Effects of 10-Week Online High-Intensity Interval Training on Body Composition and Aerobic Performance during the COVID-19 Lockdown

1. Introduction

The COVID-19 pandemic has ushered in unprecedented challenges, significantly disrupting daily routines and prompting profound changes in lifestyle patterns worldwide [1]. Government-imposed lockdowns and restrictions on mobility have led to a surge in sedentary behaviors among individuals, exacerbating concerns about health and well-being [2]. Sedentary lifestyles are associated with various health issues, including increased risks of cardiovascular diseases, obesity, and mental health disorders [3]. The pandemic's impact on physical activity underscores the critical need for innovative and accessible exercise solutions that can be implemented under restrictive conditions.

High-intensity interval training (HIIT) is estimated to be one of the most popular trends in fitness, according to the American College of Sports Medicine (ACSM) Annual Fitness Trend Forecast [4]. Indeed, HIIT is characterized by repeated short-term explosive-intensity anaerobic activities (≥85-90% maximal oxygen uptake [VO2 max] for health subjects or ≥80% VO2 max for clinical populations), interspersed by periods of passive or low-intensity exercise recovery, and it typically takes less than 30 minutes per training session [5,6]. In recent years, HIIT versus traditional moderate-intensity continuous exercise has been recognized as an alternative and popular exercise approach to further improve waist circumference, body fat (%), resting heart rate, aerobic fitness, and cardio-metabolic health in a shorter time in sedentary individuals [7,8,9].

Likewise, due to its anaerobic condition, HIIT training also offers great improvements in VO2 max which benefits both aerobically and anaerobically in a very short time compared to other methods [10]. Therefore, previous studies suggested that HIIT could be the solution to improve health and reduce morbidity in the adult population [11,12]. This type of training is tolerable and acceptable with physically sedentary people, although at first, it generated a lot of controversy in people with little training due to the high intensity that this mode of training entails. However, in recent years it has been prescribed to the elderly, young people, and adolescents [13]. Therefore, it must be taken into account that these HIIT training programs can cause a greater feeling of fatigue due to a higher ratio of perceived exertion (RPE) in comparison with the classic continuous training of moderate intensity [14].

The context of the COVID-19 pandemic, which disrupted regular exercise routines, underscores the relevance of understanding the impact of online HIIT during periods of restricted mobility and social distancing. While previous literature has predominantly delivered HIIT in a presential approach [7], the present study explores the novel approach of online delivery, considering the unique challenges and opportunities it presents. Therefore, the present study may contribute valuable insights into promoting accessible and effective exercise strategies, particularly during times of restricted mobility and social isolation.

Understanding how an online HIIT approach can impact individuals during a period of restricted mobility and social distancing is highly relevant. For the above reasons, the purpose of this study was to investigate the effects of a 10-week online HIIT intervention on body composition and aerobic performance variables in healthy physically sedentary women.

2. Materials and Methods

2.1. Experimental Approach to the problem

A parallel two-group, longitudinal (pre, post) design was used with physical tests performed before (preintervention) and after (postintervention) the 10-week intervention period. The participants were assigned and matched into two groups, an experimental group (EG) and a control group (CG). The participants from the CG were asked to maintain their daily life routines, while those from the EG were introduced to a one-hour HIIT familiarization session before the start of the intervention. Regarding the day of training, it was preceded by 48 hours of absence of high effort. The study was conducted between January and March 2021. (See Table 1, for more information).

2.2. Participants

A total of forty-eight physically sedentary women were recruited to participate in this study. There were two groups. Thus, the EG with twenty-four women (n = 24, age 21.21 ± 2.15 years, weight 61.16 ± 8.94 kg, height 163.96 ± 4.87 cm, body mass index body weight 22.69 ± 2.49 kg/m²), and the CG with another twenty-four women (n = 24, age 20.50 ± 1.29 years, weight 62.0 ± 6.65 kg, height 163.92 ± 4.89 cm, body mass index 23.04 ± 1.74 kg/m²). The randomization sequence was generated electronically (https://www.randomizer.org) and was concealed until interventions were assigned.

