Purpose: Investigating the effects of stressors and recovery regulation on the super-compensation effect of high-intensity intensive training (IT) in endurance athletes. Methods: This study will recruit 176 competitively trained endurance adult athletes. Participants will conduct a 7-week 3-stage experiment, including 3 weeks of regular training, 3 weeks of 130% progressive IT, and 1 week of 55% taper. Measurement will include training responses (performance and fatigue symptoms), stressors, recovery regulation, and mood state at baseline, during and after regular training, IT, and taper separately. According to the performance after taper, all participants will category into either the responder or non-responder of well-response to supercompensation effect. Statistic analysis: Independent t-test, Chi-squared test, and binary logistic regression will be used to compare the difference in training responses, stressors, recovery regulation, and mood state characteristics between responder and non-responder groups. P value sets at 0.15 for identifying the potential predictors. Logistic stepwise multiple regression will be used further to determine the significant predictors for the responders of well-response to the super-compensation among endurance athletes. P value sets at 0.05.

• Research problems and motivation:

  (1) Research problems: Currently, the primary challenge in sports training monitoring resides in the disparity of responses among athletes to identical high-intensity, high-volume training stimuli (e.g., total distance, frequency, intensity, workload) (Aubry et al., 2015; Aubry et al., 2014; Bellinger, 2020; Le Meur et al., 2014; Le Meur et al., 2013). Despite the widespread dissemination of preventative strategies for overtraining syndrome among coaches and athletes in current sports science research, non-functional overreaching and overtraining syndrome continue to exhibit a prevalence of approximately 19% to 30% in recent years.

  (2) Research motivation: Recent studies have found that following high-intensity, high-volume training, approximately 45.5% to 100% of individuals are categorized as athletes who fail to recover and show poor responses adequately (Aubry et al., 2015; Aubry et al., 2014; Bellenger, Karavirta, et al., 2016; Bellenger et al., 2017; Bellinger, 2020; Fuller et al., 2017; Le Meur et al., 2013; Woods et al., 2018). This phenomenon may be influenced by factors such as inadequate individual recovery capacity or excessive external stressors. Hence, there is an increased emphasis on understanding individual differences, including stressors and recovery regulatory characteristics. By observing the differences between athletes who successfully improve their performance (responders) and those who do not (non-responders) following high-intensity, high-volume training, it may be possible to enhance the precision of future applications of high-intensity, high-volume training in terms of effectiveness and safety.

• Study purpose and hypotheses:

  1. The purpose of this study are two:

     1. To investigate the training responses (sports performance and fatigue symptoms) post-intervention among endurance athletes who exhibit either compensatory training effects (responders) or fail to show such effects (non-responders) following high-intensity, high-volume training. Additionally, it will examine the differences in stressors, recovery regulatory, and emotional state characteristics between these two groups during the intervention period.
     2. To explore the impact of stressors and recovery regulatory characteristics on the presence or absence of compensatory effects following high-intensity, high-volume training among endurance athletes who either respond well or poorly to the intervention. The aim is to understand, through a simple yet comprehensive sideline measurement approach, the characteristics of athletes regarding their ability to adequately recover from fatigue states induced by high-intensity, high-volume training.

  2. The hypotheses of this study:

     1. Endurance athletes who exhibit either compensatory training effects (responders) or fail to show such effects (non-responders) in sports performance are expected to significantly differ in their training responses (sports performance and fatigue symptoms), stressors, recovery regulatory characteristics, and emotional state characteristics.
     2. Certain stressors and recovery regulatory characteristics are expected to have a significant influence on whether endurance athletes exhibit favorable compensatory training effects following high-intensity, high-volume training.

• Study procedure:

  1. Study procedures:

     The study will involve a 7-week experimental protocol divided into 3 phases: 3 weeks of regular training, 3 weeks of gradually increasing high-intensity training at 130%, and 1 week of reduced training at 55% (Bellinger, 2020). Athletes will undergo assessments before, during, and after each phase, including regular training, high-intensity training, and reduced training, to evaluate training responses (sports performance and fatigue symptoms), stressors, recovery regulatory characteristics, and emotional state. Based on athletes' sports performance after the reduced training phase, they will be categorized into two groups: those with favorable compensatory training effects (response group) and those without (non-response group). The detailed weekly training volume design is as follows:

     Stage One: 3 weeks of maintaining participants' regular training habits.

     Stage Two: 3 weeks of high-intensity training (increasing by 10% each week):

     Stage Three: 1 week of reduced training (gradually reduced to 55% of participants' regular training volume): Decrease current training volume followed by 20%, 15%, 10%, 10%, 10%, 5%, and 5% each day.

  (4) Measurement instruments: This study will utilize questionnaires and maximal exercise testing for measurement.

  1. Questionnaire Survey: Participants will be asked to complete the Chinese versions of the Profile of Mood States (POMS) (Hsu, Chang, & Lu, 2003) and the Recovery-Stress Questionnaire for Athletes (RESTQ-Sport) (Chen & Ji, 2002) after each week of training (e.g., every Saturday or Sunday). These questionnaires represent the participants' emotional states and stress-recovery regulatory characteristics. The total questionnaire will be administered 7 times, each session taking approximately 20 minutes.
  2. Maximal exercise testing: Maximal exercise testing will be conducted before and after each stage of the study, including at the beginning of the experiment, after the third week, after the fourth week, after the sixth week, and after the seventh week, totaling 5 maximal exercise tests. Each testing session will take approximately 30 minutes.

