; critical revision of the manuscript: JSL; statistical analysis: HJH, YHE, and BC; obtained funding: BO; administrative, technical, or material support: HJK and HJO; and study supervision: BO.