Effect of different inter-repetition rest intervals across four load intensities on velocity loss and blood lactate concentration during full squat exercise

Effect of different inter-repetition rest intervals across four load intensities on velocity loss and blood lactate concentration during full squat exercise Ricardo Mora-Custodio, David Rodríguez-Rosell, Juan Manuel Yáñez-García, Miguel Sánchez-Moreno, Fernando Pareja-Blanco and Juan José González-Badillo ABSTRACT This study aimed to analyze the acute effect of inter-repetition rest (IRR) intervals on mechanical and metabolic response during four resistance exercise protocols (REPs). Thirty resistance-trained men were randomly assigned to: continuous repetitions (CR), 10 s (IRR10) or 20 s (IRR20) inter-repetition rest. The REPs consisted of 3 sets of 6, 5, 4 and 3 repetitions against 60, 70, 75 and 80% 1RM, respectively, in the full squat exercise. Muscle fatigue was assessed using: percentage of velocity loss over three sets, percentage of velocity loss against the ~1 m·s−1 load (V1 m·s−1), and loss of countermovement jump(CMJ) height pre-post exercise. Blood lactate was measured before and after exercise. The percentage ofvelocity loss over three sets and lactate concentration were significantly lower (P < 0.05) for IRR groups compared to CR in all REPs. The CR group showed a significantly higher (P < 0.05) velocity loss against V1 m·s−1 load and loss of CMJ height pre-post exercise than IRR groups in REP against 60% 1RM. In conclusion, both IRR groups produced a significant lower degree of fatigue compared to CR group. However, no significant differences were found in any measured variables between IRR configurations. Introduction Muscle tissue structural and functional adaptations occur as a consequence of a particular blend of different exercise-related stressor factors including mechanical load, neuronal activation, hormonal adjustments and metabolic disturbances (Hoppeler, 2016). These stresses experienced are specific to the exercise training undertaken. Therefore, knowledge of the mechanical and physiological aspects underlying resistance training (RT) is essential to improve our understanding of the stimuli that affect adaptations (Crewther, Cronin, & Keogh, 2005; Sanchez-Medina & Gonzalez-Badillo, 2011). In this context, several studies have examined the effects of manipulating RT variables such as intensity (Aarskog, Wisnes, Wilhelmsen, Skogen, & Bjordal, 2012), number of repetitions performed in each set with respect to the maximum predicted number (García-Ramos et al., 2016; Gonzalez-Badillo et al., 2016; González-Hernádez et al., 2017; Sanchez- Medina & Gonzalez- Badillo, 2011), number of sets (González-Hernádez et al., 2017; Krieger, 2010; Tufano, Brown, & Haff, 2017), movement velocity (Pareja- Blanco, Rodriguez-Rosell, Sanchez-Medina, Gorostiaga, & Gonzalez-Badillo, 2014) and recovery time between sets (Rahimi, Qaderi, Faraji, & Boroujerdi, 2010). Another variable that is currently receiving great interest from researchers and practitioners is the introduction of interrepetition rest (IRR) periods (i.e. a rest interval after every repetition or after a certain number of repetitions), traditionally known as cluster set (Haff et al., 2003; Tufano et al., 2017). During a set performed with a traditional configuration (without IRR intervals), an involuntary decline in velocity, force and power occurs as a consequence of fatigue accumulated during previous repetitions (Enoka & Duchateau, 2008; Izquierdo et al., 2006; Lawton, Cronin, & Lindsell, 2006). The continued use of this type of efforts during RT may generate undesired neuromuscular adaptations (Denton & Cronin, 2006; Lawton et al., 2006), as it has been reported that increase in strength as a consequence of training is velocity-specific (Englund, Sharp, Selsby, Ganesan, & Franke, 2017; Jones, Bishop, Hunter, & Fleisig, 2001; Pareja-Blanco et al., 2014, 2017). In accordance with this approach, the largest increases in strength occur at or near the training velocity, while a less effective training effect occurs if training velocity deviates from the target velocity (Englund et al., 2017; Jones et al., 2001; Pareja- Blanco et al., 2014). Therefore, training methods that allow maintenance of velocity, force and power output associated with every repetition during RT may optimize neuromuscular adaptation and maximize performance gains. In fact, a recent study (Pareja-Blanco