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ORIGINAL ARTICLE
Photobiomodulation therapy as a tool to prevent hamstring strain
injuries by reducing soccer-induced fatigue on hamstring muscles
Maurício Pinto Dornelles
1
&Carolina Gassen Fritsch
1
&Francesca Chaida Sonda
2
&Douglas Scott Johnson
3
&
Ernesto Cesar Pinto Leal-Junior
4
&Marco Aurélio Vaz
2
&Bruno Manfredini Baroni
1
Received: 1 November 2018 /Accepted: 14 December 2018
#Springer-Verlag London Ltd., part of Springer Nature 2019
Abstract
Muscle fatigue is a potential risk factor for hamstring strain injuries in soccer players. The aim of this study was to verify the effect
of photobiomodulation therapy (PBMT) on the hamstrings’muscle fatigue of soccer players during a simulated match. Twelve
male amateur soccer players (~ 25 years) participated in this randomized, crossover, double-blinded, placebo-controlled trial. The
volunteers were evaluated in two sessions, with a minimum 7-day interval. At each session, volunteers received either PBMT
(300 J per thigh) or placebo treatment on the hamstrings prior to the simulated soccer match. Muscle strength and functional
capacity were evaluated through isokinetic dynamometry and countermovement jump (CMJ) tests, respectively, before and
immediately after the simulated soccer match. Players had lower reductions on hamstring eccentric peak torque [4.85% (ES =
0.31) vs. 8.72% (ES = 0.50)], hamstring-to-quadriceps torque ratio [3.60% (ES = 0.24) vs. 7.75% (ES = 0.50)], and CMJ height
[1.77% (ES= 0.09) vs. 5.47% (ES= 0.32)] when treated with PBMTcompared to placebo. Magnitude-based inference supports
that PBMT promoted 75%, 69%, and 53% chances for beneficial effects on hamstring eccentric peak torque, hamstring-to-
quadriceps torque ratio, and CMJ height, respectively, compared to placebo treatment. In conclusion, PBMT applied before a
simulated soccer match proved to be effective in attenuating the hamstrings’muscle fatigue. These findings support PBMT as a
promising tool to prevent hamstring strain injury in soccer players.
Keywords Phototherapy .Muscle injury .Prevention .Football
Introduction
The hamstring strain injury (HSI) is one of the major problems
soccer players face in sports practice. A 13-year longitudinal
analysis of European elite soccer teams reported that 22% of
the players sustained at least one hamstring injury in each
season [1]. It means that a soccer team with a 25-player squad
can expect about 5–6 injured players per season. In addition,
the recurrence rate is high (1–2 out of 10 injured players
usually sustain a re-injury within 2 months of return to play)
and the average absence time per injury is about 17 days [1].
Despite the persistent deficits after a HIS [2], players Boff the
pitch^due to injuries compromise the team performance [3]
and negatively affect the club finances [4]; hence, prevention
is the primary goal. Soccer clubs have invested in a range of
tests for screening players who might be prone to injury and in
specific prevention programs [5,6], but HSI rates have not
decreased throughout the years [1].
