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Lasers in Medical Science
ISSN 0268-8921
Volume 26
Number 3
Lasers Med Sci (2011)
26:349-358
DOI 10.1007/
s10103-010-0855-0
Effects of low level laser therapy (808 nm)
on physical strength training in humans
1 23
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ORIGINAL ARTICLE
Effects of low level laser therapy (808 nm) on physical
strength training in humans
Cleber Ferraresi &Taysa de Brito Oliveira &Leonardo de Oliveira Zafalon &
Rodrigo Bezerra de Menezes Reiff &Vilmar Baldissera &
Sérgio Eduardo de Andrade Perez &Euclides Matheucci Júnior &
Nivaldo Antônio Parizotto
Received: 3 August 2010 / Accepted: 21 October 2010 / Published online: 18 November 2010
#Springer-Verlag London Ltd 2010
Abstract Recent studies have investigated whether low
level laser therapy (LLLT) can optimize human muscle
performance in physical exercise. This study tested the
effect of LLLT on muscle performance in physical strength
training in humans compared with strength training only.
The study involved 36 men (20.8±2.2 years old), clinically
healthy, with a beginner and/or moderate physical activity
training pattern. The subjects were randomly distributed
into three groups: TLG (training with LLLT), TG (training
only) and CG (control). The training for TG and TLG
subjects involved the leg-press exercise with a load equal to
80% of one repetition maximum (1RM) in the leg-press test
over 12 consecutive weeks. The LLLT was applied to the
quadriceps muscle of both lower limbs of the TLG subjects
immediately after the end of each training session. Using an
infrared laser device (808 nm) with six diodes of 60 mW
each a total energy of 50.4 J of LLLT was administered
over 140 s. Muscle strength was assessed using the 1RM
leg-press test and the isokinetic dynamometer test. The
muscle volume of the thigh of the dominant limb was
assessed by thigh perimetry. The TLG subjects showed an
increase of 55% in the 1RM leg-press test, which was
significantly higher than the increases in the TG subjects
(26%, P=0.033) and in the CG subjects (0.27%, P< 0.001).
The TLG was the only group to show an increase in muscle
performance in the isokinetic dynamometry test compared
with baseline. The increases in thigh perimeter in the TLG
subjects and TG subjects were not significantly different
(4.52% and 2.75%, respectively; P=0.775). Strength
training associated with LLLT can increase muscle perfor-
mance compared with strength training only.
Keywords Low level laser therapy (LLLT) .High-intensity
exercise .Isokinetic Dynamometer .Leg press .
One-repetition maximum test
Introduction
Strength training, mainly high-intensity exercises, uses
energy from anaerobic metabolism and promotes changes
in the contractile characteristics of the muscle fibers
involving a transition from type I and type IIx to type IIa.
An increase in fiber recruitment, and the timing and firing
frequency of motor units also occurs with this kind of
exercise [1,2]. In addition, strength training increases
muscle cross-sectional area (hypertrophy), which is associ-
ated with neural adaptation of muscle recruitment, increas-
ing muscle strength and performance [1,2].
C. Ferraresi (*):T. de Brito Oliveira :L. de Oliveira Zafalon :
N. A. Parizotto
Laboratory of Electrothermophototherapy,
Department of Physical Therapy,
Federal University of São Carlos,
Rodovia Washington Luís, km 235,
13565-905, São Carlos, SP, Brazil
e-mail: cleber.ferraresi@gmail.com
C. Ferraresi :E. M. Júnior
Department of Biotechnology, Federal University of São Carlos,
São Carlos, SP, Brazil
R. B. de Menezes Reiff
Department of Orthopedics and Traumatology,
University of São Paulo,
Cerqueira César, SP, Brazil
V. Baldissera :S. E. de Andrade Perez
Laboratory of Physiology of Exercise,
Department of Physiological Sciences, Federal University of São
Carlos,
São Carlos, SP, Brazil
Lasers Med Sci (2011) 26:349–358
DOI 10.1007/s10103-010-0855-0
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In general, exercises can be carried out in two ways:
closed kinetic chain (CKC) and open kinetic chain (OKC)
[3,4]. CKC exercises involve multiple articulations with
body weight or random loads being unloaded on a distal
body segment that is fixed to the ground or another base,
such as in squats or leg-press exercises. OKC exercises
generally involve movement in only one articulation, and
have the workload fixed on a distal extremity of the body
segment that is free to move, such as in knee extension
fitness equipment [5].