et al., 2017) has reported that a lower degree of fatigue within the set resulted in greater improvements on the neuromuscular performance than a greater degree of fatigue. For this reason, several studies (Girman, Jones, Matthews, & Wood, 2014; Hardee, Triplett, Utter, Zwetsloot, & McBride, 2012; Iglesias-Soler et al., 2012) have suggested that introducing an IRR period could be an alternative method for attenuating the fatigueinduced performance decrements associated with traditional configuration in resistance exercise. The current knowledge on cluster set indicates that this methodology allows greater force, velocity and power output to be attained compared with traditional set configurations while reducing accumulated fatigue (Denton & Cronin, 2006; Garcia-Ramos et al., 2017; González-Hernádez et al., 2017; Haff et al., 2003; Lawton et al., 2006; Moreno, Brown, Coburn, & Judelson, 2014). However, the response to different IRR periods remains relatively unexplored. It would be relevant to know the minimal IRR period that allows the maintenance of performance during different loading conditions. In this regard, Hardee et al. (2012) reported that adding IRR intervals (20 or 40 s) induced less decrements in peak velocity, force and power output over three sets of six repetitions during the power clean exercise compared to six successive repetitions. In addition, according with these authors, because there were no significant differences between 40 s and 20 s of IRR, it can be speculated that there is no additional benefit in resting more than 20 s (Hardee et al., 2012). More recently, other studies have examined the mechanical and metabolic responses to traditional and IRR set configurations in both the bench press (Garcia-Ramos et al., 2017) and the full squat (González-Hernádez et al., 2017) exercises performed with the 10RM load and maintaining the same inter-set rest across the different IRR configurations. Taken together, these studies showed that IRR intervals of between 10–20 s induced similar degrees of fatigue, while IRR intervals of 30 s significantly reduced the velocity loss within the set and the metabolic stress (Garcia-Ramos et al., 2017; González-Hernádez et al., 2017). However, these studies were conducted only using a relative intensity (10RM, ~75% 1RM) and completing repetitions to muscle failure. Considering that (a) this type of effort causes a marked disruption of cellular homeostasis, with considerable depletion of muscle purines and very large increases in lactate (14, 15); (b) increasing scientific evidence suggests that reaching repetition failure may not necessarily improve the magnitude of strength gains (Drinkwater et al., 2007; Izquierdo et al., 2006; Izquierdo-Gabarren et al., 2010; Pareja- Blanco et al., 2017); and (c), most importantly, that ending each training set well ahead of failure seems to be a more appropriate stimulus to achieve improvements in specific athletic performance (Izquierdo et al., 2006; Izquierdo- Gabarren et al., 2010; Pareja- Blanco et al., 2017), it seems necessary to conduct further studies with training protocols that are not performed until muscle failure. RT is usually configured using multiple sets and different training loads (Crewther et al., 2005; González-Hernádez et al., 2017). However, studies analyzing the acute effects of IRR intervals have used a single training set (Garcia-Ramos et al., 2015; Iglesias-Soler et al., 2012; Lawton et al., 2006; Moir, Graham, Davis, Guers, & Witmer, 2013) and/or a unique load magnitude (Garcia-Ramos et al., 2017; González-Hernádez et al., 2017; Hansen, Cronin, & Newton, 2011; Hardee et al., 2012; Iglesias-Soler et al., 2012; Oliver et al., 2015; Rahimi et al., 2010) Therefore, it is difficult to extrapolate findings from this type of research to the responses associated with a typical RT session. Moreover, to the best of our knowledge, only four studies (Denton & Cronin, 2006; Garcia-Ramos et al., 2017; Girman et al., 2014; González-Hernádez et al., 2017) have compared the acute metabolic response induced by IRR intervals with traditional set configurations. These studies (Denton & Cronin, 2006; Garcia-Ramos et al., 2017; Girman et al., 2014 González-Hernádez et al., 2017) concluded that cluster set configurations induced lower blood lactate concentrations than traditional sets, although results relating to protocols using the full squat exercise are scarce (González-Hernádez et al., 