Effects of a 10-Week In-Season Eccentric-Overload Training Program on Muscle-Injury Prevention and Performance in Junior Elite Soccer Players

Abstract

Purpose:
To analyze the effect of an eccentric-overload training program (ie, half-squat and leg-curl exercises using flywheel ergometers) with individualized load on muscle-injury incidence and severity and performance in junior elite soccer players.

Methods:
Thirty-six young players (U-17 to U-19) were recruited and assigned to an experimental (EXP) or control group (CON). The training program consisted of 1 or 2 sessions/wk (3-6 sets with 6 repetitions) during 10 wk. The outcome measured included muscle injury (incidence per 1000 h of exposure and injury severity) and performance tests (countermovement jump [CMJ], 10-m and 20-m sprint test).

Results:
Between-groups results showed a likely (ES: 0.94) lower number of days of absence per injury and a possible decrement of incidence per 1000 h of match play in EXP than in CON. Regarding muscle performance, a substantial better improvement (likely to very likely) was found in 20-m sprint time (ES: 0.37), 10-m flying-sprint time (ES: 0.77), and CMJ (ES: 0.79) for EXP than for CON. Within-group analysis showed an unclear effect in each variable in CON. Conversely, substantial improvements were obtained in CMJ (ES: 0.58), 20-m sprint time (ES: 0.32), 10-m flying-sprint time (ES: 0.95), and injury severity (ES: 0.59) in EXP. Furthermore, a possible decrement in total injury incidence was also reported in EXP.

Conclusions:
The eccentric-based program led to a reduction in muscle-injury incidence and severity and showed improvements in common soccer tasks such as jumping ability and linear-sprinting speed.

Full text

46
www.IJSPP-Journal.com
ORIGINAL INVESTIGATION
International Journal of Sports Physiology and Performance, 2015, 10, 46-52
http://dx.doi.org/10.1123/ijspp.2013-0547
© 2015 Human Kinetics, Inc.
Effects of a 10-Week In-Season Eccentric-Overload
Training Program on Muscle-Injury Prevention
and Performance in Junior Elite Soccer Players
Moisés de Hoyo, Marco Pozzo, Borja Sañudo, Luis Carrasco,
Oliver Gonzalo-Skok, Sergio Domínguez-Cobo, and Eduardo Morán-Camacho
Purpose: To analyze the effect of an eccentric-overload training program (ie, half-squat and leg-curl exercises using ywheel
ergometers) with individualized load on muscle-injury incidence and severity and performance in junior elite soccer players.
Methods: Thirty-six young players (U-17 to U-19) were recruited and assigned to an experimental (EXP) or control group (CON).
The training program consisted of 1 or 2 sessions/wk (3–6 sets with 6 repetitions) during 10 wk. The outcome measured included
muscle injury (incidence per 1000 h of exposure and injury severity) and performance tests (countermovement jump [CMJ],
10-m and 20-m sprint test). Results: Between-groups results showed a likely (ES: 0.94) lower number of days of absence per
injury and a possible decrement of incidence per 1000 h of match play in EXP than in CON. Regarding muscle performance,
a substantial better improvement (likely to very likely) was found in 20-m sprint time (ES: 0.37), 10-m ying-sprint time (ES:
0.77), and CMJ (ES: 0.79) for EXP than for CON. Within-group analysis showed an unclear effect in each variable in CON.
Conversely, substantial improvements were obtained in CMJ (ES: 0.58), 20-m sprint time (ES: 0.32), 10-m ying-sprint time
(ES: 0.95), and injury severity (ES: 0.59) in EXP. Furthermore, a possible decrement in total injury incidence was also reported
in EXP. Conclusions: The eccentric-based program led to a reduction in muscle-injury incidence and severity and showed
improvements in common soccer tasks such as jumping ability and linear-sprinting speed.
