Effect of repeated sprint training on isokinetic strength parameters in youth soccer players

Abstract

BACKGROUND:

Sprint drills are part of the soccer training routine for their resemblance to match activities. There is a lack of information in the literature about the changes in isokinetic skeletal muscle strength in response to sprint training.

OBJECTIVE:

This study evaluates the effects of eight weeks, on-season repeated sprint training with a change of direction on isokinetic strength parameters in well-trained youth soccer players.

METHODS:

Nineteen well-trained youth soccer players volunteered to participate in the study. The training program included three sets of six, 40 m (20 $+$ 20 m) shuttle sprints twice a week for eight weeks. Isokinetic peak moment, work, and power of knee extensors and flexors were measured at angular velocities of 60, 180 and 240 ${}^{\circ}$ /s. Pre- and post-training valid isokinetic sector data were compared.

RESULTS:

Following the training period, dominant and non-dominant legs’ peak moment, work, and power values for both extensor and flexor muscle groups improved significantly at various angular velocities.

CONCLUSIONS:

Isokinetic strength enhancement may be explained with the induction of muscle hypertrophy following a prolonged period of sprint training.

Keywords

Muscle strength valid isokinetic sector

1. Introduction

Professional soccer has an intermittent characteristic in which walking and low-intensity running are interspersed with high-intensity activity periods [1]. In a typical soccer match, a player conducts 1000 to 1400 short-time actions with frequent changes every 4 to 6 seconds and sprints approximately every 90 seconds, with an average duration of 2–4 seconds for each sprint [2]. Although high-intensity activities constitute a small portion of the total match activities, they have an intense impact on the result of the game such that straight sprinting is the most dominant action when scoring goals [2, 3]. For these reasons, sprint drills are regularly included as a part of the soccer training program [4, 5]. Repeated sprint ability (RSA) is defined as the athlete’s ability to produce the best possible average sprint performance (15–40 meters) over a series of sprints (3–15 repetitions), separated by short (15–20 seconds) recovery periods [5, 6]. Coaches prefer such exercise sessions due to their resemblance to those activities during actual matches [7].

Dynamic muscle strength (concentric or eccentric) rather than static (isometric) is commonly considered an essential component in sprinting. Sprint performance is a direct result of the impulse (the product of force and time) applied by the athlete against the ground during the propulsive phase of the gait cycle. The driving force generated during the braking phase is related to the strength of the hip flexors, hip extensors, knee extensors and plantar flexors [8]. In other words, the athletes who have greater strength in these muscle groups may produce faster sprinting times. In the literature, there are studies that evaluated the correlations between isokinetic muscle strength and sprint performance [8, 9]. Those studies’ main finding is that a correlation exists between peak knee extensor and flexor moment at the measured velocities (60, 150 and 240 ${}^{\circ}$ /s). These correlations became stronger as the test velocity increased. However, there are also some studies that failed to indicate a significant relationship between strength measures and single sprint performance [10, 11, 12, 13]. The lack of a significant relationship is generally linked to the population characteristics of the subjects (gender, age, body size or their level of training experience) [12, 13]. Even though a linear relationship between muscular strength and single sprint performance is studied extensively, there is a lack of information in the literature about the changes in skeletal muscle strength in response to sprint training.

The injuries that soccer players encounter mostly occur in the lower extremity and nearly half of them are related to hamstring muscles [14]. When the lower leg decelerates in the late swing phase of the gait cycle, the eccentric contraction of the hamstring in response to rapid and active knee extension cause hamstring injuries. Also, the hamstrings are prone to injury, when they function as active hip extensors during a rapid change from their eccentric to concentric action in the early swing phase [15]. Even though there are some controversies, an imbalance in hamstring/quadriceps muscles’ strength ratio (H/Q) is accepted as one of the causes of hamstring strains [16]. Thus, any kind of intervention that has the potential to improve hamstring muscles’ strength may prevent injury by increasing the H/Q ratio.

