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Original Research

Effects of Additional Repeated Sprint Training During Preseason on Performance, Heart Rate Variability, and Stress Symptoms in Futsal Players

A Randomized Controlled Trial

Soares-Caldeira, Lúcio F.1,2; de Souza, Eberton A.3; de Freitas, Victor H.1; de Moraes, Solange M.F.4; Leicht, Anthony S.5; Nakamura, Fábio Y.1

Author Information
Journal of Strength and Conditioning Research: October 2014 - Volume 28 - Issue 10 - p 2815-2826
doi: 10.1519/JSC.0000000000000461
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Abstract

Introduction

The preseason period for team sport athletes involves very high training loads (40) with several physical capacities being developed in concomitance. Ideally, the main physical capacities determining performance in a given sport are substantially improved over the preseason, and thereafter maintained or even further improved during the competitive season (50). In most team sports, body composition, the ability to repeat sequences of sprints and intermittent high-intensity exercises, speed, and lower-limb muscle power have been identified as paramount qualities for improvement during the preseason (13,18,50). Repeated sprint ability (RSA) has been recognized as one of the most relevant components of team sport athletes' fitness (5). Because of the high demand of repetitive high-intensity running and sprinting in futsal (4), soccer (53) and Australian Rules Football (26), RSA has been highly regarded as an important capacity for competitive players (20). Subsequently, repeated sprints (RS) have been used as the main training method to improve RSA with supplemental repeated sprinting reported to improve RSA and Yo-Yo Intermittent Recovery Test, level 1 (YoYo IR1) performance to a greater extent than aerobic interval training sessions for soccer players (22). This training specificity for RSA has been recently highlighted with further studies recommended to identify the optimal volume and duration of RS training interventions (5). In contrast, others have demonstrated enhanced RSA, intermittent field test performance (resembling YoYo IR1), and postexercise parasympathetic function (assessed by heart rate variability [HRV]) after high-intensity aerobic/anaerobic interval training compared with RS training (12). Repeated sprint training during competitive season may have been less effective, as it may lead to excessive physiological and mental overloading of athletes (i.e., stress) hypothetically, suggesting fatigue accumulated that impairs RSA development (35). Because of the equivocal results of studies examining RS training, further examination is warranted regarding the value of RS training for athletes and coaches.

While traditional preseason training has involved aerobic and strength foci for futsal, soccer, and Australian Rules Football (17,35,50), the supplementation of standard training with RS training may favor improved performance without side effects such as overreaching (19,27,51) due to its relatively low volume. Nevertheless, any supplemental training loads need to be carefully dosed to avoid undesirable reduction of performance during the preseason, which can negatively affect the entire competitive season (42). Several ways to control for excessive training loads that lead to overreaching include monitoring of athlete's stress through questionnaires like the Daily Analysis of Life Demands for Athletes (DALDA) (19,27) or autonomic imbalances through HRV (30). Recent studies have demonstrated that cardiac autonomic modulation as assessed by HRV is responsive to training effects and helps to identify significant adaptations in synchrony with positive field performances (7,12,50,52). Therefore, such monitoring may provide important guidance for the development of the optimal volume and duration of RS training interventions (5).

Therefore, the aim of this study was to investigate whether supplementing regular preseason futsal training with weekly sessions of RS training would have positive effects on RSA and other performance indicators (e.g., vertical jump and YoYo IR1). It was hypothesized that supplemental RS training would improve performance in comparison with regular training alone. Furthermore, it was hypothesized that this training-induced improvement of performance would occur without inducing unnecessary and undesirable stress (as assessed by DALDA and resting cardiac autonomic modulation [HRV]).

Methods

Experimental Approach to the Problem

This study involved a parallel 2-group, randomized, longitudinal design (Figure 1). Elite futsal players of 1 team were enrolled in the study and included initially 12 outfield players and 2 goalkeepers. The 14 players were randomly assigned to a normal training group (NormT; n = 7), or a group with additional RS training sessions (AddT; n = 7) using a specifically designed Web site (www.random.org) by an independent researcher who had no direct contact with the athletes and assessments. The training intervention lasted for the full duration of the team's preseason (i.e., 4 weeks). All players were assessed for resting HRV, psychological sources and symptoms of stress, and physical performance (e.g., body compositions, speed, aerobic and anaerobic capacities, and lower-limb power) before, during, and after the preseason training period. Rest measurements and exercise testing sessions were scheduled on Monday (resting HRV and aerobic)/Tuesday (jump tests, RS test, and anthropometric assessments), after a minimum of 48 hours from the last training session, and on the same days of the week after the last intervention training week. The rest measurements and physical performance tests were assessed in the morning, whereas anthropometric assessments were conducted in the afternoon.