Keywords: maximal power output, countermovement jump, sprint ability, muscle-injury incidence, muscle-injury severity
Injuries in soccer represent the most common reason for
player unavailability in training and matches.1,2 Different authors
have reported a rate of 7 to 8 injuries per 1000 hours of practice.3,4
Muscle strains (37%) are the major type of injury and mainly occur
in the lower extremity (87%).5 Ekstrand et al4 showed that the most
common type of strain is in the thighs, representing the 17% of all
injuries, with hamstring strain (12% total thigh strains) showing a
higher incidence than quadriceps (5% of the total).
Soccer is a sport that requires repetitive kicking and sprinting
efforts,6 and hamstrings and quadriceps muscles are frequently
injured during these actions (ie, kicking and sprinting).6 Commonly,
a quadriceps muscle injury is produced when the maximal length
of the rectus femoris is reached during the early swing phase of a
sprint.7 Conversely, hamstring injuries usually appear during the
second half of swing, where hamstrings are stretched and preparing
for foot contact.8,9 Note that both injuries primarily occur during
the eccentric contractions.
There are many risk factors associated with muscle-strain
injuries in soccer.6,10 However, muscle-strength deciency has been
proposed as the main risk factor.11–13 Traditionally, strengthening
programs are based on resistance exercises where the stimulus is
provided by gravitational loads.14 However, the efcacy of these
methods is limited to concentric actions, with lower activation in
the eccentric phase.15,16 Therefore, preventive strength-training
strategies might also include exercises where muscles are activated
during the eccentric phase for an adequate muscle response.10
It seems that eccentric strengthening exercises for the quadri-
ceps and hamstrings reduce the thigh muscle strain-injury rate in
soccer.10,17,18 Regarding injury prevention in the quadriceps, only
1 study has assessed the chronic adaptations to eccentric training
with professional soccer players.18 During preseason, participants
performed a strengthening program (3 times/wk for 4 wk) includ-
ing eccentric exercises. No signicant changes were observed in
strength measurements. Nonetheless, eccentric training increased
the optimal length of both knee extensors and exors after the
intervention. It is interesting that no injuries were reported in the
intervention group, while the control group suffered 2 muscle-strain
injuries. While this is not a proper eccentric-overload intervention,
those authors, in the same line of recent studies such as Askling et
al,19 indicated that focusing the intervention program on lengthen-
ing-based exercises may be more effective in the rehabilitation and
prevention of these injuries than traditional protocols.
On the other hand, Arnason et al17 applied a preventive warm-
up that included the Nordic hamstrings exercise (NHE) combined
with stretching during 4 consecutive soccer seasons. Consequently,
hamstring strains were reduced in comparison with teams that were
not involved in the training program. In the same line, using an
eccentric-overload training program on a ywheel inertial device
for hamstring muscles (ie, leg-curl exercise), Askling et al10 showed
that the occurrence of strain injuries was lower in the training group
than in the control group. Furthermore, there was an increase in
both eccentric and concentric knee-exor strength and in maximal
de Hoyo and Domínguez-Cobo are with the Fitness Section, Sevilla Foot-
ball Club, Seville, Spain. Pozzo is MasterdeFutbol, University of Pablo de
Olavide, Seville, Spain. Sañudo, Carrasco, and Morán-Camacho are with
the Dept of Physical Education and Sport, University of Seville, Seville,
Spain. Gonzalo-Skok is with the Faculty of Health Sciences, University
of San Jorge, Zaragoza, Spain. Address author correspondence to Moisés
de Hoyo at dehoyolora@us.es.

Eccentric Training and Injury Prevention and Performance 47
sprinting speed. In this regard, it has been reported that NHE and
ywheel exercises can produce greater eccentric overload10,14 and
greater muscle activation (measured by electromyography [EMG])16
than traditional exercises. However, despite the difculty in match-
ing the intensities between the NHE and the YoYo leg-curl exercise,
it seems that the YoYo leg curl provides a much higher eccentric
overload than the NHE.20,21 Furthermore, the eccentric–concentric
transition phase is completely different in the exercises. Therefore,
due to the wide spectrum of advantages, it might be recommended
to include these exercises with ywheel devices.