HSI is typically a non-contact injury (high-speed running is
responsible for 60–80% of soccer HSIs) [7,8]. Therefore, the
scientific community has paid great attention to the intrinsic
risk factors associated with HIS [9,10]. Age and previous
injuries are well-accepted non-modifiable risk factors [9,
10]. Among the modifiable risk factors, there is great attention
on the hamstring eccentric strength [8,11,12]andthe
hamstring-to-quadriceps (H:Q) strength ratio [11,13,14]. It
is important to note that both factors are expected to be affect-
ed by fatigue during a soccer match. Studies with intermittent
exercise protocols designed to mimic demands of competitive
*Bruno Manfredini Baroni
bmbaroni@yahoo.com.br
1
Universidade Federal de Ciências da Saúde de Porto Alegre
(UFCSPA), Porto Alegre, RS, Brazil
2
Exercise Research Laboratory, Universidade Federal do Rio Grande
do Sul (UFRGS), Porto Alegre, RS, Brazil
3
Multi Radiance Medical, Solon, OH, USA
4
Laboratory of Phototherapy and Innovative Technologies in Health,
Universidade Nove de Julho (UNINOVE), São Paulo, SP, Brazil
Lasers in Medical Science
https://doi.org/10.1007/s10103-018-02709-w
soccer have found significant decreases (up to 31%) in ham-
string eccentric strength with few or no impact on quadriceps
strength [15–18], thus affecting the agonist–antagonist
strength balance. The link between eccentric weakness and
increased HSI risk suggests that players would be more sus-
ceptible to injury in fatigue situations, and this hypothesis is
supported by the fact that most HSIs sustained during matches
occurred during the last third of the first and second halves of
the match (31–45′and 76–90′,respectively)[7]. Therefore,
muscle fatigue has been commonly stated as a potential risk
factor for HSI [9].
Photobiomodulation therapy (PBMT), also known as pho-
totherapy, is a non-invasive and non-pharmacological therapy
largely employed to treat a series of musculoskeletal disorders
[19]. Positive effects on pain [20], inflammation [20], and
tissue repair [21] have been reported with application of
low-level LASERs or light-emitting diodes (LEDs) over a
target tissue [22]. Along the last decade, a growing body of
evidences has been published regarding the PBMT action on
muscle fatigue—for a review, see Vanin et al. [23]. Improved
exercise performance and/or reduced fatigue markers have
been found with a single application of PBMT immediately
before a range of exercise protocols, such as exercise with free
weights [24], isokinetic exercise [25], cycling [26], running
[27], and intermittent sprints test [28].
In summary, the ability of PBMT to reduce muscle fatigue
in different types of exercise has elevated this therapy to the
category of potential ergogenic agent in sports. Considering
the relationship between fatigue and HSI in soccer players, it
seems plausible that PBMT may also be a promising tool for
this injury prevention. The first step in the application of
preventive-based PBMT is to find therapy parameters capable
of minimizing the loss of hamstring strength generated by
soccer practice. Therefore, the aim of the current study was
to verify the effect of PBMTon the hamstrings’muscle fatigue
of amateur soccer players during a simulated match.
Methods
Study design
The study is characterized as a randomized, crossover, double-
blinded, placebo-controlled trial. The volunteers were evalu-
ated in two sessions, with a minimum 7-day interval. Both
sessions were performed at the same hour of day and with
similar climatic conditions. At each session, volunteers re-
ceived either PBMT or placebo treatment on the hamstrings
prior to a soccer match simulation protocol. The order of the
PBMT and placebo sessions was randomized through the
random.org website. Muscular performance tests were
applied before and after a simulated soccer match. The
following sequence of events was performed within each
session: (1) general warm-up, (2) countermovement jump
(CMJ) test, (3) isokinetic test, (4) PBMT or placebo therapy,
(5) simulated soccer match, (6) CMJ re-test, and (7) isokinetic
re-test (Fig. 1). The study was previously approved by the
Ethics and Research Committee of the University where the
study was conducted (no. 63299416.4.0000.5345), and all
participants signed an informed consent.
Participants
Twelve male amateur soccer players were recruited to
participate in the study (age, 25.17 ± 4.04 years; body
mass, 73.75 ± 5.85 kg; height, 1.74 ± 0.04 m). All partic-
ipants followed a minimum routine of one soccer match
per week (1.75 ± 0.86 h per week of sports practice) and
had competitive experience in amateur soccer leagues.
Exclusion criteria for participation in the study were as
follows: (1) the presence of any musculoskeletal injuries
during the study period, (2) history of muscle injuries on
lower limbs (e.g., hamstrings, quadriceps, groin) in the
6 months prior to the evaluations, (3) recent history of
any lower limbs injuries that could interfere with the stud-
ied outcomes (e.g., anterior cruciate ligament rupture,
Fig. 1 Flowchart of study
Lasers Med Sci
meniscus injury, patellofemoral pain syndrome), (4) any
contraindication to performing maximum exercises, and
(5) difficulty in understanding and/or executing the testing
protocols.
