Impact of exercise intensity during detraining on physical fitness and muscle functions in obese women post high-intensity interval training: A preliminary analysis

Abstract

Background

Regular physical exercise helps reduce body fat and improve obesity-related health issues by affecting various health indicators. However, 80% of individuals who intentionally lost weight experienced a rebound effect, commonly known as the yo-yo effect, within one year. Detraining, the cessation or reduction of exercise, can lead to rapid decreases in aerobic function and a decline in various physiological parameters.

Objective

This study aims to investigate whether varying exercise intensities during the detraining period can maintain improved body composition, physical fitness, and isokinetic muscle function in obese adult women after high-intensity interval training (HIIT).

Methods

Participants were divided into three groups: non-training (NTG, n = 6), low-intensity (LIG, n = 6), and moderate-intensity (MIG, n = 7) exercise groups. For eight weeks, the HIIT program was applied three times a week, followed by a four-week detraining period with varying exercise intensities. Body composition, physical fitness, and isokinetic muscle function were measured before and after the HIIT and detraining periods.

Results

Body composition, particularly waist circumference, hip circumference, and the waist-to-height ratio, significantly improved in the MIG compared to the control group. The MIG also positively affected muscular endurance, agility, and cardiorespiratory endurance. However, no positive effect on isokinetic muscle function was detected in any of the groups.

Conclusions

This study emphasizes the importance of continuous moderate-intensity exercise during the detraining period to maintain the anti-obesity effects of an 8-week HIIT program.

Keywords

Detraining phases exercise intensity obesity physical fitness

1 Introduction

Obesity is a condition characterized by excessive increases in the size and number of fat cells in the body caused by various factors, such as eating habits, lack of sleep, physical activity, medications, genetics, and family history. Excessive fat accumulation can lead to metabolic diseases such as heart disease, stroke, depression, diabetes, and musculoskeletal disorders.¹ Although dietary, exercise, and drug approaches have been proposed as therapeutic strategies for preventing and managing obesity, the importance of exercise therapy has been emphasized in terms of economy and effectiveness.² For women, given the unique challenges related to hormonal changes and reproductive health, special attention is required in the prevention and management of obesity to effectively alleviate these risks.

Regular physical exercise aids in weight loss by reducing body fat, and a 5–10% weight loss in obese individuals can improve obesity-related health problems by positively affecting fasting blood sugar, glycosylated, systolic and diastolic blood pressure, and blood lipids.^3,4 Previous clinical trials have demonstrated that high-intensity physical exercise increases skeletal muscle mass, leading to weight loss through the enhancement of energy metabolism.⁵ However, within one year, 80% of individuals who intentionally lost more than 10% of their body weight experienced the yo-yo effect.^6–8 Thus, maintaining regular exercise is crucial for overcoming this issue, but sports injuries and environmental changes that occur during the training period can make it difficult to continue exercise. According to strength and condition researches, detraining is defined as a phenomenon where exercise is stopped or reduced after long-term engagement.⁹

Previous studies on detraining have shown that the maximum oxygen uptake (VO₂max), a cardiopulmonary capacity indicator, decreased by 4–14% within a 4-week detraining period, and the total blood and plasma volume declined by 5–12%.^4,10 Additionally, lipoprotein lipase activity and GLUT-4 protein content in skeletal muscle decreased by 17–33% during 6–10 days of detraining.^4,11,12 It has also been suggested that lipoprotein activity in skeletal muscle decreases by 45–75%, while body fat accumulation increases by 86%.^11–13 These findings indicate that aerobic exercise functions, such as cardiovascular endurance and maximum oxygen uptake (VO₂ max), decline rapidly within two weeks of detraining, whereas anaerobic exercise capacity remains relatively stable for four weeks of detraining. Most obesity-related detraining studies have focused solely on changes in exercise effects during detraining after regular exercise. However, understanding how different exercise intensities during the detraining period can sustain health benefits remains limited. Therefore, the purpose of this study is to investigate whether varying exercise intensities during detraining can help maintain improvements in body composition, physical fitness, and isokinetic muscle function in obese adult women after high-intensity interval training (HIIT).

2 Methods

2.1 Participants and study design

To determine the optimal sample size for this study, the minimum sample size (Effect size = 0.35, Cohen's f, Power = 0.97, size = 40) was calculated using G*Power 3.1.¹⁴ The study participants were fifty-six adult women who had no history of cardiovascular or musculoskeletal diseases in the past 6 months. As shown in Figure 1, after excluding those with a body fat percentage of less than 30% (n = 6) or greater than 50% (n = 5), as measured using a body composition analyzer, forty-five individuals participated.¹⁵ During the 8-week high-intensity training, exercise was discontinued due to COVID-19 infection (n = 16) and personal reasons (n = 5), leaving 21 participants who completed the high-intensity training. The participants were divided into a non-training group (NTG, n = 7), a low-intensity exercise group (LIG, n = 7), and a moderate-intensity exercise group (MIG, n = 7) after HIIT. During the 4-week detraining period, one participant from the NTG and one from the LIG dropped out, resulting in 19 participants completing the detraining phase. With the reduced sample size of 19, the post-hoc power analysis indicated a power of approximately 0.80. Written consent was obtained from the study participants, and the study design was approved by the Institutional Review Board of Jeju National University (JJNU-IRB-2022-022). The characteristics of the participants based on exercise intensity during the 4 weeks of this study are shown in Table 1.

Figure 1.

The experimental design.

Table 1.

Characteristics of participants based on exercise intensity during the 4-week period.