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Figure 1:
Description participant recruitment and progression through each stage of the study. AddT = additional repeated sprint training group; NormT = normal training group.

Subjects

Fourteen players (21.4 ± 5.5 years, 72.2 ± 8.8 kg, 172.3 ± 5.7 cm) were recruited from a professional futsal team competing in the first division of the State of Paraná, Brazil. The study procedures were approved by the local research ethics (220/10) committee with participants and guardians of under-aged players (n = 2) providing voluntary, written informed consent before participation in the study. The inclusion criteria for the study were: (a) regular participation in more than 90% of training sessions during the period of investigation; (b) not suffering from injuries during the same period; (c) not taking any medication that could alter the outcomes of this study; and (d) minimum experience of at least 2 years of high-level futsal playing and training. During the preseason period, 1 athlete from the AddT group was transferred to another team with their data removed from all analyses.

Training Sessions

The athletes attended either 1 or 2 daily training sessions as planned by the coaching staff with Sunday used as a nontraining day for the entire preseason. The technical, tactical, and physical training sessions of the preseason (40 in total) are displayed in Table 1. The training sessions included specific and generic exercises for athletes according to their positions (i.e., goalkeepers and outfield players). The technical training involved kicking, dribbling and drills, passing, heading, and tackling exercises. The tactical training included typical activities such as attack patterns, marking pressure, half-court and small-sided games, rehearsed plays, games and cognitive corrections, counterattack with and without goalkeeper line. The physical training sessions were performed on the playing court and beach sand and included continuous and interval-based runs, sprints, jumps, and traction accelerations exercises to improve aerobic and anaerobic fitness. The sand-based sessions were designed to induce lower muscle stress (48) and improve athletes' sprint and jump performance (36) while reducing the risk of injury. Additionally, plyometric and resistance training (RT) with open and closed kinetic chain (dynamic and isometric contractions) training were performed to improve players' strength and power. Mixed training sessions were applied combining sessions such as technical and tactical training and physical and technical training.

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Table 1:
Typical 4-week preseason training program for futsal players.*

The 2 training groups performed the same training sessions with the AddT group performing an additional RS training session before some of the regular training sessions. The RS training consisted of: weeks 1 and 2, 2 bouts of 6 × 30 m sprints with 20 seconds of rest between sprints; week 3, 2 bouts of 7 × 30 m sprints with 20 seconds of rest between sprints; and week 4, 2 bouts of 8 × 30 m sprints with 20 seconds of rest between sprints. The passive recovery time between bouts of RS was 5 minutes each week. To minimize athletes' improvement of RSA performance through improved change of direction ability (9), the additional RS sessions were conducted using a straight-line running format with each sprint being performed at maximal velocity.

Procedures and Testing

Warm-up

All participants performed 5 minutes of warm-up comprising ∼1 minute of light active static stretching (10 seconds for hamstrings, quadriceps, and calf muscles) and 2 minutes jogging, followed by short distance accelerations (2 submaximal sprints, progressing to ∼90% of their maximal velocity for the shuttle distance [20 + 20 m]). This routine was supervised by the team's physical trainer before the squat jump (SJ), countermovement jump (CMJ), YoYo IR1, and RSA tests.