CMJ test
After general warm-up (5-min run at moderate intensity), low-
er extremity functional capacity was assessed through the
CMJ test [29]. The rater explained and demonstrated the
CMJ proper execution. The volunteer should be standing with
hands on hips and feet aligned respecting the same distance
from the shoulders, then squat until a position of approximate-
ly 90° of knee flexion and perform a vertical jump as high as
possible. Submaximal CMJs were performed to familiarize
the volunteer with the movement. A smartphone was posi-
tioned 1.5 m away from the volunteer to record the execution
of three maximal CMJs [29]. The volunteer was instructed to
use the hip, knee, and ankle flexion movements to dampenthe
landing. The jump height analysis was done through the mo-
bile application named Jumpo (available for free download).
This application works similar to the previously validated ap-
plication named My Jump [29]. Jumpo was chosen because it
was the only available application for the iOS operating sys-
tem when our data were collected. The highest height obtained
between the jumps was considered for analysis. The CMJ re-
test started 3 min after ending the simulated soccer match.
Isokinetic dynamometry
The volunteer was positioned on the isokinetic dynamometer
Biodex System 3 Pro (Biodex Medical Systems, USA) ac-
cording to the manufacturer’s recommendations for evalua-
tion of the knee flexion–extension movements with the dom-
inant limb. Ten submaximal repetitions of knee flexion–
extension in the concentric mode at an angular velocity of
90° s
−1
were used as specific warm-up. The volunteer was
familiarized with the execution of isokinetic concentric and
eccentric tests through submaximal contractions. Two sets of
three maximal knee extensors concentric contractions and two
sets of three maximal kneeflexors eccentric contractions were
performed [30]. The 60° s
−1
angular velocity was used for all
tests, and a 1-min rest period was observed between sets [30].
The volunteer was verbally encouraged to produce as much
strength as possible in all contractions. The highest peak
torques in each contraction type were considered for analysis.
The H:Q functional torque ratio was calculated as follows:
hamstring eccentric peak torque/quadriceps concentric peak
torque [31]. The isokinetic re-test started 5 min after the sim-
ulated soccer match end due to the time required for the vol-
unteer’s positioning at the dynamometer.
PBMT/ placebo treatments
Interventions with PBMT or placebo were administered im-
mediately before the soccer match simulation protocol. A sin-
gle researcher was responsible for the treatments’allocation
(PBMT or placebo) in each session. The researchers respon-
sible for the CMJ and isokinetic dynamometry assessments,
and for the soccer match simulation protocol, were not inside
the room during the PBMT/placebo application. The volun-
teers were blindfolded and used headphones plugged into a
music player device during treatments to avoid identification
of sounds emitted by the PBMT device. Since PBMT is a
modality that does not promote any thermic, sensitive, and/
or painful stimulus, participants did not know whichtreatment
was applied in each session.
LEAP SportsPOD (prototype by Multi Radiance Medical,
Solon, OH, USA) was used for administration of PBMT and
placebo treatments (Fig. 2). This device contains 152 infrared
LEDs (880 nm) distributed evenly over an area of 252 cm
2
(10.5 cm × 24 cm); thus, it was possible to treat most of the
posterior thigh area with a single application (differently than
conventional cluster probes that are smaller and require mul-
tiple application sites). The PBMT device used here is a flex-
ible pad, so it was coupled appropriately to the convexity of
the posterior thigh. In addition, the researcher exerted a slight
pressure with the hands to ensure the diodes contact with the
Fig. 2 Volunteer receiving PBMT/placebo treatment (top) and PBMT
parameters used in this study (bottom)
Lasers Med Sci
volunteer’s skin. The parameters used for PBMT application
are shown in Fig. 2. The PBMTwas applied for 60 s, totalizing
an energy dose of 300 J in each of the volunteer’shamstring
muscle. This pre-exercise energy dose provided the best re-
sults in a previous trial with high-level soccer players [32].