Variables	NTG(n = 6)	LIG(n = 6)	MIG(n = 7)	Total	F	p
Age (yr)	20.33 ± 1.75	22.17 ± 1.17	20.86 ± 2.27	21.11 ± 1.88	1.624	.228
Height (cm)	162.8 ± 6.41	162.5 ± 0.94	161.47 ± 7.62	162.22 ± 6.16	.076	.927
Weight (kg)	62.35 ± 7.95	62.07 ± 4.19	63.44 ± 6.02	62.66 ± 5.91	.090	.915
SMM (kg)	21.20 ± 2.70	21.10 ± 1.43	21.57 ± 2.05	21.31 ± 2.01	.090	.914
BFM (kg)	23.36 ± 4.97	21.99 ± 1.82	23.09 ± 4.59	22.82 ± 3.89	.192	.827
BMI (kg/m²)	24.55 ± 1.57	23.67 ± 1.95	22.91 ± 2.87	23.67 ± 2.23	1.504	.252
%BF	37.28 ± 4.84	35.47 ± 2.54	36.27 ± 5.28	36.34 ± 4.26	.250	.782
WC (cm)	80.67 ± 5.33	76.05 ± 4.47	80.04 ± 6.19	78.98 ± 5.52	1.298	.300
HC (cm)	98.57 ± 3.49	96.15 ± 3.16	98.50 ± 1.56	97.78 ± 2.87	1.488	.255
WHtR (%)	.50 ± .02	.47 ± .03	.50 ± .05	.49 ± 0.04	1.171	.335
WHR (%)	.82 ± .04	.79 ± .03	.81 ± .06	.81 ± 0.05	.555	.585

NTG: Non-training group; LIG: Low-intensity exercise group; MIG: Moderate-intensity exercise group; SMM: Skeletal muscle mass; BFM: Body fat mass; BMI: Body mass index; %BF: Percent body fat; WC: Waist circumference; HC: Hip circumference; WHtR: Waist-to-height ratio; WHR: Waist-to-hip ratio.

2.2 Exercise program

The exercise program of this study consisted of Zone-I and Zone-II. Zone I applied the principle of progressive load, 3 times a week for 8 weeks, a rating of perceived exertion (RPE; Borg Scale) of 8–18, an exercise-rest ratio of 1:1–3, and an exercise duration of 10–30 s. Zone II was established three times a week during the detraining period (4 weeks) by applying different exercise intensities for each group. LIG was applied with RPE 8–10, the exercise-rest ratio was 1:3, the exercise time was 20–30 s, and MIG was applied with RPE 12–14, the exercise-rest ratio was 1:2, and the exercise time was 20–30 s.¹⁶ The exercise program used in this study is shown in Table 2.

Table 2.
Exercise program.

Session Exercise Intensity Time Frequency

Warm-up Dynamic stretching 5 min 3 days a week

Running 5 min

Main exercise Squats Zone I RPE: 8–12 Exercise: 10–30 (sec)

(2 Weeks) Rest: 20–90 (sec)

Butt Kicks RPE: 12–18 Exercise: 20–30 (sec)

(6 Weeks) Rest: 20–90 (sec)

High Knee Zone II Low RPE: 8–10 Exercise: 10–30 (sec)

(4 Weeks) Rest: 60–90 (sec)

Scissors Jump Moderate RPE: 12–14 Exercise: 10–30 (sec)

(4 Weeks) Rest: 20–60 (sec)

Cool-down Static stretching 5 min

Session	Exercise	Intensity	Time	Frequency
Warm-up	Dynamic stretching			5 min	3 days a week
	Running			5 min
Main exercise	Squats	Zone I	RPE: 8–12	Exercise: 10–30 (sec)
			(2 Weeks)	Rest: 20–90 (sec)
	Butt Kicks		RPE: 12–18	Exercise: 20–30 (sec)
			(6 Weeks)	Rest: 20–90 (sec)
	High Knee	Zone II	Low RPE: 8–10	Exercise: 10–30 (sec)
			(4 Weeks)	Rest: 60–90 (sec)
	Scissors Jump		Moderate RPE: 12–14	Exercise: 10–30 (sec)
			(4 Weeks)	Rest: 20–60 (sec)
Cool-down	Static stretching	5 min

Table 3.

Comparison before and after 8-week of high-intensity exercise.