Yo-Yo Intermittent Recovery Test Level One

All participants performed the YoYo IR1 on an indoor synthetic floor court (i.e., 40 × 20 m). This test consists of running consecutive 20-m shuttle runs (back and forth) at a progressively increasing velocity with 10 seconds of active recovery after each 40 m. The test started with 4 shuttle runs at 10–13 km·h−1 (0–160 m) followed by 7 shuttle runs at 13.5–14 km·h−1 (160–440 m). Thereafter, the test continues with stepwise increase of 0.5 km·h−1 in speed for every 8 shuttle runs. The recovery zone was delineated 5 m behind the starting line with the test speed governed by a beep emitted by an amplifier coupled to a computer. The test finished when the athletes failed to reach the finish line simultaneously with the audio beep on 2 consecutive runs within the same stage or by volitional exhaustion. Up to 7 athletes performed the test simultaneously with strong verbal encouragement provided to the athletes throughout the test. Total distance was reported as the performance criterion in the YoYo IR1 (2).

Vertical Jump

To evaluate the power of the lower limbs, the SJ and CMJ tests were conducted using a jump platform (Multisprint; Hidrofit, Belo Horizonte, Brazil) to estimate jump height. For both SJ and CMJ tests, the maximum height from 3 jumps was recorded. For the SJ, no arm swing was allowed, and the athletes lowered themselves to approximately 90° flexion of the knee and remained still on the jump platform for 3 seconds before jumping. The CMJ was performed with arm swing allowed, and the jumps were performed explosively using the stretch-shortening cycle from an initial upright standing position. The interval between each jump was ∼45 seconds for all tests.

Repeated Sprint Ability Test

The RSA test was performed on an indoor synthetic floor court (40 × 20 m) and consisted of performing 6 × 40 m maximal shuttle sprints with 180° (20 + 20 m) direction change, interspersed with 20 seconds of rest between each sprint. The time of each shuttle sprint was recorded by a photoelectric cell (Multisprint). The athletes started the test 0.5 m behind the starting line and were instructed to run as quickly as possible for all sprints. Indices assessed for the RSA test were RSAbest (the quickest sprinting time), RSAmean (mean time of the 6 sprints) and RSAworst (the slowest sprinting time). The degree of fatigue experienced by athletes during the RSA was calculated as the RSAdecrement using the equation proposed by Fitzsimons et al. (23):

where ideal time = 6 × RSAbest (s); and total time = the sum of all sprints (s).

Heart Rate Variability

Heart rate variability at rest was calculated from the recording of R-R intervals using a RS800 heart rate monitor (Polar Electro Oy, Kempele, Finland). The athletes remained at rest for 10 minutes in the sitting position with the last 5 minutes analyzed for resting HRV (i.e., during signal stabilization). The reliability of the RS800 for R-R interval measurements has been reported to be very good with a low standard error of measurement (61–79 milliseconds) and a high intraclass correlation coefficient (ICC) (0.80–0.86) for short-term repeated measures (49). Furthermore, the reliability of all HRV assessments (i.e., repeat assessment of 50 recordings by the same researcher) was determined as excellent for the current study with an ICC (90% confidence intervals [CIs]) of 0.998 (0.995–0.999). Correction of ectopic beats and/or erroneous signals was performed automatically using the manufacturer's software (i.e., Polar Pro Trainer 5.0, Polar Electro Oy, Kempele, Finland) with the degree of correction <3% for all recordings. In addition, artifacts were manually removed and replaced by the interpolation of adjacent R-R intervals. The resulting R-R intervals were analyzed in the time domain, in the frequency domain using spectral analysis (Fast Fourier Transform), and nonlinearly through the Poincaré plot (Kubios HRV Analysis v 2.0, Biosignal Analysis and Medical Imaging Group at the Department of Applied Physics, University of Kuopio, Kuopio, Finland). The time domain indices analyzed included the standard deviation of all normal R-R intervals (SDNN) and the root mean square difference of successive normal R-R intervals (rMSSD). In the frequency domain, oscillations of RR intervals were examined within the low-frequency (LF: 0.04–0.15 Hz) and high-frequency bands (HF: >0.15–0.40 Hz). The frequency domain indices were expressed in both natural logarithm-transformed absolute (ln m2) and normalized units (n.u.). The sympatho-vagal balance was obtained by the ratio of the LF to HF (LF/HF) bands. The Poincaré plot was examined, and the transversal (SD1) and longitudinal (SD2) axes of the ellipse-like dispersion were calculated. The SD1, rMSSD, and HF indices have been reported to reflect vagal modulation, whereas SD2, SDNN, and LF comprise both sympathetic and vagal cardiac modulations (46).

