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A proposed model to test the hypothesis of exercise-induced localized fat reduction (spot reduction),
including a systematic review with meta-analysis
Short title: Exercise-induced spot reduction
Rodrigo Ramirez-Campillo1,2, David Andrade3, Filipe Manuel Clemente4,5, José Afonso6, Alejandro Pérez-
Castilla7, Paulo Gentil8
1 Department of Physical Activity Sciences. Universidad de Los Lagos. Santiago, Chile.
2 Exercise and Rehabilitation Sciences Laboratory, School of Physical Therapy, Faculty of Rehabilitation
Sciences, Universidad Andres Bello, Santiago 7591538, Chile.
3 Centro de Fisiología y Medicina de Altura, Facultad de Ciencias de la Salud, Universidad de Antofagasta,
Antofagasta, Chile.
4 Escola Superior Desporto e Lazer, Instituto Politécnico de Viana do Castelo, Rua Escola Industrial e
Comercial de Nun’Álvares, 4900-347 Viana do Castelo, Portugal.
5 Instituto de Telecomunicações, Delegação da Covilhã, Lisboa 1049-001, Portugal.
6 Centre of Research, Education, Innovation, and Intervention in Sport (CIFI2D), Faculty of Sport, University
of Porto, Portugal.
7 Departamento de Educación Física y Deportiva. Facultad de Ciencias del Deporte. Universidad de Granada.
España.
8 Faculdade de Educação Física e Dança, Universidade Federal de Goiás, Brasil.
Corresponding author
Rodrigo Ramirez-Campillo, PhD.
ORCID: 0000-0003-2035-3279
r.ramirez@ulagos.cl
A proposed model to test the hypothesis of exercise-induced localized fat reduction (spot reduction),
including a systematic review with meta-analysis
ABSTRACT
Background: The notion that specific exercises reduce localized adipose tissue depots (i.e., targeted fat loss)
and modify fat distribution is commonly termed spot reduction. According to this long-held popular belief,
exercising a limb would lead to greater reduction in the adjacent adipose tissue in comparison to the
contralateral limb. Aside from popular wisdom, scientific evidence from the 20th and 21th century seems to
offer inconclusive results. Objective: To summarize the peer-reviewed literature assessing the effects of
unilateral limb training, compared to the contralateral limb, on the localized adipose tissue depots on healthy
participants, and to meta-analyse its results. Methods: We followed the guidelines of the Preferred Reporting
Items for Systematic Reviews and Meta-Analyses (PRISMA). We searched PubMed, Web of Science, and
SCOPUS electronic databases, using several relevant keywords combinations. Independent experts were
contacted to help identify additional relevant articles. Following a PICOS approach, we included controlled
studies that incorporated a localized exercise intervention (i.e., single-limb training) to cohorts of healthy
participants (i.e., no restriction for fitness, age, or sex), compared to a control condition (i.e., contralateral
limb), where the main outcome was the pre-to-post intervention change of localized fat. The methodological
quality of the studies was assessed using the Physiotherapy Evidence Database scale. Pre- and post-intervention
mean ± standard deviation for fat-related outcome from the trained and control groups (limbs) were converted
to Hedges’ g effect size (ES; with 95% confidence intervals [CI]), using a random-effects model. The impact
of heterogeneity was assessed using the I2 statistic. The risk of reporting bias was explored using the extended
Egger’s test. The statistical significance threshold was set at p < 0.05. Results: From 1,833 search records
initially identified, 13 were included in the meta-analysis, involving 1,158 male and female participants (age,
14-71 years). The 13 studies achieved a high methodological quality, and results with low heterogeneity (I2 =
24.3%) and no bias (Egger’s test p = 0.133). The meta-analysis involved 37 comparisons, with 17 of these
favouring (i.e., greater reduction of localized fat) the trained limb, and 20 favouring the untrained limb, but the
ES ranged between -1.21 to 1.07. The effects were consistent, with a pooled ES = -0.03, 95% CI -0.10 to 0.05,
p = 0.508, meaning that spot reduction was not observed. Conclusion: Localized muscle training has no effect
on localized adipose tissue depots, i.e., no spot reduction, regardless of the characteristics of the population
and of the exercise program. The popular belief on spot reduction is probably derived from wishful thinking,
and convenient marketing strategies, such as influencers seeking increased popularity and procedures’ sellers
interested in increasing advertising. Keywords: exercise; human physical conditioning; resistance training;
high-intensity interval training; body composition; subcutaneous fat.
1. INTRODUCTION
“A man may box and fence, and even walk, without losing
his terrible abdominal accumulation; but if he centres his
efforts at muscular exertion on the abdomen itself the fat
cannot stand the attack and will gradually disappear”.
Checkley, E. (1895) [1].
Since (at least) the 19th century, the notion that specific exercises can reduce localized adipose tissue depots
(i.e., targeted fat loss) and modify fat distribution remains a very popular belief, commonly termed spot
reduction [2]. From the mid to nearly the end of the 20th century, several studies were performed on the subject,
suggesting that spot reduction might be feasible [3-6]. However, during the same period several studies
disproved the notion of spot reduction [2, 7-13]. Toward the end of the 20th century there seemed to be a
consensus among the scientific community that spot reduction is a myth. Nonetheless, during the 21th century
new studies [14-24] relaunched the debate.