The placebo application was carried out in the same way as the
active therapy, but with the equipment turned off.
Simulated soccer match
Based on movement analysis of the English Football League,
Small et al. [18] designed and validated the protocol entitled
soccer-specific aerobic field test (SAFT
90
). The protocol con-
sists of a sequence of activities that mimics the physiological
demands of a soccer match within a 15-min module of inter-
mittent exercise. To represent the 90 min of a soccer match,
this module is repeated six times with a 15-min interval be-
tween the 3rd and 4th execution. Therefore, SAFT
90
accurate-
ly represents the physiological load associated with a soccer
match, including multi-directional movements with different
intensities and requiring changes in direction and speed (ac-
celerations and decelerations) [18].
Since the present study included only amateur soccer
players, the original protocol of SAFT
90
was adapted to a
45-min protocol (i.e., three executions of the intermittent ex-
ercise module that represents the first half of a soccer match).
A single researcher provided commands and verbal encour-
agement during the simulated soccer match for all volunteers.
At the end of each 15-min module, the volunteers ingested
regular water ad libitum to avoid any influence of
hypohydration on their fatigue level. The rating of perceived
exertion (RPE) provided by the volunteer after completing the
simulated soccer match was multiplied by the time duration
(~ 45 min) to determine the internal workload generated by
the exercise protocol in each volunteer [33].
Statistical analysis
Sample size was calculated through G*Power software (ver-
sion 3.1.9.2) using results provided by a previous study that
assessed the effect of simulated soccer match on the hamstring
eccentric peak torque of amateur soccer players [15]. A sam-
ple of nine participants was estimated to provide a statistical
power over 0.90. As we expected a dropout rate of approxi-
mately 20%, 12 subjects were initially included in the trial.
Data normality was verified using the Shapiro–Wilk test.
Internal workloads promoted by the simulated soccer match
in placebo and PBMT sessions were compared through a
paired sample Student’sttest.
Subjects’behaviors within each session (PBMT and place-
bo) were analyzed with the following calculations: mean per-
cent change (Δ% =post-exercise / pre-exercise 1); paired
sample ttest (pre-exercise vs. post-exercise), with a
significance level set as 5% (α< 0.05); and effect size (ES)
through Cohen’sd[ES = (M
post
−M
pre
)/SD
pooled
,whereM
post
is the mean post-exercise measure, M
pre
is the mean pre-
exercise measure, and SD
pooled
is the pooled standard devia-
tion of the pre- and post-exercise]. Effect sizes were consid-
ered as Btrivial^(ES < 0.2), Bsmall^(ES > 0.2), Bmoderate^
(ES > 0.5), or Blarge^(ES > 0.8) [34].
Longitudinal percent changes (pre- to post-exercise) were
used for comparison between PBMT and placebo treatments.
Data were analyzed for practical significance using
magnitude-based inferences because traditional statistical ap-
proaches often do not indicate the magnitude of an effect,
which is typically more relevant to sports medicine than sta-
tistical significance [35]. The magnitude of between-
treatments differences (PBMT vs. placebo) was calculated
and expressed as standardized mean differences, considering
the Cohen’s criteria for the analysis (> 0.2 = small; > 0.5 =
moderate; > 0.80 = large). The chances that the true
(unknown) mean changes were trivial, harmful, or beneficial
[i.e., greater than the smallest worthwhile change (0.2 multi-
plied by the between-participant standard deviation)] were
determined. Quantitative chances of a beneficial or harmful
effect were assessed qualitatively, as follows: < 1%, almost
certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–
75% = possibly; 75–95% = likely; 95–99% = very likely; and
> 99% = almost certain [36]. When the harmful and beneficial
values were both > 5%, the inference was classified as unclear
[36].
Results
The simulatedsoccer match represented similar internal work-
loads in PBMT (368.18 ± 60.06 a.u.) and placebo (363.34 ±
85.08 a.u.) sessions (p=0.863).