Anthropometry

To assess body composition, skinfold measurements were obtained by an experienced anthropometrist using a caliper (Cescorf, Porto Alegre, Brazil) with an accuracy of 0.1 mm. Body mass was determined by an electronic scale with an accuracy of 0.1 kg and height assessed by a wooden stadiometer with an accuracy of 0.1 cm. Three measurements per skinfold site were obtained in a rotational fashion, and the median value was recorded. The skinfold measures were assessed on the left side of the body and included abdominal, suprailiac, triceps, subscapular, pectoral, axillary, and medial thigh with body density estimated using the Jackson and Pollock (39) equation. Body fat percentage was calculated using the Brozek et al. (8) equation, whereas lean body mass (LBM) in kilograms (kg) was calculated as follows:

Training Load

Participant's training loads were monitored daily using the session rating of perceived exertion (RPE) method (24). The training loads of all athletes were quantified by the session-RPE method involving the product of training duration (minutes) by RPE using the CR-10 Borg scale (6). Players were asked to rate how hard they found the training session, approximately 15–20 minutes after its completion. On days with 2 training sessions, the daily training load was taken as the sum of the sessions performed. The total weekly training load (Σ session-RPE), the average daily loads (mean session-RPE), monotony, and strain were determined for each athlete. Monotony was calculated by dividing the mean session-RPE by its SD, whereas strain was calculated by multiplying the Σ session-RPE by monotony (25).

Psychometric Responses

After the last training session of the week (1), players were asked to identify their subjective ratings of well-being (SRWB) that comprised quality of sleep, fatigue, stress, and muscle soreness, using a scale of 1–7, ranging from very, very low, or good (1) to very, very high, or bad (7) (32). Additionally, the sources and symptoms of stress for athletes were also quantified on a weekly basis using the DALDA questionnaire (55). The athletes were given the DALDA questionnaire on the same day as the SRWB scale. The number of the “worse than normal” responses in part A and B of the DALDA were used to quantify the sources and symptoms of stress experienced by athletes, respectively. To evaluate weekly symptoms of training stress, the DALDA questionnaire and scales of subjective ratings of well-being as proposed by Hooper et al. (32) were completed by participants once per week (i.e., every Saturday) after the last training session of the day.

Statistical Analyses

The Gaussian distribution of data was verified by the Shapiro-Wilk test. Mean and SDs were presented when data were normally distributed, whereas median with interquartile intervals was presented when the data were not normally distributed. Differences between groups (AddT vs. NormT) and over time (pre- vs. posttraining) were examined through 2-way analysis of variance for repeated measures and Bonferroni post hoc comparisons. For nonparametric data (i.e., weekly responses from the DALDA questionnaire and SRWB scale), within-group comparisons were examined using a Friedman test and Wilcoxon post hoc test. The Mann-Whitney's test was used to compare questionnaire responses between groups (i.e., AddT vs. NormT) each week. The independent Student's t-test was used to compare the percentage change {% change = ([Post − Pre]/Pre) × 100} for both the AddT and NormT groups. Pearson's correlation was performed to assess the relationship between pretraining rMSSD and % change of rMSSD. Significance level was set at 5% (p ≤ 0.05) for all analyses that were performed in SPSS version 17.0 software for Windows. The Cohen's effect size (ES) with a 90% CI was determined and based on the pre- and posttraining variations for both AddT and NormT. The ES was calculated using a spreadsheet available at www.sportsci.org/resource/stats with the following threshold values used: <0.2: trivial; 0.2–0.6: small; 0.6–1.2: moderate; >1.2: large (33). The quantitative chances of higher or lower differences were estimated through magnitude-based inferences as follows using qualitative threshold inferences: <1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely 95–99%, very likely; 99%, almost certain. If the chance of having beneficial or unbeneficial performances and HRV were both >5%, the true difference was assessed as unclear (33). The Cohen's ES principle was applied to estimate the probabilities of whether differences were lower, similar, or higher than the smallest worthwhile difference or change (0.2 multiplied by the between-subject SD of pretraining value), as follows:

where SD is the pooled SD, and M is the mean of the pretraining values considering AddT and NormT. The constant 0.2 represent the Cohen's ES principle.

