The main result of the present study was a very large correlation (r = − .91) between HRR and v4, implying a link between the ability to recover from SSGs and the general aerobic endurance performance of semi-professional soccer players. Thus, besides being associated with endurance capacity, v4 may also help to evaluate soccer players’ specific ability to recover. Furthermore, the present investigation proved that soccer players with a higher v4 accumulated less lactate during SSGs (explained variance, 35%). Simultaneously, a better performance in the ITT was related with a higher number of accelerations (nacc) during SSGs (explained variance, 36%). Interestingly, TD and distances in predefined speed zones (WAL, JOG, RUN, HIR) did not show any relevant linear association to v4.
In a recent study, Schwesig et al. [11] investigated the differences in running velocities at lactate thresholds of 2, 4, and 6 mmol L−1 among male soccer players (n = 152; 3rd and 4th German league). Significant performance differences between playing positions were detected only between field players and goalkeepers. In comparison with the total sample of Schwesig et al. [11], the endurance of the investigated soccer players at v4 ranged between the 25th (14.4 km/h) and 50th percentile (15.0 km/h). Consequently, the aerobic endurance performance level of the recruited sample can be classified as normal to good. In addition, Altmann et al. [28] analyzed the physical capacity of soccer players from the 2nd German league utilizing a similar test design (start speed, 6 km/h; duration per speed level, 3 min; increment, 2 km/h). These authors found a slightly higher v4 (15.1 km/h) in field players (regardless of playing position) compared to the current study. For the 3rd league of Greece, Kalapotharakos et al. [10] showed a clearly lower endurance performance (v4, 12.3–13.7 km/h) using identical increments and speed level durations.
In the majority of research investigating the physiology of SSGs in soccer ØHR, BLC and the rating of perceived exertion (RPE) were used as indicators of exercise intensity [22, 23, 29]. In a representative study, Rampinini et al. [30] examined the effects of different modalities (e.g., number of players, pitch size, coach encouragement) on the exercise intensity in repeated bouts (3 × 4 min with 3 min recovery) of SSGs without goalkeepers. In the most physically intense variant (3 vs. 3 on a pitch of 18 × 30 m), RPE was 8.5 ± 0.4, BLC was 6.5 ± 1.5 mmol L−1, and ØHR was 91 ± 2%. A similar load protocol (6 × 2 min with 2 min recovery) to that used in the present study was applied by Köklü et al. [31]. Besides other forms, the authors investigated a 4 vs. 4 (without goalkeepers; game objective: keep ball possession; pitch dimensions, 25 × 32 m) and determined values of 5.2 ± 1.3, 7.9 ± 2.2 mmol L−1, and 85 ± 3% for RPE, BLC, and ØHR, respectively. Furthermore, the authors reported that around 77% of TD was covered with running velocities below 13 km/h. In comparison, the players in the current study covered only 61% of the distance at speeds below 13 km/h, indicating a clearly higher intensity, which in turn explains the greater ØHR and BLC (Table 1). BLC at a similar level as in the current study (11.4 ± 3.31 mmol L−1) has been reported by Castagna et al. [32] in repeated bouts (4 × 30 s with 150 s recovery) of 1 vs. 1 SSGs with mini-goals (1.5 × 2 m) played on a relatively large pitch (30 × 20 m). The SSGs were performed all-out; ergo, the players were told to cover as much distance as possible to create maximal effort (RPE, 8 ± 1). In total, the players covered a distance of 601 ± 54 m, resulting in an average velocity of about 18 km/h and corresponding to more than twice the mean velocity of our study (8.7 km/h). This high average velocity may have been due to Castagna et al. [32, 33] focus on the development of players’ extended sprint ability. For this purpose, the authors selected an exercise density of 300 m2 per player and a work to rest ratio of 1:5. Given the fact that all available studies on SSGs reported clearly lower values for BLC, the current study’s chosen load protocol appears to be particularly suited to provoke a very high BLC in a soccer-specific test situation. Thus, this also enables the evaluation of players’ anaerobic capacity that is an important precondition to cope with high-intensity phases during matches more frequently and for longer periods of time [32]. All things considered, the SSG protocol utilized in the present study caused a very high internal load for the players involved. Since the investigation has been carried out during the pre-season period, it is to be assumed that the fitness level was comparably low. For example, Rampinini et al. [30] collected their data from September to June (except for the period between December and January).
As can be seen in Fig. 2, the individual differences in TD were comparably small (CV < 8%), whereas the running performance was more variable among the players in the upper speed zones (RUN and HIR) resulting in CV values higher than 30%. This finding may represent divergent individual player profiles (continuous vs. explosive) or playing styles (playmaker vs. dribbler) that can only be affected to a limited extent by the modalities of the SSGs. Furthermore, the activity profiles of the players were completely independent from their individual aerobic performance. This fact becomes more notable, since the worst (P1) and the best player (P14) with respect to v4 showed nearly identical activity profiles. However, the acceleration efforts (nacc) showed a moderate linear association to v4. Considering that accelerating is more energetically demanding than moving at a constant velocity [34], better aerobic and anaerobic capabilities may enable players to execute accelerations more frequently within a defined timeframe. Moreover, peaks in the acceleration also reflect soccer-specific movements (i.e., starts, turns, directional changes) and thus maybe more relevant for the purpose of performance diagnostics in SSGs than distances in defined speed zones.
The ability to recover (as expressed by HRR) following a soccer-specific test (especially in SGGs) has not been investigated until now. Consequently, no evidence is available for the relationship between the ability to recover and endurance performance. Faster HRR is due to the faster reactivation of vagal activity [15]. Seiler et al. [35] pointed out that highly trained runners showed a higher vagal activity after intensive exercise over several hours compared to those with less running experience. Furthermore, individual resilience strongly affects the ability to recover. In the present study, an inverse association of v4 and LSSGs was observed. This is in line with results reported after repeated-sprint exercise [36]. In a previous investigation, Buchheit et al. [36] concluded that an increased concentration of plasma metabolites of anaerobic metabolism causes a delay in post-exercise vagal reactivation, which also delays HRR.
The reader should be aware of the limitations of the current study. First of all, it has to be remarked that the here presented outcomes and relationships are based on only one observation among a relatively small sample of 14 players. Secondly, the study was conducted during the first week of pre-season preparation. Hence, different results might be expected during the competitive season when the physical fitness level of players may be improved. To avoid overinterpretation, the preliminary findings of the current study should be re-evaluated (external validation). The external validation should be performed with different samples and performance levels (e.g., 1st, 2nd, and 3rd leagues). Further, it should be noted that the evaluation of internal load and recovery was only at the cardiac level (HR, HRR). However, recovery and fatigue are multidimensional (e.g., metabolic, neural, hormonal) phenomena. Thus, in future studies, different outcomes (e.g., creatine kinase, urea, muscular stiffness) should also be considered.