Despite these positive results, the evidence of the preventive
effects associated with these devices is scarce. Therefore, this study
aimed to assess the effect of an eccentric-overload training program
(half-squat and leg-curl exercises using ywheel ergometers) with
individualized load on muscle-injury incidence and severity and
performance in junior elite soccer players. Considering that this
type of program can enhance muscle strength in a short period of
time,22 we hypothesized that it might prove effective in reducing
muscle-injury incidence and severity in this population.
Methods
Subjects
Thirty-six young elite Spanish soccer players (U-17 to U-19, age
17 ± 1 y, height 178.11 ± 2.34 cm, weight 71.76 ± 4.56 kg, body-
mass index 19.1 ± 2.4 kg/m2) voluntarily accepted to participate in
this study. Data collection took place during the fth month after
starting the season (Table 1). Athletes belonged to a rst Spanish
soccer division (ie, Liga BBVA) club academy squad. None of
them had previously used ywheel devices. A physician reviewed
the medical histories and assessed the suitability for the study. Par-
ticipants with severe lower-limb muscle injuries (strains for more
than 27 d) in the previous 2 months were excluded. The study was
conducted according to the Declaration of Helsinki, and the protocol
was fully approved by the local research ethics committee before
recruitment. After a detailed explanation about the aims, benets,
and risks involved in this investigation, all participants gave their
written informed consent.
Study Design and Procedures
Using a controlled nonrandomized study design, players from 2
different teams were divided into an eccentric-overload training
group (EXP, n = 18) and a control group (CON, n = 15). One team
was assigned to EXP and another team to CON. Both teams were
involved in soccer training with a similar weekly training volume
and methodology (4 or 5 sessions/wk of 60–90 min and 1 match/
wk). Participants in CON continued with their usual technical/tac-
tical training, avoiding strength training during the whole season.
EXP players, in addition to their usual soccer training, underwent
an additional concentric–eccentric training program 1 or 2 times
per week for 10 weeks (Figure 1). Furthermore, a familiarization
session with the ywheel devices and exercises (half squat and leg
curl) used in the study was developed in EXP. In the rst week, the
EXP group performed a test where the inertia used during the inter-
vention (rst 5 training wk) in the ywheel devices was selected.
In these sessions, a full explanation of the experimental protocol
and recommendations were given to the participants, and they were
permitted to practice all the tests.
Table 1 Descriptive Data of the Participants, Mean ± SD
Age (y) Height (cm) Weight (kg) Body-mass index (kg/m
2
)
Experimental group 18 ± 1 177.86 ± 3.12 70.87 ± 3.87 18.8 ± 2.2
Control group 17 ± 1 178.24 ± 1.25 73.12 ± 2.56 19.3 ± 2.5
Figure 1 — Project-design timeline.

48 de Hoyo et al
With these devices the “load” is given by the inertia of a rotating
mass (ywheel), which in turn depends on its geometrical (diameter,
thickness) and physical properties (density of the material). The
ywheel provides an inertial resistance during coupled concentric
and eccentric actions.23,24 The total number of ywheels installed
adjusts the overall inertia. Given their properties (material: PVC;
density: 1.4 kg/cm3, diameter: 380 mm; thickness: 20 mm), the
resulting inertia of each ywheel was 0.11 kg/m2. To determine
the inertia that was used during the intervention, an assessment
with inertia 2 and 4 (4 repetitions per inertia with 180-s interinertia
recovery) was performed. The inertia that achieved higher power
output was selected. The protocol was performed using a specic
analysis feature in a performance-measurement system compatible
with ywheel devices (SmartCoach Power Encoder, SmartCoach
Europe AB, Stockholm, Sweden) with associated SmartCoach
software (v 3.1.3.0). For each participant, the inertia corresponding
to the higher concentric power output was used in the intervention.
This inertia was individually readjusted after 5 training weeks.