	Variable	Base	8-week test	Total	t	p
Body composition	Weight (kg)	64.81 ± 6.28	62.66 ± 5.91	63.74 ± 6.10	11.071	.001
	SMM (kg)	22.04 ± 2.14	21.31 ± 2.01	21.67 ± 2.07	11.128	.001
	BFM (kg)	23.99 ± 4.29	22.82 ± 3.89	23.41 ± 4.09	3.861	.001
	BMI (kg/m²)	24.81 ± 2.61	23.98 ± 2.39	24.40 ± 2.50	10.276	.001
	%BF	36.91 ± 4.48	36.34 ± 4.26	36.62 ± 4.37	1.324	.202
	WC (cm)	81.80 ± 6.01	78.98 ± 5.52	80.39 ± 5.76	7.500	.001
	HC (cm)	99.53 ± 2.74	97.78 ± 2.87	98.66 ± 2.81	5.100	.001
	WHtR (%)	.51 ± .04	.49 ± .04	.50 ± .04	5.895	.001
	WHR (%)	.82 ± .04	.81 ± .05	.81 ± .04	3.059	.007
Physical fitness	GS (kg)	26.07 ± 5.72	28.17 ± 6.57	27.12 ± 6.15	−3.814	.001
	BS (kg)	55.80 ± 15.12	69.65 ± 16.02	62.73 ± 15.57	−7.586	.001
	TF (cm)	14.44 ± 8.53	18.26 ± 9.29	16.35 ± 8.91	−2.591	.018
	SU (rep)	15.26 ± 9.29	21.26 ± 8.81	18.26 ± 9.05	−5.948	.001
	VJ (cm)	23.21 ± 3.41	26.47 ± 3.69	24.84 ± 3.55	−5.815	.001
	STS (rep)	33.00 ± 5.73	34.89 ± 7.87	33.95 ± 6.80	−1.511	.148
	PEI (%)	47.50 ± 3.87	49.97 ± 4.14	48.74 ± 4.00	−3.479	.003
Knee peak torque 60˚/sec	Right extensors (%BW)	140.63 ± 42.41	161.68 ± 28.17	151.16 ± 35.29	−3.013	.007
	Left extensors (%BW)	147.68 ± 31.09	167.32 ± 31.44	157.50 ± 31.27	−2.862	.010
	BBRE (%)	13.37 ± 9.78	7.79 ± 6.86	10.58 ± 8.32	1.989	.062
	Right flexors (%BW)	63.53 ± 24.81	82.26 ± 20.57	72.89 ± 22.69	−4.371	.001
	Left flexors (%BW)	67.84 ± 23.09	80.47 ± 19.33	74.16 ± 21.21	−2.554	.020
	BBRF (%)	16.00 ± 15.11	8.42 ± 8.58	12.21 ± 11.84	1.785	.091
	Right H: Q Ratio (%)	45.95 ± 8.13	51.05 ± 10.14	48.50 ± 9.13	−2.391	.028
	Left H: Q Ratio (%)	45.79 ± 13.08	48.53 ± 9.18	47.16 ± 11.13	−1.009	.326
Trunk peak torque 30˚/sec	Extensors (%BW)	160.42 ± 41.87	154.42 ± 38.52	157.42 ± 40.20	.971	.345
	Flexors (Nm)	165.42 ± 52.36	177.79 ± 41.64	171.61 ± 47.00	−1.894	.074
	Flexors (%BW)	257.32 ± 69.65	279.63 ± 54.02	268.47 ± 61.83	−2.154	.045
	F: E Ratio (%)	63.79 ± 18.86	55.74 ± 15.19	59.76 ± 17.03	2.442	.025

SMM: skeleton muscle mass; BFM: body fat mass; BMI: body mass index; %BF: percent body fat; WC: waist circumference; HC: hip circumference; WHtR: waist to height ratio; WHR: waist to hip ratio; GS: grip strength; BS: back strength; TF: trunk flexion; SU: sit-up; VJ: vertical jump; STS: sit-to-stand; PEI: physical efficiency index; Nm: newton meter; % BW: percent body weight; BBRE: bilateral balance ratio for extensor; BBRF: bilateral balance ratio for flexor; H: Q ratio: hamstring to quadriceps ratio; F: E ratio: flexor to extensors ratio

2.3 Measurements

Physique and body composition were measured using equipment such as the height and weight scale (DS-103 M, Dong San Jenix, Seoul, Korea) and Inbody 770 (Inbody, Seoul, Korea), respectively. The participants visited the laboratory at 9:00 am in a fasted state on three occasions: at baseline, after 8 weeks of training, and after an additional 4 weeks of detraining, to conduct the measurements. Circumferences were obtained using a “Picco” tape measure (Hoechstmass Balzer GmbH, Sulzbach, Germany). Waist circumference (WC) was measured at the narrowest point between the rib cage and the top edge of the iliac crest, specifically during exhalation. Hip circumference (HC) was recorded at the level of the greater trochanters, where the hips are the widest. The waist-to-height ratio (WHtR) was calculated by dividing WC (cm) by height (cm), while the waist-to-hip ratio (WHR) was determined by dividing WC (cm) by HC (cm).

Grip and back strength were measured using a digital dynamometer (T.K.K. 5101 and T.K.K. 5402, TAKEI, Japan). Two measurements were conducted, and the maximum value was recorded in units of 0.1 kg. Muscular endurance and flexibility were assessed using a sit-up device (T.K.K. 5505, TAKEI, Japan) and a sit-and-reach device (T.K.K. 5111, Takei, Japan), respectively. The sit and reach test involved two trials, and the maximum distance traveled was recorded in cm after the participants held their posture for 3 s. Cardiorespiratory endurance was measured using the Physical efficiency index (PEI) score through the Harvard step test. Participants first measured resting heart rate, and then repeatedly step up and down on a 45 cm box at a speed of 120 bpm for 5 min. After the end of the test, heart rates were measured at 1.5, 2.5, and 3.5 min.¹⁷ The PEI value was calculated using the long-form equation - fitness index formula as follows: Fitness index = (100 × test duration in seconds) / (2 × sum of heartbeats in the recovery periods).

Isokinetic strength of the knee extensors and flexors were evaluated at 60°/s¹⁸ while trunk muscle strength was measured at 30°/sec using the HUMAC NORM (Humac Norm 776, CSMI, Boston, USA).¹⁹ To minimize interference from other muscle groups, belts secured the chest and femur, and fixation devices stabilized the back, chest, waist, and thighs. For isokinetic maximal strength of the knee, participants completed over three preliminary movements to understand the process and then they were tested through three repetitions of extension and flexion at 60°/sec.

Vertical jump height was recorded twice with a digital vertical jump device (DW 771A, SKARO, Korea). In the sit-to-stand test, participants sat on a chair (height: 43.2 cm), arms crossed at chest level, and hands on their shoulders, and they repeatedly sat and stood for 30 s while fully extending their knees. Each stand-up, starting from a sitting position, was counted as a repetition, and the maximum number of repetitions was recorded.²⁰

2.4 Statistical analysis

For the measurement data of this study, the normality test was conducted using the Shapiro-Wilk test with the SPSS for Windows statistical program (Version 22; IBM, Armonk, NY), and the mean and standard deviation for all variables were calculated. To confirm the effect of high-intensity exercise, a paired-sample t-test was conducted comparing results before participation in exercise (0 weeks) and after high-intensity exercise (8 weeks). Isokinetic muscular functions, body composition and physical fitness during detraining were analyzed by two-way repeated-measures ANOVA at the end of high-intensity exercise (8 weeks) and after detraining (4 weeks) to confirm the main effects of group, time, and the group × time interaction according to exercise intensity and differences between groups. Variables that exhibited significant differences between groups after 8 weeks of high-intensity exercise were controlled for prior variables using analysis of covariance (ANCOVA). One-way ANOVA was conducted to confirm the amount of change within the group, and the effect size was confirmed through a post-hoc test (Scheffe-HSD). The statistical significance level of all analyses was set at P < 0.05.

