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This study aimed to evaluate the acute fatigue effects of different volume-equated agility-like SC sprint protocols, differing in W:R (1:2 vs. 1:3) and sprint distance per interval (20 m vs. 30 m), on metabolic load as well as on measures of cognitive and neuromuscular performance. We found significantly higher BLac values post-exercise in protocols that used a shorter (P1 and P3) compared to a longer relief duration (P2 and P4) at the same sprint distance, while total metabolic load was moderate across all protocols. Contrary to our initial hypothesis, none of the applied SC protocols induced fatigue on the cognitive level (reaction time) or on the physical level (i.e., jump and sprint performance) in elite female soccer players. Instead we surprisingly observed a significantly increased cognitive function and neuromuscular performance post-exercise. Irrespective of the outcome of this study, the present data may be useful as reference data for improved RTS decision-making for elite female soccer players.
Metabolic load
In general, there was low exercised-induced metabolic fatigue reflected by BLaC values ranging from 3.12 to 4.52 mmol/L across all protocols. This indicates a contribution from the anaerobic glycolytic energy pathway to a lesser extent and suggests a metabolic balance near the lactate steady-state at the systemic level [37]. It has been reported that COD sprints are metabolically less demanding than linear sprints due to the low metabolic demand of the deceleration phase that may not be offset by the increased energy requirement of the reacceleration phase [38]. Since the players performed 7–10 CODs per interval during short-distance multidirectional sprinting, this could be a possible explanation.
Additionally, research indicate sex-dependent differences in metabolic and neuromuscular properties, with females reported to be more resistant to fatigue [39]. This may be related to a lower glycolytic enzyme activity and greater reliance on aerobic metabolism combined with a likely greater distribution of type I fibres compared to males, who have a higher initial power output and therefore a greater involvement of anaerobic glycolysis during repeated-sprint exercise [39]. This could also explain the moderate BLaC levels in our study. To compare our results with the current body of evidence is hard since we are not aware of any similar research. Most of the studies have primarily been carried out on male soccer players [40, 41] or used a different methodological approach (e.g., repeated sprint cycling) [42, 43]. In summary, the design of the applied SC protocols was not sufficient to induce metabolic fatigue in elite female soccer players.
There were significantly higher BLaC values post-exercise in protocols that used a shorter (P1 and P3) compared to a longer relief duration (P2 and P4) at a constant sprint distance. Contrary to this, we found no differences in BLaC levels between protocols that covered a longer (P1 and P2) or shorter sprint distance (P3 and P4) at a constant W:R. Thus, the major driver for the higher BLaC values in P1 and P3 seems to be the shorter relief duration as opposed to the manipulation of sprint distance travelled. It can be assumed that the shorter recovery time in P1 and P3 more strongly impair adequate phosphocreatine resynthesis that slightly taxes more anaerobic glycolysis to meet the energy demands of repeated sprinting [4]. This is supported by previous research showing that a gradual reduction of the relief duration (6 × 40 m repeated shuttle sprint test separated by 25/20/15 s recovery for each test) is associated with increasing BLaC values, at least in young male soccer players [44]. Based on the conditions of this study, it therefore can be assumed that female players need a shorter relief duration during repeated-sprint exercise (e.g., W:R = 1:1) because of a greater fatigue resistance in order to amplify blood lactate accumulation and metabolic fatigue. However, this needs to be confirmed in further research.
Cognitive function
Irrespective of protocol design, there was an improved cognitive function post-exercise. None of the protocols induced mental fatigue associated with decreased stimulus processing speed (as measured by RT). Research indicates that acute exercise basically has an inverted U-shaped effect on the performance of a cognitive task with respect to exercise intensity. In particular, improvements in RT performance were usually observed at exercise intensities ranging from 40 to 60% of maximal oxygen uptake (VO2max) or below the lactate threshold, respectively [45]. This means that the aerobic metabolism is predominantly targeted without blood acidosis or accumulation of metabolic waste products [46]. In this study, we did not determine VO2max or the lactate threshold, since the intensity of all protocols was defined as all-out efforts. Considering the low exercise-induced metabolic load with BLaC values close to the lactate-steady state in all protocols, it can be assumed that the aerobic and phosphocreatine metabolism were the main contributors to energy supply during the repeated-sprint exercise, thus possibly explaining the increase in post-exercise RT performance.
