High-intensity muscle work is often required under any competitive sport conditions, no matter whether it is an endurance, sprint/power event or a combination (sport games). Since most sports performances last several minutes to hours, both aerobic and anaerobic capabilities in varied proportions are always necessary. Each athlete is unique, however, and can be characterized from a number of points of view, including the capacity to perform mostly aerobic or mostly anaerobic events as well as the ability to successfully repeat and maintain high-intensity exercise for a prolonged period (Spencer et al., 2005). Muscle fibre type distribution, metabolic and cardiorespiratory regulation and aerobic capacity enable endurance athletes to have an advantage in prolonged endurance events, whereas sprint/power athletes are better suited for highi-ntensity, short-term, and explosive activities (Kenney et al., 2015). This simple classification is challenged, however, when exercise is described as a combination of both a high-intensity workload and prolonged duration.
High-intensity interval training (HIIT) is a widely used and effective training method in various sports, including both endurance and sprint/power events (Milanovic et al., 2015). HIIT requires an integration of a number of physiological systems. The contributions of ATP-phosphocreatine (PCr) and glycolytic metabolic pathway are necessary for achieving high exercise intensity whereas an oxidative metabolic pathway is crucial for maintaining high exercise intensity as long as possible (Buchheit and Laursen, 2013; Tschakert and Hofmann, 2013) . Ufland et al. (2013) have demonstrated that sprinters have a lower mean repeated sprint time, but simultaneously also a lower repeated sprint ability. In addition, innate endowment, training history and a consequent ability to perform endurance or power/speed exercise certainly entails that even identical muscle work can be performed with more aerobic and less anaerobic contribution and vice versa. Therefore, a cross-sectional study could be useful in order to assess the manner in which each training background (endurance vs. sprint) influences the response to HIIT, providing important information to assist coaches in adjusting training programs to athlete-specific metabolic characteristics and developing strategies to improve performance.
The response to any stress, including exercise, is complex, highly variable, and involves a myriad of adaptive responses in multiple organ systems (Zierath and Wallberg-Henriksson, 2015). We therefore focused on multiple physiological variables of acute as well as post-exercise response to HIIT. Apart from the description of the acute cardiorespiratory (heart rate, oxygen consumption and carbon dioxide production) and metabolic (lactate) response, post-exercise heart rate variability (HRV) was assessed. HRV is considered a tool for cardiac autonomic regulation assessment which provides information about exercise load and post-exercise recovery (Buchheit, 2014).
Biochemical markers of exercise-induced inflammation (interleukin-6, leucocytes) and muscle damage (creatine kinase, myoglobin) were also evaluated (Paulsen et al., 2012). Interleukin-6 (IL-6) has been reported to have pro--as well as anti-inflammatory effects and might play an important role in metabolic and musculoskeletal adaptation to exercise (Pedersen and Febbraio, 2012). The high intensity character of HIIT can potentially lead to muscle fibre impairment which can be manifested by increases in concentration of creatine kinase (CK) and myoglobin in plasma (Paulsen et al., 2012). The direct relationships between IL-6, CK, myoglobin versus exercise intensity and duration have been previously noted (Chen et al., 2007; Cullen et al., 2016).
The primary aim of this study was to compare the acute cardiorespiratory and metabolic response to various modes of HIIT between endurance and sprint trained athletes. We assume that endurance trained athletes perform HIIT interventions with lower acute cardiorespiratory and metabolic responses despite a high peak workload intensity, primarily due to their faster oxygen uptake kinetics (Berger and Jones, 2007) and greater maximal muscle oxidative capacity (Dubouchaud et al., 2000) associated with a greater reliance on fat as fuel for the energy supply, more effective acid-base status control (Hawley, 2002) and lactate removal ability (Thomas et al., 2004). Additionally, potential exercise-induced changes in HRV, IL-6, leucocytes, and muscle damage markers may provide unique holistic insight into the question of differences between endurance and sprint type athletes in response to a single bout of HIIT.
