Ten cadet karate athletes (age 15.2±1.6years; height 164.8±7.7cm; body mass 64.0±14.5kg) who compete annually in national and international level tournaments completed this study. They were invited to participate in the study during the annual planning transition period (January 2020) and randomly assigned into experimental group (EG; n=5; age 16.1±1.12years; height 168.8±7.6cm; body mass 68.5±20.9kg) and control group (CG; n=5; age 14.5±2.0years; height 160.8±7.1cm; body mass 59.6±6.5kg). Each group consisted of two females and three males (for details see Figure 1). To participate in the study, all athletes had to meet the following inclusion criteria (i) three years or more of karate experience; (ii) no history of disease and medication; (iii) no injuries or fractures during at least the last six months; (iv) consistently training at least three times per week for at least 6h per week; (v) membership in the National Karate Federation; (vi) not undergoing a period of body mass reduction; and (vii) participation in at least 85% of the intervention sessions. All athletes and/or family members of athletes under 18years of age were previously informed of the study purposes, associated benefits, experimental procedures, and potential by informed consent or informed assent before the assessments and training sessions. The study was conducted in compliance with the ethical standards for sport science studies (Harriss and Atkinson, 2015) and implemented after approval by the university ethics committee Autónoma university following the Helsinki declaration for work with humans (General Assembly of the World Medical Association, 2014).

The training program had a duration of 10 sessions (4weeks) of 90min each session and was applied on 3 non-consecutive days (Monday, Wednesday, and Friday). The training load distribution was oriented to technical-tactical development with the coach’s permanent intervention during the training sessions. The HIIT was performed in front of a partner who did not participate in the study. Specifically, the protocol mimicked the official combat duration (3min). In addition, the HIIT intervals were based on the documented temporal structure for this sport (1:2; Tabben et al., 2018). Previously, both groups were instructed to use the rating of perceived exertion scale (RPE 0–10) to internal load control (Tabben et al., 2015; Slimani et al., 2017; Ouergui et al., 2020; Ojeda-Aravena et al., 2021a). The All-out HIIT format was used (Laursen and Buchheit, 2018). The training load was increased by decreasing the density volume during the last week, without modifying the high-intensity time. Briefly, the first 2weeks the athletes recovered in 3min between rounds and performed HIIT at a frequency of twice a week. In the last 2weeks, the density decreased to 2min of recovery and performed three times per week. Specifically, each training session started with a standardized 15-min warm-up group consisting of circle jogging (5min) and lower and upper body dynamic stretching (10min). Subsequently, the EG group was separated from the total group of athletes to execute the HIIT with specific techniques (∼20min; Laursen and Buchheit, 2018; Franchini, 2020). Particularly, athletes executed three rounds of 15 sets of 4s all-out efforts of straight punch and circular kick combinations in front of a partner followed by 8s of low intensity by performing a combat stance (imitating the combat stance). The striking sequence included an initial straight punch with the front hand or “oi tsuki,” followed by a circular kick “mawashi geri” with the back leg or “giaku mawashi geri,” a straight punch with the backhand or “giaku tsuki” and a kick with the front leg “oi mawashi geri” (Chaabene et al., 2015). In parallel, the CG continued with their usual training.

Subsequently, all athletes participating in the study were reintegrated to the usual training by continuing with three blocks of dynamic tasks of exercises for 40min with an RPE of 5–6. Specifically, the first block (15min) consisted of the application of attack techniques with hands (four sets of 20 repetitions of attack techniques with straight punches with the front hand, and later with the backhand with a recovery of 3min between series). The second block (15min), consisted of the application of attack techniques with kicks (four sets of 20 “mawashi geri” with the front leg, and then with the back leg with a recovery of 3min between sets). The third block consisted of free combats (15min) with the permanent intervention of the coach to point out technical and tactical aspects. The training sessions finished with stretching (10min; Figure 2).