Moreover, a priori sample size calculation was performed using a free online tool, G*Power (www.gpower.hhu.de), with a power level of 95% and an α level of 0.05 and based on previous and similar studies [15] revealed that a sample size of >36 would be sufficient for conducting a randomized controlled trial. The sample group for this study was selected using the criterion sampling method, which falls under the category of purposive sampling techniques [16]. The inclusion criteria of the participants in this study were: i) being physically sedentary (not engaging in 150 minutes per week or moderate intensity, or 75 minutes per week of vigorous intensity, or a combination equivalent to these two different intensities); ii) not presenting any injury; iii) giving their consent, and iv) participating in at least 90% of the training sessions during the intervention.

Finally, the participants obtained information on the main objectives of the research and signed the informed consent. All the participants in this research were treated according to the guidelines of the American Psychological Association (APA), therefore the anonymity of the participants' responses was guaranteed. The study was carried out in accordance with the ethical principles of the Declaration of Helsinki for research in humans and was approved by the Research Ethics Committee of the Pontifical University of Comillas (2021/85).

2.3. Procedures

The forty-eight physically sedentary women enrolled in this study visited the laboratory twice, both for pre-test and post-test assessments, consistently between 9:30 am and 5:00 pm. Heart rate (HR) was monitored throughout all sessions. During each session, participants underwent a sub-maximal incremental fitness test on a stationary ergometer. Adhering to the guidelines outlined by the American College of Sports Medicine (2018), measures were taken to ensure the safety of physically sedentary women. Body mass, measured without shoes, utilized a bioelectrical impedance analysis (BIA) device (Tanita BC-730) accurate to the nearest 0.1 kg. Height was measured using a stadiometer (Type SECA 225, Hamburg, Germany) accurate to the nearest 0.1 cm. Body mass index was computed as mass (in kilograms) divided by the square of height (in meters).

All participants performed a sub-maximal incremental fitness test on a cycle ergometer (Viasprint 150 P cycle-ergometer) connected Jaeger Master Screen gas analyzer. Determination of the ventilatory anaerobic threshold (VAT) was based on the respiratory gas exchange method (RER) (RER = CO2 production/O2 consumption), which detected the VAT at the point at which RER exceeds the cut-off value of 1.0 [17]. First, an RS800CX Polar monitor (Polar Electro, Finland) was used to monitor and record heart rate (HR) during both sessions. In this sense, the protocol consisted of a submaximal incremental test with a fixed cadence of 60 revolutions per minute (rpm). The warm-up started at 0 W and the workload was increased by 10 W every min until min 5. The participant began the exercise phase pedaling at 50 W, and the workload was increased to 25 W every two min. After each increase, the workload remained stable for the next 2 minutes. The submaximal test ended once the VAT was reached. The highest power output (W) reached during the cycle ergometer test was registered. Finally, the highest power output reached was relativized by each participant's body weight (W/kg).

2.4. Training program

Concerning the training regimen, the Experimental Group (EG) engaged in a 10-week online program, involving three sessions per week. The sessions comprised High-Intensity Interval Training (HIIT) following the Tabata method, involving 4-minute intervals with 8 intensive training blocks, each followed by 1 minute of recovery [18]. The exercises involved body weight and functional movements, aligning with a protocol that demonstrated favorable outcomes in female university students [8]. Notably, the last session of each week focused on continuous running, as outlined in Table 2.

In the first month, they exercised for 20 seconds of intense intervals with 10 seconds of recovery, repeating it for 4 minutes. In the second month, they increased the technical difficulty of the exercises and increased the volume of the sessions until they achieved 20 minutes of Tabata in the last week of the intervention.