Results

The anthropometric and body composition variables did not significantly change over time and were similar between groups with no time × group interactions evident. Accordingly, magnitude-based inferences did not detect any worthwhile changes in body mass, % fat, and LBM for either group (Table 2).

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Table 2:
Physical characteristics, anthropometric profile, and performance results of athletes, before and after preseason training.*

No significant main or interaction effects were noted for SJ, CMJ, and RSAbest (Table 2). Although no group or interaction effect was detected, a significant time effect was identified for YoYo IR1 performance (F = 41.4; p < 0.01), RSAmean (F = 57.4; p < 0.01), RSAworst (F = 22.24; p < 0.01), and RSAdecreament (F = 8.61; p < 0.02). Nonetheless, the change (% change) for AddT and NormT was not significantly different for YoYo IR1 (p = 0.41), RSAmean (p = 0.50), RSAworst (p = 0.30), or RSAdecrement (p = 0.29). The only performance index exhibiting a moderate ES over the preseason was SJ (−0.62) in favor of NormT. The AddT group presented a “likely unbeneficial” effect for SJ while presenting a “possibly beneficial” effect for RSAbest.

No significant main effects for time, group, or group × time interaction were evident for HF (ln ms2 and n.u.), LF (n.u.), and LF/HF. Significant differences were observed over time for HR (F = 50.5; p < 0.01), iRR (F = 64.1; p < 0.01), SDNN (F = 11.9; p < 0.01), rMSSD (F = 11.6; p < 0.01), LF (ln) (F = 14.9; p < 0.01), SD1 (F = 9.5; p < 0.02), and SD2 (F = 11.2; p < 0.01) for both groups (Table 3). Significantly greater iRR (F = 4.6; p = 0.05) and less LF (ln) (F = 4.8; p ≤ 0.05) were observed for AddT compared with NormT at posttraining (time × group interaction). A small ES was identified for the HF (ln and n.u.), LF (n.u), LF/HF ratio, SD1, and SD2 indices, whereas moderate ES were observed for HR, iRR, SDNN, rMSSD, and LF (ln) indices (Table 3). The effects of training for both groups were rated as “unclear” for HF (n.u.), LF (n.u.), LF/HF, and SD2. For the AddT group a “likely beneficial” effect was observed for HR, iRR, SDNN, rMSSD, HF (ln), LF (ln), and SD1 compared with NormT (Table 3).

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Table 3:
Resting heart rate variability parameters for futsal athletes before and after preseason training.*

The mean session-RPE, ∑ session-RPE, monotony, and strain were significantly altered during the preseason with the lowest values at week 2 (Table 4). There were no differences between groups or group × time interaction for any workload variable (Table 4).

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Table 4:
Weekly indices of training load using session rating of perceived exertion method for additional training and normal training groups during preseason.*

During the preseason, no significant group differences were identified for DALDA part A or part B (Table 5). For SRWB, similar responses were observed between AddT and NormT for fatigue, muscle soreness, and sleep (Table 5). In contrast, NormT exhibited greater stress in week 2 compared with AddT (Table 5).

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Table 5:
Frequency of “worse than normal” responses for parts A and B from the DALDA and the fatigue, muscle soreness, quality of sleep, and stress ratings scales of the SRWB (median and interquartile intervals).*

When both groups were combined, pretraining rMSSD and the training-induced change of rMSSD (%) during the preseason were significantly correlated (r = −0.70, p ≤ 0.05; Figure 2). No significant correlations for any variable were identified for AddT or NormT, separately.

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Figure 2:
Relationship between initial or pretraining rMSSD values and the training-induced change of rMSSD (%) during the preseason. rMSSD = the root mean square difference of successive normal R-R intervals.

Discussion

The main results of the study were that additional RS training resulted in greater changes in parasympathetic cardiac autonomic modulation without higher training loads or higher psychometric responses (DALDA and SRWB) compared with normal preseason training. However, both groups improved their YoYo IR1 and RSA performance similarly, and thus, our hypothesis that additional RS training would improve performance was not confirmed.