3 Results

3.1 Changes in physiological and physical factors after 8 weeks of HIIT

Eight weeks after HIIT, significant changes were observed in physical fitness (Table 3), except for body fat percentage (t = 1.324, p = .202) and the sit-to-stand test (t = -1.511, p = .148). Regarding the isokinetic knee strength, most variables showed significant differences, excluding the bilateral balance ratio for the extensor (BBRE, t = 1.989, p = .062), the bilateral balance ratio for the flexor (BBRF, t = 1.785, p = .091), and the left hamstring: quadriceps ratio (H:Q ratio, t = -1.009, p = .326). For trunk muscle strength, significant differences were noted in both flexor strength (t = -2.154, p = .045) and the H:Q ratio (t = 2.442, p = .025).

3.2 Changes in body composition by exercise intensity during detraining period

During the detraining period, as shown in Table 4, no significant differences were noted between groups for body weight (F = 1.959, p = .173), skeletal muscle mass (F = 1.955, p = .174), body fat mass (F = 2.372, p = .125), body mass index (F = .853, p = .445), or body fat percentage (F = .914, p = .421). NTG and LIG showed a trend of maintaining or increasing these measures post-HIIT, while MTG showed a decrease. Waist circumference (F = 6.008, p = .011) and hip circumference (F = 9.634, p = .002) were significantly lower in LIG and MIG compared to NTG. The waist-to-height ratio (F = 4.587, p = .027) was significantly decreased in MIG compared to NTG, while no significant difference was found in the waist-hip ratio (F = 1.993, p = .169).

Table 4.
Changes in body composition and circumference with exercise intensity during 4-week detraining.

Variable Group 8-week test 12-week test ANOVA(p), η² Post-hoc t p

Body weight (kg) NTG^a 62.35 ± 7.95 65.27 ± 7.85 G .722, .040 − −10.561 .001

LIG^b 62.07 ± 4.19 62.40 ± 4.46 P .017, .306 −1.356 .233

MIG^c 63.44 ± 6.02 58.59 ± 5.63 G × P .001, .944 10.981 .001

Skeletal muscle mass (kg) NTG^a 21.20 ± 2.70 22.19 ± 2.67 G .723, .040 − −10.610 .001

LIG^b 21.10 ± 1.43 21.22 ± 1.52 P .016, .311 −1.363 .231

MIG^c 21.57 ± 2.05 19.92 ± 1.92 G × P .001, .944 11.062 .001

Body fat mass (kg) NTG^a 23.36 ± 4.97 24.28 ± 4.50 G .531, .076 − −1.838 .126

LIG^b 21.99 ± 1.82 21.90 ± 2.48 P .005, .399 .206 .845

MIG^c 23.09 ± 4.59 19.83 ± 3.75 G × P .001, .771 8.908 .001

Body mass index (%) NTG^a 23.44 ± 1.60 24.55 ± 1.57 G .957, .005 − −8.958 .001

LIG^b 23.54 ± 1.86 23.67 ± 1.95 P .032, .255 −1.371 .229

MIG^c 24.83 ± 3.28 22.91 ± 2.87 G × P .001, .917 8.552 .001

Percent body fat (%) NTG^a 37.28 ± 4.84 37.09 ± 4.44 G .692, .040 − .261 .805

LIG^b 35.47 ± 2.54 35.13 ± 3.54 P .006, .347 .469 .659

MIG^c 36.27 ± 5.28 33.79 ± 4.96 G × P .224, .153 4.699 .003

Waist circumference (cm) NTG^a 80.67 ± 5.33 83.72 ± 4.68 G .135, .222 b,c < a −6.437 .001

LIG^b 76.05 ± 4.47 76.90 ± 4.60 P .677, .011 −3.437 .018

MIG^c 80.04 ± 6.19 75.60 ± 4.10 G × P .001, .780 4.230 .006

Hip circumference (cm) NTG^a 98.57 ± 3.49 100.80 ± 2.72 G .082, .268 b,c < a −3.686 .014

LIG^b 96.15 ± 3.16 97.02 ± 2.71 P .640, .014 −1.872 .120

MIG^c 98.50 ± 1.56 94.97 ± 1.77 G × P .001, .815 7.345 .001

Waist to height ratio (%) NTG^a .50 ± .02 .52 ± .02 G .207, .179 c < a −5.398 .003