The measurement of muscle performance in CKC and
OKC exercises usually involves isotonic tests including
the one-repetition maximum leg-press (1RMleg) test
(CKC) and isokinetic muscle performance in isokinetic
dynamometry (MPID), especially in activities involving
knee extension (OKC) [6,7]. These tests allow more
complete evaluations and assist in directing training
programs [8].
The desire to increase and/or accelerate the gains in
physical performance, such as muscle hypertrophy and
enhancement of aerobic and anaerobic capacities, often
leads athletes and sportsmen to improve their nutrition [9].
Androgenic substances can also can be used but they may
pose risks to health [10]. The potential of low level laser
therapy (LLLT) for improving performance in exercises,
such as strength and resistance to fatigue has been tested
[11,12].
LLLT is mainly used for local pain control and tissue
repair [13,14]. It interacts with the cellular mitochondria,
promoting structural changes (appearance of giant mito-
chondria) and metabolic changes (increased oxidative
enzyme activity), increasing energy synthesis (ATP) for
metabolic processes [15,16]. Thus, the few recent studies
with laser therapy in men during physical exercise have
concentrated on investigating fatigue and muscle damage
after acute exercise of high intensity, and have involved the
determination of the concentrations and kinetics of bio-
chemical markers such as lactate and muscle creatine kinase
[12,17,18]. However, some studies reported in the
literature are divergent as to the effectiveness of LLLT in
increasing muscle performance in humans [11,18]. In these
studies, LLLT parameters such as dose and wavelength are
defined in terms of the depth of tissue reached by the
energy and consequently its attenuation, which directly
influences the therapeutic effect in the target tissue [13].
Furthermore, infrared laser radiation seems to be better for
stimulating muscle tissue because it can penetrate the skin
layers and reach greater depths without significant loss of
energy [13].
The purpose of this study was to determine whether
LLLT is able to optimize the effects of chronic strength
training. It was hypothesized that a chronic strength training
program associated with LLLT would promote a greater
increase in muscle performance than strength training
alone. The study was a randomized controlled clinical trial
with three tools to measure muscle performance: (1)
1RMleg test, (2) MPID test (knee torque extensor), and
(3) thigh perimetry as a measure of changes in thigh
volume.
Materials and methods
This study was designed as a randomized controlled clinical
trial. All procedures were approved by the Ethics in Human
Research Committee of the Federal University of São
Carlos (approval no. 342/2008) and the study was regis-
tered with NIH ClinicalTrials (NCT01113021). The sub-
jects were recruited from among graduate students at the
university. All volunteers were informed about the study
purposes and procedures. After inclusion in the trial, all
subjects signed a consent form.
Subjects
The study participants were 36 male subjects who reported
being clinically healthy.
Inclusion criteria
The inclusion criteria were: healthy males aged between 18
and 28 years with a body mass index (BMI) equal to or less
than 26 kg/m
2
, and with a beginner or moderately trained
pattern of physical activity, i.e. performed some physical
activity with a noncompetitive aim one to three times a
week, in accordance with previous studies [7,19].
Exclusion criteria
The exclusion criteria were: previous injury to the femoral
quadriceps or hamstring muscles (within 6 months prior to
study), osseous or articular disorder in the lower limbs,
cardiovascular system disorders, systemic disease, and
taking prescription medicines or using dietary supplements
(such as muscle mass builders).
After entering the study, subjects who did not comply
properly with the training routine, missed two consecutive
training sessions or developed any osseous or muscle or
articular injuries were excluded.
Randomization
Randomization was performed by a simple drawing
procedure and the subjects were distributed equally into
three groups: training with LLLT group (TLG), training
alone group (TG) and control group (CG).
350 Lasers Med Sci (2011) 26:349–358
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Study groups
TG and TLG subjects were submitted to a dynamic strength
training program involving the leg-press exercise twice a
week for 12 consecutive weeks. Soon after the end of each
session, the TLG subjects underwent LLLT to both femoral
quadriceps muscles. CG subjects did not carry out any form
of intervention and did not receive any treatment. Thus, this
group was evaluated only at the beginning and at the end of
the study.