Why is the notion of spot reduction so appealing across centuries [1, 25, 26]? Why have researchers not reached
a definitive answer to the problem? This might be explained by three main factors. A first factor may be the
difficulty inherent to address the hypothesis of spot reduction. There are complex interactions between i)
different exercise programming characteristics (e.g., exercise modality; periodization; load management;
adherence to the program), ii) differential regional responses of adipose tissue depots to exercise (i.e., lipolysis;
re-esterification; mobilization of free fatty acids), and iii) inter-individual differences in the modulators of the
fat metabolism in response to exercise (e.g., sex; obesity) [19, 27-32]. A second factor of controversy may
arise from different concepts of spot reduction [16, 33]. Different models of study were used to test the
hypothesis of spot reduction, such as cross-sectional studies [7, 14, 34, 35], as well as long-term intervention
studies involving exercise compared to nutrition [22, 23], trunk-localized exercise [8, 20], limb-localized
exercise [5, 9, 18, 21], and whole-body exercise [15, 16]. A third factor is the difficulty to conduct rigorous
experimentation to test such a hypothesis (e.g., control the participants’ diet and their compliance to the
program; use valid measurement techniques [18]). The difficulty encountered by scientists is in contrast with
the ease with which personal beliefs (or publicity) can be communicated [1, 25, 26]. Marketing and science
often collide [33], and marketing the notion of exercise-based spot reduction to persons seeking a desperate
solution to their problems [36] may be very appealing.
If the notion of spot reduction is correct, then performing a regimen of unilateral exercise should lead to higher
reduction in adipose content in that region than that in the contralateral limb. To the best of our knowledge, the
debate regarding exercise-based spot reduction seems to be active, even after (at least) three centuries [1]. To
contribute to settle dawn the debate, a systematic review with meta-analysis was conducted to qualitatively
assess and quantitatively summarize the evidence in the field, but also circumvent the problem of most
exercise-sport related studies: a reduced sample size [37]. Our aim was to summarize the peer-reviewed
literature assessing the effects of unilateral limb training, compared to the contralateral limb, on the localized
adipose tissue depots on healthy participants across the life span, and to meta-analyse its results.
2. METHODS
We followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses
(PRISMA) [38, 39]. The methods were established before initiating the research, and protocol registration
preceded the search.
2.1. Search strategy
We searched through PubMed, Web of Science, and SCOPUS electronic databases from the inception of
indexing to June 2021, with no restriction on language. Potentially relevant keywords were collected through
authors’ consensus based on previous studies conducted in relation to spot reduction, and organized vocabulary
(i.e., Medical Subject Headings: MeSH) was also incorporated. As a result, the following key words were
introduced in the electronic databases in different combinations using Boolean search syntax with the operators
“AND”, “OR”: activity, arm, body, clinical, composition, conditioning, controlled, distribution, dominant,
elbow, exercise, extension, fat, flexion, forearm, high, human, intensity, interval, knee, leg, local, localized,
loss, mass, modalities, model, motor, movement, muscle, musculoskeletal, non-dominant, phenomena,
physical, physiological, reduction, regional, resistance, running, single, sport, spot, strength, subcutaneous,
targeted, therapy, thigh, training, treatment, trial, unilateral. Electronic searches were conducted according to
the specific characteristics of each electronic database search engine. For example, in the PubMed database,
the following search syntax was used: controlled clinical trial [Publication Type] AND training [Title/Abstract]
OR single-leg [Title/Abstract] AND body composition [MeSH Terms] AND fat [Title/Abstract].
After the initial search in June 2021, we created accounts in the respective databases. Through these accounts,
the lead investigator received weekly automatically generated emails for updates regarding the search terms
used (if available). All studies that were published before August 2021 were considered for inclusion. We
excluded studies based on the review of the title, abstract, or (when needed) after the full text was read.
Conference proceedings were considered if the full-text was available. The reference list of included studies
was searched for potentially relevant studies. Two authors (RRC, DCA) conducted the process independently,
with potential discrepancies resolved by consensus.
Thereafter, the list of included articles and the inclusion criteria were sent to two independent world experts in
the field of body composition (https://www.expertscape.com/ex/body+composition) to help identify additional
relevant articles. Additionally, the experts (i) hold a Ph.D. in Sports Sciences or related field (e.g., Health
Sciences); (ii) have peer-reviewed publications in body composition, in journals with impact factor according
to the Journal Citation Reports®. The experts were not provided with our search strategy, to avoid biasing their
own searches. Upon completion of all these steps, the databases were again consulted in search for errata or
retractions of any included study.
2.2. Eligibility criteria
To elaborate the PICOs eligibility criteria, we first elaborated a definition for the investigated problem.
Accordingly, spot reduction (in humans) is defined as a greater reduction of the non-intramuscular fat-related
depot(s) (e.g., subcutaneous fat) adjacent to a voluntary exercised muscle compared to the same depot from
the contralateral non-exercised muscle, after an intervention period.