LIG^b .47 ± .03 .47 ± .03 P .547, .023 −1.581 .175

MIG^c .50 ± .05 .47 ± .04 G × P .001, .802 4.861 .003

Waist to hip ratio (%) NTG^a .82 ± .04 .83 ± .04 G .400, .108 − −1.941 .110

LIG^b .79 ± .03 .79 ± .03 P .822, .003 −.307 .771

MIG^c .81 ± .06 .80 ± .03 G × P .122, .232 1.297 .242

Variable	Group	8-week test	12-week test		ANOVA(p), η²	Post-hoc	t	p
Body weight (kg)	NTG^a	62.35 ± 7.95	65.27 ± 7.85	G	.722, .040	−	−10.561	.001
LIG^b	62.07 ± 4.19	62.40 ± 4.46	P	.017, .306	−1.356	.233
MIG^c	63.44 ± 6.02	58.59 ± 5.63	G × P	.001, .944	10.981	.001
Skeletal muscle mass (kg)	NTG^a	21.20 ± 2.70	22.19 ± 2.67	G	.723, .040	−	−10.610	.001
LIG^b	21.10 ± 1.43	21.22 ± 1.52	P	.016, .311	−1.363	.231
MIG^c	21.57 ± 2.05	19.92 ± 1.92	G × P	.001, .944	11.062	.001
Body fat mass (kg)	NTG^a	23.36 ± 4.97	24.28 ± 4.50	G	.531, .076	−	−1.838	.126
LIG^b	21.99 ± 1.82	21.90 ± 2.48	P	.005, .399	.206	.845
MIG^c	23.09 ± 4.59	19.83 ± 3.75	G × P	.001, .771	8.908	.001
Body mass index (%)	NTG^a	23.44 ± 1.60	24.55 ± 1.57	G	.957, .005	−	−8.958	.001
LIG^b	23.54 ± 1.86	23.67 ± 1.95	P	.032, .255	−1.371	.229
MIG^c	24.83 ± 3.28	22.91 ± 2.87	G × P	.001, .917	8.552	.001
Percent body fat (%)	NTG^a	37.28 ± 4.84	37.09 ± 4.44	G	.692, .040	−	.261	.805
LIG^b	35.47 ± 2.54	35.13 ± 3.54	P	.006, .347	.469	.659
MIG^c	36.27 ± 5.28	33.79 ± 4.96	G × P	.224, .153	4.699	.003
Waist circumference (cm)	NTG^a	80.67 ± 5.33	83.72 ± 4.68	G	.135, .222	b,c < a	−6.437	.001
LIG^b	76.05 ± 4.47	76.90 ± 4.60	P	.677, .011	−3.437	.018
MIG^c	80.04 ± 6.19	75.60 ± 4.10	G × P	.001, .780	4.230	.006
Hip circumference (cm)	NTG^a	98.57 ± 3.49	100.80 ± 2.72	G	.082, .268	b,c < a	−3.686	.014
LIG^b	96.15 ± 3.16	97.02 ± 2.71	P	.640, .014	−1.872	.120
MIG^c	98.50 ± 1.56	94.97 ± 1.77	G × P	.001, .815	7.345	.001
Waist to height ratio (%)	NTG^a	.50 ± .02	.52 ± .02	G	.207, .179	c < a	−5.398	.003
LIG^b	.47 ± .03	.47 ± .03	P	.547, .023	−1.581	.175
MIG^c	.50 ± .05	.47 ± .04	G × P	.001, .802	4.861	.003
Waist to hip ratio (%)	NTG^a	.82 ± .04	.83 ± .04	G	.400, .108	−	−1.941	.110
LIG^b	.79 ± .03	.79 ± .03	P	.822, .003	−.307	.771
MIG^c	.81 ± .06	.80 ± .03	G × P	.122, .232	1.297	.242

NTG^a: non-training group; LIG^b: low-intensity exercise group; MIG^c: moderate-intensity exercise group.

3.3 Changes in physical fitness by exercise intensity during detraining period

As shown in Table 5, upon examining the changes in physical fitness attributes based on exercise intensity during the detraining period, no significant differences were found between groups in grip strength (F = .124, p = .884), back strength (F = 2.281, p = .134), and flexibility (F = 1.442, p = .266). However, compared to the NTG, the MIG showed a significant increase in sit-ups (F = 4.945, p = .021), vertical jumps (F = 12.195, p = .001), and sit-to-stand tests (F = 5.421, p = .016). Furthermore, the body efficiency index (F = 6.742, p = .008) was significantly higher in both the LIG and MIG than in the NTG.

Table 5.
Changes in physical fitness with exercise intensity during 4-week detraining.

Variable Group 8-week test 12-week test ANOVA(p), η² Post-hoc t p

Grip strength (kg) NTG^a 27.77 ± 9.66 28.23 ± 8.76 G .421, .102 − −.714 .057

LIG^b 27.50 ± 3.11 29.17 ± 3.04 P .487, .031 −1.326 .242

MIG^c 29.09 ± 6.51 29.94 ± 5.47 G × P .107, .244 −1.238 .262

Back strength (kg) NTG^a 64.18 ± 22.89 60.57 ± 21.38 G .905, .012 − 1.968 .106

LIG^b 69.58 ± 10.23 70.20 ± 12.13 P .071, .189 −.446 .674

MIG^c 74.40 ± 13.78 78.97 ± 11.67 G × P .646, .053 −3.386 .015

Trunk flexion (cm) NTG^a 17.62 ± 14.59 13.80 ± 8.28 G .294, .142 − .994 .366

LIG^b 17.47 ± 9.12 24.98 ± 18.24 P .561, .022 −1.210 .280

MIG^c 19.49 ± 3.04 19.13 ± 3.61 G × P .005, .480 .713 .503

Sit-up (reps) NTG^a 17.17 ± 8.89 15.33 ± 7.71 G .141, .217 a < c 2.015 .100

LIG^b 23.67 ± 4.32 22.67 ± 3.33 P .211, .096 1.369 .229

MIG^c 22.71 ± 11.24 27.71 ± 8.67 G × P .001, .665 −4.494 .004

Vertical jump (cm) NTG^a 24.33 ± 4.50 22.17 ± 3.87 G .083, .267 a < c 7.050 .001

LIG^b 26.17 ± 2.23 25.67 ± 1.63 P .094,.165 .745 .490

MIG^c 28.57 ± 3.16 29.71 ± 2.36 G × P .001, .809 −2.489 .047

Sit to stand (reps) NTG^a 33.17 ± 9.11 30.00 ± 6.03 G .010, .434 a < c 1.591 .172

LIG^b 35.67 ± 8.87 36.83 ± 6.21 P .098, .162 −.442 .677

MIG^c 35.71 ± 6.85 41.57 ± 6.66 G × P .001, .585 −4.330 .005

Physical efficiency index (%) NTG^a 17.17 ± 8.89 15.33 ± 7.71 G .132, .202 a < b,c 3.632 .015