Instruments
The following instruments were used: a 45° leg press
(ReForce, São Paulo, Brazil) for the 1RMleg test; a
goniometer (ISP, São Paulo Institute, São Paulo, Brazil) to
determine knee flexion angle in the 1RMleg test; a digital
Qwik Time QT5 metronome to standardize the timing of
concentric and eccentric muscle contractions during training;
a computerized isokinetic dynamometer (Multi-Joint System
3; Biodex, New York,NY) to record the isokinetic variables in
the MPID test; and a metric tape (3M, model Sanny, Brazil) to
measure the thigh perimeter of the subjects.
Procedures
The baseline assessments were carried out in the morning
and consisted primarily of recording the subjects’thigh
perimeter, followed by the MPID test. The MPID test
recorded the values for PT.ext. (knee peak torque extensor
of two series of evaluation) and Avg.PT.ext. (knee peak
torque extensor, average of two evaluation series). The
afternoon of the same day the 1RMleg test was performed.
The results of these muscle performance assessments were
normalized to the individual body mass (BM) and multi-
plied by 100, following the procedure described previously
[20]. All subjects were instructed not to change their usual
physical routine or eating habits during the study, not to
ingest alcohol, and to sleep well (both in quantity and
quality).
A pilot study was also conducted to establish the
reliability of the 1RMleg test, the MPID test and thigh
perimetry. The two tests were applied by the same
investigator randomly to six subjects who were not part of
the study on two separate occasions and separated by a 5-
day interval. The intraclass correlation coefficient (ICC 3,1)
was used to assess intraexaminer reliability and the standard
error of measurement (SEM) to describe measurement
accuracy. The results were: ICC 0.92, SEM (5.00 Nm/
BM)×100, for Avg.PT.ext.; ICC 0.93, SEM (5.17 Nm/
BM)×100, for PT.ext.; ICC 0.99, SEM (0.71 kg/BM)×100,
for the 1RMleg test; and ICC 0.99, SEM 0.01 cm, for
thigh perimetry.
Protocols for assessments, training and LLLT
All protocols for muscle performance assessments and
workload adjustment were performed by the same evalu-
ator. It is important to note that the assessment at baseline
and after 12 weeks were conducted on different training
days and that the assessment results were normalized by
subject BM at both the beginning and the end of the study.
Protocol I (thigh perimetry) The thigh perimeter was
measured midway between the anterior/superior iliac spine
and the base of the patella of the subject’s dominant lower
limb. The dominant lower limb was determined as that used
to kick a ball with greater accuracy. This assessment was
performed in orthostatic position and with the thigh
muscles relaxed. The thigh perimeter was measured only
at baseline and after 12 weeks of strength training.
Protocol II (isokinetic dynamometry) A brief 5-min warm-
up was carried out on a cycle ergometer (Ergo 167 Cycle;
Ergo-FIT, Pirmasens, Germany) with a load of 100 W and a
speed in the range 60–70 rpm. Next, the subjects were
positioned on the isokinetic dynamometer which had been
previously calibrated. The subjects stood properly aligned
and stabilized with straps in order to avoid possible
compensatory movements, in accordance with the guide-
lines for the device. The evaluation was performed only on
the subject’s dominant lower limb, and the dynamometer
rotation axis was adjusted to the knee axis of the subject
being assessed (at the lateral epicondyle of the femur;
Fig. 1a). The hip was stabilized in 80° flexion and the lever
arm of the equipment was set approximately 1 cm above
the tibial malleolus. Parameters such as chair height,
backrest distance, seat level and dynamometer base were
adjusted for each subject.
Before starting the recording of isokinetic variables,
there was a familiarization period with the apparatus that
consisted of three submaximal voluntary concentric muscle
contractions in the full range of standardized and preprog-
rammed motion (90–20°), with a constant angular velocity
of 60°/s. After a 3-min rest, the test began with two sets
(separated by a 3-min interval) of five maximal voluntary
concentric and reciprocal quadriceps and hamstring con-
tractions in all ranges of standardized and preprogrammed
knee motion in flexion and extension (Fig. 1b). The
subjects were encouraged verbally and visually to achieve
maximum effort. This evaluation was performed only at the
baseline and after 12 weeks of strength training. Only those
findings with a coefficient of variation less than 10% were
accepted [21].