2017). In light of the aforementioned considerations, the aim of this study was to compare acute mechanical (losses of movement velocity and CMJ height) and metabolic (blood lactate concentration) responses to twelve different RT sessions, using four loading magnitudes (60, 70, 75 and 80% 1RM) and three IRR periods (0, 10 and 20 s) in resistance-trained men. Methods Participants Thirty resistance-trained men (age: 22.8 ± 3.1 years; height: 1.80 ± 0.10 m; weight: 71.9 ± 10.9 kg) volunteered to participate in this study. Their estimated one-repetition maximum (1RMest) strength (for more details, see testing procedure section) for the full squat exercise was 103.1 ± 12.7 kg (1.45 ± 0.24 normalized per kg of body mass). All participants were physically active sports science students with an RT experience ranging from 1.5 to 4 year (1–3 sessions per week) and were accustomed to performing the squat with correct technique. Participants were asked not to perform any strenuous RT during the course of the present study. After the first testing session, participants were matched according to their performance in 1RMest in the full squat exercise and then randomly assigned to continuous repetitions (CR, n = 10), 10 s (IRR10, n = 10) or 20 s (IRR20, n = 10) interrepetition rest training protocols. After being informed about the purpose, testing procedures and potential risks of the investigation, participants gave their voluntary written consent to participate. No physical limitations or health problems were found after a medical examination. This study was conducted according to the Declaration of Helsinki and was approved by the research ethics committee of Pablo de Olavide University. Experimental design A descriptive design was used to analyze the acute mechanical and metabolic response evoked by twelve different resistance exercise protocols (REPs) performed with four loading conditions (60, 70, 75 and 80% 1RM) and three different IRR configurations: 0 s (CR), 10 s (IRR10) and 20 s (IRR20) in the full squat exercise. To address this, each group undertook 5 testing sessions separated by 7 days: 1) an initial test with increasing loads for the individual determination of 1RMest strength and full load–velocity relationship in the full squat exercise (Sánchez-Medina, Pallarés, Pérez, Morán-Navarro, & González- Badillo, 2017); and 2) four different REPs differing in the rest between repetitions (0, 10 or 20 s). Thus, for each REP, all participants used the same number of sets (3 sets), repetitions per set (6, 5, 4 and 3 repetitions), loading magnitude (60, 70, 75 and 80% of 1RM, respectively), and inter-set rest (4 min). Obviously, since (i) the IRR interval was different between groups (0, 10 and 20s), and (ii) the inter-set rest was exactly the same (4 min) for all three experimental groups, the total resting time in the session and, consequently, the work-to-rest ratios were different between groups. Thus, according to the different training configurations carried out in the present study, the CR group performed training sessions with the highest work-to-rest ratio and the IRR20 group had the lowest work-to-rest ratio. The REPs were configured as follows (set x repetitions [% 1RM]): 3 × 6 [60%] (REP 1); 3 × 5 [70%] (REP 2); 3 × 4 [75%] (REP 3); 3 × 3 [80%] (REP 4). The number of repetitions per set was selected based on the number of repetitions required to reach a velocity loss of ~20% within the set during traditional configurations, since a recent study has reported that this degree of fatigue produces greater improvements in neuromuscular performance than ~40% velocity loss within the set (near to muscle failure). The REPs were performed in random order and separated by 6 – 7 days. In order to compare the mechanical and metabolic responses following each REP, participants undertook a battery of measurements at pre-exercise (Pre) and post-exercise (Post): countermovement jump (CMJ) height; the individual load (kg) that elicits a ~ 1.00 m·s−1 (V1 m·s−1 load) mean propulsive velocity (MPV); and blood lactate concentration. During the course of the present study, participants did not perform any other RT. In addition, participants refrained from any strenuous physical activity for at least 2 days before each REP trial. All REPs were performed at the same time of day (± 1 h) for each participant and under controlled environmental conditions (20°C and 55% humidity). Testing procedures Familiarization