Where does this approach come from? Is it evidence-based?

Some previous studies (Bachero-Mena et al., 2020; Gorostiaga et al., 2010; Jiménez-Reyes et al., 2013, 2016, 2019) monitored CMJ performance over different running sessions for sprinters and those authors concluded that monitoring CMJ height may allow a more accurate setting of training loads during this type of workouts using an individualized dose rather than a fixed number of repetitions. In the current review, we analyzed the potential application of monitoring CMJ performance during typical running sessions in endurance athletes. We also suggest that the attempt to literally transfer the findings obtained in those previous studies with sprinters, during workouts not typically included in training of endurance runners, may induce misinterpretation.

Therefore, the potential application of CMJ monitoring for deciding when to interrupt an endurance running session in order to individualize the training load is based on two questionable assumptions extracted from the aforementioned studies (Bachero-Mena et al., 2020; Gorostiaga et al., 2010; Jimenez-Reyes et al., 2013, 2016, 2019). First, it is assumed that the CMJ height loss occurs during typical interval training for endurance runners in the same way as it has been observed with sprinters (Gorostiaga et al., 2010; Jimenez-Reyes et al., 2013, 2016, 2019) and 800-m athletes (Bachero-Mena et al., 2020). Secondly, it is also assumed that a significant relationship between physiological stress and the neuromuscular strain induced during endurance workouts exists since that relationship was strong and significant in sprinters. These two assumptions will be questioned and examined for endurance runners in the next sections.

Jump height loss during running events. Does it even happen?

Findings of previous studies revealed that the effects of continuous vs. interval training sessions on CMJ performance are controversial (Table 1). While some studies have reported reductions in jumping height after a marathon (~13%, Nicol et al., 1991, and ~22%, Del Coso et al., 2013), a half-marathon mountain race (4.1-7.9%, Rousanoglou et al., 2016) and after a half-ironman triathlon (~23%, Del Coso et al., 2012), one study did not find a reduction in CMJ height immediately after a simulated sprint-distance triathlon (García-Pinillos et al., 2015). Moreover, another study (Vuorimaa et al., 2006) reported an improvement (8.5%) in CMJ performance immediately after 40-min of a continuous run at an intensity of 80% of velocity associated with maximal oxygen uptake (VO_2max) in elite endurance runners. Furthermore, Del Rosso et al. (2016) observed an enhancement in CMJ height during a 30-km trial in well-trained distance runners (14%).

Previously, Boullosa and colleagues (2009, 2011) examined the effect of two exhausting infield protocols, the University of Montreal Track Test and the time limit test at maximum aerobic speed, on CMJ performance. Those authors found an increment in jumping height after both running protocols (12.7% and 4.9%, Boullosa and Tuimil, 2009; and 3.6%, Boullosa et al., 2011). Likewise, in a related study (García-Pinillos et al., 2018), a group of endurance runners completed the Léger-Lambert test and the CMJ height was also improved (5.5%). In line with other studies (Boullosa et al., 2018; Wilson et al., 2013), these works highlight that there are several factors which could mediate the effect of running sessions on jumping capacity (e.g., neuromuscular evaluation methods, running protocol or recovery time), but the authors suggested that an elastic energy enhancement might explain the improvement in CMJ height after running under the tested conditions.