As mentioned above, coaches prefer training drills that show similarities with match activities on-season. When the relationship between sprint performance and muscle strength is taken into consideration, training modalities such as sprint may constitute a very valuable and practical drill to improve the players’ muscle strength in addition to an increase in speed and agility. This study evaluates the effects of eight weeks on-season repeated sprint training with a change of direction on isokinetic strength parameters in well-trained youth soccer players.

2. Methods

2.1 Subjects

Nineteen well-trained youth soccer players (17.5 $\pm$ 0.5 years, 175.1 $\pm$ 4.1 cm and 67.4 $\pm$ 5.2 kg) competing in the Turkish regional youth championship, participated in the study. The participants had a soccer training history of at least four years, having 4–6 weekly soccer training sessions with their team, and they regularly trained 8–12 h/week. They also played at least one match per week. The participants were all free from chronic health problems. Also, none of the participants was excluded because of irregular participation in the training program. The Ethics Committee of Çukurova University Faculty of Medicine has approved this study (91/17/04.09.2019) and, carried out under the Declaration of Helsinki.

2.2 Anthropometric measurements

The participants visited the laboratory after 12-h of overnight fasting. Before the exercise, the same person performed anthropometric measurements and used a scale, stadiometer, and a nonelastic measuring tape for body mass, height, and circumference measurements, respectively. A Holtain calliper was used to measure skinfold thickness. Thigh circumference and skinfold were measured from the anterior aspect of the thigh midway between the inguinal crease and the proximal border of the patella. Anthropometric data were used to calculate body fat percentage and muscle mass [17, 18].

Table 1
Repeated sprint training program

Week	Training/week	Training volume (set $\times$ repetitions)	Recovery time between repetitions (seconds)	Recovery time between sets (minutes)
1	1	1 $\times$ 6	20	–
2	1	2 $\times$ 6	20	4
3	2	3 $\times$ 6	20	4
4	2	3 $\times$ 6	20	4
5	2	3 $\times$ 6	20	4
6	2	3 $\times$ 6	20	4
7	2	3 $\times$ 6	20	4
8	2	3 $\times$ 6	20	4
9	2	3 $\times$ 6	20	4
10	2	3 $\times$ 6	20	4

2.3 Maximal cardiopulmonary exercise test

The test was performed on a treadmill (HP Cosmos, Nussdorf – Traunstein, Germany), and breath-by-breath gas measurements were taken throughout the exercise using an indirect calorimetric system (PFT Cosmed, Rome, Italy). The system’s volume and gas calibrations were performed using a 3 L calibration syringe and calibration gases, respectively (16% O ${}_{2}$ and 5% CO ${}_{2}$ ). The heart rate was recorded continuously by telemetry using a heart rate monitor (Cosmed, Rome, Italy). The participants started the test at 6 km/h and 1% slope. The speed was increased by 1 km/h every min until exhaustion. The test was terminated if one or more of the following criteria were fulfilled: reaching up to 90% of the maximum heart rate according to the 220-age formula, formation of an oxygen uptake plateau (defined as an increase of no more than 2 ml/min/kg), continuation of a non-protein respiratory quotient (npRQ) value at and over 1.15 [19]. Cardiopulmonary parameters were averaged on 10 s periods, and the peak oxygen uptake was determined.

2.4 Experimental protocol

The study lasted for twelve weeks. Pre- and post-training tests were performed one week before and one week after the repeated sprint training program. The final evaluation was performed at least 24-h after the last training session. The first two weeks of the ten-weeks training program was used for preparation, in which the volume of the training increased progressively (Table 1). Following the preparation period, the number of sets increased to three for every training session (weeks 3–10). Every set consisted of six, 40 m (20 $+$ 20 m) shuttle sprints with 20-s rest between each repetition and 4-min rest between each set.