Protocol III (1RMleg test) There was a brief warm-up
period of 5 min on a cycle ergometer (Ergo 167 Cycle;
Lasers Med Sci (2011) 26:349–358 351
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Ergo-FIT, Pirmasens, Germany) with a load of 100 W and a
speed in the range 60–70 rpm. Next, the load-lifting
technique was demonstrated by the evaluator. The test was
standardized by defining the subject’s lower limb extension,
identifying 90° knee flexion (using the goniometer) and
marking the position (in centimeters) corresponding to this
angle on the leg-press machine. The proposed range of motion
was 0° (full knee extension, start) to 90° (finish). The
anatomical references for the identification of the desired
angle were the greater trochanter of the femur, lateral
epicondyle of the femur and the malleolus of the fibula of
the same lower limb (Fig. 1c). Before beginning the test,
there was a familiarization period with the apparatus
consisting of ten repetitions with a load estimated less than
60% of 1RM. This subjective load was identified in
accordance with the level of physical effort by the subject
during the familiarization period, following the OMNI scale
(0 equal extremely easy and 10 equal extremely hard) [22].
The load increments for identifying the 1RMleg were in
terms of percentage of the load in the familiarization period,
and depended on the subject's score on the OMNI scale. The
load choices were limited to five attempts, separated by 5-
min intervals to avoid metabolic disorders and impairment of
test quality. The subjects were encouraged verbally to
achieve maximum effort.
Protocol IV (training) TG and TLG subjects began the
strength training program based on specific scientific data
in the literature [23,24] after 2 days of baseline assess-
ments. The training program consisted of twice-weekly
training sessions for the leg-press exercise at 45° on
nonconsecutive days. The total training period was 12
consecutive weeks (3 months), giving a total of 24
sessions. The training intensity was always 80% and
the training volume was 50 repetitions divided into five
sets of ten repetitions each. If the subject could not
complete ten repetitions in each set, he would continue
until concentric muscle failure and then rest. The rest
interval between sets was 2 min and the exercise speed
was governed by the metronome: 2 s eccentric muscle
action for each second of concentric action [23]. During
all training sessions (the leg-press exercise and the
1RMleg test) the heart rate of subjects and the range of
motion of the lower limbs were monitored to validate the
training and load in the 1RMleg test. The room temper-
ature was maintained between 23°C and 26°C. Adjust-
ments in workload were made by retesting the 1RMleg
every eight sessions during normal training (thus replacing
the session). Two days after the 24th session, subjects
underwent a final thigh perimetry assessment, followed by
final MPID and the 1RMleg test.
Protocol V (LLLT) TLG subjects underwent a LLLT
protocol immediately after each training session. A contact
technique was used for the infrared laser treatment. The
beam was kept stationary and perpendicular to the skin
during the 24 sessions in seven areas distributed over the
belly of the femoral quadriceps muscle of each subject in
previously demarcated areas. The first area was 10 cm
below the superior-anterior iliac spine and the others were
every 5 cm below the initial marks (Fig. 5, part A). The
pattern in each areas was recorded to make the laser
applications uniform between sessions. A near-infrared
laser device (GaAlAs, 808 nm) with six obliquely arranged
diodes of 60 mW power each was used operating in
continuous mode with the following parameters: beam area
0.0028 cm
2
; energy per point (diode) 0.6 J; per-session total
energy in each lower limb 25.2 J (for a total of 50.4 J);
application points 42 (for a total of 84 points); diode energy
density (fluency) 214.28 J/cm
2
; diode power density
21.42 W/cm
2
; and an application time in each lower limb
of 70 s (for a total time of 140 s, both lower limbs).
Fig. 1 a Subject positioning for the MPID test. bRange of motion developed in the MPID test. cSubject positioning in the 1RMleg test and
definition of knee angle flexion
352 Lasers Med Sci (2011) 26:349–358
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Statistical analysis
The normality of the data distribution was analyzed using
the Shapiro-Wilk test and the homogeneity of variances
using Levene’s test. The effects of training on 1RMleg,
MPID and thigh perimetry were evaluated by two-way
analysis of variance (ANOVA) with repeated measures only
on one factor. The independent factors were group (with
three levels: TLG, TG and CG) and time (with two levels;
baseline and after 12 weeks), which was also considered as
a repeated measurement. When significant differences were
found, Tukey’s post-hoc test was applied. The training effect
was also analyzed in terms of the percentage change in the
variables studied in relation to baseline (considered 100%)
and was compared among the groups by the Kruskal-Wallis
ANOVA test. Significance was set at P<0.05.