The similar YoYo IR1 and RSA improvements between groups observed in our study partly contrasts with the results reported by Ferrari-Bravo et al. (22), who showed that RS training with changes of direction (COD) performed twice a week resulted in greater improvements for YoYo IR1 and RSA performance than high-intensity interval training in soccer players. According to others (9,11), the advantage of additional RS training demonstrated in some studies seems to be related to an increased motor coordination to perform COD and consequently the RSA shuttle test. It has also been shown that for soccer players, RS training improved RSA performance and maintained maximal speed compared with explosive strength training (11). However, it should be noted that our study applied straight-line RS training in contrast to the aforementioned studies of RS training with COD. In the current study, both training groups exhibited similar lower-limb stretch-shortening power improvement, assessed by CMJ gain, reinforcing the suggestion that RS training without COD (AddT) was not sufficient to induce further improvements in performance (9). Similarly, additional RS training did not improve 10- and 20-m sprint test performance compared regular training in elite badminton players (57). Collectively, these previous results and similar improvement of RSAbest for all in the current study suggest that sprinting performance is not easily changed in highly trained athletes with additional RS training, even during the preseason when most fitness components are developed (50). The only negative result found for performance was for SJ (reduced by 6%), which was identified as “likely unbeneficial” for AddT compared with NormT. In contrast, a “possibly beneficial” effect for AddT was noted for RSAbest. Nevertheless, when both groups were combined, significant improvements in RSAmean (∼2.5%), RSAworst (∼4%), and RSAdecreament (∼25%) resulted from preseason training, whereas no differences for RSAbest, SJ, and CMJ performances were noted over the preseason.

High-intensity intermittent activities and high physiological demands are required during futsal games (4) and have been related with YoYo IR1 performance in other sports (43). Additionally, futsal players perform 5–12% of game-time at high-intensity running (speed >15 km·h−1) (4). In the present study, the average of the final velocity for the YoYo IR1 test was ∼17 km·h−1 posttraining for all athletes and required athletes to perform high-intensity shuttle running (>15 km·h−1) similar to that experienced during competition (4). In the present study, additional RS training and regular training induced a similar improvement in YoYo IR1 performance during the preseason. The similar changes in YoYo IR1 performance (∼27%) was likely due to an increase in aerobic (2) and anaerobic capacities (16,31) incorporated into both training regimes during the preseason period. In a similar way, performance improvements have been reported by others. For instance, Oliveira et al. (50) showed a 20% increase in YoYo IR1 after a short (3 weeks) preseason of futsal players and 28% increase after a 7-week preseason period for elite futsal players (3). Finally, our results suggest that using regular futsal training programs (including friendly games, physical, strength, tactical, and technical training, and also RS training) that match with the high physiological demands of futsal play (15) is sufficient to result in positive adaptations associated with performance in RSA and YoYo IR1 tests (34).

Currently, there is limited information regarding additional RS training following preseason on the cardiac autonomic responses in futsal players. Significant positive changes in HRV after a 4-week preseason in this study suggest improvements in athlete's cardiac autonomic modulation, a finding previously identified in futsal players after a short 3-week preseason (50). Indeed, positive changes in cardiac autonomic responses after training have been associated with improvements in maximal aerobic performance (10,29,30) and with training load (31,56). Consequently, HRV indices have been used as a training-monitoring tool (52) to identify adaptations in athletes (7). In the present study, additional RS training resulted in positive changes of HRV (i.e., SDNN, rMSSD, HF, LF, and SD1 indices) compared with NormT. At first glance, this positive response of AddT may indicate several complex physiological mechanisms for the preseason change (21,28,30), most likely due to a higher training volume (14,41). However, caution should be taken when interpreting this result as the AddT group tended to have lower vagal-related HRV indices (e.g., rMSSD) at pretraining. Therefore, the greater increase in vagal-related HRV indices for AddT group may reflect the initial HRV values with a strong inverse correlation (r = −0.70; P ≤ 0.05) identified between baseline HRV and its preseason change (Figure 2). The major changes observed in vagal-related indices with additional RS training may well be due to the greater training intensity needed for HRV change in healthy young and fit participants (44). Therefore, future studies are needed to elaborate on the trainability of the cardiac autonomic system with and without RS training for team sports.