2.5 Repeated sprint ability (RSA) test

The RSA test consisted of six, 40 m (20 $+$ 20 m) shuttle sprints interspersed with 20-s of passive recovery [7]. The participants started from a line set 50 cm behind the photocell (Microgate, Bolzano, Italy), sprinted for 20 m, touched a line with one foot, turned their direction backwards, and returned to the starting line as fast as possible. After twenty seconds of passive recovery, the participants began to sprint again. The direction of rotation preference was left to the participants. The RSA tests (pre- and post-training) and sprint training were performed on a natural grass soccer field, and strong verbal encouragement was provided to each participant during all sprints.

Before the testing procedure, the participants completed a standardized warm-up session consisting of 15-min low-intensity running striding and three sub-maximal shuttle sprints. Following the warm-up session each participant completed a single preparatory 40 m (20 $+$ 20 m) sprint and rested for 5 min. This single preparatory sprint time was used as the criterion score during the subsequent RSA test. If the time of the first sprint in the RSA test was 2.5% greater than the criterion score, the test was terminated immediately, and the participants were rested for about 5-min. After resting, the participants were asked to sprint with a maximum effort to have a time duration less than the criterion score [7]. The best, mean sprint and percentage decrement (Sdec) for all six runs were calculated (Eq. (2.5)) and used for further analysis [20, 21].

$\displaystyle\text{Sdec}=100-(\text{total time/ideal time}\times 100),$

(1) $\displaystyle\text{where ideal time}=6\times\text{best time}.$
2.6 Isokinetic strength evaluation

Concentric peak moment (PM) was measured with a Cybex Norm dynamometer (Computerized Sports Medicine Inc., USA). The backrest angle of the dynamometer’s chair was 90 ${}^{\circ}$ throughout the measurements. Stabilization straps were placed over the participants’ pelvis and chest to fix their torso. The axis of knee rotation was aligned with the axis of the dynamometer’s armature rotation, and the lateral femoral epicondyle was used as the bony landmark. The ankle pad was attached approximately 3 cm above the dorsal surface of the foot at the level of the lateral malleolus to allow full ankle dorsiflexion. Gravity correction was performed before contraction bouts.

Before the isokinetic strength measurements the participants warmed-up with a standardized session for 10-min with a bicycle ergometer (Monark 894 E, Sweden) (five minutes cycling 1 kg load at 65 $\pm$ 5 rpm followed by two minutes cycling 0.05 gr/kg load at 65 $\pm$ 5 rpm. In the last part of the 10-min warm-up session, the participants performed three, 10-s submaximal sprinting and 50-s active recovery, with their individual final load). The participants performed stretching exercises before each isokinetic strength measurements.

The first set of the test protocol involved seven contractions of 240 ${}^{\circ}$ /s angular velocity for the participants’ familiarization to the device. The participants performed the same maximal contraction protocol from slow (60 ${}^{\circ}$ /s) to fast (240 ${}^{\circ}$ /s) angular velocities, as shown in Table 2. The range of motion of the knee joint was adjusted from full extension (0 degrees) to 90 degrees of flexion. The participants rested for 1-min between the sets. Following the training period, initial muscle strength measurement protocol was repeated to evaluate the effect of repeated sprint training. The same person from the research team gave strong verbal encouragement to ensure that maximum effort was exerted by the participants

Table 2
Muscle strength testing protocol (angular velocity $\times$ number of repetitions)

Isokinetic strength evaluation protocol
(240 ${}^{\circ}$ /s $\times$ 7) – 60-s rest (Data not used)
(60 ${}^{\circ}$ /s $\times$ 7) – 60-s rest
(180 ${}^{\circ}$ /s $\times$ 7) – 60-s rest
(240 ${}^{\circ}$ /s $\times$ 20) – 60-s rest

2.7 Valid Isokinetic Sector (VIS) data calculation

Knee extensor and flexor muscle groups’ pre- and post-training VIS data for PM, work, and power values were compared. The detailed procedure for calculating the VIS data was given in elsewhere [22]. Briefly, the acceleration and deceleration phases of contraction were determined by taking the first derivative of velocity with respect to the range of motion. The data points near zero in the first derivative curve (fluctuations equal to 0 $\pm$ 0.2) were accepted as isokinetic (VIS) and used to calculate VIS PM, work, and power. The values obtained by the analysis of the data were normalized with respect to body weight.