Given the lack of consensus between those previous studies, it seems justified to further examine the effect of the potential mediators on the balance between potentiation and fatigue influences after a running protocol, which could be the intensity of the running exercise (i.e., maximal vs. submaximal) and the training background of the athletes (i.e., sprinters vs. endurance runners). Previously, Jiménez-Reyes et al. (2016, 2019) examined the acute effect of two different habitual training routines for sprinters on metabolic (i.e., blood lactate and ammonia) and neuromuscular variables (i.e., CMJ and sprint performance). The protocols consisted of: (i) 40-m maximal sprints with 4 min rest intervals between sets until sprint performance decreased by 3% (Jimenez-Reyes et al., 2016), and (ii) 60-m maximal sprints with 6 min rest intervals between sets, up to a 3% speed loss (Jiménez-Reyes et al., 2019). After both protocols, ~14-16% impairment of CMJ performance was observed thus supporting the use of CMJ height as a tool for monitoring sprint training and quantifying mechanical and metabolic fatigue (Jiménez-Reyes et al., 2013). A more recent study with 800-m athletes (Bachero-Mena et al., 2020) reported similar results during a typical high-intensity training session based on 5 × 200 m with a 4 min rest interval. Similarly, another study (Morcillo et al., 2015) tested the acute effect of repeated sprints on vertical jump performance and the results were consistent with the data above. Meanwhile, what does occur during interval running sessions at submaximal intensities? Does CMJ performance decrease with the onset of and increases in fatigue? The current evidence does not support such a decrease. Furthermore, although the evidence might seem surprising, it is consistent. Two different studies by García-Pinillos et al. (2015, 2016) described the acute effects of three different high-intensity interval running sessions on jumping performance in endurance runners: (i) 12 runs of 400 m (4 sets of 3 runs) with a passive recovery of 1 min between runs, and 3 min between sets (4 × 3 × 400 m) (García-Pinillos et al., 2015); (ii) 10 runs of 400 m with 90 s of recovery (10 × 400 m) (García-Pinillos et al., 2016); (iii) 40 runs of 100 m with 30 s of recovery (40 × 100 m) (García-Pinillos et al., 2016). None of those protocols reduced CMJ height in endurance runners, despite high levels of the RPE (~17) and blood lactate (~13 mmol/l). In fact, the results showed a CMJ performance enhancement after the 4 × 3 × 400 m protocol (7.89%) (García-Pinillos et al., 2015), and after the 10 × 400 m protocol (5.18%) (García-Pinillos et al., 2016), whereas the CMJ height remained stable during the 40 × 100 m protocol. Another previous study (Vuorimaa et al., 2006) also examined the effect of an interval running workout on CMJ height in trained endurance runners based on alternating 2 min at 100% of velocity associated with VO_2max and 2 min of recovery. An increase in jumping height after each running bout was reported thus reinforcing the notion that CMJ height is not reduced during high-intensity interval running protocols at submaximal intensities. In fact, most of those previous studies (García-Pinillos et al., 2015, 2016; Vuorimaa et al., 2006) did not find any impairment in CMJ height during the running session, but an improvement in vertical jump performances despite high levels of central and peripheral fatigue (i.e., when considering the RPE and blood lactate responses).

Therefore, according to the current evidence, it seems that, when running speed is maximal (e.g., sprint interval session), a CMJ height loss would be expected. However, during submaximal running bouts (e.g., high-intensity intermittent sessions) for endurance runners, CMJ performance remains unchanged or even is improved. Although there is not a single explanation for that phenomenon, there are a number of potential justifications. On the one hand, there is a direct relationship between phosphocreatine resynthesis following intense exercise and maximal oxygen uptake (McMahon and Jenkins, 2002), and the adaptations of the muscle cells to endurance training showing an increased content of fast myosin light chains in type I muscle fibers (Hamada et al., 2000). This might explain the between-athlete differences in response to endurance running protocols. On the other hand, it has been suggested that a faster running pace imposes higher neural demands, which may lead to a failure to fully activate the contracting musculature in the subsequent activity (Ross et al., 2001), because the higher intensities during training protocols would induce the recruitment of additional fast-twitch motor units for relatively short duration (Enoka and Duchateau, 2008).

Consequently, such findings reinforce the notion that CMJ height may not be an adequate marker of internal training loads during this type of workouts in endurance runners lacking its sensitivity to help in the regulation of the external training load. Moreover, based on previous research (Boullosa et al., 2011; García-Pinillos et al., 2015), the authors would suggest that the analysis of mechanical variables and force-time relationships during vertical jumping might be more useful.

Post-activation performance enhancement during running protocols

As already mentioned, previous studies have demonstrated the lack of reduction in CMJ height during high-intensity intermittent running protocols in endurance athletes, despite high levels of perceptual and metabolic fatigue. These specific responses of endurance runners may be understood when considering the post-activation performance enhancement (PAPE) phenomenon which refers to the improvement of muscular performance as a result of previous muscular work (Blazevich and Babault, 2019). In this context, when endurance athletes experience post-running jump potentiation (Boullosa et al., 2020), this means that the level of muscle potentiation is greater than neuromuscular fatigue associated with the running exercise.

After muscular work is completed in a conditioning protocol, mechanisms of muscular fatigue and PAPE coexist with subsequent mechanical power output and performance depending on the balance between these two factors. As recently indicated by Boullosa et al. (2018), PAPE practices have been traditionally associated to maximal intensity and short duration efforts (i.e., power demanding sports) (Bachero-Mena et al., 2020; Jiménez-Reyes et al., 2013, 2019), whereas endurance practitioners are unaware of the potential of this phenomenon to optimize endurance performance in both training and competition settings.