A week prior the investigation began the athletes completed a familiarization session and EG practiced the HIIT protocol to reduce the learning effect. In addition, both the coach and athletes received an induction on the RPE 0–10 scale. All assessments were scheduled between 9:00 and 11:00AM, completed in the same order, at the same location (gymnasium with wooden floor), with the same sports clothing, and by the same sports science professional before and after the intervention, previously blinded to the intervention. Previously, all participants were instructed to (i) sleep 8h between each assessment session, (ii) not to modify their usual eating and hydration habits during the days before the assessments, and (iii) not to consume caffeinated beverages. The tests were assessed according to exercise intensity in the following order: SJ, CMJ, and T-test. Before the execution of the tests, a general warm-up of ~10min (e.g., submaximal running with a change of direction, 10 vertical and 10 horizontal submaximal jumps) was performed. This was followed by a specific warm-up with potentiation exercises, including stretching, and two submaximal jumping attempts (~5min). The best of the two attempts was considered for performance for each assessment. A 2-min rest interval was performed between each trial, and a 5–10-min rest interval was applied between each test to reduce the effects of fatigue.

Data analysis was performed with SPSS version 26 for Mac (SPSS Institute, Chicago, IL, United States). Data are presented as mean±SD. Homoscedasticity of variance and normality was checked by Levene’s test and the Shapiro-Wilk test, respectively. The unpaired t-test was used to examine for possible gender biases. The interaction of group (inter-subject factor) EG vs. CG and time (intra-subject factor) pre-intervention vs. post-intervention was analyzed by a repeated-measures mixed ANOVA. If significant effects or interactions were observed, the Bonferroni post hoc test was applied to adjust for differences between the means of the two groups. For ANOVA outcomes, effect sizes (ES) were calculated using partial eta squared (η²_p). Complementarily, post-intervention changes within and between groups were calculated using Cohen’s d following the classification proposed by Rhea for recreationally trained participants (individuals training consistently for 1–5years; trivial <0.25; small 0.25–0.50; moderate 0.50–1.0; large >1.0; Rhea, 2004). Subsequently, the sample was classified into Rs and NRs using the two-technical error (TE) criterion according to a previously established equation (Bonafiglia et al., 2016). NRs were identified and defined as individuals who were unable to demonstrate an increase or decrease (in favor of beneficial changes) in sport-related fitness that was greater than twice the TE away from zero (Ramirez-Campillo et al., 2018). For the current study, two replicates of all outcomes analyzed were used to calculate TE. A change beyond twice the TE was representative of a high probability (i.e., 12–1 odds) that the observed response was a true physiological adaptation beyond what might be expected as a result of technical and/or biological variability (Ramirez-Campillo et al., 2018). Therefore, the TEs were as follows: [SJ; 3.10 (cm)×2; CMJ, 3.32 (cm)×2; T-test, 0.28 (s)×2]. All assessments showed acceptable reliability coefficient of variation or CV<5% and intraclass correlation or ICC>0.90 (Hopkins, 2000). The level of statistical significance used was set at p<0.05.

Table 1 presents the summary of the time factor analysis independently in each group and group-by-time interaction for SJ, CMJ, and CODS. Specifically, for SJ no significant effect was reported in the group factor (F_1,8=1.03; p=0.33; η²_p=0.11) and time factor (F_1,8=4.53; p=0.06; η²_p=0.36). Neither for CMJ in the group factor (F_1,8=0.15; p=0.70; η²_p=0.01) and time factor (F_1,8=0.19; p=0.67; η²_p=0.02). In addition, the CODS showed no significant effect in the group factor (F_1,8=0.65; p=0.44; η²_p=0.07) and time factor (F1,8=0.45; p=0.51; η²_p=0.05).

Table 1 presents the changes based on inference after the intervention. Particularly, in EG increases in jumping ability were reported for SJ with a moderate increase (15.2%; ES=0.91). In contrast, in CG a trivial increase in this outcome (16.2%; ES=0.28). For CODS, an increase small in performance was reported in EG (−1.7%; ES=−0.35). On the other hand, a trivial decreased performance in CG (0.48%; ES=0.03).

On the other hand, for CMJ performance a decrease was reported trivially in EG (1.7%% ES=0.35) and CG (0.48% ES=0.03).

Figure 3 and Table 1 show the inter-individual variability analysis of jumping ability and CODS in athletes from both groups analyzed. In particular, in EG athlete Rs were reported for SJ and T-test (n=2; 40%). Additionaly, for CMJ in the CG (n=1; 20%).