• Data collection:

  1. Measurement of Personal Data and Other Relevant Factors:

     1. Personal Data: Before the commencement of the experiment, participants will be asked to complete a personal information form in paper format. This form will record all participants' basic information, including age, height, weight, gender, education level, exercise level, sports specialization, sports performance, medical history, dietary habits, and sleep habits. Additionally, information on risk factors for overtraining syndrome (including exercise performance and fatigue symptoms) and other similar symptomatic diseases (including primary viral diseases, bacterial infections, related inflammatory diseases, diabetes, and hyperthyroidism) will be collected to gather participants' baseline characteristics and exclude overtraining syndrome and other similar symptomatic diseases.
     2. Training Records: During the experiment, all participants' daily training records (including training activities, intensity, and volume) will be electronically recorded to ensure that the expected training intensity is achieved during high-intensity training sessions.

  2. Primary Outcome Measures:

     1. Training response:

        - Performance: This study will conduct progressive maximal exercise tests, including before the experiment starts and after the third, fourth, sixth, and seventh weeks, totaling 5 tests. The results will record participants' resting heart rate, peak power output, maximum heart rate, maximal oxygen uptake, anaerobic threshold, and perceived fatigue index. The tests will be conducted using a progressive resistance cycle ergometer (Corival CPET, LODE, Netherlands). During the test, participants will undergo a 5-minute warm-up at 50 watts with a self-selected pace, followed by a progressive maximal cycle ergometer test. The test will start at 50 watts and increase by 20 watts every minute until the participant cannot maintain a pedaling speed above 10 seconds. After the test, the intensity will be reduced to 50 watts, and participants will pedal at a self-selected pace for 2 minutes, followed by an inquiry about their perceived fatigue index.

     2. Symptom Response:

        • Fatigue Symptoms: Participants will fill out the Chinese version of the Profile of Mood States (POMS) questionnaire after each week's training (e.g., on Sundays), recording their feelings over the past week. The abbreviated Chinese version of the POMS contains 37 mood-related adjectives and takes approximately 3 to 7 minutes to complete. The fatigue subscale score will reflect participants' fatigue symptoms.
        • Emotional State: Participants will complete the Chinese version of the Profile of Mood States (POMS) questionnaire, and the total score for emotional disturbance (= tension + depression + anger + confusion + fatigue - vigor) will be calculated to indicate their overall negative emotional disturbance state.

     3. Stress Recovery Regulation:

        • Stress Sources: Participants will fill out the Chinese version of the Recovery-Stress Questionnaire for Athletes (RESTQ-Sport) after each week's training (e.g., on Sundays), recording their feelings over the past three days. The abbreviated Chinese version of the RESTQ-Sport contains 52 items and takes approximately 15 minutes to complete. General stress and specific sport-related stress scores will reflect participants' stress source characteristics.
        • Recovery Regulation: Participants will complete the Chinese version of the Recovery-Stress Questionnaire for Athletes (RESTQ-Sport), and scores for general recovery and specific sport-related recovery will be calculated to reflect their recovery regulation characteristics.

• Statistical analysis

  1. Firstly, this study will utilize independent t-tests, chi-square tests, and binary logistic regression analyses to compare potential differences between the response (exercise performance and fatigue symptoms), stress sources, recovery regulation, and emotional states of the response and non-response groups. The significance level (P-value) will be set at 0.15.
  2. Secondly, logistic stepwise regression analysis will be employed to identify stress sources and recovery regulation characteristics that significantly influence endurance athletes with or without responses after high-intensity interval training. The significance level (P-value) will be set at 0.05.

Inclusion Criteria:

• Athletes aged 18 or above with endurance-type competitive sports experience. For instance, athletes who have participated in track events of 400 meters or longer, marathons, road cycling, swimming, rowing, race walking, triathlons, orienteering, and similar competitive events.
• A habitual training regimen of at least 6 hours per week. For instance, training three times a week for at least 2 hours each session.
• Minimum of 3 years of training experience, such as being part of a school team or a specialized athlete.
• No severe injuries within the past year. For instance, severe non-regenerative anemia, craniotomy due to cerebral aneurysm, acute myocardial infarction, severe hepatic cirrhosis, severe systemic lupus erythematosus nephritis, severe rheumatoid arthritis, severe head trauma, and similar conditions.
• Not taking substances affecting athletic, physiological, and psychological performance. For instance, unauthorized drugs, prohibited substances, growth hormones like somatotropin and related substances, β2 agonists, hormone and metabolic modulators, diuretics, masking agents, stimulants, anesthetics, cannabis, adrenal corticosteroids, and similar substances.

Exclusion Criteria:

• Presence of an illness resembling symptoms of overtraining syndrome in the week before the start of the experiment. This includes primary viral illnesses, bacterial infections, associated inflammatory diseases, diabetes, or hyperthyroidism.
• Suspected symptoms of high-risk overtraining syndrome in the week preceding the experiment, such as persistent fatigue or exhaustion, unexplained significant decrease in athletic performance exceeding 10%.

Prior to the commencement of the experiment and during the experimental period, both the participants and the investigator will be unaware of whether the participants exhibit a good response to compensatory training effects. After the participants complete the entire experimental procedure, they will be grouped post hoc based on their outcomes following the intervention, according to predetermined criteria.

The criteria for determining whether there is a good response to compensatory training effects:

1. Non-response group: Exercise performance during the tapering period (week 7) shows a significant increase compared to post-intervention (week 6) and is higher than pre-intervention (week 3) exercise performance.
2. Response group: Exercise performance during the tapering period (week 7) shows a significant increase compared to post-intervention (week 6) and is higher than the change in exercise performance during normal training (= week 3 - week 1).