LIG^b 23.67 ± 4.32 22.67 ± 3.33 P .019, .269 .557 .602

MIG^c 22.71 ± 11.24 27.71 ± 8.67 G × P .017, .363 −6.688 .001

Variable	Group	8-week test	12-week test		ANOVA(p), η²	Post-hoc	t	p
Grip strength (kg)	NTG^a	27.77 ± 9.66	28.23 ± 8.76	G	.421, .102	−	−.714	.057
LIG^b	27.50 ± 3.11	29.17 ± 3.04	P	.487, .031	−1.326	.242
MIG^c	29.09 ± 6.51	29.94 ± 5.47	G × P	.107, .244	−1.238	.262
Back strength (kg)	NTG^a	64.18 ± 22.89	60.57 ± 21.38	G	.905, .012	−	1.968	.106
LIG^b	69.58 ± 10.23	70.20 ± 12.13	P	.071, .189	−.446	.674
MIG^c	74.40 ± 13.78	78.97 ± 11.67	G × P	.646, .053	−3.386	.015
Trunk flexion (cm)	NTG^a	17.62 ± 14.59	13.80 ± 8.28	G	.294, .142	−	.994	.366
LIG^b	17.47 ± 9.12	24.98 ± 18.24	P	.561, .022	−1.210	.280
MIG^c	19.49 ± 3.04	19.13 ± 3.61	G × P	.005, .480	.713	.503
Sit-up (reps)	NTG^a	17.17 ± 8.89	15.33 ± 7.71	G	.141, .217	a < c	2.015	.100
LIG^b	23.67 ± 4.32	22.67 ± 3.33	P	.211, .096	1.369	.229
MIG^c	22.71 ± 11.24	27.71 ± 8.67	G × P	.001, .665	−4.494	.004
Vertical jump (cm)	NTG^a	24.33 ± 4.50	22.17 ± 3.87	G	.083, .267	a < c	7.050	.001
LIG^b	26.17 ± 2.23	25.67 ± 1.63	P	.094,.165	.745	.490
MIG^c	28.57 ± 3.16	29.71 ± 2.36	G × P	.001, .809	−2.489	.047
Sit to stand (reps)	NTG^a	33.17 ± 9.11	30.00 ± 6.03	G	.010, .434	a < c	1.591	.172
LIG^b	35.67 ± 8.87	36.83 ± 6.21	P	.098, .162	−.442	.677
MIG^c	35.71 ± 6.85	41.57 ± 6.66	G × P	.001, .585	−4.330	.005
Physical efficiency index (%)	NTG^a	17.17 ± 8.89	15.33 ± 7.71	G	.132, .202	a < b,c	3.632	.015
LIG^b	23.67 ± 4.32	22.67 ± 3.33	P	.019, .269	.557	.602
MIG^c	22.71 ± 11.24	27.71 ± 8.67	G × P	.017, .363	−6.688	.001

NTG^a: non-training group; LIG^b: low-intensity exercise group; MIG^c: moderate-intensity exercise group.

3.4 Changes in isokinetic muscle function by exercise intensity during detraining period

As shown in Table 6, upon examining the changes in isokinetic knee and trunk functions based on exercise intensity during the detraining period, a significant difference was found only in the relative value of the left knee extensor muscle strength (F = 4.062, p = .037). The post-test results showed a significantly greater increase in the MIG than in the LIG. The NTG showed a significant tendency to increase the relative value of the right extensor (t = -3.067, p = .028) at maximum knee strength, and the trunk extensor (t = -2.803, p = .038) post-intervention. Additionally, the MIG tended to increase the relative value of the right extensor (t = -2.723, p = .035) at maximum knee strength, and the relative values of the right flexor (t = -2.480, p = .048) and the trunk extensor (t = -2.823, p = .030) were significantly increased after the intervention compared to before. However, no significant change was found in the LIG.

Table 6.
Changes in isokinetic muscle function with exercise intensity during 4-week detraining.

Variable Group 8-week test 12-week test ANOVA(p), η² Post-hoc t p

Knee peak torque 60˚/sec

Right extensors (%BW) NTG^a 155.83 ± 12.42 133.50 ± 57.11 G .332, .115 − .892 .413

LIG^b 145.00 ± 32.95 156.00 ± 20.93 P .831, .003 −1.344 .237

MIG^c 181.00 ± 24.47 194.43 ± 26.18 G × P .567, .061 −2.723 .035

Left extensors (%BW) NTG^a 98.50 ± 19.52 105.33 ± 12.71 G .038, .335 b < c −1.433 .211

LIG^b 96.83 ± 25.86 100.83 ± 23.34 P .083, .176 −.698 .516

MIG^c 122.00 ± 26.37 125.00 ± 23.30 G × P .848, .020 −1.680 .144

Bilateral balance ratio for extensor (%) NTG^a 8.83 ± 1.83 7.33 ± 4.08 G .130, .225 − .681 .526

LIG^b 10.00 ± 10.20 6.33 ± 3.67 P .252, .081 .780 .471

MIG^c 5.00 ± 6.06 3.71 ± 1.80 G × P .842, .021 .619 .559

Right flexors (%BW) NTG^a 83.50 ± 20.50 92.33 ± 16.45 G .123, .231 − −3.067 .028

LIG^b 69.67 ± 20.95 76.83 ± 17.07 P .002, .452 −1.403 .220

MIG^c 92.00 ± 16.79 100.14 ± 23.53 G × P .956, .006 −2.480 .048

Left flexors (%BW) NTG^a 77.83 ± 11.75 85.33 ± 16.15 G .016, .404 − −1.973 .106

LIG^b 66.00 ± 18.13 76.00 ± 9.74 P .030, .261 −1.908 .115

MIG^c 95.14 ± 16.25 100.57 ± 22.06 G × P .173, .843 −.815 .446

Bilateral balance ratio for flexor (%) NTG^a 10.17 ± 8.08 10.17 ± 4.17 G .349, .123 − .000 1.000