On the other hand, in EG for CMJ, 100% of the athletes were classified as NRs.

Current evidence shows inconsistencies to the effect of HIIT with specific techniques on this physical ability in karate athletes. In this regard, a previous study on HIIT in karate did not document significant effects for SJ and CMJ after comparing a HIIT based on repeated-CMJ vs. repeated-sprints incorporated during the usual training session after 6weeks (Ojeda-Aravena et al., 2019). Although, the authors reported percentage and ES increases for SJ in the repeated-CMJ group (5%; ES=0.30) and repeated-sprints group (8.3%; ES=0.82). Additionally, in another combat sport such as taekwondo, not reported significant increases in CMJ after adding two sessions of simulated combat in different areas sizes (4×4m; 6×6m; and 8×8m) to regular training after 8weeks (Ouergui et al., 2021). On the other hand, in the same sport, Ouergui et al. (2020) documented significant increases for CMJ height in both groups independently after comparing a HIIT with repeated-sprints vs. technical-specific efforts (three sets of 10 repetitions of 6s of repeated kicks with 10s of rest between repetitions and 3min of recovery between sets) after 4weeks (Ouergui et al., 2020).

According to the above, through the results analyzed and given the heterogeneity of the HIIT protocols applied, it is still not possible to be conclusive about the increase in performance in this physical ability in karate. In this sense, the lack of volume of training load applied may have influenced the results. In this regard, studies in combat sports such Taekwondo, that report significant increases in jumping ability add independent training sessions (Ouergui et al., 2020, 2021). Furthermore, the different motor patterns used during HIIT (running, jumping, and specific techniques) involve different muscle tension for the lower limbs and potentially different adaptations. The above, considering the growing evidence in other collective sports such as ice hockey (Kinnunen et al., 2019), highly trained running athletes (Kohn et al., 2011), and recreationally active individuals (Schaun et al., 2019; Moghaddam et al., 2020) who primarily use HIIT based on running. In these studies, neuromuscular (improvement of lower body muscular power, increase of the force-velocity curve and decreased maximal voluntary contraction times), histological (increases in type 2 fiber pool, increases in muscle cross-sectional area), and biochemical (increases in lactate dehydrogenase and decreased lactate levels; Kohn et al., 2011; Kinnunen et al., 2019; Schaun et al., 2019; Moghaddam et al., 2020) adaptations are reported.

Based on the current background, it would be necessary to increase the weekly training volume, either with a higher number of rounds during the training session, by adding independent HIIT sessions or by increasing the training weeks. In addition to HIIT, other specific training strategies such as high-intensity functional training, plyometric training or speed-based training could be applied in order to optimize the dynamic strength components (Loturco et al., 2014; Neto and Kennedy, 2019; Franchini, 2020; Quinzi et al., 2020).

According to the CODS results, an increase in percentage and ES (−1.7%; −0.35, respectively) was observed post-intervention. About this, the evidence regarding the improvement of CODS using HIIT in karate athletes is still controversial. In this regard, for example, previously in this sport, percentage and ES increases (−11.6%; ES=1.20) are observed, although without significant decreases in the performance of this ability after HIIT intervention based on repeated-CMJ vs. repeated-sprints (Ojeda-Aravena et al., 2019). On the other hand, in taekwondo, significant increases (p=0.04) in this ability are documented in favor of the group that performed simulated bouts in the 4×4m vs. 6×6m and 8×8m area size after 8weeks (Ouergui et al., 2021). In addition, this same group of researchers in youth taekwondo athletes reported significantly greater performance in the HIIT with specific-techniques vs. HIIT with repeated-sprints (p<0.01) after 4weeks of training (Ouergui et al., 2020).

However, despite the growing positive evidence of HIIT on this physical ability, it is still not possible to state this with certainty, considering the disparity of protocols and the number of athletes analyzed. Also, the lack of specific neuromuscular stress between HIIT and CODS has likely influenced the results obtained. In this sense, it may be that the lack of accelerations and decelerations in the motor patterns used influenced the observed response. In this regard, it is important to emphasize that in acute terms the evidence shows that muscle power expressed indirectly through jumping ability and including indirect eccentric indexes, in this sport is significantly related to and influences CODS performance (Herrera-Valenzuela et al., 2020; Ojeda-Aravena et al., 2021a). Furthermore, these results are consistent when examining the relationship between jumping ability and specific CODS in male youth karate athletes (Herrera-Valenzuela et al., 2020).

