Participants

This cross-sectional repeated measures cross-over study was registered at ClinicalTrials.gov (ID = NCT04531891), approved by the Ethics Committee of the Ghent University Hospital (approval code: BC-07679, date of approval: 07 August 2020), and written informed consent was obtained from each participant. An a priori sample size of ≥13 players was estimated to observe a reduction in jump performance (i.e., jump height) of 2.8 ± 2.2 cm after fatigue (power = 0.80, α = 0.004, d = 0.61) (Scanlan et al., 2018). To be included in this study, participants had to meet the following criteria: (1) male, (2) playing volleyball or basketball ≥3 times per week, (3) ≥18 years of age, and (4) no history of lower extremity injuries in the past 6 months. Finally, we included 10 volleyball and 10 basketball players (Table 1).

Procedures

Players visited the lab twice to complete either the HIIP or the HIIP-5. The order of both sessions was a priori randomly assigned and sessions were separated by more than 48 h. Before each session, players were asked to (1) avoid heavy meals for ≥2 h, (2) refrain from caffeine, energy drinks and smoking at the day of testing, and (3) avoid strenuous exercise for ≥48 h. Each session started with a 10-min warm-up (execution of the fatigue protocol at low speed without inducing any fatigue). To minimize potential accumulation of fatigue, players were instructed to execute one circuit of the fatigue protocol at low speed after the baseline tests and sit quietly for 15 min between the post-fatigue measures.

Fatigue Protocol

The HIIP fatigue protocol consisted of intermittent bouts of high-intensity exercise during the execution of functional circuits (i.e., sprints, directional changes, jumps and side-stepping tasks, performed at the highest possible movement speed), interspersed with passive rest periods of 30 s (Whyte et al., 2018a−c). During the circuits, participants were asked to sprint forwards 5 m, cut off at a 90° angle, sprint forwards another 5 m and sprint backwards 5 m. This was repeated until arriving at the hurdles, where 10 jumps and 10 side-steps were performed. Finally, participants side-shuffled around the cones back to the starting point (Figure 1) (Whyte et al., 2018a−c). During the rest periods after each circuit, participants were asked to score their perceived exertion on a Borg scale from 6 (“no exertion at all”) to 20 (“maximal exertion”), indicating the degree to which they felt the body was working (e.g., breathing rate, muscular effort). As such, both the rate of perceived exertion for breathlessness (RPE-B) and legs (RPE-L) were measured (Borg et al., 2010). According to previous HIIP studies (Whyte et al., 2018a−c), the HIIP was terminated when participants achieved a score of ≥18/20 (“very hard”) on the RPE-B and/or the RPE-L scale. This score was suggested to be ‘safe’ while still eliciting muscular fatigue (Abergel et al., 2021). The HIIP-5 was terminated when a fixed number of five circuits was completed. Failure to complete five circuits during the HIIP-5 resulted in exclusion of the data from statistical analysis.

Figure 1

Fatigue protocol.

Data Collection

The timing of examining the outcome variables before, during and after the fatigue protocols is presented in Table 2. For physiological variables, the heart rate (HR) was monitored with a Polar system (Polar, Electro, Finland). The HR obtained at the end of the protocols can be considered a marker of cardiovascular stress during the protocol (Stojanović et al., 2018), whereas the recovery of the HR in the timeframe following exercise can reflect the recovery from the homeostatic stress induced by the exercise (Mann et al., 2014). Blood lactate levels were registered using a Lactate Pro 2 Meter (Samcon, Belgium). First, capillary blood samples were drawn from the fingertip after cleaning and drying the finger from excessive sweat (Hertogh et al., 2005). Thereafter, blood lactate levels were determined from these samples and peak values were compared to baseline in order to establish the metabolic perturbation and reflect the metabolic acidosis induced by the protocol (Goodwin et al., 2007; Marcinek et al., 2010).

For the physical characteristics, jump height and maximal muscle strength of the quadriceps were determined to assess muscular fatigue (Bossuyt et al., 2016). Jump height was measured using Optojump Photo-electric cells (Microgate Optojump Next, The Netherlands) in which the average height of three maximal counter-movement jumps was determined (Claudino et al., 2017). Therefore, participants were instructed to keep their arms akimbo and to jump as high as possible without pulling their knees up during the flight. Second, concentric (CONC) quadriceps muscle strength was assessed using isokinetic dynamometry (IKD, Biodex System 3, Shirley, NY). Therefore, players performed five maximal knee extensions at moderate (joint angular) velocity (120°/s) and five at high velocity (300°/s). These velocities were chosen since a previous study demonstrated high knee angular velocities (>300°/s) during stop-jump tasks, which have been associated with increased knee injury risk (Bisseling et al., 2008). During the IKD assessments, participants flexed their knee with sub-maximal effort to avoid hamstring muscle fatigue. Strength evaluations were always performed on the participants’ leading leg when performing a stop-jump task, since the pilot work showed higher impact forces for this leg. Two practice trials were allowed before the baseline physical tests. Additionally, circuit time during the protocols was registered with infrared timing gates (Microgate, The Netherlands).