Accordingly, and following PICOS criteria, we incorporated studies that: (i) included cohorts of healthy (e.g.,
with medical/ERB clearance to participate in a training programme) participants (humans), with no restriction
for fitness/sport background, age or sex. Excluded participants were those with physical trauma (e.g., limb
amputation) [40], health diseases (e.g., stroke leading to paretic limb [41]; genetic conditions or syndromes
potentially affecting adipose tissue or its response to training [24, 42, 43]); (ii) incorporated a localized exercise
intervention (without restriction from the mode of exercise, e.g., resistance training; endurance training) were
one limb was trained and contralateral limb was the control. Interventions with a minimum of 2 weeks were
considered [44, 45]. Studies that incorporated a non-localized exercise intervention (e.g., running; bilateral leg
press) were excluded. Cross-sectional studies were also excluded. Studies were not excluded if they lacked
dietary control and/or included nutritional supplementation, as this is not a critical factor for experimental
models using a contralateral limb as a control condition [46]; (iii) localized exercise was compared with a
control condition (i.e., contralateral limb), with the only difference between the conditions being the exercise
intervention; (iv) the study included a pre-to-post intervention assessment of at least one fat-related parameter
(e.g., fat mass; fat volume) using DEXA, magnetic resonance imaging, computerized tomography, skinfold
callipers, ultrasound, and microscopic method (i.e., subcutaneous fat biopsy). Secondary outcomes were
considered, including potential adverse effects derived from the intervention (e.g., injury); and (v) utilized a
randomised or non-randomised controlled design, as long as at least one comparator group existed.
2.3. Data extraction
Two authors of the review (RRC, DCA) performed the data extraction independently, using a pre-defined form
created in Microsoft Excel (Microsoft Corporation, Redmond, WA, USA). If there were any discrepancies
between the authors in the extracted data, the accuracy of the information was re-checked in the studies. We
extracted the following data: participants’ sex, age (years), body mass (kg), height (m), and previous experience
with training. If applicable, information about the type and level (e.g., professional, amateur) of sport practice
was also retrieved. Regarding training characteristics, extracted data included training frequency (days/week)
and training duration (weeks), intensity level and marker of intensity (e.g., % of one repetition maximum
[1RM]), total volume (e.g., repetitions; minutes), types of exercises performed, combination of exercise with
diet, and progressive overload techniques (if any).
The means and standard deviation of dependent variables were extracted at pre- and post-intervention time
points from included studies. In cases where the required data were not clearly or completely reported, the
authors of the study were contacted for clarification. If no response was obtained from the authors (after two
attempts), or if the authors could not provide the requested data, the study outcome was excluded from the
analysis. However, even when no numerical data were provided by the authors upon contact, in cases where
data were displayed in a figure [9], the meta-analysis used validated (r = 0.99, p <0.001) [47] software
(WebPlotDigitizer; https://apps.automeris.io/wpd/) [48] to derive the relevant numerical data.
2.4. Methodological quality assessment
The Physiotherapy Evidence Database (PEDro) scale was used to assess the methodological quality of the
included studies [49]. There are 11 items on the PEDro checklist, but item 1 is not included in the total score.
Therefore, the methodological quality of the included studies was rated from 0 (lowest quality) to 10 (highest
quality). This scale evaluates different aspects of the study design, such as participant eligibility criteria,
randomization, blinding, attrition, and reporting of data. The validity and reliability of the PEDro scale has
been established previously [49-51]. Additionally, its agreement with other scales (e.g., Cochrane risk of bias
tool) has been reported [52]. Moreover, the PEDro scale is probably one of the most frequently used scales in
the literature, helping to make comparisons between meta-analyses. According to cut-off scores, the
methodological quality was rated as ‘poor’ (<4), ‘fair’ (4-5), ‘good’ (6-8) and ‘excellent’ (9-10) in some sub-
fields, however, it is not possible to satisfy all scale items in some areas of physiotherapy practice [53].
Moreover, in the context of this study, and according to the definition of spot reduction and the proposed
experimental model to test the hypothesis of spot reduction, is not possible to blind the participants regarding
whether they trained or not one of their limbs, making the item 5 from the PEDro scale an unfair criteria to
assess the methodological quality of studies in the context of our review. Therefore, as outlined in previous
systematic reviews in some sub-fields of physiotherapy [54, 55], the methodological quality of studies was
interpreted using the following convention, based on the summary score: studies that scored ≤ 3 points were
considered as being of “poor quality”, studies scoring 4 or 5 points were considered as being of “moderate
quality”, and studies that scored 6 – 10 points were considered as being of “high quality”. Two authors (RRC,
DCA) performed the methodological quality assessment independently. Disagreements in the assessments
between the reviewers were resolved through discussion and consensus.
2.5. Statistical analysis
Pre- and post-intervention mean ± standard deviation (SD) for a given fat-related outcome from the trained and
control groups were converted to Hedges’ g effect size (ES). A meta-analysis for a given fat-related outcome
was conducted if at least three studies provided sufficient data for the calculation of ES [56-58]. The data were
standardized using post score SD. For studies that reported standard errors, standard deviations were calculated
by multiplying the standard error with the square root of the sample size [59]. In all analyses, we used the
random-effects model to account for differences between studies that might affect the treatment effect [60, 61].
The ES values are presented alongside their respective 95% CIs. Calculated ES were interpreted using the
following scale: < 0.2, trivial; 0.2 – 0.6, small; > 0.6 – 1.2, moderate; > 1.2 – 2.0, large; > 2.0 – 4.0, very large;
> 4.0, extremely large [62]. The impact of heterogeneity was assessed using the I2 statistic, with values of <
25%, 25 - 75%, and > 75% considered to represent low, moderate, and high levels of heterogeneity,
respectively. The risk of reporting bias was explored using the extended (two-tailed) Egger’s test [63]. To
adjust for publication bias, a sensitivity analysis was conducted using the trim and fillmethod (Duval and
Tweedie, 2000), with L0 as the default estimator for the number of missing studies (Shi and Lin, 2019). All
analyses were carried out using the Comprehensive Meta-Analysis program (version 2; Biostat, Englewood,
NJ, USA). The statistical significance threshold was set at p < 0.05.