Quadriceps concentric peak torque was not affected by the
simulated soccer match in any experimental condition
(p> 0.05; trivial effect size; Table 1). Hamstring eccentric
peak torque decreased significantly in both conditions
(p< 0.05), while a significant fall of H:Q torque ratio was
observed only in placebo session (p< 0.05). Greater effect
sizes were observed with placebo treatment for both hamstring
eccentric peak torque and H:Q torque ratio (Table 1). There
was no statistically significant reduction on jump height
(p> 0.05 for both conditions), but a small effect size was ob-
served only with placebo treatment (Table 1).
As shown in Fig. 3, PBMT promoted 75%, 69%, and 53%
chances for beneficial effects on hamstring eccentric peak
torque, H:Q torque ratio, and CMJ height, respectively, com-
pared to placebo treatment. PBMT had no beneficial or harm-
ful effects compared to placebo treatment on quadriceps con-
centric peak torque (Fig. 3).
Lasers Med Sci
Discussion
The main finding of the current study was that PBMTapplied
before a simulated soccer match reduced the losses of ham-
string eccentric strength, H:Q functional ratio, and CMJ
height. Our results become relevant to clinical practice from
the following rationale: hamstring fatigue generated by a soc-
cer match is a potential risk factor for HSI, and PBMT can
reduce soccer-induced fatigue on hamstring muscles; thus,
PBMT may be an innovative tool for HSI prevention.
Fatigue is frequently pointed out as a strong risk factor for
non-contact injuries in soccer players [5,6], specially the HSI
[9]. Fatigue perhaps explain, at least partially, why most HSIs
sustained during matches occurred during the last third of the
first and second halves of the match [7]. However, the rela-
tionship between soccer-induced fatigue and HSI is only sup-
ported by indirect evidences, such as the fall of hamstring
maximal eccentric strength promoted by simulated soccer
matches [15,16,18]. Like our findings, Small et al. [18]and
Jones et al. [15] found 5–8% reduced hamstring eccentric
strength after 45 min of the SAFT
90
in semi-professional
and amateur soccer players, respectively. That strength loss
seems to be higher than 16% after the full SAFT
90
protocol
[15,18], and up to 31% with another simulated soccer match
protocol called Loughborough intermittent shuttle test [16].
Prospective studies have evidenced that eccentric knee flexor
weakness increases the risk of hamstring injury in soccer [8,
11] and other sports [12]. For instance, Timmins et al. [8]
found a significant inverse relationship between pre-season
hamstring eccentric strength and the incidence of HSIs along
the season in premier league soccer players; for every 10-N
increase in hamstring eccentric strength, the risk of HSI was
reduced by 9%. Therefore, it is expected that a player with
greater deficits on hamstring eccentric strength during a soccer
Fig. 3 Standardized mean
differences and 95%CI for PBMT
effect compared to placebo effect
(forest plot); and chances of
harmful, trivial, or beneficial
effects with PBMT treatment
(right column—percent values
and magnitude-based inferences)
Table 1 Soccer players’isokinetic and jump performance in photobiomodulation (PBMT) and placebo sessions
Pre Post Δ%pvalue ES
Quadriceps CON PT (Nm)
PBMT 229.95 ± 40.56 226.74 ± 43.72 −1.40 0.407 0.08
Placebo 229.06 ± 41.31 224.21 ± 41.11 −2.12 0.200 0.12
Hamstring ECC PT (Nm)
PBMT 188.33 ± 29.20 179.19 ± 32.32 −4.85 0.005* 0.31
†
Placebo 190.78 ± 35.01 174.13 ± 34.85 −8.72 0.001* 0.50
†
H:Q torque ratio (a.u.)