Results
The study began with 36 male subjects who met all the
inclusion criteria and signed a consent form. However, six
subjects were excluded during the study for the following
reasons: one subject did not agree with the group to which
he was randomly allocated, three were injured during
training and two CG subjects began a physical training
program during the study. Thus, our final sample size was 30
subjects, ten in each group. TLG subjects had a mean age of
19.7±0.8 years, a mean weight 76.6±11.5 kg, a mean height of
1.78±0.06 m and a BMI of 23.3±2.1 kg/m
2
.TGsubjectshad
a mean age of 21.2±2.5 years, a body weight of 75.7±6.3 kg,
a mean height of 1.78±0.05 m and a BMI of 23.7±1.9 kg/m
2
.
CG subjects had a mean age 21.8±2.1 years, a body weight
of 77.1±13.5 kg, a mean height of 1.80±0.05 m and a BMI
of 22.4±3.1 kg/m
2
.
The baseline 1RMleg, MPID and thigh perimetry
assessments were compared among the three groups to
identify any statistically significant differences. No signif-
icant difference was observed in any variable (P> 0.05) at
baseline. BM used for muscle performance normalization
of the 1RMleg and MPID test results changed after the
training program, but not significantly (P> 0.05). The TLG
subjects showed an increase in BM of 1.30%, the TG
subjects an increase of 1.50% and the CG subjects an
increase of 0.12%. The TLG and TG subjects showed
significant increases (P<0.001) in the 1RMleg after the
strength training program. The 1RMleg in the TLG subjects
was higher (P<0.001) than that in the CG subjects and not
significantly different from that in the TG subjects (P=
0.748). The TG subjects had a higher 1RMleg (P=0.008)
than the CG subjects. In terms of average percentage, the
1RMleg in the TLG subjects increased by 55.59%, in the
TG subjects by 26.83% and in the CG subjects by 0.27%.
Comparing the groups, the TLG subjects had a higher
percentage gain than the TG subjects (P=0.033) and the
CG subjects (P<0.001). The TG subjects had a higher
percentage gain than the CG subjects (P= 0.033). These
changes in the load in 1RMleg test are summarized in
Fig. 2and the percentages in Table 1.
The MPID test results were higher in the TLG and TG
subjects after the strength training program but only the
TLG subjects showed statistically significant increases in
Avg.PT.ext. (P=0.003) and PT.ext. (P=0.036). The com-
parison among the groups did not identify significant
differences (P>0.05). In terms of percentages, the TLG
subjects showed an increase in Avg.PT.ext. of 7.38% and in
PT.ext. of 4.67%. In terms of percentage comparisons, the
values of Avg.PT.ext. and PT.ext. in the TLG subjects were
significantly higher than the values in the CG subjects
(P<0.001, P=0.001, respectively). There were no significant
differences (P>0.05) between the TLG and TG subjects or
between the TG and CG subjects. These changes in the
MPID test results are summarized in Fig. 3and the
percentages are shown in Table 1.
The thigh perimeter of the dominant lower limb
increased significantly in TLG and TG subjects (P<
0.001). CG subjects showed a decrease in thigh perimeter
without statistical significance (P=0.999). In terms of
percentage change in thigh perimeter, the TLG subjects
showed an increase of 4.52%, the TG subjects an increase
of 2.75% and the CG subjects a decrease of 0.53%.
Comparing these percentages among groups, the thigh
perimeter in the TLG subjects was significantly higher than
in the CG subjects (P<0.001) but not significantly different
from that in the TG subjects (P= 0.775). The thigh
perimeter in the TG subjects was significantly higher than
in the CG subjects (P=0.006). These changes in the thigh
Fig. 2 Loads in the 1RMleg test in the three study groups at baseline
and after 12 weeks of strength training (TLG training and LLLT group,
TG training only group, CG control group; BM body mass; *P<0.05)
Lasers Med Sci (2011) 26:349–358 353
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perimeter are summarized in Fig. 4and the percentages are
shown in Table 1.