3. RESULTS
3.1. Study selection
A total of 1,833 search records were initially identified. After excluding the duplicates and studies based on
title, abstract, 83 studies remained, and the full text was read. From these, 13 were included in the meta-analysis
[5, 9, 10, 18, 21, 46, 64-70]. Figure 1 provides a diagram of the study selection process. The included studies
involved 1,158 participants (acting as both experimental and control groups). The characteristics of the
participants from the included studies, the programming parameters of the training interventions, and the fat-
related outcomes (for both the control and experimental limbs) are presented in Table 1.
***Figure 1***
***Table 1***
Briefly, training interventions were applied during 2 up to 20 weeks, with a training frequency of 3 sessions
per week, up to 7 sessions per week (i.e., daily training). The training intensity (i.e., single-limb) varied from
10% up to 90% of 1RM for those interventions that applied resistance training exercises, and ~40% of peak
oxygen consumption (VO2peak) in the intervention that applied endurance (i.e., cycling) training. Of note, the
interventions that applied resistance training exercises commonly used elbow flexors- extensors-related
exercises (e.g., dumbbell bicep concentration curls; overhead triceps extension), or knee extensors-related
exercises (e.g., seated leg press; seated leg extension), although none of the included studies applied knee
flexors-related exercises. No major adverse effects were reported among the included studies, other than mild-
moderate delayed onset of muscle soreness. However, most of the included studies in this meta-analysis failed
to report specific information regarding adverse health effects. This reflects a larger problem in sports sciences
and produces unbalanced accounts, as authors report the main effects, but not the potential adverse health
effects.
3.2. Methodological quality
Using the PEDro checklist, the 13 studies achieved 6-8 points and were classified as being of “high”
methodological quality (Table 2).
***Table 2***
3.3. Meta-analysis results
The meta-analysis included 13 controlled studies, involving 37 comparisons, with 17 of these favouring (i.e.,
greater reduction of localized fat) the trained limb, and 20 favouring the untrained limb, but the ES ranged
between -1.21 to 1.07. The effects were consistent, with a pooled ES = -0.03, 95% CI -0.10 to 0.05, p = 0.508,
I2 = 24.3%, Egger’s test p = 0.133 (Figure 2), meaning that spot reduction was not observed.
***Figure 2***
4. DISCUSSION
According to the definition for our proposed model to test the hypothesis of spot reduction, our aim was to
summarize the peer-reviewed literature assessing the effects of unilateral limb training, compared to the
contralateral limb, on the localized adipose tissue depots on healthy participants across the life span, and to
meta-analyse its results. From the 13 studies included in our meta-analysis, all achieved six or more points in
the PEDro scale. This may increase the perceived quality of research included in our analyses and the
confidence in evidence. Further, results were obtained with low impact of heterogeneity (I2 = 24.3%) and no
significant risk of reporting bias (Egger’s test p = 0.133). In addition, a total of 1,158 participants were included
in the 13 studies, a strength when compared to the relatively reduced number of participants involved in sport
sciences literature [37]. Although exercise is a potent contributor to fat reduction [71], our meta-analysis
indicated no significant (trivial) effect of localized muscle training on localized adipose tissue depots, i.e., no
spot reduction was observed. Therefore, long-term exercise-based localized adipose tissue reduction would not
be an expected result from an adequately planned exercise intervention. The result from our meta-analysis is
based on interventions with a mean duration of 11 weeks (range, 2 to 20 weeks), involving different training
approaches (e.g., cycling; resistance training), in participants of different sex, age, and physical fitness level
(e.g., sedentary; physically active). Despite the heterogeneity in sample, protocols, and study designs, the lack
of an effect was consistent, denoting a robust phenomenon that is largely independent of the characteristics of
the population or of the exercise program. It is indeed intriguing from a physiological and anatomical
perspective how exercise-based interventions may induce a localized effect on skeletal muscle tissue [72], bone
tissue [73], or even skin tissue [74], but not on adipose tissue.
Such an intriguing phenomenon generated controversy since (at least) the 19th century [1, 25, 26], with several
studies performed on the subject from the mid of the 20th century up to recently [2-24]. It is possible that the
controversy regarding spot reduction relates to its definition. For example, if spot reduction considers the intra-
muscular fat stores, a localized reduction may occur, contrary to the subcutaneous fat depot [24]. According to
our definition of voluntary exercise-based localized fat reduction (i.e., spot reduction, see methods section,
sub-section eligibility criteria), a valid model to test the hypothesis of spot reduction would be one in which,
essentially, the muscles in one part of the body are trained, whereas the muscles in the contralateral side are
not. Indeed, the use of an appropriate research model is fundamental to researchers avoid flawed experiments
that may lead them to inappropriate (or even intended) results. For such definition and proposed model we
considered [19, 27-30, 32, 69] i) fat depots from different body regions are not equally comparable within a
given individual (i.e. comparing arms and legs); ii) for the same body fat depot, significant inter-individual
differences might occur. For example, abdominal fat may respond differently to exercise in male compared to
female [28, 75, 76]; iii) contrary to neuromuscular-related outcomes, there is no evidence for a cross-education
between subcutaneous fat depots through exercise; iv) the effects of exercise training on one limb compared to
the contralateral non-exercised limb allow a tight control for dietary (even if this is not manipulated) and other
possible intervening factors (e.g., methodology; seasonal variation; genetic; biology; variations in attention
and motivation between experimental and control groups) [46, 69]; v) studies seeking to validly test the
hypothesis of spot reduction should consider the size of the adipose tissue depots adjacent to the trained and
respective non-trained muscles before and after an intervention period (with a relatively high volume of work
to impact fat tissue) not just after an acute exercise bout [14]; vi) valid studies should use valid measurement
techniques, avoiding techniques that may provide biased results due to changes in muscle mass [18] or other
factors not related to biological changes in fat content [77]. For example, reductions between 3-14% (mean
7.5%) were noted in the trained arm compared to the non-trained arm when subcutaneous fat was measured in
the biceps using a skinfold calliper [18]. In contrast, when MRI was used to measure arm subcutaneous fat
volume, the reduction was nearly three-fold lower (range 0-7%; mean 2.8%) [18]. Additionally, valid studies
should report the reliability of measurement (e.g., coefficient of variation; total error of measurement), as not
all studies in this field have reported such essential element [4-6, 15].