Sixteen highly-trained males volunteered in this study (Table 1). All the participants were deliberately approached and chosen in order to match the specification of the study subgroups, i.e. regular sport training with the aim of preparing for official national or international competitions in endurance or sprint sport disciplines. Endurance athletes participated in at least one of the following: 5 km run (1 athlete), tower-running (1 athlete), sky/trail-running (3 athletes), triathlon (2 athletes), long-track in-line skating (1 athlete), cross-country skiing (1 athlete). Sprint athletes participated in 100-400 meters track run.
None of the participants were clinically diagnosed with any chronic or acute cardiovascular, metabolic, respiratory, immunological or musculoskeletal system disorders. None of the participants used any medication. Prior to the participant's involvement, the local Ethics Committee of the University approved the experimental protocol and the investigation conformed to the principles outlined in the Declaration of Helsinki. All participants were fully-informed about the study details and provided written informed consent.
The participants visited the laboratory on four separate occasions over a 1-2 week interval. During this time, they firstly performed a maximal incremental treadmill test. They consequently performed a short HIIT, a long HIIT, and one constant load exercise (CE) session matched for mean load and total duration. The order of the exercise sessions was chosen at random. All sessions were performed in the morning and were conducted by the same researchers in a thermally-controlled laboratory room.
All the participants were informed about the experimental procedure during the first laboratory visit. They also completed a short questionnaire about physical activity, acute or chronic diseases and the use of dietary supplements/medication. Anthropometric assessment and a body composition analysis then followed (Tanita BC418MA; Japan).
In order to determine their maximum aerobic capacity ([??][O.sub.2max]), the minimal running speed required to elicit [??][O.sub.2max] ([??][O.sub.2max]), as well as the first and second ventilatory thresholds ([VT.sub.1], [VT.sub.2]), participants performed a graded exercise test (GXT) as previously described (Cipryan et al., 2016). Expired air was continuously monitored for an analysis of [O.sub.2] and C[O.sub.2] concentrations during the GXT by the use of a breath-by-breath system (ZAN600Ergo; Germany). It was determined that the participants had reached their [??][O.sub.2max], when at least two of the following criteria were met: (A) a plateau in the [??][O.sub.2] or an increase less than 2.1 mL.[kg.sup.-1].[min.sup.-1] despite the increasing running speed, (B) a final respiratory exchange ratio (RER) higher than 1.10; (C) an attainment of 95% of the age-predicted maximal heart rate (HR). The [??][O.sub.2max] was based on the highest average [O.sub.2] consumption measured during a 30 s period. Gas-exchange measurements were also used to quantify the first and second ventilatory thresholds ([VT.sub.1] and [VT.sub.2]) (Hofmann and Tschakert, 2011). [VT.sub.1] was defined as the first increase in ventilation and equivalent for oxygen consumption (VE/V[O.sub.2]), without an increase of the equivalent for the carbon dioxide production (VE/VC[O.sub.2]). [VT.sub.2] was defined as the second increase in VE with an increase in both VE/V[O.sub.2] and VE/VC[O.sub.2]. The final incremental test speed reached at the end of the test ([v.sub.inc.t.]) and at the [??][O.sub.2max] were calculated (Kohn et al., 2011). Heart rate was measured using a chest belt (Polar Electro; Finland).
The maximal countermovement jump height (Haff and Dumke, 2012) and the 30s Bosco test (Bosco et al., 1983) were performed 30-40 min before GXT in order to precisely distinguish the differences between the endurance and sprint study groups.
The following visits to the laboratory consisted of interval and continuous exercise interventions. Participants always arrived at the laboratory between 7 and 9 a.m., after a night of fasting (i.e., no breakfast was consumed). Detailed prescriptions for long and short HIIT and CE are shown in Table 2. The 8 min warm-up at 50 % v[??][O.sub.2max] was performed before both HIIT. The exercise interventions were ended by 3 min of walking at 5 km.[h.sup.-1]. All three exercise tests were matched for the total duration and mean power (Tschakert and Hofmann, 2013). Long (3 min) and short intervals (30 s) were identical for work/relief ratio as well as for the relative work and relief intensity. Ventilatory parameters and HR were monitored during the exercise. Blood lactate concentrations obtained from...