LIG^b 11.67 ± 11.64 8.83 ± 8.13 P .695, .010 .407 .701

MIG^c 4.14 ± 4.30 10.43 ± 6.63 G × P .422, .102 −1.781 .125

Right H: Q Ratio (%) NTG^a 53.17 ± 9.60 53.33 ± 7.58 G .685, .046 − −.092 .930

LIG^b 48.17 ± 13.96 49.33 ± 8.62 P .828, .003 −.377 .721

MIG^c 51.71 ± 7.48 51.29 ± 8.24 G × P .890, .015 .208 .842

Left H: Q Ratio (%) NTG^a 49.00 ± 5.22 50.00 ± 10.39 G .123, .231 − −.385 .716

LIG^b 44.83 ± 12.69 49.33 ± 9.56 P .002, .452 −.765 .479

MIG^c 51.29 ± 8.56 52.57 ± 10.60 G × P .956, .006 −.358 .733

Trunk peak torque 30˚/sec

Extensors (%BW) NTG^a 149.33 ± 27.31 166.67 ± 24.97 G .128, .227 − −2.803 .038

LIG^b 133.17 ± 51.69 147.00 ± 38.45 P .024, .279 −1.359 .232

MIG^c 177.00 ± 23.02 183.43 ± 38.64 G × P .661, .050 −.712 .503

Flexors (%BW) NTG^a 264.50 ± 24.34 297.00 ± 40.50 G .115, .237 − −3.527 .017

LIG^b 266.17 ± 70.03 274.17 ± 44.62 P .006, .382 −4.19 .693

MIG^c 304.14 ± 55.34 343.14 ± 49.91 G × P .313, .135 −2.823 .030

F: E Ratio (%) NTG^a 264.50 ± 24.34 297.00 ± 40.50 G .610, .060 − −.056 .957

LIG^b 266.17 ± 70.03 274.17 ± 44.62 P .780, .005 −1.364 .231

MIG^c 304.14 ± 55.34 343.14 ± 49.91 G × P .086, .264 1.795 .123

Variable	Group	8-week test	12-week test		ANOVA(p), η²	Post-hoc	t	p
Knee peak torque 60˚/sec
	Right extensors (%BW)	NTG^a	155.83 ± 12.42	133.50 ± 57.11	G	.332, .115	−	.892	.413
		LIG^b	145.00 ± 32.95	156.00 ± 20.93	P	.831, .003		−1.344	.237
		MIG^c	181.00 ± 24.47	194.43 ± 26.18	G × P	.567, .061		−2.723	.035
	Left extensors (%BW)	NTG^a	98.50 ± 19.52	105.33 ± 12.71	G	.038, .335	b < c	−1.433	.211
		LIG^b	96.83 ± 25.86	100.83 ± 23.34	P	.083, .176		−.698	.516
		MIG^c	122.00 ± 26.37	125.00 ± 23.30	G × P	.848, .020		−1.680	.144
	Bilateral balance ratio for extensor (%)	NTG^a	8.83 ± 1.83	7.33 ± 4.08	G	.130, .225	−	.681	.526
		LIG^b	10.00 ± 10.20	6.33 ± 3.67	P	.252, .081		.780	.471
		MIG^c	5.00 ± 6.06	3.71 ± 1.80	G × P	.842, .021		.619	.559
	Right flexors (%BW)	NTG^a	83.50 ± 20.50	92.33 ± 16.45	G	.123, .231	−	−3.067	.028
		LIG^b	69.67 ± 20.95	76.83 ± 17.07	P	.002, .452		−1.403	.220
		MIG^c	92.00 ± 16.79	100.14 ± 23.53	G × P	.956, .006		−2.480	.048
	Left flexors (%BW)	NTG^a	77.83 ± 11.75	85.33 ± 16.15	G	.016, .404	−	−1.973	.106
		LIG^b	66.00 ± 18.13	76.00 ± 9.74	P	.030, .261		−1.908	.115
		MIG^c	95.14 ± 16.25	100.57 ± 22.06	G × P	.173, .843		−.815	.446
	Bilateral balance ratio for flexor (%)	NTG^a	10.17 ± 8.08	10.17 ± 4.17	G	.349, .123	−	.000	1.000
		LIG^b	11.67 ± 11.64	8.83 ± 8.13	P	.695, .010		.407	.701
		MIG^c	4.14 ± 4.30	10.43 ± 6.63	G × P	.422, .102		−1.781	.125
	Right H: Q Ratio (%)	NTG^a	53.17 ± 9.60	53.33 ± 7.58	G	.685, .046	−	−.092	.930
		LIG^b	48.17 ± 13.96	49.33 ± 8.62	P	.828, .003		−.377	.721
		MIG^c	51.71 ± 7.48	51.29 ± 8.24	G × P	.890, .015		.208	.842
	Left H: Q Ratio (%)	NTG^a	49.00 ± 5.22	50.00 ± 10.39	G	.123, .231	−	−.385	.716
		LIG^b	44.83 ± 12.69	49.33 ± 9.56	P	.002, .452		−.765	.479
		MIG^c	51.29 ± 8.56	52.57 ± 10.60	G × P	.956, .006		−.358	.733
Trunk peak torque 30˚/sec
	Extensors (%BW)	NTG^a	149.33 ± 27.31	166.67 ± 24.97	G	.128, .227	−	−2.803	.038
		LIG^b	133.17 ± 51.69	147.00 ± 38.45	P	.024, .279		−1.359	.232
		MIG^c	177.00 ± 23.02	183.43 ± 38.64	G × P	.661, .050		−.712	.503
	Flexors (%BW)	NTG^a	264.50 ± 24.34	297.00 ± 40.50	G	.115, .237	−	−3.527	.017
		LIG^b	266.17 ± 70.03	274.17 ± 44.62	P	.006, .382		−4.19	.693
		MIG^c	304.14 ± 55.34	343.14 ± 49.91	G × P	.313, .135		−2.823	.030
	F: E Ratio (%)	NTG^a	264.50 ± 24.34	297.00 ± 40.50	G	.610, .060	−	−.056	.957
		LIG^b	266.17 ± 70.03	274.17 ± 44.62	P	.780, .005		−1.364	.231
		MIG^c	304.14 ± 55.34	343.14 ± 49.91	G × P	.086, .264		1.795	.123