In contrast to our proposed definition and model to test the hypothesis of exercise-induced localized fat
reduction, two cross-sectional studies [14, 17] found acute localized lipolysis. However, the studies did not
demonstrate spot reduction (i.e., localized reduction of adipose tissue). Moreover, the two aforementioned
cross-sectional studies [14, 17], although found that exercising one leg promoted an increase in lipolysis in the
subcutaneous fat adjacent the muscles being exercised (e.g., anterior thigh), the effect was highly local,
meaning that any significant long-term effect (i.e., fat reduction) would be unlikely. Further, compared to the
aforementioned cross-sectional studies [14, 17], some authors have found contradicting findings, with intense
exercise (e.g., resistance training) reducing subcutaneous adipose tissue blow flood and lipolysis [31]. Aside
the controversial findings, the fact that an acute increase in lipolysis does not translate in chronic reduction in
fat depots, is analogue to the fact that exercise at a given intensity may allow maximal acute rate of fat oxidation
(i.e., FATmax) [78], without long-term effect on body composition [79]. Indeed, even if acute localized
lipolysis occurs during exercise, several additional physiological processes are needed before free fatty acids
enter the blood stream for later oxidation in tissues [28, 29, 80]. Moreover, the authors from one of the
aforementioned cross-sectional studies [14] indicated that “More calories are expended during aerobic, whole
body exercise than by exercise with local muscle groups, and, accordingly, a person seeking to loose fat must
be advised to perform whole body exercise”. Indeed, high intensity exercise have been found to promote large
reduction in body fat in different body parts, with many different activities [81]. From a practical point of view,
if the main aim of a training program were to improve body composition, including reductions of adipose
tissue, the most logically defendable approach would be to include a training programme allowing a
considerable energy expenditure density. To this aim, compared to localized exercise, non-localized exercise,
involving large muscles groups, would be preferable. Of course, localized exercise may still offer important
practical relevance, such as to improve the endurance of trunk muscles (e.g., abdominal muscle training), to
induce a cross-education effect on injury limbs, to improve localized-peripheral adaptations with a
minimization of central responses (e.g., blood pressure), among others. But the current literature does not
support its use for regional fat reduction.
4.1. Limitations
According to our definition, a valid model to test the hypothesis of spot reduction would be one in which the
muscles in one limb are trained, whereas the muscles in the contralateral limb are not. To our knowledge, this
model is less prone to bias compared to the rest of the models (e.g., cross-sectional; exercise compared to
nutrition; trunk-localized exercise; whole-body exercise) currently proposed in the scientific literature to test
the hypothesis of spot reduction through exercise training. According to our proposed definition, and model,
we conducted a systematic review with meta-analysis that included studies with participants across a wide
range of ages, with no restriction to sex or training status, and that included different protocols (e.g., training;
assessment techniques). Due to the high heterogeneity between included studies, a high heterogeneity in results
might been expected. However, the meta-analysis clearly denotes that lack of spot reduction is ubiquitous, i.e.,
the effect is very strong and seems to be sample-independent and protocol-independent. Although our results
seem highly consistent, we discuss some potential limitations.
Firstly, exercising one limb might induce a partial activation of the contralateral limb [82], and contralateral
strength gains have been reported [83-85]. How much activation of the control limb might have occurred and
to what extent this affected study outcomes is unclear. Additionally, studies usually controlled for the correct
technical execution of training exercise by proper spotters and researchers. Therefore, it is assumed that
participants recruited for exercise interventions have an adequate exercise technique and supervision that made
them able to activate the target muscle while maintaining the contralateral muscle relatively inactive. Secondly,
the lack of nutritional control was not considered as an exclusion criterion in our meta-analysis. Nonetheless,
the effects of exercise training on one limb compared to the contralateral non-exercised limb allow a tight
control for dietary (even if this is not manipulated) and other possible intervening factors (e.g., seasonal
variation; genetic; biology) [46, 69]. Thirdly, we only considered voluntary training protocols in this meta-
analysis. Therefore, non-voluntary muscle activation strategies and their potential to affect the trained limb
[86, 87] were not considered. Fourthly, the studies included in our meta-analysis consisted of training
programmes between 2-20 weeks of duration. Therefore, longer-term interventions were not addressed.
However, based on current findings, and those derived from some cross-sectional studies involving athletes
with several years of training using one limb more than the contralateral limb (e.g., tennis) [7, 35, 88], longer-
term interventions would probably help to confirm current findings.