2.8 Statistical analysis

The results are presented as the mean $\pm$ standard deviation (SD). Statistical analysis was performed using Statistical Package for the Social Sciences (SPSS for Windows Version 22.0, USA). The Shapiro-Wilk test verified the Gaussian distribution of data. $T$ -tests were used to detect pre- and post-training differences. Paired samples $t$ -test and Wilcoxon signed-rank test were used for normal and non-normal distributed data, respectively. The meaningfulness of the effects is presented using Cohen’s d effect sizes (ES) with a statistical power analysis software (Gpower Ver. 3.1.9.4.) [23]. The effect sizes calculated according to the procedure proposed by Cohen [24], considering the following criteria: $>$ 0.2 (small), $>$ 0.5 (medium), and $>$ 0.8 (large). The level of significance was set as $p<$ 0.05.

3. Results

3.1 Physical characteristics

The physical characteristics of the participants are given in Table 3. Even though the participants’ body weights, body fat percentage, and peak oxygen uptake did not change, thigh circumference ( $p=$ 0.000, ES $=$ 1.49, Table 3) and percentage of muscle mass ( $p=$ 0.001, ES $=$ 0.96, Table 3) increased significantly.

Table 3
Pre- and post-training characteristics of the participants

	Pre-training		Post-training
Body weight (kg)	67.4	$\pm$ 5.2	67.3	$\pm$ 5.6
Height (cm)	175.1	$\pm$ 4.1	175.6	$\pm$ 4.4
BMI (kg/m ${}^{2}$ )	21.9	$\pm$ 1.4	21.9	$\pm$ 1.5
$\dot{V}$ O ${}_{\text{2peak}}$ (mL.min ${}^{-1}$ .kg ${}^{-1}$ )	57.4	$\pm$ 5.8	55.6	$\pm$ 3.8
Thigh circumference (cm)	50.9	$\pm$ 2.4	52.9	$\pm$ 2.9 ${}^{\ast}$
Thigh skinfold (mm)	11.0	$\pm$ 3.4	11.0	$\pm$ 3.4
% fat	10.9	$\pm$ 2.9	10.8	$\pm$ 2.7
% muscle	41.1	$\pm$ 2.0	42.6	$\pm$ 1.8 ${}^{\ast}$

Body mass index (BMI); peak oxygen uptake ( $\dot{V}$ O ${}_{\text{2peak}}$ ). ${}^{\ast}$ signifi- cantly different that pre-training values ( $p<$ 0.05).

Table 4

Repeated sprint ability test results of the participants before and after the training period

	Pre-training		Post-training
Repeated sprint ability test
Percentage decrement (%)	$-$ 5.49	$\pm$ 2.22	$-$ 5.7	$\pm$ 1.4
Best sprint time (seconds)	6.94	$\pm$ 0.16	7.02	$\pm$ 0.16 ${}^{\ast}$
Mean sprint time (seconds)	7.32	$\pm$ 0.2	7.42	$\pm$ 0.15 ${}^{\ast}$

${}^{\ast}$ , significantly different that pre-training values ( $p<$ 0.05).