sessions and initial strength assessment In the two weeks prior to the study, four familiarization sessions were carried out in order to ensure the use of proper technique in the full squat and CMJ exercises. These familiarization sessions consisted of 3 sets of 6, 5, 4 and 3 repetitions against loads of 60, 70, 75 and 80% 1RM, respectively, for the full squat exercise and five maximum CMJs in each session. The percentages of 1RM were estimated from the velocity-load relationship for this exercise (Sánchez-Medina et al., 2017). The first testing session was used to determine the participants’ anthropometric data and to assess the individual loadvelocity relationships and 1RMest using a progressive isoinertial loading test in the full squat exercise. The warm-up and testing protocols were identical to those described elsewhere (Sánchez-Medina et al., 2017). The full squat was performed starting from the upright position with the knees and hips fully extended. Each participant descended in a continuous motion until the top of the thighs was below the horizontal plane, then immediately ascended back to the upright position. Participants were required to always execute the concentric phase of full squat at maximal intended velocity. This execution technique was carefully reproduced in all REPs performed in the study. Initial load was set at 30 kg and was progressively increased in 10 kg increments until the attained mean propulsive velocity (MPV) was <0.6 m·s−1 (which corresponds to ~85% 1RM) (Sánchez- Medina et al., 2017). Three repetitions were executed for light (MPV ≥ 1.10 m·s−1), two for medium (1.10 m·s−1 > MPV ≥ 0.80 m·s−1) and only one for heavy loads (MPV < 0.80 m·s−1). Inter-set recoveries ranged from 3 min (light) to 5 min (heavy loads). Strong verbal encouragement was provided to motivate participants to give a maximal effort in each repetition. A Smith machine with no counterweight mechanism (Multipower Fitness Line, Peroga, Murcia, Spain) was used for all sessions. A dynamic measurement system (T-Force System, Ergotech, Murcia, Spain) was used to measure movement bar velocity in every repetition during all testing sessions. The reliability of this system has been reported elsewhere (Sanchez-Medina & Gonzalez-Badillo, 2011). All reported repetition velocities in this study correspond to the MPV of the concentric phase (Sanchez-Medina, Perez, & Gonzalez-Badillo, 2010). The MPV was used in the present study for several reasons: 1) recent studies (Banyard, Nosaka, Vernon, & Haff, 2017; Garcia-Ramos, Pestana-Melero, Perez-Castilla, Rojas, & Haff, 2018) have shown that this variable has a very high intra- and inter-participant reliability, similar to mean velocity and peak velocity; 2) referring the mean values to only the propulsive phase (i.e. MPV) when assessing the velocity with which a load is lifted in a concentric action avoids underestimating an individual’s neuromuscular ability, especially when lifting light and medium loads (Sanchez-Medina et al., 2010). In fact, it has been shown that when mean values are referred to the entire concentric portion of a lift (i.e. mean velocity), the higher the strength level (1RM) of a subject, the more underestimated his/ her true strength potential gets when compared to that of another subject with a lower strength level (Sanchez-Medina et al., 2010). In other words, the MPV allows better discrimination between subjects with different performance levels compared to the mean velocity and peak velocity. For these reasons, the assessment of strength performance in a single trial and the analysis of the training effect should preferentially be done by referring mean values to the propulsive phase (MPV), especially when using loads ≤ 70% 1RM (Sanchez-Medina et al., 2010). Resistance exercise protocols The characteristics of each REP are displayed in Table 1. The 12 types of REP were performed always using three sets and 4-min inter-set recovery periods. The number of repetitions per set was 6 (60% 1RM), 5 (70% 1RM), 4 (75% 1RM) and 3 (80% 1RM), respectively. The load magnitude for each participant in each REP was determined from the load-velocity relationship for the full squat exercise, as it has recently been shown that there is a very close relationship between percentage of 1RM and MPV (Sánchez- Medina et al., 2017). Thus, the target MPV to be attained in the first (usually the fastest) repetition of the first exercise set in each REP