4.2. Conclusions
Localized muscle training has no effect on localized adipose tissue depots, i.e., no spot reduction, regardless
of the characteristics of the population and of the exercise program.
4.3. Other information
The protocol for this systematic review with meta-analysis was registered with the International Platform of
Registered Systematic Review and Meta-Analysis Protocols (INPLASY) on 28 June 2021 (registration
number: INPLASY202160103).
Acknowledgments: Authors state no conflict of interest. No author has any financial interest from this
research. The datasets generated during and/or analysed during the current study are available from
the corresponding author on reasonable request.
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Figure 1. PRISMA 2020 flow diagram.
Reports assessed for
eligibility
(n = 6)
Records identified from
databases (n = 1,833).
Records removed before
screening:
Duplicate records
removed (n = 517)
Records screened
(n = 1,316)
Records excluded
(reasons
provided in the manuscript) (n =
1,233)
Reports sought for retrieval
(n = 83)
Reports not retrieved
(n = 0)
Reports assessed for
eligibility (n = 83)
Reports excluded:
Participants (n = 1)
Intervention (n = 6)
Comparator (n = 50)
Records identified from:
Manual searches in reference lists of included articles (n = 5).
Suggestions from independent experts (n = 1).
Errata/correction/corrigenda/retraction (n = 0).
Reports excluded:
Repeated studies (n = 5)
Intervention (n = 1)
Studies included in review
(n = 13)
Identification of studies via databases and registers
Identification of studies via other methods
Identification
Screening
Include
d
Reports sought for retrieval
(n = 6)
Reports not retrieved
(n = 0)
Figure 2. Forest plot for changes in localized fat (spot reduction) in trained compared to untrained limbs. Negative values denote that the trained
limb reduced more fat than the untrained limb. Values shown are effect sizes (Hedges’s g) with 95% confidence intervals (CI). The size of the
plotted squares reflects the statistical weight of each study. The white diamond reflects the overall result.
Study name Statistics for each study Hedges's g and 95% CI
Hedges's Standard Lower Upper
g error Variance limit limit Z-Value p-Value
Brinkworth et al., 2004, bovine colostrum 0.044 0.335 0.112 -0.613 0.700 0.130 0.896
Brinkworth et al., 2004, whey protein -0.108 0.335 0.112 -0.765 0.549 -0.321 0.748
Devries et al., 2015 0.045 0.255 0.065 -0.454 0.545 0.177 0.859
Hanson et al., 2009 (all) subcutaneous -0.222 0.205 0.042 -0.625 0.180 -1.083 0.279
Hanson et al., 2009 (men) subcutaneous 1.072 0.317 0.101 0.450 1.694 3.378 0.001
Hanson et al., 2009 (women) subcutaneous -1.209 0.304 0.092 -1.804 -0.614 -3.982 0.000
Hanson et al., 2009 (all) intermuscular fat -0.242 0.205 0.042 -0.644 0.161 -1.176 0.239
Hanson et al., 2009 (men) intermuscular fat 0.255 0.297 0.088 -0.328 0.838 0.857 0.391
Hanson et al., 2009 (women) intermuscular fat -0.240 0.279 0.078 -0.788 0.307 -0.860 0.390
Kostek et al., 2007 (men and women) biceps -0.096 0.149 0.022 -0.387 0.196 -0.644 0.520
Kostek et al., 2007 (men) biceps -0.216 0.228 0.052 -0.662 0.230 -0.948 0.343
Kostek et al., 2007 (women) biceps -0.046 0.195 0.038 -0.428 0.335 -0.237 0.813
Kostek et al., 2007 (men and women) arm -0.023 0.138 0.019 -0.294 0.248 -0.164 0.869
Kostek et al., 2007 (men) arm -0.173 0.209 0.044 -0.584 0.237 -0.828 0.408
Kostek et al., 2007 (women) arm -0.020 0.183 0.033 -0.378 0.339 -0.108 0.914
Krotkiewski, et al., 1979, subcutaneous -0.751 0.444 0.198 -1.622 0.120 -1.690 0.091
Krotkiewski, et al., 1979, cell fat -0.735 0.444 0.197 -1.604 0.135 -1.655 0.098
Miura et al., 2009 0.005 0.473 0.223 -0.921 0.932 0.011 0.991
Nickols-Richardson et al. 2007 (concentric), arm -0.033 0.230 0.053 -0.484 0.418 -0.141 0.887
Nickols-Richardson et al. 2007 (eccentric), arm 0.034 0.243 0.059 -0.442 0.511 0.142 0.887
Nickols-Richardson et al. 2007 (concentric), leg 0.010 0.230 0.053 -0.441 0.460 0.042 0.967
Nickols-Richardson et al. 2007 (eccentric), leg 0.053 0.243 0.059 -0.424 0.530 0.217 0.828
Olson and Edelstein, 1968 -0.504 0.251 0.063 -0.996 -0.012 -2.008 0.045
Orkunoglu-Suer et al., 2008 (women) 0.015 0.079 0.006 -0.140 0.170 0.190 0.849
Orkunoglu-Suer et al., 2008 (men) 0.033 0.101 0.010 -0.164 0.231 0.333 0.739
Ramirez-Campillo et al., 2013, fat mass 0.046 0.380 0.144 -0.699 0.791 0.121 0.903
Ramirez-Campillo et al., 2013, fat percentage 0.000 0.380 0.144 -0.744 0.744 0.000 1.000
Roby, 1962 0.080 0.355 0.126 -0.616 0.777 0.226 0.821
Walts et al., 2008 (men), subcutaneous 0.000 0.155 0.024 -0.305 0.305 0.000 1.000
Walts et al., 2008 (women), subcutaneous 0.101 0.142 0.020 -0.178 0.380 0.707 0.480
Walts et al., 2008 (Caucassians), subcutaneous 0.150 0.132 0.017 -0.110 0.409 1.131 0.258
Walts et al., 2008 (African American), subcutaneous 0.153 0.195 0.038 -0.229 0.535 0.783 0.434
Walts et al., 2008 (men), intermuscular fat 0.119 0.156 0.024 -0.186 0.424 0.767 0.443
Walts et al., 2008 (women), intermuscular fat -0.096 0.142 0.020 -0.375 0.183 -0.677 0.498
Walts et al., 2008 (Caucassians), intermuscular fat 0.000 0.132 0.017 -0.259 0.259 0.000 1.000
Walts et al., 2008 (African American), intermuscular fat -0.065 0.195 0.038 -0.446 0.317 -0.332 0.740
Yao et al., 2007 0.122 0.142 0.020 -0.157 0.401 0.857 0.391
-0.025 0.037 0.001 -0.098 0.048 -0.661 0.508
-2.00 -1.00 0.00 1.00 2.00
Favours trained Favours untrained
Table 1. Included studies characteristics.