Table 5

Statistical parameters of changes in isokinetic knee extensor and flexor muscle groups’ strength

	Angular velocity	60 ${}^{\circ}$ /s				180 ${}^{\circ}$ /s				240 ${}^{\circ}$ /s
		Dominant		Non-dominant		Dominant		Non-dominant		Dominant		Non-dominant
		Ext	Flex	Ext	Flex	Ext	Flex	Ext	Flex	Ext	Flex	Ext	Flex
Moment	$p$ value	NS	0.003	0.00	0.006	0.00	0.00	0.00	0.002	NS	0.049	0.014	0.001
	Effect size	–	0.98	1.02	0.71	1.31	1.10	1.21	0.84	–	0.50	0.65	0.94
Work	$p$ value	0.003	0.001	0.002	0.019	NS	0.012	NS	NS	NS	NS	NS	0.037
	Effect size	0.80	0.87	0.85	0.59	–	0.64	–	–	–	–	–	0.52
Power	$p$ value	0.017	0.005	0.001	0.018	0.00	0.008	0.00	0.006	NS	NS	NS	0.009
	Effect size	0.60	0.73	0.88	0.64	1.05	0.68	1.20	0.71	–	–	–	0.67

Ext: Extensors, Flex: Flexors.

Figure 1.

Comparison of pre- and post-training valid isokinetic sector peak moment, work and power data of the dominant and non-dominant leg extensor muscle groups. A. Peak Moment, B. Work, C. Power ( ${}^{*}$ significantly different than the pre-training value, $p<$ 0.05).

Figure 2.

Comparison of pre- and post-training valid isokinetic sector peak moment, work and power data of the dominant and non-dominant leg flexor muscle groups. A. Peak Moment, B. Work, C. Power ( ${}^{*}$ significantly different than the pre-training value, $p<$ 0.05).

After the training period, the participants’ best sprint time ( $p=$ 0.012, ES $=$ 0.64, Table 4) and mean sprint time ( $p=$ 0.041, ES $=$ 0.50, Table 4) had increased significantly. On the other hand, pre- and post-training percent decrement ratio did not show any significant difference.

3.2 Isokinetic knee extensor and flexor muscle groups’ strength evaluation

After the training period, the participants’ dominant leg extension PM values increased significantly at 180 ${}^{\circ}$ /s. Although PM at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s were increased, this difference was not significant. However, non-dominant legs’ extension PM values were increased significantly for all the angular velocities (Fig. 1A, Table 5). Following the training period, the work values of the dominant and non-dominant legs’ extensor muscles increased significantly at 60 ${}^{\circ}$ /s. Training did not induce any significant difference for the work values at 180 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s for both legs (Fig. 1B, Table 5). The dominant leg extensor muscle power had increased significantly at 60 ${}^{\circ}$ /s and 180 ${}^{\circ}$ /s. Similar to the dominant leg; the participants’ non-dominant leg extension power values were also increased significantly at 60 ${}^{\circ}$ /s and 180 ${}^{\circ}$ /s (Fig. 1C, Table 5).

The dominant leg flexor group muscle PM values increased significantly for all three angular velocities after the training period. Non-dominant leg peak flexion moment values increased as well for all the angular velocities (Fig. 2A, Table 5). The dominant leg flexor muscle work values increased significantly after the training period at 60 ${}^{\circ}$ /s and 180 ${}^{\circ}$ /s. On the other hand, non-dominant leg flexor muscle work value had increased significantly after the training period at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s (Fig. 2B, Table 5). A similar increase was apparent for the dominant flexor muscle power values and increased significantly at 60 ${}^{\circ}$ /s and 180 ${}^{\circ}$ /s. In addition to that, the non-dominant leg flexor muscle power increased significantly at all of the angular velocities as well (Fig. 2C, Table 5).

4. Discussion

The main finding of this study is that eight weeks of on-season repeated sprint training significantly increased the isokinetic strength parameters of soccer players despite the lack of an improvement in their sprinting performance. Changes in muscle mass together with repeated maximal stimulation throughout the sprinting activity, may explain the isokinetic strength enhancement. Moreover, to our knowledge, this is the first study that evaluates the change in isokinetic strength parameters in response to sprint training.

