Regular physical activity is essential for the prevention of cardiovascular and metabolic diseases (Fealy et al., 2018) and high-intensity interval training (HIIT) is an effective training method to elicit rapid improvements in cardiorespiratory fitness (CRF; expressed as maximal oxygen consumption (V[O.sub.2]max)) (Astorino et al., 2012; Daussin et al., 2008; Gist et al., 2014b). Recent data suggests that repeated maximal to supramaximal exercise bouts have a similar, or even greater influence on CRF and metabolic adaptions than traditional moderate-intensity continuous training (MICT) (Gist et al., 2014b). Indeed, Tabata et al. (1996) demonstrated that short duration (7-8 sets of 20 s exercise, interspersed with 10 s rest; the "Tabata protocol") high-intensity intermittent exercise caused the same, or even greater improvements in aerobic (V[O.sub.2]max) and anaerobic power as moderate-intensity endurance training (60 min; intensity 70% of V[O.sub.2]max). These data indicated that short duration exercise, which is of a sufficiently high intensity, is capable of inducing favorable training adaptions. Considering adherence to classic MICT is typically low, HIIT is a more time-efficient training modality and may therefore be the method of choice for increased encouragement in exercise participation (McRae et al., 2012).
Endurance athletes perform HIIT training to improve sport specific performance (Gist et al., 2015). Typically, this involves using classical exercise modalities such as, running, cycling and rowing (Buckley et al., 2015). For recreationally active individuals, these traditional exercise modalities may be perceived as boring due to little or no variation, which could have a negative impact on training adherence, as "lack of enjoyment" is a commonly cited barrier for engaging in regular exercise (Bartlett et al., 2011).
In recent times, functional training, mostly executed with the individual's own bodyweight, is increasing in popularity. High-intensity functional training (HIFT) comprises a variety of functional movements and exercises executed at a high intensity (Haddock et al., 2016). An important advantage of HIFT is that it can be undertaken with minimal equipment, minimal space, and in various locations (i.e., indoor/outdoor) (Gist et al., 2015). While the "classical" HIIT predominantly targets the aerobic system (central adaptions), HIFT incorporates both endurance and resistance training, providing multiple training benefits within the same training session (Feito et al., 2018).
HIFT has also been shown to induce aerobic improvements to the same extent as traditional endurance exercise, but with the added benefit of improved muscle performance (Buckley et al., 2015; McRae et al., 2012). McRae et al. (2012) demonstrated that four minutes of Tabata style training utilizing whole body aerobic exercises (e.g., burpees, jumping jacks, mountain climbers) conducted four times per week for four weeks, elicited similar improvements in V[O.sub.2]max (+7% and 8% for MICT and HIFT, respectively) as MICT (30 min treadmill running, 4x/week). Moreover, Myers et al. (2015) demonstrated that circuit-based whole-body aerobic training using only body-weight exercises, elicited greater CRF responses when compared to a traditional training program. Nonetheless, while these data confirm that HIFT matches, or in some instances appears superior in terms of CRF adaptations to MICT, the question remains whether the improvement in CRF from HIFT can match those achieved through high-intensity running.
To the best of our knowledge, only one study (Buckley et al., 2015) has compared the chronic effects of a traditional HIIT (rowing) with a multi-modal HIIT incorporating multiple exercise modalities, with the observation that both training modalities induced similar improvements in aerobic and anaerobic capacity. Albeit, only the multimodal training resulted in greater muscle performance (e.g. squat strength). While these initial data are intriguing, extension to other types of exercise programs incorporated by the general public such as those executed with one's own body weight are important.
Therefore, the purpose of the current study was to compare two different low-volume HIIT modalities including running vs. functional training on V[O.sub.2]max and muscular endurance in moderately trained female and male participants. Based on the literature discussed, we hypothesized that V[O.sub.2]max will improve in both groups, but that muscular endurance will only significantly improve after functional HIIT. Our secondary aim was to determine if HIFT (body weight exercises) could generate the same degree of cardiorespiratory strain as high-intensity running.
The study was designed as a randomized controlled training study including two different training groups (running and functional training) and two measurement times (baseline vs. post-training). Participants were instructed to refrain from intense exercise and alcohol 24 h before the baseline and post-training measurements and to appear fully hydrated on the test days. Baseline measurements included a laboratory treadmill test, muscular endurance tests and the assessment of body composition. Both, baseline and post-training consisted of two testing days, separated by at least 48 hours. On the first visit, body composition measures were completed prior to the graded exercise test (V[O.sub.2]max test). On the second visit, the muscular endurance test was performed. After baseline measurements, the participants were randomly assigned, stratified by gender and V[O.sub.2]max (determined in the laboratory treadmill test) to either the running high-intensity interval training group (HIIT-R) or the functional high-intensity interval training group (HIIT-F). After a break of at least four days, both the HIIT-R and HIIT-F started the four week training program. Post-training measurements were the same as for baseline conditions and were conducted three to five days after the last HIIT session.
Eighteen healthy male (n = 6) and female (n = 12) sport students were recruited for the study. Participants reported to regularly exercise 8.3 [+ or -] 4.2 h/week, predominantly running, cycling, fitness training, ball sports as well as alpine sports (alpine skiing, ski mountaineering, hiking and climbing). One participant of the HIIT-F dropped out before the start of the training intervention because of time constraints. During the training intervention, one participant of the HIIT-R and one from the HIIT-F dropped out due to illness, unrelated to the study intervention. Finally, 15 participants completed the study and were incorporated into the current dataset. Age and physical characteristics of the participants are presented in Table 1. All study participants underwent a routine pre-participation screening prior to the baseline testing. Exclusion criteria were pre-existing acute or chronic diseases, pregnancy and lactation period. Before providing their verbal and written informed consent to participate in the study, participants were provided detailed information about the procedure and potential risks of the study. The study was carried out according to the Declaration of Helsinki and was approved by the Board for Ethical Questions in Science of the University (Certificate of good standing, 15/2018).
Maximum Oxygen uptake: Participants initially performed a graded exercise test on an electrically driven treadmill (h/p/cosmos pulsar, h/p/cosmos Sports and Medical, Nussdorf-Traunstein, Germany). A treadmill protocol as described in detail by Burtscher et al. (2008), was used to assess V[O.sub.2]max. Briefly, exercise started at 5% inclination and 5 km/h, after 2 min inclination was set at 10% for another 2 min. Subsequently, running speed was increased to 6 km/h and inclination was elevated by 2% every minute until 20%. Lastly, inclination was kept at 20% and the speed was increased by 1 km/h per minute. Ratings of perceived exertion (RPE) were documented at the end of every work load (Borg, 1982). The test was completed when the participant reached volitional exhaustion despite verbal encouragement. Directly after terminating the treadmill test, a capillary blood sample was collected from the hyperaemized earlobe to assess the maximal blood lactate concentration (BLAmax; Biosen C line, EKF Diagnostics, Germany). Gas analysis was performed using an open spirometric system (Oxycon Pro, Care Fusion, Germany) which was calibrated before each measurement, as per the...