The HIIT sessions were carried out twice a week and their intensity were monitored daily using both HR. Participants were instructed to perform the exercise at maximum intensity while maintaining the correct technique. The structure of the session consisted of a 10–15-minute warm-up, a 35-minute main part, and a 10-minute cooldown. On the third day of each training week, the participants completed a moderate-intensity session outdoors, combining continuous running with walking. In the initial weeks of the exercise program, the intensity was within the 65-75% HRmax range. As the weeks progressed, the intensity gradually increased, and in the final week, it was within the 80-90% HRmax range, and the intensity of the walking was always at 40-45% HRmax during the 10 weeks.

Moreover, the rate of perceived exertion (RPE) was measured using the Borg’s Scale (6-20) as a supplement of the HR during each training session. Each participant was individually asked: "How did you perceive exertion during the exercise execution?". The first four weeks had an RPE of 12-13, indicating a somewhat hard level. The following four weeks were at an RPE of 15-16, signifying a hard level. In the last two weeks, the RPE was 18-19, representing an extremely hard level.

Table 3. Example of HIIT and running session and work set progression during the HIIT program.

Example of weekly session
	1st session	2nd session	3rd session
Exercises	HIIT. Example 1	HIIT. Example 2	Running
1st	Push-ups	Jumping jacks	5 minutes of running at 65-75% HRmax
2nd	Mountain Climbers	Box squat	5 minutes of walking at 40-45% HRm
3rd	Forearm plank	Bench step-ups	5 minutes of running at 65-75% HRmax
4th	Dips using a bench or chair	Deadweight	5 minutes of walking
Training progression during the HIIT program
Week	Set/session	Recovery between sets (s)
1st and 2nd	2	60
3rd to 5th	3	60
6th to 8th	4	60
9th and 10th	4	45

Table 3. Example of HIIT and running session and work set progression during the HIIT program.

Example of weekly session
	1st session	2nd session	3rd session
Exercises	HIIT. Example 1	HIIT. Example 2	Running
1st	Push-ups	Jumping jacks	5 minutes of running at 65-75% HRmax
2nd	Mountain Climbers	Box squat	5 minutes of walking at 40-45% HRm
3rd	Forearm plank	Bench step-ups	5 minutes of running at 65-75% HRmax
4th	Dips using a bench or chair	Deadweight	5 minutes of walking
Training progression during the HIIT program
Week	Set/session	Recovery between sets (s)
1st and 2nd	2	60
3rd to 5th	3	60
6th to 8th	4	60
9th and 10th	4	45

Technical progressions in exercise complexity were systematically implemented throughout the experimental period. The progression for push-ups involved starting with hands against the wall, followed by positioning knees on the floor, and advancing to performing the exercise without resting the knees. Planks transitioned from forearms on the floor to arms fully extended. Similarly, jumping jacks initially began as static and progressed to dynamic, while squats advanced from using a box to incorporating a jump. This systematic approach extended to all exercises. In the initial two weeks, participants engaged in 8 minutes of Tabata, gradually increasing the volume as their physical condition improved, ultimately reaching a 20-minute Tabata duration.

Due to the COVID-19 confinement regulations mandated by the Spanish government, the training program had to be conducted inside participants' homes, where the absence of equipment led to the selection of calisthenics exercises.

2.5. Statistical procedures

Data were analyzed using Statistica software (version 10.0; Statsoft, Inc., Tulsa, OK, USA). Finally, the significance level was set at p < 0.05. Normal distribution and homogeneity tests (Kolmogorov–Smirnov and Levene’s, respectively) were conducted on all metrics. A paired sample t-test was used for determining differences as a repeated measures analysis (pre and post). Cohen’s d was the effect size indicator. To interpret the magnitude of the effect size, we adopted the following criteria: d ≤ 0.20, small; d ≤ 0.50, medium; and d ≤ 0.80, large. To elucidate between-group differences, an ANCOVA test was performed using the pretest as a covariate and the times pre and post as factors. To interpret the magnitude of the effect size of ANCOVA we adopted the following criteria: ηp² = 0.02, small; ηp² = 0.06, medium; and ηp² = 0.14, large.