Study
Participants
Training
Outcomes
Brinkworth et
al., 2004
Healthy physically active men
supplemented with bovine
colostrum (n=17; age, 21.4 y;
height, 179 cm; body mass,
77.8 kg) or whey protein (n=17;
age, 23.8 y; height, 179 cm;
body mass, 81.5 kg).
8 weeks, 4 sessions per week. Muscle: elbow flexors non-dominant arm.
Exercises: dumbbell bicep concentration curls. Velocity: controlled (slower
during lengthening). Sets/repetitions/intensity: 6 sets to failure at 80% 1RM.
Progressive overload: yes.
Arm skin and
subcutaneous fat
(cm2; MRI).
Devries et al.,
2015
30 healthy men (age, 70 y;
height, 1.8 m; body mass, 84
kg)
2 weeks, 3 sessions/week. Unilateral leg press and leg extension. Equipment:
air-resistance strength machines. Sets, intensity: 3, 30% 1RM until volitional
fatigue.
Leg fat mass (g;
DEXA).
Hanson et al.,
2009
Sedentary (without medical
condition) women (n=25; age,
71 y; height, 161 cm; body
mass, 75.5 kg; BMI, 29.2 kg·m-
2) and men (n=22; age, 71 y;
height, 174 cm; body mass,
86.4 kg; BMI, 28.4 kg·m-2).
10 weeks, 3 sessions per week. Knee extensions for the dominant leg
(pneumatic [air powered] knee extension machine). Sets: 4-5 (4 for
participants >75 y of age and 5 for those <75 y of age). First set: 5 repetitions,
50% 1RM. Second set: 5RM value (initially, 85% of basal 1RM). Third set:
5RM, then a drop-set of 1-2 repetitions until reaching 10 repetitions. Fourth
set: 5RM, then a drop-set of 1-2 repetitions until reaching 15 repetitions.
Fifth set: 5RM, then a drop-set of 1-2 repetitions until reaching 20
repetitions. Full ROM was required during repetitions. Repetition duration:
2-3 (shortening-lengthening). A seat belt was worn throughout the exercise
session, with arms across their chest. Progressive overload was monitored
session by session.
Knee extensor
subcutaneous fat
(cm2; computed
tomography).
Knee extensor
intermuscular fat
(cm2; computed
tomography).
Kostek et al.,
2007
45 men and 59 women,
Caucasian (94%); age, 24.1 y;
BMI, 24.2 kg·m-2
12 weeks; 2 sessions per week (45-60 min per session). Progressive,
supervised resistance training of their nondominant arm. Exercises: biceps
preacher curl, overhead triceps extension, biceps concentration curl, triceps
kickback, and standing biceps curl. Dose per exercise: 3 sets of 12 repetitions
at 65–75% 1RM (i.e., 12RM). Each contraction involved 2 s for the
concentric phase and 2 s for the eccentric phase. A 2-min rest followed each
set. At week 5, the number of repetitions was decreased to eight (i.e., 8RM),
and then to six (i.e., 6RM) at week 10. Consequently, the exercise intensity
at weeks 5 and 10 increased to 75–82% and 83–90% 1RM, respectively.
Experienced investigators supervised the training sessions and adjusted the
weight accordingly.
Biceps
subcutaneous fat
(mm; skinfold
callipers).
Arm
subcutaneous fat
volume (mL;
MRI).
Krotkiewski,
et al., 1979
10 women; age, 24-29 y;
height, 166.2 cm; body mass,
72-81 kg; body fat, 19-28 kg
5 weeks, performed daily. Three sets of 10 maximal voluntary isokinetic
right knee extensions (constant angular velocity of 60 deg·s-1).
Thigh adipose
tissue thickness
(cm; ultrasound).
Fat cell weight
(µg; microscopic
method).
Miura et al.,
2009
8 women, Japanese, sedentary;
ages 21–23 y; height, 157 cm;
body mass, 49.4 kg; VO2max,
32.4 mL·min-1·kg-1
12 weeks, 3 sessions per week (60 min per session). The right or left leg was
assigned to cycling at 40% of single leg peak VO2 (i.e., below lactate
threshold), equivalent to 25.3 W and a heart rate of 90–110 bpm.