4.1 Morphological adaptations to sprint training

Skeletal muscle exhibits morphological and metabolic plasticity in response to different forms of physical activity. A mutual relationship exists between athletic performance and skeletal muscle’s morphology. Muscle cross-sectional area and muscle fiber distribution determine the contraction speed and strength of the muscle. Morphological characteristics of the skeletal muscle, which may determine athletic performance are influenced by the training. However, Esbjornsson et al. demonstrated muscle fiber transformations towards IIA from both IIB and I at the end of six weeks of sprint training performed three times a week [25].

Furthermore, it was shown that sprint training ranging in duration from 8 weeks to 8 months has a significant influence on fiber size [26]. Similar to fiber size, significant increases in total muscle volume have been observed following nine weeks of sprint cycle training [27]. Thus, our participants’ muscle mass increase might be explained by the induction of muscle hypertrophy following a prolonged period of sprint training. Maughan et al. evaluated the correlations of muscle strength with lean body mass and cross-sectional muscle area and found a significant relationship [28]. Masuda et al. examined the relationship between cross-sectional muscle area and muscular strength in terms of isokinetic knee extension and flexion among well-trained soccer players and found a positive correlation [29]. In our study, the increase in the thigh circumference without a change in thigh skinfold may point out an increase in the cross-sectional area of the thigh musculature. Thereby, these findings may explain the isokinetic strength improvement with the increase in muscle mass.

4.2 Muscular activation throughout sprinting performance

Evaluation of muscle activation pattern during gate cycling may help investigators to understand the series of events during sprinting. Electromyography signals are frequently used to analyze the sequence and duration of muscle activity [30]. This effort is also crucial for the interpretation of the effects and effectiveness of sprint training. Analysis of the hamstring muscle mechanics during a straight-line sprinting showed that peak eccentric contraction speed of the muscle was significantly higher during the late swing phase than the late stance phase. This indicates the importance of hamstring muscle in generating forward ground reaction forces during the propulsive part of the stance in sprinting [30]. On the other hand Hader et al. found that the acceleration-deceleration dynamics associated with repeated sprints with a change of direction require high levels of eccentric loading [31]. These investigators had shown that the EMG activity to be phase-dependent and change of direction sprints caused greater quadriceps and hamstring muscle activities during and just after the turn, which is generally associated with the highest deceleration (breaking force) and the highest re-acceleration (propelling force) respectively [31].

In our study, eight weeks of repeated sprint training improved isokinetic extensor and mainly flexor muscles’ strength parameters of the dominant leg (Figs 1 and 2). However, the flexor muscles’ PM was significantly higher for all the three angular velocities following the training period. PM reflects a certain angular point in the range of motion, whereas work value is the area underneath the moment/range of VIS. Therefore, work data may be more informative to evaluate the progress in muscle contractile properties following a training period. During the sprinting with a change of direction, quadriceps muscles contract eccentrically in deceleration and contracts concentrically for the acceleration of the body in the propulsive phase. On the other hand, the hamstring muscles function as a knee stabilizer for decelerating the centre of gravity during the impact phase of the gait cycle and absorb forces associated with directional change [32]. Also, Besier et al. showed greater activation of biceps femoris during sprints with a change of direction and concluded that this activation is essential for coping with the internal rotation moments applied to the knee [33]. Moreover, as the hamstrings are the prime movers of hip extension, changing hip from a flexed to an extended position in the propulsion phase causes a higher muscle activation of the hamstring group during a sprint with a change of direction [31]. Thus, as shown in this study, the higher improvement in hamstring muscles strength could be due to their continuous stimulation during the directional changes in sprints.