The session-RPE method to determine the magnitude of internal load for intermittent sports has been well established (25,37,46). High training loads and monotony scores (>2 a.u.) have been related to a greater incidence of overtraining in athletes (24). In our study, athletes reported low monotony scores and average weekly training loads. Previously, Milanez et al. (47) reported that average weekly training loads for futsal players were 2,876–5,035 a.u., values that were similar or slightly lower than the current futsal players [AddT: ∼5,064 (4,027–6,134) a.u.; NormT: ∼4,385 (3,617–4,904) a.u., p > 0.05]. The training loads observed in the second week were unusual and likely reflected the inclusion of public holidays and less training days compared with the other weeks of the preseason. Interestingly, in the present study additional RS training did not lead to higher session-RPE for futsal players compared with regular futsal training throughout the preseason using traditional statistical procedures. Although, the mean session-RPE and the ∑ session-RPE of the entire 4-week preseasons were “likely higher” for AddT with strain and monotony also “possibly higher” for AddT. The group difference in ∑ session-RPE could be explained by the higher training volume of AddT group which performed ∼20 minutes of additional RS training.

During the preseason, weekly stress symptoms were similar for both AddT and NormT groups, demonstrating that additional RS training did not lead to higher stress symptoms in futsal players. Several studies have suggested that the DALDA (Part B) was sensitive to changes in training loads (19,54), although we observed differences in training loads, with no changes in preseason DALDA responses. In this study, most responses were ≤5 points, which are considered below critical values for stress accumulation in SRWB (32). Furthermore, a reduction in fatigue, muscle soreness, and stress for athletes indicated training-induced adaptations (38). Previously, the improvement of quality of sleep was associated with positive adaptations in parasympathetic control after applying cold-water immersion as a recovery method in highly trained swimmers (1). We found positive effects in field test performance and vagal-related indices by HRV despite the impairment in quality of sleep within the last week (for AddT and NormT) compared with the first week of preseason. Stress symptoms can be affected by several factors that result in a complex outcome. For instance, in the present study both groups, throughout preseason, reported similar DALDA responses despite significant differences in SRWB scales and no noticeable anecdotal signs of overtraining. Further studies examining stress and training loads in elite athletes may clarify the role of monitoring tools (e.g., HRV) to assist athletes and coaches in optimizing preseason loading patterns.

In summary, the major finding from the present study was that regular futsal training applied during the preseason led to improvements in performance in the YoYo IR1, RSAmean, RSAworst, and RSAdecreament, but not in RSAbest, SJ, or CMJ. Compared with normal futsal training, additional RS training conducted in a straight-line improved cardiac autonomic modulation to a greater extent with similar improvements in performance tests. These findings suggest that the cardiac autonomic responses may be an important and sensitive tool to assess adaptations for different types of training compared with performance testing (7) in futsal players. Furthermore, additional RS training did not result in greater weekly stress symptoms evaluated by psychometric tools. Therefore, addition of RS training may provide a simple and effective method to enhance cardiac autonomic control with minimal negative impact on athletes' performance and stress during the preseason. Studies examining additional RS training, with and without COD, may clarify the long-term benefits of RS to enhance HRV and performance in futsal players.

Practical Applications

Additional RS training can be used as an alternative strategy to apply high-intensity exercises with low volume to improve physical conditioning and performance in futsal athletes during preseason. Nonetheless, no major improvement in field test performance and no stress accumulation were observed with additional RS training in comparison with futsal regular training, which also requires performing repeated sprints. Subsequently, coaches can incorporate additional RS sessions for athletes who need to improve RS ability without risking the development of nonfunctional overreaching or sympathetic-vagal imbalance during the preseason. Possible positive changes for vagal-related indices assessed by HRV indicated a major physiological improvement of athletes to assess training-induced effects. In this way, HRV monitoring may be a critical tool during routine training to identify important physiological adaptations in athletes (7,50,52,56). In conclusion, additional RS training, twice a week, provided a simple and effective training regime for futsal coaches and athletes with possible positive effects on cardiac autonomic regulation during preseason without impairment in psycho-physiological stress.

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Keywords:

intermittent field test performance; cardiac autonomic control; psychometric assessments; session rating of perceived exertion

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