Data Analysis

Raw joint torque data were extracted from the IKD and imported in Matlab (MathWorks, Inc., Natick, MA, USA) for individual curve analysis using IKD1D (www.ikd1d.org). During this process, only gravity-corrected torque-angle data measured within 10% velocity tolerance at 120°/s or 300°/s were selected. Afterwards, a fourth order polynomial fit curve was calculated for ≥3 valid trials. The common divider for the knee joint range was eventually calculated for all participants, resulting in torque data analysed between 95° and 35° of knee flexion for the IKD at 120°/s, and between 85° and 45° of knee flexion for the IKD at 300°/s. These ranges appeared to be similar to those observed during the greatest part of the landing phase of the stop-jump task (Vermeulen et al., 2023b). All torque data were normalized to body mass (Nm/kg).

Statistical Analysis

The statistical analysis of the discrete variables was performed in IBM SPSS statistics 26. Isokinetic torque-angle profiles were compared using Statistical Parametric Mapping (SPM, www.spm1d.org) in Matlab (MathWorks, Inc., Natick, MA, USA) (Pieters et al., 2022). For all discrete variables, normality was first checked with the Shapiro-Wilk test and corresponding normality plots. Two-way repeated measures ANOVA was performed for all normally distributed outcome variables (or the Friedman test for non-normally distributed variables) to investigate any interaction effect (protocol*fatigue), or any main effect of the protocol (HIIP vs. HIIP-5) or fatigue (PRE vs. POST protocol). Post-hoc paired sample t-tests (or Wilcoxon signed-rank tests) with Bonferroni correction were applied when a statistically significant effect was found (α = 0.05). Partial eta squared (np²), which is the effect size measuring the proportion of the total variance in the dependent variable (i.e., outcome variable) that is partially explained by the independent variable (i.e., protocol, fatigue or interaction of protocol*fatigue) (Richardson, 2011), was calculated for the omnibus ANOVA and classified into small (0.01−0.06), medium (0.06−0.14) and large effects (>0.14). For the post-hoc tests, Cohen’s d effect sizes, which measure the magnitude of the differences, were calculated and classified as small (0.20−0.50), medium (0.50−0.80) and large (>0.80) (Sullivan and Feinn, 2012).

Fatigue Protocol Characteristics

The data of the fatigue protocol characteristics met the assumptions of normal distribution, except for the number of completed circuits and protocol completion time. Participants completed a median number of four circuits during the HIIP (range = 1−23; <5 circuits: n = 12, 5 circuits: n = 3, >5 circuits: n = 5). Protocol completion time did not significantly differ between both protocols (HIIP: median = 4.6 min, range = 0.7−27.8; HIIP-5: median = 5.5 min, range = 5.1−6.6; p = 0.296). Circuit time significantly increased from the first to the final lap for both protocols (HIIP: +3.8 ± 3.0 s, p < 0.001, d = 1.28; HIIP-5: +4.5 ± 3.8 s, p < 0.001, d = 1.18). Since one volleyball player was not able to complete the HIIP-5, data from 19 vs. 20 players were included for the HIIP-5 and the HIIP in the analysis, respectively.

Responses Following the Fatigue Protocols

The blood lactate data of one participant were withdrawn from the statistical analysis due to an experimental error during data collection. The data of all responses following the protocols fulfilled the assumptions of normal distribution. Responses following the protocols are presented in Figures 2 and 3 and Table 3, and post-hoc statistics are shown in Table 4. For the total group, no significant interaction and protocol main effects were found for all variables, except for the protocol on the HR (p = 0.005, np² = 0.41) with significantly higher HRs 10−30 min after the HIIP-5 compared to the HIIP (p = 0.003−0.024, d = 0.47−0.84). A significant main effect of fatigue was found for all variables (HR: p < 0.001, np² = 0.95; blood lactate: p < 0.001, np² = 0.95; jump height: p < 0.001, np² = 0.59; quadriceps muscle strength at 120°/s: p < 0.001, np² = 0.60, quadriceps muscle strength at 300°/s: p = 0.005, np² = 0.55; RPE-B: p < 0.001, np² = 0.91; RPE-L: p < 0.001, np² = 0.81). Post-hoc comparisons showed that the HR and RPE-L scores significantly increased and remained elevated up to 30 min after both protocols. RPE-B scores also significantly increased, but only up to 20 min after both protocols. Peak blood lactate levels significantly increased after both protocols. Jump height and quadriceps muscle strength at 120°/s and 300°/s significantly decreased up to 30 min after both protocols, except for quadriceps strength at 120°/s immediately after the HIIP and for quadriceps strength at 300°/s at 30 min after the HIIP-5.