The underlying mechanism has been linked to enhanced central noradrenergic activation, with a significant relationship between the concentration of plasma catecholamines and RT performance during moderate exercise [47]. However, we did not measure blood noradrenaline or adrenaline levels, so this remains a plausible speculation. Another possible factor for the improvement in post-exercise RT may be related to exercise specificity that induced an enhanced stimulus processing, since the players received the same visual stimuli during the repeated-sprint exercise and the RT test. Consequently, the applied SC protocols are capable to significantly improve RT performance, making them valuable for implementation in specific warm-up routines before training or competition in elite female soccer players, for example. To induce mental fatigue by means of deterioration in RT, it can be speculated that it is necessary to increase exercise intensity or the degree of post-exercise physical exertion [45]. During SC repeated-sprint exercise this could be achieved by increasing the sprint distance or the number of intervals as well as by decreasing the relief duration between intervals. Whether the manipulation of these variables has an effect on RT performance needs to be shown in future studies.
Neuromuscular performance
There were significant increases in jump and sprint performance after completing all of the SC sprint protocols. From a metabolic view, contractile function seemed to be little affected due to the relatively low level of exercise-induced metabolic fatigue and the correspondingly low accumulation of metabolic by-products from anaerobic metabolism [48]. From a performance perspective, we also did not record any fatigue-related increases in split times from the first to the final third of the SC sprint protocols. However, this does not explain the increase in jump and sprint performance, but it does provide favourable conditions for post-activation performance-enhancing (PAPE) effects that rely on other mechanisms and typically peak between 5 to 10 min after the conditioning activity [49]. These potential PAPE effects may be associated with increased muscle temperature (i.e., temperature-sensitive ATPase reaction increases the rate of force development and contraction velocity), alterations in muscle water content (i.e., the resulting decrease in ionic strength increases muscle fibre force) and increased neural drive (i.e., increased spinal-level synaptic excitability heightens net motoneuron output, partially through enhanced motivation) [50]. It is likely that these and potentially other factors contributed to a composite effect.
The present results surprised us, as we primarily wanted to investigate the extent of neuromuscular fatigue induced by the different SC protocols. We did not expect a completely opposite and performance-enhancing effect. This makes a comparison to other studies difficult, since heavy strength or plyometric rather than repeated-sprint exercises are typically used as pre-conditioning activities to investigate their potential PAPE effects on jump or sprint performance [49, 51]. In this respect, it has been reported that the coupling of a biomechanically similar high-load and high-speed exercise during a so-called complex training program (e.g., alternating squats and countermovement jumps, CMJ) is likely to induce a PAPE effect in the neuromuscular system, resulting in increased power output during subsequent jump and sprint exercises [52, 53]. Since the repetitive short-distance sprints and reactive CODs of the SC protocols have similar biomechanical demands as the subsequent MRJs and linear sprints, the increase in performance could be explained through a possible PAPE effect.
In addition, one study investigated the potentiating effects of heavy squat exercise on CMJ performance immediately after the conditioning activity as well as before and after each of five match-sets in elite female volleyball players. In most cases the authors found a significantly higher CMJ performance between baseline and all other measurement times with CMJ heights being consistently higher in the experimental compared to the control group [54]. However, there also was an increased CMJ performance between baseline and match-set 3 and 5 in the control group, possibly suggesting that sport-specific movements themselves could elicit a PAPE effect. This may well also support our findings. In summary, all SC protocols induced an increase in jump and sprint performance that likely is associated with a PAPE effect. In order to cause fatigue with a relevant decline in neuromuscular performance, a further reduction of the recovery time between repeated-sprint efforts seems to be promising [55]. Future research will have to prove this assumption.
The different protocols were designed to intentionally induce fatigue under ecologically more valid conditions, similar to those of game sports. As already mentioned, we failed to induce fatigue effects with the existing protocols and surprisingly observed an increase in cognitive and neuromuscular performance post-exercise. Nevertheless, we believe that the protocols could still be useful for RTS decision-making. For example, if a female player undergoing RTS testing showed a decrease in performance after completing one of the protocols (in particular P1 or P3), this would possibly indicate a low resistance to fatigue and poor recovery ability between repeated-sprint efforts. As a result, it can be assumed that the tolerance to intense training and competition loads is limited and the risk of re-injury is increased.
Some methodological limitations are to be considered. Most of the players completed the testing and training procedures before and during the pre-season preparatory period. However, due to time and organisational reasons, some players carried out the test and training programs during the season. Consequently, we cannot rule out differences in fitness levels between players which may have affected our results. Additionally, the running paths were automatically generated in a randomized form by the SC software, corresponding to a sprint distance of either 20 or 30 m travelled per interval. However, due to technical reasons, these were only approximate values that deviated from the actual sprint distances covered. The mean variability in sprint distances travelled ranged from 4.5 to 6.5 m across all protocols, which resulted in total differences of 16 to 27 m between players. Therefore, small differences in exercise volume could have had an impact on our results. Finally, we did not record ratings perceived exertion (i.e., RPE) nor did we measure VO2max and blood-borne markers (e.g., creatine kinase) to verify our explanations from a mechanistic perspective. Body fat was also not recorded. However, this should be taken into account in future approaches.
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