NTG^a: non-training group; LIG^b: low-intensity exercise group; MIG^c: moderate-intensity exercise group; % BW: percent body weight; H: Q ratio: hamstring to quadriceps ratio; F: E ratio: flexor to extensors ratio.

4 Discussion

This study aimed to determine the minimum exercise intensity required during a 4-week detraining period to maintain the physiological adaptations achieved by high-intensity interval training (HIIT) in obese adult women. High-intensity circuit exercises are adaptable to various fitness levels and effective for obese individuals, as shown in methods like Gibala or Tabata intervals.^21,22 Numerous studies have verified that high-intensity circuit exercise positively affects body composition and fitness, improving cardiovascular and muscular functions.^23–25 In this study, eight weeks of high-intensity circuit training significantly improved body composition and muscle function in obese women.^23,26

While many previous studies have demonstrated the effectiveness of regular circuit exercise, our study provides a novel approach to maintaining improved muscle functions during training cessation in obese women. This approach is particularly relevant for inactive populations after the cessation of HIIT. Previous studies suggest aerobic capacity decreases quickly after stopping high-intensity exercise, but muscle strength might last longer.⁹ Given that the detraining research findings primarily analyzed elite athletes, their applicability to obese individuals is limited. Thus, this study examines how different exercise intensities affect the retention of physiological adaptations in obese women, a demographic underrepresented in research. As a result, only the MIG during the detraining period maintained the improvements in body weight (BW), body fat mass (BFM), body mass index (BMI), body fat percentage (%BF), waist circumference (WC), hip circumference (HC), and skeletal muscle mass (SMM) in obese women after HIIT, unlike other groups. WC, HC, and the waist-to-height ratio (WHtR) were significantly lower in the MIG than in the NTG. According to a previous study,²⁴ changes in body composition after high-intensity exercise did not deteriorate within two weeks after training cessation. Moreover, moderate-intensity exercise might prevent aggravation in BW and %BF during a 6-week detraining period, which is consistent with our findings.

Regular physical exercise is widely recognized for its efficacy in preventing and improving various metabolic syndromes, including cardiovascular disease and diabetes. A high level of physical fitness achieved through exercise is a major index for predicting these metabolic diseases.^27–29 Our study demonstrated that moderate-intensity exercise during a 4-week detraining period effectively sustained muscular endurance, power, and cardiovascular fitness improvements achieved through HIIT compared to the control group. Both low- and moderate-intensity exercises preserved cardiorespiratory function, preventing decreases in VO₂max and cardiac output. This suggests practical exercise regimens for maintaining cardiovascular health in women. Future research should use graded exercise testing (GXT) for scientific verification.

The study indicated that moderate-intensity exercise during the detraining period significantly enhanced knee and trunk functions, critical for overall mobility and independence, particularly in obese women.” Interestingly, the NTG also showed an increase in the relative value of the right extensor at maximum knee strength and of the trunk extensor, suggesting benefits even when exercise is halted following HIIT.^30–33 The MIG further displayed a trend toward increased maximum knee strength and an increase in both the right flexor and trunk extensor post-intervention. These findings underline the importance of moderate-intensity exercise for enhancing knee and trunk function. However, the LIG showed no significant change, suggesting that low-intensity exercise may not suffice to induce notable improvements in these areas during detraining.^34,35 Thus, moderate-intensity exercise appears more effective than low-intensity exercise in maintaining muscular function during detraining. Overall, these findings highlight the critical role of exercise intensity during detraining, suggesting that moderate-intensity exercise is superior to low-intensity or unstructured training in improving these muscular functions. Such insights are vital for developing tailored exercise programs that ensure the maintenance of physical capabilities and prevent regression in muscle function during periods of detraining. Nonetheless, more research is needed to confirm the long-term effects of varying exercise intensities on overall physical performance and injury prevention.

This study suffers from two major limitations. First, due to the eruption of the COVID-19 epidemic that took place within the experiment period, the sample size was significantly reduced. Second, the detraining period was relatively short. This combination may have well affected the generalizability of the results. For these reasons, the study is described as ‘preliminary’.

5 Conclusions

With the above qualifications untaken into consideration, this research emphasizes the importance of regular exercise during the detraining period to maintain the physiological characteristics and physical fitness gains achieved through high-intensity training. It is recommended that continuing moderate-intensity exercise is vital for preserving the improved body composition and physical fitness obtained through HIIT during the detraining period.

Footnotes

Ethics approval and consent to participate

This study was approved by Jeju National University Institutional Review Board (JJNU-IRB-2022-022).

Author contributions

CONCEPTION: Joo-In, Yu

PERFORMANCE OF WORK: Joo-In, Yu, Yeong-Hyun Cho

INTERPRETATION OR ANALYSIS OF DATA: Joo-In, Yu, Yeong-Hyun Cho

PREPARATION OF THE MANUSCRIPT: Joo-In, Yu

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Tae-Beom Seo

SUPERVISION: Tae-Beom Seo

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ORCID iD

Tae-Beom Seo

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