Injury incidence and severity were registered during the whole
season (10 mo, with the intervention carried out during the last 5
mo). Muscle performance was assessed through a countermove-
ment-jump (CMJ) test and a 20-m sprint test (10-m split time). All
tests were administered 1 week before and 1 week after the interven-
tion in the following order: CMJ and 20-m sprint test.
Eccentric-Training Intervention
Two YoYo isoinertial ywheel training devices (YoYo Technology
AB, Stockholm, Sweden) were used in the strengthening program
(half squat in YoYo Squat and leg curl in YoYo Prone Leg Curl).
During the concentric phase of the muscle action, the athlete imparts
rotation to a ywheel by means of a strap connected to its shaft. At
the end of the range of motion the strap is completely unwound,
and the ywheel keeps spinning by virtue of its inertia and recoils
the strap, demanding that the athlete decelerate it during the subse-
quent eccentric action. By controlling the execution technique (ie,
delaying the braking action in the eccentric phase), these devices
enable achievement of a given degree of eccentric overload.10,16,21,25
In the leg-curl exercise, participants performed a bilateral knee
exion in a prone position (Figure 2) by accelerating and decelerat-
ing the ywheel through a concentric, and subsequently eccentric,
action of the hamstrings muscle group. Participants were encouraged
to apply the maximum effort from the knee-extension position (0°)
to a 130° to 140° exion and then asked to resist the movement
during the eccentric phase when the 90° position was reached. In
the half-squat exercise, participants had to bend the knees up to 90°
during the eccentric phase and then perform the concentric phase as
fast as possible (Figure 3). Participants performed both exercises in
1 session during the rst 2 weeks and 2 sessions per week during
Figure 2 — Leg-curl exercise performed in the study.
Figure 3 — Half-squat exercise performed in the study.

Eccentric Training and Injury Prevention and Performance 49
the other 8 weeks. The volume was increased as follows: 3 × 6
repetitions in weeks 1 to 4, 4 × 6 repetitions in weeks 5 and 6, 5 × 6
repetitions in weeks 7 and 8, and 6 × 6 repetitions in weeks 9 and 10.
Muscle-Injury Incidence and Severity
Medical staff recorded the number of training sessions and matches
and registered the number of muscle injuries per 1000 hours to
exposure (match and training). In addition, injury severity was
determined considering the number of training or match days of
absence per injury.26 In the current study, a player was considered
fully rehabilitated when the medical team allowed full participation
in team training or matches.
Performance Tests
CMJ Test. A CMJ test was assessed using an infrared-light
platform built into the OptoJump system (Microgate, Bolzano,
Italy). The CMJ test starts with a preparatory movement of knee
extension going down to 90° knee exion and, without pause,
jumping upward as high as possible. It was performed without the
use of the arms; subjects were asked to keep their hands on their
hips. The subjects performed 3 jumps; the best result was used for
later analyses. Participants were allowed to recover for 90 seconds
between jumps.
10-m and 20-m Sprint Tests. Acceleration and sprint tests were
measured using dual-beam electronic timing gates (Race Time
2 Light Radio System; Microgate, Bolzano, Italy). All subjects’
sprint times were assessed on 10-m and 20-m distance. The starting
position was standardized with the left toe 1 m back from the starting
line and the right toe approximately in line with the heel of the left
foot. All assessments were performed on a natural grass surface,
and subjects wore specic soccer shoes. All participants performed
3 sprint tests, with the best score used for subsequent analysis. A
recovery time of 120 seconds between actions was allowed. The
next variables were used for posterior analysis: 10-m sprint time
(0–10 m), 20-m sprint time (0–20 m), and 10-m ying-sprint time
(10–20 m).