was used as an estimation of percentage of 1RM, as follows: ~0.98 m·s−1 (60% 1RM), ~0.82 m·s−1 (70% 1RM), ~0.75 m·s−1 (75% 1RM), and ~0.68 m·s−1 (80% 1RM) (Sánchez-Medina et al., 2017). Consequently, before starting each REP, adjustments in the proposed load (kg) were made when needed so that the velocity of the first repetition matched the programmed velocity (± 0.03 m·s−1). In each testing session the warm-up consisted of 5 min of jogging at a self-selected easy pace, 5 min of joint mobilization exercises, two 30-m running accelerations, two sets of 10 squats with no external load (own body weight) and five CMJs with increasing intensity. Then, three maximal CMJs, separated by 20 s rests, were performed and the mean jump height (cm) was taken as the pre-exercise reference value. The CMJ height was determined using an infrared timing system (Optojump, Microgate, Bolzano, Italy). This was followed by the determination of the V1 m·s−1 load. For this purpose, participants performed three sets of six down to three repetitions (3-min rests) with increasing loads (from 20 kg) up to V1 m·s−1 load (± 0.03 m·s−1). This value was chosen because it is a sufficiently high velocity, which is attained against a medium load (~60%RM in the SQ), and it allows a good expression of the effect of loading on velocity, besides being a relatively easy-to-move and well tolerated load (Sanchez-Medina & Gonzalez-Badillo, 2011). The V1 m·s−1 load (kg) was thus taken as a pre-exercise reference measure against which to compare the velocity loss experienced after the three exercise sets (Sanchez-Medina & Gonzalez-Badillo, 2011). Finally, after load adjustments in the proposed load (kg), the three sets of the corresponding REP were performed next. In those REPs with IRR intervals the barbell rested on the supports of the Smith machine during the rest periods. Immediately after completing the last repetition of the third training set, participants performed three maximal CMJs, separated by 20-s rests. In addition to CMJs, the participants also executed three maximal-effort consecutive repetitions against the V1 m·s−1 load (load was changed within 10–15 s with the help of trained spotters). The CMJ and V1 m·s−1 load mean values were taken as the immediate postexercise measures. After 1 min rest, blood samples were taken for the analysis of lactate. Measurements of fatigue Three different methods were used to quantify the extent of fatigue induced by each REP (Sanchez-Medina & Gonzalez- Badillo, 2011). The first method analyzed the decline in repetition velocity during the three consecutive exercise sets and was calculated as the percentage loss in MPV from the fastest to the slowest repetition of each set and averaged over the 3 sets. The second method examined the pre-post exercise change in velocity attained against the V1 m·s−1 load. The third method involved the calculation of percentage change in CMJ height after the three exercise sets. It was calculated as the percentage change in CMJ height from pre- to post-exercise (see Figure 1). Blood lactate concentration Capillary whole blood samples were drawn from the earlobe before exercise and again 1 min after completing the last repetition of the V1 m·s−1 load in each REP. The Lactate Pro 2 LT-1730 (Arkray, Kyoto, Japan) portable lactate analyzer was used for lactate measurements. The suitability and reproducibility of this analyzer has been previously established through out the physiological range of 1.0–18.0 mmol.l−1 (Pyne, Boston, Martin, & Logan, 2000). Statistical analysis Values are reported as mean ± standard deviation (SD). Homogeneity of variance across groups (CR vs. IRR10 vs. IRR20) was verified using Levene’s test, whereas the normality of distribution of the data was examined with the Kolmogorov-Smirnov test. A one-way analysis of variance (ANOVA) was conducted on the MPV attained in the first repetition of each set configuration, the CMJ height reached at the beginning of each testing session, and blood lactate concentration, in order to examine possible differences in these variables between the different set configurations (CR vs. IRR10 vs. IRR20) against each training load. A two-way factorial ANOVA with an inter-subject factor (Group: CR, IRR10, IRR20) anda repeated measures factor (Repetitions: 6, 5, 4, 3) was used to compare MPV of each repetition in each REP with Bonferroni post-hoc comparisons to clarify the interaction. A oneway ANOVA was used