PBMT 0.83 ± 0.13 0.80 ± 0.13 −3.60 0.056 0.24
†
Placebo 0.85 ± 0.17 0.78 ± 0.12 −7.75 0.008* 0.50
†
CMJ height (cm)
PBMT 35.06 ± 7.04 34.44 ± 7.46 −1.77 0.531 0.09
Placebo 35.29 ± 5.87 33.36 ± 6.65 −5.47 0.085 0.32
†
CON concentric, ECC eccentric, PT peak torque, CMJ countermovement jump, Δ%mean percent change, ES effect size
*Significant difference (p<0.05)
†
Small effect size (ES> 0.2)
Lasers Med Sci
match may be closer to experiencing a HSI, although there is
no data available in the literature to consistently support this
hypothesis.
In contrast to the consistent loss of hamstring strength, the
soccer match leads to a small (or even insignificant) fall on
quadriceps maximal strength [16,18]. Consequently, the H:Q
torque ratio is also negatively affected throughout the soccer
match, as supported by our results. It means that a player with
proper strength balance between knee flexor and extensor
muscles during laboratory tests (performed at a non-fatigued
condition) may present an H:Q ratio increasingly poor
throughout the soccer match. Although prospective studies
have found conflicting results regarding the predictive value
of the H:Q ratios for HSIs in soccer players [11,13,37],
isokinetic dynamometry is largely used as a screening tool
and muscle imbalance is considered one of the main risk fac-
tors for non-contact injuries by premier league soccer teams
[5,6]. The H:Q functional ratio (i.e., hamstring eccentric peak
torque divided by quadriceps concentric peak torque) has been
considered a Bmore functional^way to screen for injury risk in
soccer players compared to the H:Q conventional ratio (i.e.,
hamstring concentric peak torque divided by quadriceps con-
centric peak torque). However, the usual landmark of 1.0 is
rarely reached by professional male soccer players when
assessed at the most traditional testing angular velocity (i.e.,
60° s
−1
)—for a review, see Baroni et al. [31]. Independently of
the accuracy and predictive value of any landmark, a poor
H:Q functional ratio suggests a reduced hamstring capacity
of Bbreaking^knee extension and hip flexion during the ter-
minal swing phase of running [9], precisely the movement
responsible for most HSIs in soccer [7,8].
To the best of our knowledge, there is no evidence to sup-
port the CMJ performance as predictive of HSI. Jump tests are
largely used to assess lower extremity power, and we added
CMJ to the current study to verify the PBMTeffect on a motor
task closer to the multi-articular complex movements required
in a soccer match. The primary hip extensor during CMJ is the
gluteus maximus, but other hip extensors (including the ham-
string muscles) are also activated during the CMJ stretch-
shortening cycle [38]. Hamstrings execute a rapid eccentric
contraction during squat phase, when energy is stored within
the elastic components of the muscle-tendon unit and muscle
spindle is activated; next, a powerful concentric contraction
during jump phase is generated through a sum of muscle vol-
untary activation, myotatic reflex response, and elastic energy
release [38]. Fatigue impairs all these factors, and consequent-
ly reduces the CMJ height [39]. Our findings support that
PBMT minimized the fatigue effects on hamstring muscles
and, consequently, the soccer match-induced impair on CMJ.
Fatigue is classically defined as an exercise-induced reduc-
tion in the muscle’s maximal force capacity [40], so reduction
in the maximal strength that a person can produce provides a
straightforward demonstration of fatigue level [41]. However,
other fatigue-related deficits (not assessed in the current study)
may contribute to a player experiencing a HSI. Animal exper-
iments demonstrated that fatigued muscles absorbed less en-
ergy before failure when compared with unfatigued muscles
[42], making fatigued hamstring muscles more likely to suffer
a strain injury due to a reduced capacity to resist to over-
lengthening. At the same time, fatigue negatively affects knee
joint position sense [43] and execution of soccer-specific mo-
tor tasks [44]. These findings suggest decreased function in
afferent output of the sensory organs, including the muscle
spindles and their protective action against over-lengthening.