Statistical Analysis
Data are presented as mean ± SD. All data were rst log-trans-
formed to reduce bias arising from nonuniformity error. The stan-
dardized difference or effect size (ES, 90% condence limit) in
the selected variables was calculated using the pooled pretraining
SD. Threshold values for Cohen ES statistics were >0.2 (small),
>0.6 (moderate), and >1.2 (large).27 For within/between-groups
comparisons, the chances that the differences in performance and
measures of injury prevention were better/greater (ie, greater than
the smallest worthwhile change [0.2 multiplied by the between-
subjects SD, based on the Cohen d principle]), similar, or worse/
smaller were calculated. Quantitative chances of benecial/better
or detrimental/poorer effect were assessed qualitatively as fol-
lows: <1%, almost certainly not; 1% to 5%, very unlikely; 5% to
25%, unlikely; 25% to 75%, possibly; 75% to 95%, likely; 95%
to 99%, very likely; and >99%, almost certainly.27 A substantial
effect was set at >75%.28 If the chances of having benecial/
better and detrimental/poorer performances were both >5%, the
true difference was assessed as unclear. Otherwise, we interpreted
that change as the observed chance.27 Participants not attending
80% of training sessions (ie, due to muscle injury) were omitted
from the analysis.
Results
Relative changes and qualitative outcomes resulting from the within-
group analysis are shown in Tables 2A and 2B. No substantial
improvements were found in any measure of injury prevention (ie,
severity and incidence) in any group compared with the pretest,
with the exception of severity in the EXP group, which obtained
a substantial enhancement. Similarly, no substantial improvement
in muscle performance (ie, CMJ and 0 to 20 m) was achieved in
the CON group. Nevertheless, players in EXP showed substantial
better results in CMJ and 10 to 20 m.
Results from between-groups analysis are illustrated in Figure
4. The decrement in injury severity was substantially greater (65%
[90%CL: 10.1; 86.4] with chances for better/similar/lower values
of 93/6/2%, respectively) in EXP than in CON. A possibly lower
(23.7% [90%CL: –115.0; 72.9], 69/27/4%) incidence per 1000 hours
of match play was provided in EXP compared with CON. There
were no substantial between-groups differences in the rest of the
injury-prevention variables. Substantially better performance was
found in CMJ (9.4% [90%CL: 3.8; 15.4], 98/2/0%), 0 to 20 m (1.6%
[90%CL: –0.2; 3.5], 75/23/1%), and 10 to 20 m (3.1% [90%CL:
0.9; 5.2], 96/4/0%) in EXP than in CON at posttest.
Table 2A Changes in Injury-Prevention Markers and Performance After Eccentric-Overload Training, Mean ± SD
Difference
Variable Pretest Posttest % (90% CL) Standardized (90% CL)
a
Chances
b
QA
Training incidence (n)c4.4 ± 5.7 2.6 ± 3.9 2.1 (–23.6; 36.3) 0.03 (–0.35; 0.41) 21/65/15% Unclear
Match incidence (n)c7.0 ± 14.9 7.0 ± 10.6 13.3 (–31.5; 87.3) 0.08 (–0.25; 0.41) 25/68/7% Unclear
Total incidence (n)c4.7 ± 5.8 3.4 ± 4.1 14.2 (–2.3; 28.0) 0.18 (–0.03; 0.39) 43/56/1% Possibly
Severity (d)d5.9 ± 8.2 1.9 ± 1.8 47.7 (23.4; 64.3) 0.59 (0.24; 0.94) 97/3/0% Very likely
CMJ (cm) 35.7 ± 4.1 38.3 ± 4.2 7.6 (4.1; 11.2) 0.58 (0.32; 0.85) 99/1/0% Very likely
10-m sprint (s) 1.73 ± 0.12 1.71 ± 0.08 1.0 (–1.5; 3.4) 0.15 (–0.21; 0.50) 40/55/5% Possibly
20-m sprint (s) 3.03 ± 0.14 2.99 ± 0.12 1.5 (0.1; 3.0) 0.32 (0.02; 0.61) 75/25/0% Possibly
10-m ying sprint (s) 1.30 ± 0.04 1.26 ± 0.05 3.3 (2.1; 4.5) 0.95 (0.60; 1.30) 100/0/0% Almost certainly
Abbreviations: CL, condence limits; QA, qualitative assessment; CMJ, countermovement jump.