Thigh fat cross-
sectional area
(cm2; ultrasound).
Nickols-
Richardson et
al. 2007
70 women, white (95%); age,
20.2 y; BMI, 22.1 km·m-2
20 weeks; 3 sessions per week. Concentric (or eccentric) slow-velocity
(60°·s-1) isokinetic training of the non-dominant leg and arm. During week
1, one set of 6 repetitions was performed for knee extension and elbow
Arm fat mass (kg;
DEXA).
flexion. From weeks 2 to 5, one set was added each week, so by week 5
participants completed 5 sets of 6 repetitions. From week 6 to week 20, the
volume was maintained. Torque output was not controlled during training,
but was free to vary (i.e., increase) as participants performed each repetition
at maximal volitional effort. Of note, one group of women (n=37) performed
concentric training, and the other group (n=33) eccentric training.
Leg fat mass (kg;
DEXA).
Olson and
Edelstein,
1968
32 boys, with no experience in
weight training; age, 14-16 y.
6 weeks, 3 or 5 days per week (half of the participants exercised 5 days a
week and the other half exercised 3 days per week; however, data from all
the participants was mixed). Right arm curl with dumbbell and triceps
extension with dumbbell, for 3 sets of 7RM each exercise (with as many
repetitions as possible in the second and third sets). When a sufficient gain
in strength allowed seven repetitions to be performed in all three sets, the
resistance was increased. There was no warm-up prior to the exercises. The
boys did not participate in physical education or in intramural or
interscholastic athletics during the study.
Triceps
subcutaneous fat
(mm; skinfold
callipers).
Orkunoglu-
Suer et al.,
2008
320 women (age, 22.9 y; body
mass, 64.7 kg; height, 164.2
cm; BMI, 23.7 kg·m-2) and 197
men (age, 23.9 y; body mass,
78.8 kg; height, 178.5 cm; BMI,
24.7 kg·m-2). All European
descents (White).
See: Kostek et al., 2007
Arm
subcutaneous fat
volume (mm3;
MRI).
Ramirez-
Campillo et
al., 2013
11 physical education students
(7 men and 4 women;
12 weeks; 3 sessions per week (80 min per session). Localized muscle
endurance resistance training for the non-dominant leg muscles; Subjects
completed one set of leg press per session, at 10–30% 1RM (10% during
Leg fat mass (kg;
DEXA).
Latinamerican); age, 23.0 y;
BMI, 25.0 kg·m-2
weeks 1–4, 20% during weeks 5–6, and 30% during weeks 7–12). Subjects
completed 960–1,200 consecutive repetitions for their set (no rest between
repetitions), with 4–5 seconds per repetition.
Leg fat percentage
(DEXA).
Roby, 1962
15 male college students; age,
21.1 y
10 weeks, 3 sessions per week. Dominant arm triceps extension, for 3 sets of
10-15 repetitions at 50% 1RM. Overload was applied when participants were
able to perform 15 repetitions in all three sets.
Triceps
subcutaneous fat
(mm; skinfold
callipers).
Walts et al.,
2008
Men (n=78-82) and women
(n=95-98), relatively healthy,
physically inactive; age, 63.0 y.
Self-reported Caucasians
(n=114) or African Americans
(n=52).
See: Hanson et al., 2009
Knee extensor
subcutaneous fat
(cm2; computed
tomography).
Knee extensor
intermuscular fat
(cm2; computed
tomography).
Yao et al.,
2007
Men (n=46; age, 64.4 y; height,
174 cm; body mass, 84 kg; %
body fat, 27.4) and women
(n=52; age, 62.7 y; height, 163
cm; body mass, 73.2 kg; %
body fat, 38.8)
See: Hanson et al., 2009
Knee extensor
intermuscular fat
(cm2; computed
tomography).
Abbreviations (listed alphabetically): 1RM: one repetition maximum; BMI: body mass index; BPM: beats per minute; DEXA: dual-energy x-ray
absorptiometry; MRI: magnetic resonance image; ROM: range of motion; VO2: volume of oxygen consumption.
Table 2. Methodological quality for included studies using the PEDro rating scale.
Study name
Q1
Q2
Q3a
Q4
Q5
Q6
Q7
Q8
Q9
Q10
Q11
Total*
Study quality
Brinkworth et al., 2004
1
0
1
1
0
1
1
1
1
1
1
8/10
High
Devries et al., 2015
1
1
1
1
0
0
0
1
1
1
1
7/10
High
Hanson et al., 2009
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Kostek et al., 2007
1
0
1
1
0
1
1
1
1
1
1
8/10
High
Krotkiewski, et al., 1979
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Miura et al., 2009
1
1
1
1
0
0
0
1
1
1
1
7/10
High
Nickols-Richardson et al. 2007
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Olson and Edelstein, 1968
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Orkunoglu-Suer et al., 2008
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Ramirez-Campillo et al., 2013
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Roby, 1962
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Walts et al., 2008
1
0
1
1
0
1
1
1
1
1
1
8/10
High
Yao et al., 2007
1
0
1
1
0
0
0
1
1
1
1
6/10
High
Note: A detailed explanation for each PEDro scale item can be accessed at https://www.pedro.org.au/english/downloads/pedro-scale. *:
from a possible maximal punctuation of 10. a: in the context of this study, Q3 was considered to be attained even if concealed allocation
was not reported, since the decision about whether or not to include a person in a trial could not be influenced by knowledge of whether
the subject was to receive treatment or not.