Discussion
This study investigated whether LLLT combined with
physical strength training (80% of 1RM) would promote a
higher increase in muscle performance in CKC and OKC
exercises and in the higher thigh perimetry volume in
young men when compared with strength training without
LLLT. The 30 subjects who completed the program were
randomly distributed into three groups of equal size with no
significant differences at baseline. BM was used to
standardize the force measured in the CKC and OKC tests,
and it did not change significantly from baseline values (P>
0.05). Thus, it did not significantly affect muscle perfor-
mance in the final assessment.
After the 12 weeks of strength training, the 1RM load in
the leg-press test was significantly higher than the baseline
load in subjects of both the TLG and TG. However, there
was no significant difference in the CG subjects (P>0.999)
since there was no intervention involved. An increase in
muscle strength following a strength training program has
been well established [1,2].
There were no statistically significant differences be-
tween the TLG and TG subjects in terms of their means and
variances in the 1RMleg test after training (P= 0.748). Only
the differences between TLG and CG subjects (P< 0.001)
and between TG and CG subjects (P=0.008) were
significant. This may have been due to the initial average
Fig. 4 Thigh perimetry in the three study groups at baseline and after
12 weeks of strength training. *P<0.05
Fig. 3 Results of MPID test for the three study groups at baseline and
after 12 weeks of strength training. Avg.PT.ext. knee peak torque
extensor, average of two evaluation series; PT.ext. knee peak torque
extensor of two evaluation series. *P<0.05
Table 1 Percentage change (gains or losses) in muscle performance in the MPID test, 1RMleg test and perimetry, and comparisons among the
groups (Pvalues in the Kruskal-Wallis test)
Variable Percentage change after training Pvalues
TLG TG CG TLG × CG TLG × TG TG × CG
1RMleg 55.59 26.83 0.27 <0.001* 0.033* 0.033*
Avg.PT.ext. 7.38 3.16 −2.97 <0.001* 0.639 0.092
PT.ext. 4.67 1.82 −2.98 0.001* 0.401 0.126
Perimetry 4.52 2.75 −0.5 <0.001* 0.775 0.006*
Avg.PT.ext. knee peak torque extensor, average of two evaluation series; PT.ext. knee peak torque extensor of two evaluation series; *P<0.05.
354 Lasers Med Sci (2011) 26:349–358
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1RMleg load in the TLG subjects being smaller than the
initial load in the TG subjects (a difference of 49 kg),
although the difference was not significant (P=0.919).
Thus, despite the final load average in the 1RMleg test
being higher in the TLG subjects than in the TG subjects,
the difference was not significant (P=0.748). However,
considering the percentage increase, the TLG subjects
showed an increase of 55.5% in the 1RMleg test which
was significantly higher (P=0.033) than the increase of
26.8% in the TG subjects after the training program (see
Table 1).
When these groups were compared in the OKC MPID
test, only the TLG subjects showed an increase in average
knee peak torque extensor and the knee peak torque
extensor (Avg.PT.ext. and PT.ext.; see Fig. 3). Only the
TLG subjects showed higher MPT.ext. and PT.ext. values
in comparison with the CG subjects. However, no signif-
icant difference was observed between TLG and TG
subjects for MPID (see Fig. 3and Table 1). The low
percentage transfer of muscle performance between isotonic
CKC exercises and isokinetic OKC exercises found in this
study have been reported previously [4] and is due the
specificity of the training [8,25] and the form of evaluation
[6].
Some studies that have investigated muscle performance
in men undergoing physical exercise associated with LLLT
did not find significant improvements [11,18]. However,
other studies with similar methodologies have found highly
relevant results for fatigue reduction and improvement in
the muscle performance [12,17,18]. Since the fluency,
number of application points and total energy delivered to
muscle in previous studies have differed [11,12,17,18],
we decided to use a total energy similar to that used in the
study by Leal Junior et al. [18], which was 40 J. So the
fluency used was lower (214.28 J/cm
2
vs 1,428.57 J/cm
2
)
and the total number of application points was greater (84
points vs 10 points) to get the best energy distribution in the
femoral quadriceps muscle (Fig. 5, part A).
The evaluation methods used in previous studies
analyzed the kinetics of biochemical markers of fatigue
(lactate) and muscle damage (creatine kinase) [12,17,18].