Note: For clarity, all differences are presented as improvements (positive), so that negative and positive differences are in the same direction.
a Effect size. b Percentage chance of having better/similar/poorer values. c Per 1000 h. d Days of absence per injury.

50 de Hoyo et al
Discussion
This study aimed to assess the effect of a10-week eccentric-overload
training on lower-limb muscle-injury prevention and performance in
young elite soccer players. Our results showed a greater incidence
per 1000 hours of match time in CON but also a lower injury sever-
ity in EXP after the intervention.
Previous studies have suggested that hamstrings and quad-
riceps strains are the most common injuries in soccer players.4,29
The incidence of muscle strains in soccer players was low during
the preseason period but 2 or 3 times higher during the competitive
season.29 This is in agreement with the results observed in our study
for the CON group, although a decrement was observed during
the competitive season (during and after the intervention) in the
EXP group. In addition, the injury severity (days of absence) was
substantially lower (ES: 0.59) in this group. Thus, it may also be
attributed to an effect of the intervention.
In this line, Arnason et al17 studied the effects of different mus-
cle-injury-prevention programs consisting of warm-up stretching,
exibility, and/or eccentric strength training on hamstring strains
Table 2B Changes in Injury-Prevention Markers and Performance After Training for the Control Group, Mean ± SD
Difference
Variable Pretest Posttest % (90% CL) Standardized (90% CL)
a
Chances
b
QA
Training incidence (n)c3.7 ± 3.4 3.7 ± 3.9 –9.8 (41.9; –40.0) –0.25 (1.32; –0.82) 22/24/53% Unclear
Match incidence (n)c8.3 ± 13.4 12.5 ± 14.0 –48.4 (43.2; –287.8) –0.32 (0.45; –1.08) 12/27/61% Unclear
Total incidence (n)c4.3 ± 4.0 5.9 ± 6.6 –22.0 (31.4; 116.7) –0.38 (0.72; –1.48) 18/21/62% Unclear
Severity (d)d7.1 ± 6.9 8.5 ± 12.1 –49.3 (–260.5; 38.1) –0.34 (–1.09; 0.41) 11/26/63% Unclear
CMJ (cm) 36.8 ± 3.4 36.2 ± 3.2 –1.7 (–5.9; 2.6) –0.18 (–0.61; 0.26) 8/46/46% Unclear
10-m sprint (s) 1.71 ± 0.08 1.71 ± 0.08 –0.3 (–1.3; 1.8) –0.05 (–0.38; 0.28) 10/68/22% Unclear
20-m sprint (s) 2.99 ± 0.11 3.00 ± 0.13 –0.1 (–1.4; 1.2) –0.03 (–0.38; 0.32) 13/66/21% Unclear
10-m ying sprint (s) 1.27 ± 0.05 1.26 ± 0.06 0.2 (–1.7; 2.1) 0.05 (–0.40; 0.50) 28/55/17% Unclear
Abbreviations: CL, condence limits; QA, qualitative assessment; CMJ, countermovement jump.
Note: For clarity, all differences are presented as improvements (positive), so that negative and positive differences are in the same direction.
a Effect size. b Percentage chance of having better/similar/poorer values. c Per 1000 h. d Days of absence per injury.
Figure 4 — Efciency of the eccentric-overload training group in comparison with control group to improve incidence per 1000 h of training (training
incidence), incidence per 1000 h of match time (match incidence), incidence per 1000 h of training and matches (total incidence), days of absence per
lower-limb muscle injury (severity), countermovement-jump performance (CMJ), 10-m sprint time (0–10 m), 20-m sprint time (0–20 m), and 10-m
ying-sprint time (10–20 m). Bars indicate uncertainty in the true mean changes with 90% condence intervals. Trivial areas were calculated from the
smallest worthwhile change (see the Methods section).