to examine the differences between training configurations (CR, IRR10, IRR20) in mechanical and metabolic responses in each REP. Bonferroni post-hoc comparisons were used to analyze the differences between experimental groups. Intra-group effect sizes (ES, 95% confidence interval) were calculated using Hedge’s g (Hedges & Olkin, 1985). Threshold values for assessing magnitudes of standardized effects (changes as a fraction or multiple of baseline SD) were 0.20, 0.60, 1.20, and 2.00 for small, moderate, large, and very large, respectively (Hopkins, Marshall, Batterham, & Hanin, 2009). Statistical significance was established at P < 0.05. Statistical analyses were performed using SPSS version 18.0 (SPSS Inc., Chicago, IL, USA). Results All variables were normally distributed and homoscedastic across REPs. No significant differences between groups were found prior to the experimental protocols for any of the variables analyzed. Resistance exercise protocols characteristics Averages of the best MPV and averages of the MPV over the three sets for each REP in each group are reported in Table 1. No significant differences between-groups were found in the best MPV in any REP. The average MPV over the three sets was significantly greater for IRR20 compared to CR in all REPs, whereas differences between IRR10 and CR only were significant for REP 1 (Table 1). No significant differences between IRR10 and IRR20 were observed in the average MPV over the three sets in any of the four REPs performed. There was significant ‘group x repetition’ interaction (P < 0.001) in the velocity decline pattern within each set configuration for all the four REPs under study. Bonferroni posthoc comparisons revealed significant differences between CR and IRR10 (P < 0.05 – 0.001) and CR and IRR20 (P < 0.05 – 0.001) in MPV of each repetition, mainly in the last two repetitions of each training set Figure 2). No significant differences for MPV were observed between both IRR configurations. The intra-group analysis of velocity decline pattern within each set configuration (Figure 2) showed that the MPV significantly decreased (P < 0.05 – 0.01) from the second repetition in all three sets of any REP for CR. For IRR10, significant decrements (P < 0.05 – 0.01) in MPV were found from the second (REP 4) and third (REP 1, 2 and 3) repetition of each set, while for IRR20 the MPV declines were statistically significant (P < 0.05) from the third (REP 4), fourth (REP 3) and fifth (REP 1 and 2) repetition of each set. Mechanical and metabolic measurements of fatigue Comparison between groups showed that the percentage of MPV loss over the three sets and post-exercise blood lactate concentration were significantly (P < 0.05 – 0.001) greater for CR in all REPs compared to IRR10 and IRR20, whereas no significant differences between IRR10 and IRR20 were found (Table 2). The CR group showed significantly (P < 0.01) greater loss of CMJ height than IRR20 in REP 1, and significantly higher loss of MPV against V1 m·s−1 load than IRR10 (P < 0.05) and IRR20 (P < 0.01) in REP 1. For REP 2, REP 3 and REP 4, although there were no significant differences between groups, CR showed a greater percentage of change (16.8 – 22.1%) and ES (1.05 – 2.35) compared to IRR10 (Δ: 12.8 – 18.4%; ES: 1.08 –2.47) and IRR20 (Δ: 13.5 – 18.1%; ES: 0.91 – 2.64) in these variables (Table 2). No statistically significant or practical differences were observed in loss of CMJ and MPV against V1 m·s−1 load between IRR10 and IRR20 for any REP (Table 2). Discussion The aim of our study was to compare mechanical and metabolic responses to four REPs performed with three different IRR periods (0, 10 and 20 s) during full squat exercise in resistance-trained men. As expected, CR presented a significant greater velocity loss within the set and a higher lactate concentration compared to both IRR10 and IRR20 in all REPs analyzed. Despite this, loss of CMJ height and loss of MPV against the V1 m·s−1 load was significantly higher for CRcompared to IRR10 and IRR20 only in the REP performed against 60% 1RM. In contrast, no significant differences were observed between IRR10 and IRR20 configurations in any measured variables. Therefore, the results of the present study confirm that for a training session with the same volume and relative load, introducing a recovery time between repetitions reduces the degree of induced fatigue during RT. However, despite these findings, further studies