These studies were randomized, double-blind, placebo-
controlled trials and showed significantly lower levels of
these markers and also an increase in the number of
maximal voluntary contractions, indicating a reduction in
the fatigue induced by exercise [12,17,18]. The authors
considered that the improved physical performance provid-
ed by the action of LLLT was a result of lower creatine
kinase activity, increased antioxidant levels and improve-
ment in microcirculation and lactate removal.
Adding to previous hypotheses, the present study
investigated three more possible physiological mechanisms
for the improvement in physical performance in humans
when exercise is associated with LLLT, all based on the
Fig. 5 AApplication points for LLLT on the femoral quadriceps
muscle. BMitochondrial creatine shuttle mechanism. In this mecha-
nism, creatine (Cr) is transported from the ATP-utilizing sites (e.g.,
myofibrils) to the mitochondria, and the phosphocreatine (PCr)is
transported in the opposite direction. Due to the presence of creatine
kinase (CK) in the inner mitochondrial membrane, the creatine reacts
with ATP produced during oxidative phosphorylation and resynthe-
sizes phosphocreatine. This process increases the ADP concentration
and it in turn stimulates respiration. However, the phosphocreatine
decreases the ADP concentration and respiration (ANT adenine
nucleotide translocase) (modified from Tonkonogi and Sahlin [27]).
CLactate oxidation in the mitochondrial pathway. The lactate is
transported to the intermembrane space or directly to the mitochon-
drial matrix, where it is oxidized to pyruvate by NAD
+
and this
reaction is catalyzed by lactate dehydrogenase mitochondrial enzyme
(mLDH). The reduced NAD (NADH) is oxidized in the electron
transport chain (ETC) and provides electrons and protons for the
aerobic production of ATP (mMCT mitochondrial monocarboxylate
transporters) (modified from Brooks et al. [31])
Lasers Med Sci (2011) 26:349–358 355
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importance of the cellular mitochondria in energy produc-
tion. There is strong evidence in the scientific literature that
LLLT has a close relationship with the mitochondria,
promoting their growth and/or fusion of smaller mitochon-
dria to form giant mitochondria consequently increasing the
mitochondrial density in the tissues [15,16]. Greater
mitochondria have been associated with higher enzymatic
machinery for aerobic ATP production [15,16]. Although
these organelles play a fundamental role in the energy
production necessary in endurance training and in low-
intensity exercise [26–28], they may also contribute to the
greater energy availability in high-intensity exercise, such
as that investigated in the present study. This is based on
the hierarchical and ramp recruitment of the muscle fibers.
It has been found that in increasing exercise intensity, the
order of muscle fiber recruitment necessarily follows the
following order: type I (oxidative), type II (glycolytic and
oxidative), and finally type IIx (glycolytic) [29]. Therefore,
aerobic energy production (oxidative) is supplemented by
anaerobic energy production (glycolytic) as exercise
becomes more intense [30].
The second hypothesis for the muscle performance
improvement in the TLG subjects is the integration between
the aerobic and anaerobic ATP production described by
Tonkonogi and Sahlin [27]. This mechanism involves
phosphocreatine resynthesis which is dependent on the
shuttling of mitochondrial creatine in the greatest quantities
into muscle fibers with oxidative characteristics [27]. The
creatine shuttle system captures ADP and inorganic
phosphorus that result from the use of ATP in muscle
contraction, and transports them to the mitochondrial
matrix through the inner membrane organelle by means of
adenine nucleotide translocase. The ATP produced by
oxidative phosphorylation takes the same route, although
in the opposite direction, providing energy for the phos-
phocreatine resynthesis reaction in the active muscle. This
reaction is catalyzed by muscle creatine kinase. Concom-
itantly, the use of phosphocreatine energy generates muscle
creatine, ADP and inorganic phosphorus. While the ADP
and inorganic phosphorus follow the above-mentioned
path, creatine is transported to the mitochondrial intermem-
brane spaces of the muscle, and then mitochondrial creatine
kinase catalyzes the phosphocreatine resynthesis reaction,
also using ATP produced through oxidative phosphoryla-
tion. Ultimately, the phosphocreatine is transported to the
muscle contraction site, supplies the energy necessary to
continue the contractile activity and increases the ATP/ADP
ratio [27] (Fig. 5, part B).