Figure 2

Heart rate and perceived exertion before and after the HIIP and HIIP-5 protocols (mean and 95% CI).

* Statistically significant difference between pre- and post-fatigue conditions (p < 0.05). † Statistically significant difference between the HIIP and the HIIP-5 (p < 0.05)

Figure 3

Quadriceps concentric torque-angle profiles at 120°/s and 300°/s before and after the HIIP and HIIP- 5 protocols, presented as mean and standard deviation clouds.

PRE = black; POST0 = red; POST15 = blue; POST30 = green. The transparent areas under the curves indicate those knee joint angles that were significantly affected by fatigue (p < 0.05; effect of fatigue = grey bar; POST0 vs. PRE = red bar; POST15 vs. PRE = blue bar; POST30 vs. PRE = green bar).

Synthesis of the Results

To the authors’ knowledge, this is the first study that investigates the utility and validity of sports-specific, high-intensity, intermittent fatigue protocols in jump-landing sports. Our study showed that both HIIP versions induced fatigue and cardiovascular stress, as several significant main effects for fatigue were found. Moreover, the HIIP-5 seems to be a suitable alternative for the HIIP, as few to no significant main effects for the protocol were found.

This study observed acute and long-term responses for physiological, physical and perceived variables after both HIIP versions in volleyball and basketball players. Regarding physiological responses, the HR increased during the protocols up to values that were close to maximal, which reflects profound cardiovascular stress induced by the two protocols. Similar responses were observed in previous HIIP studies (Whyte et al., 2018a−c), and during volleyball and basketball game play, with average physiological intensities above 60−80% of the maximal HR (Rodríguez-Marroyo et al., 2014; Stojanović et al., 2018). Furthermore, the HR remained elevated above resting values for up to 30 min after the protocol completion. This indicates that both fatigue protocols induce extensive homeostatic stress that needs to be restored following exercise (e.g., increased core temperature, removal of lactate, restoration of phosphocreatine) and that a certain timeframe is needed to recover the resting autonomic tone (i.e., activation of parasympathetic activity and withdrawal of sympathetic activity) (Mann et al., 2014). Higher HR values were monitored 10−30 min after the HIIP-5 compared to the HIIP, indicating slower recovery from homeostatic stress after the HIIP-5 in this sample. Blood lactate concentrations increased on average to 19−20 mmol/l after both fatigue protocols, reflecting the strong metabolic perturbation induced by the protocols. It has been shown that an increase in blood lactate concentration is strongly related to the increase in protons (H+) and thus, also to the occurrence of metabolic acidosis (Marcinek et al., 2010). This metabolic acidosis strongly contributes to the occurrence of muscle (peripheral) fatigue (Fitts, 2016). As such, the significant increase in blood lactate concentrations, found in the present study, might be a reflection of this phenomenon. Small increases in blood lactate concentrations have also been demonstrated during volleyball and basketball games (Mroczek et al., 2011; Stojanović et al., 2018), showing that these sports are indeed characterized by high-intensity actions causing metabolic perturbations.

Regarding physical responses, jump height decreased until 30 min after both protocols, which is consistent with the effects observed after the SAFT-5 (Bossuyt et al., 2016). Decreases in jump height have been already demonstrated in volleyball and basketball players when fatigued (Brazo-Sayavera et al., 2017; Lesinski et al., 2016; Scanlan et al., 2018). Quadriceps muscle strength also decreased until 30 min following both protocols, which has previously been associated with decreased jump height (Scanlan et al., 2018). No differences were observed for quadriceps muscle strength at moderate velocity immediately after the HIIP and at high velocity 30 min after the HIIP-5 compared to baseline. However, there was a trend towards a decreased performance at these points (large effect size). Moreover, fast-twitch fibres are more easily fatigued and less quickly recovered than slow-twitch fibres following short-term, high-intensity exercise (Lievens et al., 2020). Therefore, acute and long-term performance decline could be observed after both fatigue protocols.

Regarding perceived responses, RPE-scores at protocol completion were similar for both the HIIP and the HIIP-5, being consistent with previous studies implementing the HIIP (Whyte et al., 2018a−c). Despite acute responses being found, RPE-B scores already returned to baseline values 20 min after both protocols (medium to large effect size). Researchers and coaches need to be aware that objective, physiological and physical responses may still be present when subjective feelings of tiredness appear to already have normalized. Although no long-lasting (perceived) responses were demonstrated in our study, similar RPE-scores at 30 min have been observed after volleyball or basketball training and/or a match (Aoki et al., 2017; Rodríguez-Marroyo et al., 2014).