need to be conducted in order to determine whether RT programs with IRR configurations may result in greater improvements in neuromuscular performance than CR training programs. A unique and important aspect of this investigation was that, by monitoring repetition velocity and adjusting the actual loads to be lifted from the load-velocity relationship for the full squat exercise (Sánchez-Medina et al., 2017), we made sure that all participants in each experimental group used a very similar relative load in each REP (Table 1). Because the movement velocity and power output decline, and consequently induced fatigue, are influenced by the relative load (% 1RM) used (Izquierdo et al., 2006), it appears essential in studies analyzing the acute effect of different RT configurations that all participants are tested with the same relative load in order to truly understand the impact of varying the IRR interval. However, previous studies in this area (Garcia-Ramos et al., 2015, 2017; Girman et al., 2014; González-Hernádez et al., 2017; Haff et al., 2003; Hansen et al., 2011; Hardee et al., 2012; Lawton et al., 2006; Moir et al., 2013; Oliver et al., 2015) did not use the load-velocity relationship (the fastest MPV or mean velocity of the set) to determine loading magnitude (Sánchez-Medina et al., 2017). Protocols used in most previous studies were prescribed according to the maximal load (kg) that could be lifted in a determined number of maximum repetitions (e.g. 4, 6, 8, 10 and 12RM) (Garcia-Ramos et al., 2017; González-Hernádez et al., 2017; Lawton et al., 2006) or asa given percentage of 1RM measured in an initial session (Garcia- Ramos et al., 2015; Girman et al., 2014; Oliver et al., 2015), i.e. these studies did not check whether the actual relative load used for participants corresponded with the proposed load (Gonzalez- Badillo & Sanchez-Medina, 2010; Gonzalez-Badillo, Yanez-Garcia, Mora-Custodio, & Rodriguez-Rosell, 2017; Sánchez-Medina et al., 2017). Therefore, to the best of our knowledge, this is the first study analyzing the acute effect of different IRR configurations compared to CR configurations where the actual relative load used for all participants was equated across the load-velocity relationship in the full squat exercise. According to our results, most studies (Garcia-Ramos et al., 2015, 2017; González- Hernádez et al., 2017; Haff et al., 2003; Hansen et al., 2011; Hardee et al., 2012; Lawton et al., 2006; Oliver et al., 2015) analyzing the effect of adding a rest interval after every repetition on changes in different kinetic and kinematic variables have reported significantly lower velocity and power output repetition declines across the set for IRR compared to CR configurations. These results are consistent in a variety of resistance exercises, including squat (Oliver et al., 2015), bench press (Garcia-Ramos et al., 2015, 2017; Lawton et al., 2006), power clean (Haff et al., 2003; Hansen et al., 2011), and jump squat (Hardee et al., 2012), and indicate that the inclusion of IRR periods may allow an increase in the number of repetitions performed at greater velocities. In connection with the loss of MPV over the set, CR also showed greater intra-group ES and percentages of losses of MPV against V1 m·s−1 load and CMJ height (mainly in REP1), and higher blood lactate concentrations (in all REPs) compared to both IRR10 and IRR20 (Table 2). Losses in CMJ height and velocity attained against V1 m·s−1 load, along with lactate concentration, have been considered as reliable and effective methods to determine the acute and delayed (72 h post-effort) degree of fatigue produced by different RT protocols using CR (Gonzalez-Badillo et al., 2016; Sanchez-Medina & Gonzalez- Badillo, 2011). However, only a few studies (Denton & Cronin, 2006; Garcia-Ramos et al., 2017; Girman et al., 2014; González- Hernádez et al., 2017; Oliver et al., 2015) have analyzed the degree of fatigue or metabolic stress after RT with different IRR configurations. In line with our results, these studies using the squat exercise reported that RT protocols conducted with IRR configurations showed significantly lower losses of CMJ height (Girman et al., 2014; González-Hernádez et al., 2017) and blood lactate concentrations (Denton & Cronin, 2006; Girman et al., 2014; González-Hernádez et al., 2017; Oliver et al., 2015) in comparison with CR configurations. It appears that velocity and power performance are primarily