Eccentric Training and Injury Prevention and Performance 51
in elite soccer players from Iceland and Norway. Results revealed
that the incorporation of eccentric strength training of hamstrings
during the warm-ups before training sessions signicantly reduced
the incidence of hamstring strains during training, in comparison
with baseline data and other types of intervention. However, the
training program was not effective at reducing match-play injury
risk. These results are in contrast with the ndings reported in our
study, which suggest a lower injury rate during match play in com-
parison with CON (ES: 0.35). Differences in the training devices,
eccentric overload imposed in the exercises, or volume of training
(ie, sessions and repetitions) might explain these between-studies
differences.
Training paradigms comparable to the ones used in the current
study reached similar conclusions. After 10 weeks of eccentric
strength training using ywheel devices (1 or 2 times a week),
Askling et al10 aimed to determine whether preseason training
affected the occurrence and severity of hamstring injuries during
the subsequent competition season in elite Swedish soccer players.
The intervention was effective to reduce the injury incidence in the
training group. These results are in agreement with the results found
in our study. Regarding the effects of eccentric-overload training for
quadriceps muscles in soccer players, only 1 previous study assessed
a 4-week program (3 times/wk) together with habitual soccer train-
ing.18 The authors found that players in the control group suffered
more quadriceps injuries than those in the eccentric-based interven-
tion group. It may be possible that the improvements in strength
and mainly in the eccentric component could explain these results.10
In the current study, improvements in strength can be observed
in the results of CMJ and sprint tests. Thus, EXP participants
showed an increase in jumping ability and 0- to 20-m and 10- to
20-m sprint tests. Numerous studies have shown an improvement
in muscle performance after power training.30–32 Most of these pro-
grams lack a decelerating phase during exercise. In our study, we
analyzed the combined effect of power training in both concentric
and eccentric phases on muscle performance. Our improvements
in CMJ (ES: 0.58) are not in agreement with other studies14,33 that
used eccentric training programs. These differences could be due to
the exercises performed or training subjects (ie, soccer players vs
patellar tendinopathy). Therefore, in addition to the aforementioned
benets (injury incidence and severity), this program can obtain
benets not just in CMJ but also in acceleration tasks. Authors such
as Askling et al10 observed an improvement (ES: 0.80) in a 30-m
sprint test after hamstrings eccentric-overload training. These results
are in line with those found in our study in 10 to 20 m (ES: 0.95).
Nonetheless, in spite of enhancing 0 to 20 m, the effect reported
(ES: 0.32) is lower than that found in 30-m sprint time in Askling
et al.10 It may be possible that the hamstrings are more involved in
longer than shorter linear-sprinting distances, and, in consequence,
it may explain these between-studies differences and the lack of
improvement in 0 to 10 m (ES: 0.15). Several limitations of the study
include assessing the effect of eccentric-overload training by means
of strength measurements (ie, 1-repetition maximum, maximum
power output at different inertias, force impulse). Furthermore, it
would be of interest to analyze the effect of this intervention on
the most representative performance tests in soccer, such as hop
tests and change-of-direction tests, and to include more functional
training exercises. Further studies are needed to analyze the effect
of eccentric-overload training in subjects of different ages, genders,
sports, and training backgrounds and to compare this protocol with
a well-designed conventional strength-training program in both
reducing injuries and improving performance.
Conclusions
In conclusion, a 10-week exercise program based on maximal-power
concentric load and eccentric overload is effective in reducing
muscle-injury incidence and severity in junior elite soccer players.
In addition, improvements in jumping and sprinting abilities were
obtained. These nding may have implications for future injury-
prevention programs aiming to reduce the risk of hamstrings and
quadriceps strain injury, as they are the main cause of absence from
matches in soccer players. Moreover, it seems that training at the
inertia that optimizes individual maximal power for these particular
exercises was effective for improving sprinting and jumping ability,
which are explosive strength-related tasks usually associated with
performance in soccer.
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