Consequently, the player may perceive a normal hamstring
muscle action during running, while in reality, repeated
over-lengthening of the hamstrings is occurring [9]. That re-
peated over-lengthening leads to microscopic muscle damage
that may accumulate to become macroscopic damage (i.e.,
muscle strain injury) [45].
To date, the usage of PBMT as a preventive tool against
sport injuries remains unexplored. Previous studies have al-
ready evidenced that a single PBMTsession, applied immedi-
ately before exercise, can minimize the muscle damage levels
in animals [46] and humans [47], but the preventive goal of
PBMT suggested here is totally new. Of course, a long-term
prospective study is needed to verify if PBMT can reduce the
HSI rate along a soccer season. The current study was just the
kick-off for employing PBMT as a preventive tool in sports
medicine, and our findings support that PBMT parameters
used here may be applied to reduce the soccer-induced fatigue
on hamstring muscles, exactly the primary effect to seek HSI
prevention through PBMT.
The reduced fatigue after PBMT was firstly demonstrated
in animal model [48], and a robust body of evidence has
supported this ergogenic effect in humans [23]. The mecha-
nisms responsible for this effect are not fully understood, and
the current study had no intention to investigate how PBMT
affects muscle fatigue from a physiological point of view.
Evidences suggest that the absorption of light energy by spe-
cific chromophores at mitochondria lead to intracellular reac-
tions that increase the ATP production and cellular metabo-
lism [49], as well as improve microcirculation (and, conse-
quently, oxygen supply to irradiated tissue) due to nitric oxide
release [50], among other responses that may optimize the
muscle cell functioning [22].
Given the typical biphasic dose–response effect of PBMT
treatments [22], finding the optimal parameters of PBMT is a
key factor for the treatment success. We cannot state that
PBMT parameters used in the current study are the most ef-
fective ones because we tested a single dosage, which can be
pointed out as a study limitation. However, the total energy
dose applied per muscle group (i.e., 300 J) is within the range
recommended to treat large muscle groups by the most recent
meta-analysis [23]. In addition, previous findings support that
pre-exercise PBMT with 300 J is better than lower dosages
Lasers Med Sci
(60–180 J) to improve performance and biochemical markers
related to skeletal muscle damage and inflammation in soccer
players [32]. Finally, the PBMT device used in the current
study required 60 s to deliver that energy amount to each
hamstring muscle (i.e., a 2-min treatment per player), so it is
a feasible therapy for application before a soccer match or
even during the half-time interval. The applicability of
PBMT as proposed here to the real-world of competitive soc-
cer (and other sports) is a strength of the current study.
Conclusion
PBMT applied on the posterior thigh muscles immediately
before a simulated soccer match proved to be effective in
attenuating fatigue-related impairments on hamstring maxi-
mal eccentric strength, H:Q strength ratio, and CMJ perfor-
mance. Given the relation between soccer-induced ham-
strings’fatigue and HSI, this study is pioneer to highlight
PBMT as a promising tool to prevent HSI in soccer players.
Long-term prospective studies are needed to verify if PBMT
will be effective in reducing the HSI rate along a soccer
season.
Acknowledgements Marco Aurélio Vaz and Ernesto Cesar Pinto Leal-
Junior thank CNPq-Brazil for the research productivity fellowships.
Maurício Pinto Dornelles and Carolina Gassen Fritsch thank CAPES-
Brazil for the scholarships.
Compliance with ethical standards
The study was previously approved by the Ethics and Research
Committee of the Universidade Federal de Ciências da Saúde de Porto
Alegre (no. 63299416.4.0000.5345), and all participants signed an in-
formed consent.
Conflict of interest Professor Ernesto Cesar Pinto Leal-Junior receives
research support from Multi Radiance Medical (Solon - OH, USA) and
Douglas Scott Johnson is an employee and shareholder of Multi Radiance
Medical, a photobiomodulation/laser device manufacturer. They didn’t
have any participation in data collection or data analysis in this study.
Furthermore, Multi Radiance Medical didn’t have any participation in
any aspect related to this study. The remaining authors declare that they
have no conflict of interests.
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