Generally, PAPE is not expected to occur after prolonged running in the presence of high levels of fatigue. However, as previously mentioned, CMJ improvements have been also reported after prolonged continuous runs (García-Pinillos et al., 2015; Rousanoglou et al., 2016; Vuorimaa et al., 2006), incremental running protocols to exhaustion (Boullosa et al., 2009, 2011; García-Pinillos et al., 2018), and after high-intensity intermittent running exercises (García-Pinillos et al., 2015, 2016; Vuorimaa et al., 2006). Although there are different proposed mechanisms, most studies from animal to human models (Boullosa et al., 2018; Hamada et al., 2000; Wilson et al., 2013) suggest the regulatory myosin light chain phosphorylation as the prominent underlying mechanism (Grange et al., 1993). It is known that endurance training simultaneously induces phosphorylation of regulatory myosin light chains in slow-twitch fibers and resistance to fatigue, which may explain the typical PAPE responses observed in endurance athletes after endurance running exercises (Boullosa et al., 2018; Hamada et al., 2000). Currently, there are some gaps in the literature about the effective practical applications of the PAPE phenomenon in the training context of endurance runners and thereby some uncertainty among endurance coaches have been raised. Meanwhile, it seems clear that the presence of PAPE during interval workouts of endurance runners makes CMJ height loss a useless index of the internal load in terms of determining when the external training load should be reduced or stopped as it would occurs with power trained athletes.

Does neuromuscular strain correlate with metabolic stress responses during endurance running?

Previous studies with different populations (Bachero-Mena et al., 2020; Gorostiaga et al., 2010; Sánchez-Medina and González-Badillo, 2011) found an almost perfect correlation between CMJ height loss and metabolic stress (e.g. blood lactate concentration) after different training protocols (Table 2). Previously, Sanchez-Medina et al. (2011) identified a strong and significant relationship between mechanical (i.e., CMJ height loss) and metabolic response (i.e., blood lactate and ammonia concentrations) during different resistance exercise protocols (r = 0.93-0.97). Likewise, similar results have been reported during different sprint-based running protocols with athletes simultaneously impairing their CMJ performance while increasing blood lactate and ammonia levels during testing trials. More recently, Bachero-Mena et al. (2020) reported a negative relationship (p < 0.001, r = -0.083) between the individual values of jumping height and blood lactate concentration, whereas Gorostiaga et al. (2010) found a significant curvilinear negative relationship between the individual blood lactate and ammonia concentrations and jumping height (p < 0.05; R² = 0.68 and 0.51, respectively) during typical workouts for elite 400-m runners. Furthermore, Jiménez-Reyes et al. reported in a number of studies (2013, 2015, 2016, 2019) an inverse relationship between CMJ height and metabolic measures of fatigue after different sprint-based running protocols in which athletes impaired CMJ performance and increased blood lactate and ammonia concentrations (r = 0.92-0.99 for all cases). Thus, these aforementioned studies suggested the use of CMJ height as a tool for monitoring sprint training and quantifying mechanical and metabolic fatigue. However, it was unclear whether those findings may be extrapolated to the neuromuscular and metabolic responses of endurance runners during workouts.

Subsequent studies with endurance athletes (García-Pinillos et al., 2015, 2016; Vuorimaa et al., 2006) examined the acute metabolic and neuromuscular response of different high-intensity intermittent running protocols and found that, despite the high levels of fatigue, the CMJ height was not impaired but even potentiated. For instance, a previous study (García-Pinillos et al., 2015) reported a 7.89% improvement in CMJ height after 12 repetitions of a 400 m run despite accumulating a high level of blood lactate (i.e., 13.8 mmol/l) and a near maximum RPE score (i.e., 18.3). Likewise, another study in endurance runners (García-Pinillos et al., 2016) found a 5.18% increase in CMJ height after an interval running protocol even in the presence of high blood lactate concentrations (i.e., 12.9 mmol/l) and RPE values (i.e., ~16). Therefore, the current evidence suggests that neuromuscular strain does not exhibit an association with metabolic stress during running protocols in endurance runners. Therefore, monitoring CMJ height loss to estimate metabolic stress during endurance workouts seems unfeasible for this population.

The present review aimed to critically analyze the literature regarding the use of CMJ height as a tool to monitor the internal load among endurance runners during running sessions for the manipulation of an athlete’s external load. Current evidence shows that fatigue induced by common running workouts of endurance runners does not counterbalance the potentiation effect on CMJ height. Additionally, findings reported in different studies are consistent regarding the lack of association between CMJ height loss and metabolic stress during high-intensity interval running sessions in endurance runners.