Another aspect that may have affected the analyzed results is the phenomenon of interference about muscle hypertrophy and/or power or force rate development adaptations resulting from concurrent training (strength and endurance) performed during the same session or as part of the training program (Wilson et al., 2012; Coffey and Hawley, 2017; Fyfe and Loenneke, 2018; Neto and Kennedy, 2019). However, this phenomenon is currently debated and associated factors (including exercise volume, intensity, and nutritional status, among others) must be taken into account (Neto and Kennedy, 2019). Evidence points out that such an effect could also depend on the participants’ general fitness level, their training experience, and the frequency of sessions in a week (Fyfe and Loenneke, 2018). On the other hand, HIIT has been shown to reduce the phenomenon of concurrent training interference (Methenitis, 2018). Additionaly, it is important to mention that this study did not apply strength training with external loads, therefore, it is pertinent to question whether bodyweight training could be sufficient to generate this phenomenon or whether it is due to the aforementioned factors.

Another purpose was to analyze the athletes inter-individual variability. Among the main results, Rs were reported for the two groups in SJ and only for the T-test in EG. Meanwhile, athletes’ NRs were reported for all the analyzed outcomes. These results are similar to those reported recently in taekwondo athletes after 4weeks of HIIT with specific-techniques (Ojeda-Aravena et al., 2021b). In this study, the authors documented Rs for SJ (n=2) and CODS (n=3; Ojeda-Aravena et al., 2021a). In another combat sport such as boxing, recently the authors Herrera-Valenzuela et al. (2021) interestingly documented after the application of a HIIT with specific-techniques after 4weeks a higher proportion of athletes Rs in outcomes related to specific actions and performance of bipodal and unipodal CMJ of both limbs (Herrera-Valenzuela et al., 2021).

Accordingly, the inter-individual variability of observed responses to training, including HIIT, according to Walsh et al. (2020) is a combination of (i) individual responses to perseverative exercise training (subject-training interaction), (ii) day-to-day biological variation, and technical error (random variation), and (iii) physiological responses associated with behavioral/maturational changes not attributable to exercise (e.g. within-person variability; Walsh et al., 2020). This includes genetic (Mann et al., 2014; Sparks, 2017; Bonafiglia et al., 2020; Del Coso et al., 2020), climatic (Corbett et al., 2018), cognitive (Atkinson and Batterham, 2015), stress and sleep status (Mann et al., 2014), gender, age, time of day variation (Mann et al., 2014; Sparks, 2017), training status (Pickering and Kiely, 2019), physiological (Williamson et al., 2017; Atkinson et al., 2019), and statistical (Swinton et al., 2018; Chrzanowski-Smith et al., 2020).

However, it is important to mention that the results should be analyzed for their merit, as they could be influenced by (i) the small sample size, (ii) the menstrual cycle of females (Schmitz et al., 2020); (iii) the lack of neuromuscular stress applied; and (iv) the homogeneity of the athletes according to their biological age. Nevertheless, considering the above, the incorporation of HIIT with specific-techniques in combat sports fitness is an early-stage research topic in applied sports science reflected in growing evidence (Franchini et al., 2016, 2017; Kamandulis et al., 2018; Ouergui et al., 2020, 2021; Herrera-Valenzuela et al., 2021; Ojeda-Aravena et al., 2021a). In this sense, future research could use a greater number and experience level of athletes and verify the results by gender. Also, could verify the physiological and neuromuscular effect of HIIT protocols with specific-techniques, in addition to verifying the efficient interval for this sport.

Although it requires further study, the incorporation of HIIT protocols with specific-techniques and using the time structure of combat could be an alternative as part of the training session during inter-competitive periods (e.g., during a shock microcycle) due to the limited time available to athletes to cope with the demands of this period. In addition, these HIIT protocols can be performed in reduced places. In turn, coaches could use inter-individual response analysis as a practical monitoring tool to follow the training progress of each athlete.