4.3 Changes in sprinting performance after the training period

Our participants’ sprint performance did not improve following eight weeks of repeated sprint training towards the end of the season (Table 4). In the literature, there are controversial findings on the effectiveness of repeated sprint training. Some investigators found significant performance improvements [4, 34] whereas, others had shown that similar training did not improve sprint performance [35, 36]. A possible explanation of this discrepancy might be the sprint training’s seasonal timing. Haugen et al. concluded that off-season or early pre-season sprint training could generate the more substantial effects [5]. On the other hand, Campos-Vasquez et al. [37]performed repeated-sprint training with twenty-one U19 category soccer players for eight weeks and assessed their sprint performance at the end of the season. They found that the training did not change the best and mean sprint times significantly, which is in agreement with our study. They stated that the accumulation of fatigue throughout the season might reduce repeated sprint performance in the post-competition period [37]. Since our study took place towards the end of the season, the lack of improvement in sprinting performance could have resulted from the training’s timing concerning the season. Future studies are necessary to evaluate the effectiveness of repeated sprint training on performance throughout the season.

4.4 Limitations of this study

In this study, the concentric contraction characteristics of the extensor and flexor thigh muscle groups were evaluated. Since the eccentric strength properties of the mentioned muscle groups were not measured, any possible effect of repeated sprint training on this type of contraction was not evaluated. It may be valuable to investigate the changes in eccentric contractile properties following a similar training protocol. Beside that, high precision analyses of muscle mass may give more informative data about changes in strength parameters.

4.5 Conclusion

Our results suggest that isokinetic strength parameters of well-trained youth soccer players increased significantly following eight weeks of on-season repeated sprint training even though their sprint performance did not improve. The player’s muscle mass increased at the end of the training period. Isokinetic strength enhancement may be explained with the induction of muscle hypertrophy following a prolonged period of sprint training. Therefore, this type of training may help players to improve muscle strength favoring the hamstring muscles independent of improving sprint performance.

Author contributions

CONCEPTION: Kerem Özgünen, Ümit Adaş and Nedim Askeri.

PERFORMANCE OF WORK: Kerem Özgünen, Ümit Adaş, Abdullah Kilci, Cumhur Boyraz, Nedim Askeri and Özgür ünaşti.

INTERPRETATION OR ANALYSIS OF DATA: Kerem Özgünen, Ümit Adaş, Abdullah Kilci, Cumhur Boyraz and Özgür ünaşti.

PREPARATION OF THE MANUSCRIPT: Kerem Özgünen, Çiğdem Özdemir, Özgür ünaşti and Selcen Korkmaz Eryilmaz.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Kerem Özgünen, Çiğdem Özdemir, Özgür ünaşti, Selcen Korkmaz Eryilmaz and Sadi Kurdak.

SUPERVISION: Kerem Özgünen, Çiğdem Özdemir, Özgür ünaşti, Selcen Korkmaz Eryilmaz and Sadi Kurdak.

Ethical considerations

The Ethics Committee of Çukurova University Faculty of Medicine has approved this study (91/17/04.09. 2019), carried out under the Declaration of Helsinki. The study was explained to all participants in detail, and informed consent forms were acquired.

Funding

The authors report no funding.

Footnotes

Acknowledgments

The authors have no acknowledgements to report.

Conflict of interest

The authors have no conflicts of interest to report.

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Effect of repeated sprint training on isokinetic strength parameters in youth soccer players

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Subjects

2.2 Anthropometric measurements

Table 1 Repeated sprint training program

2.4 Experimental protocol

2.5 Repeated sprint ability (RSA) test

(1) where ideal time = 6 × best time . 2.6 Isokinetic strength evaluation

Table 2 Muscle strength testing protocol (angular velocity × number of repetitions)

2.8 Statistical analysis

3. Results

3.1 Physical characteristics

Table 3 Pre- and post-training characteristics of the participants

4. Discussion

4.1 Morphological adaptations to sprint training

4.2 Muscular activation throughout sprinting performance

4.3 Changes in sprinting performance after the training period

4.4 Limitations of this study

4.5 Conclusion

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Repeated sprint training program

(1) $\displaystyle\text{where ideal time}=6\times\text{best time}.$
2.6 Isokinetic strength evaluation

Table 2
Muscle strength testing protocol (angular velocity $\times$ number of repetitions)

Table 3
Pre- and post-training characteristics of the participants