Aerobic capacity is critical for many athletes, especially for endurance athletes. Endurance training leads to cardiopulmonary and musculoskeletal adaptations, which in turn lead to improvement of endurance performance. Several training methods have been established for the improvement of aerobic capacity and performance, including long distance and low speed training, and high intensity interval training (HIIT). Training methods are used depending on the kind of endurance that aim to improve, and the specific characteristics and energy demands of the event. Especially regarding middle- and long-distance runners, the energy comes mainly from the oxidative system, however, the contribution of the glycolytic pathway is equally important. Thus, improvement mainly of the low-intensity endurance, but also high-intensity endurance is important for these athletes. Additionally, both continuous endurance and HIIT are effective training methods for improving cardiorespiratory and metabolic function, and athletic performance, while evidence also exists in favor of HIIT being more effective. Thus both training methods are used by coaches to improve aerobic capacity and performance of their athletes.

Coaches should be careful regarding the frequency of HIIT training during a microcycle, to provide adequate recovery between training sessions to avoid muscle injuries and overtraining. Existing evidence suggests that endurance exercise (continuous or HIIT) may result in exercise-induced muscle damage (EIMD), inflammatory responses, oxidative stress, and performance deterioration, yet, the timeframe of recovery of physiological and biochemical indices following different endurance training protocols has not been adequately studied. However, such information is crucial for coaches to effectively design the most favorable training program for their athletes.

This study aims to examine the acute effect of different continuous and HIIT training protocols on indices of metabolism, EIMD, neuromuscular fatigue and performance in middle- and long-distance runners.

According to a preliminary power analysis (a probability error of 0.05, and a statistical power of 80%), a sample size of 8 subjects per group was considered appropriate in order to detect statistically meaningful changes between groups. Thus, 10 men and female middle- and long-distance runners, will participate in the study.

The study will be performed in a randomized, cross over, repeated measures design. During their first 1st and 2nd visit, all participants will sign an informed consent form after they will be informed about all the benefits and risks of the study and they will fill in and sign a medical history questionnaire. Fasting blood samples will be collected in order to estimate muscle damage concentration markers. Participants will be instructed by a dietitian how to record a 7-days diet recalls to ensure that they do not consume to greater extent nutrients that may affect EIMD and fatigue (e.g. antioxidants, amino acids, etc.) and to ensure that the energy intake during the trials will be the same. Assessment of body mass and body height, body composition, and aerobic capacity (VO2max), will be performed. Using a photocells system, countermovement jump will be performed to assess jump height, and 30 sec Bosco test to assess mean jump height, peak power, mean power, and fatigue index. The peak concentric, eccentric and isometric isokinetic torque of the knee flexors and extensors, in both limbs will be evaluated on an isokinetic dynamometer at 60°/sec. Maximal voluntary isometric contraction (MVIC) of the knee extensors at 65o in both limbs, as well as the fatigue rate during MVIC through the percent drop of peak torque between the first and the last three seconds of a 10-sec MVIC, will also be evaluated. Afterwards, participants will be randomly assigned into, and perform one of the three different conditions of the study design: a) Continuous running (CT) for 40 min at lactic threshold, b) High intensity interval training (HIIT): 10x2min running at vVO2max with active recovery at 40% της VO2max (interval:recovery 1:1) with a load of 10% of body weight (BW), and c) control condition, no training (measurements only). The training protocols will be matched for mean power and total duration (Tschakert and Hofmann 2013). Prior and immediately after each experimental trial, delayed onset of muscle soreness (DOMS) in the knee flexors (KF) and extensors (KE) of both limbs, as well as blood lactate will be assessed. Additionally, DOMS of KF and KE, peak concentric, eccentric and isometric isokinetic torque, CMJ height, as well as mean jump height, peak power, mean power, and fatigue index during a 30 sec Bosco test, will be assessed 24h, 48h and 72h after the end of the trial. MVIC of the knee extensors of both limbs, as well as the fatigue rate during MVIC will also be assessed at 1h, 2h and 3h, as well as 24h, 48h, and 72h after the end of the trial. Creatine kinase will be assessed at 24h, 48h, and 72h after the end of the trial. The exact above procedures will be repeated by the participants during the remaining two experimental trials. A 2-weeks wash-out period will be implemented between trials.