dependent upon the phosphagen system (Girman et al., 2014). It has been observed that the phosphocreatine (PCr) stores progressively diminish as the number of repetitions increases, reaching nearcomplete depletion when performing a single set to muscle failure using a CR configuration (Gorostiaga et al., 2012). Therefore, it appears that the inclusion of short IRR periods would provide a partial replenishment of PCr, which is important for velocity and power output maintenance during repeated bouts of exercise (Bogdanis, Nevill, Boobis, Lakomy, & Nevill, 1995). In this regard, previous studies have speculated that at least a 15–20 s IRR interval between explosive exercises may be needed for PCr resynthesis (Harris et al., 1976) and maintenance of blood lactate concentration and physical performance (Hardee et al., 2012; Pereira et al., 2008). However, in contrast with this hypothesis, our results suggest that lower IRR intervals (10 s) could be sufficient to Figure caption Figure 1. Example of performing resistance exercise protocol 1 for a representative participant of CR. Relative MPV loss over three sets (21.5%), MPV pre-post exercise against the V1 m•s-1 load (21.6%) and loss of CMJ height pre-post exercise (20.5%) were calculated. Figure 2. Average mean propulsive velocity during each repetition for each load magnitude used (60%, 70%, 75% and 80% 1RM) in the different set configurations (CR, IRR10 and IRR20). Statistically significant differences respect to IRR10: † P< .05, †† P< .01, ††† P< 0.001. Statistically significant differences respect to IRR20: ‡ P<.05, ‡‡ P< .01, ‡‡‡ P< .001. Table 1. Descriptive characteristics of the resistance exercise protocol performed in each session. Scheduled REP 1 REP 2 REP 3 REP 4 Sets x rep 3 x 6 3 x 5 3 x 4 3 x 3 Target MPV (m·s−1) 0.98 0.83 0.75 0.68 (~60% 1RM) (~70% 1RM) (~75% 1RM) (~80% 1RM) Actually Performed Reference MPV (m·s −1 ) CR 1.00 ± 0.02 0.84 ± 0.02 0.75 ± 0.03 0.69 ± 0.02 (~58.8% 1RM) (~69.3% 1RM) (~75.1% 1RM) (~79.0% 1RM) IRR10 1.00 ± 0.03 0.83 ± 0.03 0.76 ± 0.02 0.68 ± 0.02 (~58.8% 1RM) (~69.9% 1RM) (~74.5% 1RM) (~79.6% 1RM) IRR20 0.99 ± 0.03 0.84 ± 0.02 0.75 ± 0.02 0.68 ± 0.01 (~59.4% 1RM) (~69.3% 1RM) (~75.1% 1RM) (~79.6% 1RM) Average MPV over three sets (m·s−1) CR 0.88 ± 0.05 0.71 ± 0.05 0.64 ± 0.05 0.59 ± 0.05 IRR10 0.92 ± 0.04 * 0.73 ± 0.04 0.68 ± 0.04 0.60 ± 0.04 IRR20 0.92 ± 0.05 * 0.78 ± 0.03 ** 0.69 ± 0.03 * 0.63 ± 0.02 * REP: Resistance exercise protocol; MPV: Mean propulsive velocity; CR: continuous repetition group; IRR10: 10 seconds interrepetition rest group; IRR20: 20 seconds interrepetition rest group; 1RM: One-repetition maximum. Statistically significant differences with respect to CR: * P < .05, ** P < .01. Table 2. Mechanical and metabolic measurements of fatigue after each effort for each group Loss of CMJ Loss of MPV with V1 m·s −1 Load Training Configuration Loss of MPV over three sets (%) Lactate (mmol·L −1 ) Δ (%) ES (95% IC) Δ (%) ES (95% IC) REP 1 (3x6 at 60% 1RM) CR 22.1 ± 5.7 ***††† 6.7 ± 2.9 ***††† 21.3 ± 5.3 * 1.56 (0.71 – 2.41) 22.8 ± 8.8 *† 2.94 (1.18 – 4.71) IRR10 11.2 ± 5.4 3.4 ± 0.9 17.2 ± 4.5 1.38 (0.50 – 2.26) 13.6 ± 8.1 1.93 (0.76 – 3.10) IRR20 9.1 ± 3.3 2.9 ± 0.8 13.6 ± 5.0 0.86 (0.32 – 1.41) 12.6 ± 8.5 1.89 (0.67 – 3.12) REP 2 (3x5 at 70% 1RM) CR 27.4 ± 8.6 ***††† 6.6 ± 2.7 ***†† 21.5 ± 4.7 1.62 (0.75 – 2.50) 21.2 ± 10.1 2.35 (0.92 – 3.78) IRR10 13.3 ± 4.8 3.7 ± 0.9 16.4 ± 6.0 1.44 (0.65 – 2.24) 12.8 ± 7.0 1.86 (0.85 – 2.87) IRR20 12.1 ± 4.3 2.9 ± 0.7 18.1 ± 4.1 1.34 (0.66 – 2.03) 13.5 ± 9.0 2.16 (0.97 – 3.35) REP 3 (3x4 at 75% 1RM) CR 26.9 ± 9.0 ***††† 5.5 ± 2.1 ***† 19.5 ± 5.2 1.23 (0.57 – 1.89) 22.1 ± 11.3 1.70 (0.49 – 2.92) IRR10 14.1 ± 5.9 3.9 ± 1.2 14.6 ± 5.1 1.08 (0.50 – 1.65) 15.7 ± 7.5 2.01 (0.97 – 3.04) IRR20 11.3 ± 3.3 2.9 ± 0.9 17.0 ± 3.6 1.17 (0.53 – 1.81) 15.8 ± 6.9 2.16 (1.06 – 3.26) REP 4 (3x3 at 80% 1RM) CR 22.1 ± 8.8**† 4.9 ± 1.6 *** 18.2 ± 5.8 1.05 (0.45 – 1.65) 16.8 ± 9.7 1.96 (0.77 – 3.14) IRR10 13.2 ± 5.6 3.6 ± 1.2 16.6 ± 4.8 1.08 (0.41 – 1.75) 18.4 ± 7.7 2.47 (1.19 – 3.76) IRR20 12.0 ± 5.3 2.6 ± 0.5 16.2 ± 3.8 0.91 (0.44 – 1.38) 16.7 ± 7.9 2.64 (1.19 – 4.09) CR: Continuous repetition group; IRR10: 10 seconds interrepetition rest group; IRR20: 20 seconds interrepetition rest group; statistically significant differences respect to IRR20: * P < 0.05, ** P< 0.01, *** P < 0.001; statistically significant differences respect to IRR10: † P < 0.05, †† P < 0.01, ††† P < 0.001.