Considering the effects of LLLT on mitochondria, and
the greater mitochondrial density and/or the enzymatic
machinery for ATP production [15,16], a bigger phospho-
creatine re-synthesis possibly must be occurs. Phosphocre-
atine resynthesis, that mainly occurs in the rest intervals
during high-intensity exercise, would be able to supply
some of the necessary energy for the next series of muscle
contractions, by providing the resynthesis of the ATP used
during session training or during maximal CKC and OKC
tests.
The third hypothesis for the muscle performance
improvement on the TLG subjects is the removal and
oxidation of lactic acid produced anaerobically during
exercise, because metabolic acidosis may induce muscle
fatigue [31,32]. Lactic acid formation in the cytosol of
muscle fibers is due to the reduction of the pyruvate to
lactate, which is catalyzed by cytosolic lactate dehydroge-
nase, and occurs mainly in anaerobic and strength exer-
cises. Next, lactate is transported to the mitochondrial
matrix via monocarboxylic acid transporters and, by means
of the NAD
+
and mitochondrial lactate dehydrogenase, is
oxidized to pyruvate. The reduced NAD (NADH) is
oxidized in the electron transport chain and provides the
necessary electrons and protons for the aerobic production
of ATP. The pyruvate in turn is oxidized to acetyl-CoA and,
in the Krebs cycle, it continues to be oxidized to produce
ATP aerobically through the electron transport chain [31,
32] (Fig. 5, part C).
Regarding the thigh perimetry results in the TLG, TG
and CG subjects, only the TLG and TG subjects showed a
significant increase in thigh perimeter after the training
program. However, the change in thigh perimeter was not
significantly different among all groups when compared
with their the mean and variances. With regard to the
percentage changes in thigh perimeter, TLG and TG
subjects showed a greater increase than the CG subjects
(see Table 1).
Numerous studies have shown increases in cross-
sectional area of muscle tissue following physical strength
training (for review see references [2,8,23]). A hypothesis
that could explain this physiological adaptation to strength
training is the association between the degree of muscle
injury and the possible greater activation of muscle satellite
cells, i.e. situations where microdamage occurring in the
muscle structure needs repair [33–35]. Microdamage
stimulates mononuclear inflammatory cells (neutrophils
and macrophages), attracts satellite cells to the injury site
through chemotactic mechanisms and activates their prolif-
eration and differentiation through molecular mechanisms
[36–38]. After cell differentiation, satellite cells are called
myotubes and can fuse with damaged muscle fibers, or
originate new contractile proteins [36–38]. In this process,
LLLT seems to modulate satellite cell metabolism, thus
directly influencing muscle tissue regeneration [39–41].
We suggest that future work should investigate the
possible action of LLLT on gene expression in humans.
The main genes that specifically encode transcription
factors for satellite cells in the quiescent state include
356 Lasers Med Sci (2011) 26:349–358
Author's personal copy
Pax7 (paired box 7) and c-met (hepatocyte growth factor
receptor), and in the activated (proliferative) state for
myoblast formation include Myf-5 (myogenic factor 5),
and for myoblast differentiation into myotubes include
myogenin (myogenic factor 4), MyoD (myogenic differen-
tiation) and MRF4 (muscle-specific regulatory factor 4),
which is known as Myf-6 (myogenic factor 6) [36–38]. In
addition, the gene for expression of myostatin (GDF-8)
may be modulated by LLLT, since this gene is a major
atrophy marker and acts against the process of muscle
regeneration, leading to an inhibition of satellite cell
proliferation and less hypertrophy [36]. Furthermore, in the
context of gene expression, LLLT could alter the expression of
genes for muscular hypertrophy, such as mTOR (mechanistic
target of rapamycin) and/or the genes responsible for
mitochondrial biogenesis, including NRF-1, NRF-2 (nuclear
respiratory factor 1 and 2), Tfam (transcription factor A,
mitochondrial) and PCG-1α(peroxisome proliferator-
activated receptor gamma, coactivator 1 alpha) [42,43].
Conclusion
The results of this study suggest that strength training
combined with LLLT may be superior to strength training
only. We emphasize that care is needed in generalizing the
results. Further studies, especially those involving LLLT
and gene expression, are necessary to elucidate the
interaction between laser radiation and the molecular
mechanisms of recovery and muscle performance.
Acknowledgments The authors would like to thank the Depart-
ments of Physical Therapy and Physiological Sciences of the Federal
University of São Carlos for assistance with this study, the research
subjects, and also the Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq) for partial funding of the research.
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