Effect of Functional Training on Physical Fitness Among Athletes: A Systematic Review

Abstract

There is evidence that functional training is beneficial for the overall physical fitness of athletes. However, there is a lack of a systematic review focused on the effects of functional training on athletes' physical fitness. Thus, the aimed of the present review is to clarify the effects of functional training on physical fitness among athletes. In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes (PRISMA) Statement guidelines, the systematic search of PubMed, SCOPUS, EBSCOhost (SPORTDiscus), and CINAHL Plus databases was undertaken on the 2nd November 2020 to identify the reported studies, using a combination of keywords related to functional training, physical fitness, and athletes. From the 145 studies, only nine articles met all eligibility criteria and were included in the systematic review. The assessment was performed on the Pedro scale, and the quality of the study included in the nine studies was fair (ranging from 3 to 4). The results showed that speed (n = 6) was the aspect of physical fitness studied in functional training interventions, followed by muscular strength (n = 5), power (n = 4), balance (n = 3), body composition (n = 3), agility (n = 3), flexibility (n = 1) and muscular endurance (n = 1). Existing evidence concludes that functional training significantly impacts speed, muscular strength, power, balance, and agility. Furthermore, there are still limit numbers of evidence showing effect of functional training on flexibility and muscular endurance. In contrast, no significant improvement was found in body composition where functional training was conducted.
Systematic Review Registration: https://www.crd.york.ac.uk/prospero, identifier: CRD4202123092.

Full text

SYSTEMATIC REVIEW
published: 06 September 2021
doi: 10.3389/fphys.2021.738878
Frontiers in Physiology | www.frontiersin.org 1September 2021 | Volume 12 | Article 738878
Edited by:
Vassilis Mougios,
Aristotle University of
Thessaloniki, Greece
Reviewed by:
Ugur Ödek,
Nev ¸sehir Haci Bekta ¸s Veli
University, Turkey
Patrik Drid,
University of Novi Sad, Serbia
Bilal Biçer,
Mustafa Kemal University, Turkey
*Correspondence:
Kim Geok Soh
kims@upm.edu.my
Specialty section:
This article was submitted to
Exercise Physiology,
a section of the journal
Frontiers in Physiology
Received: 09 July 2021
Accepted: 13 August 2021
Published: 06 September 2021
Citation:
Xiao W, Soh KG, Wazir MRWN,
Talib O, Bai X, Bu T, Sun H,
Popovic S, Masanovic B and
Gardasevic J (2021) Effect of
Functional Training on Physical Fitness
Among Athletes: A Systematic
Review. Front. Physiol. 12:738878.
doi: 10.3389/fphys.2021.738878
Effect of Functional Training on
Physical Fitness Among Athletes: A
Systematic Review
Wensheng Xiao 1, Kim Geok Soh 2
*, Mohd Rozilee Wazir Norjali Wazir 2, Othman Talib 3,
Xiaorong Bai 2, Te Bu 4, He Sun 2, Stevo Popovic 5,6 , Bojan Masanovic 5,6,7 and
Jovan Gardasevic 5, 6
1Department of Sports Studies, Faculty of Educational Studies, Universiti Putra Malaysia, Seri Kembangan, Malaysia,
2Department of Sports Studies, Faculty of Education Studies, Universiti Putra Malaysia, Seri Kembangan, Malaysia,
3Department of Science and Technical Education, Faculty of Educational Studies, Universiti Putra Malaysia, Seri Kembangan,
Malaysia, 4Department of Sports Studies, Faculty of Education Studies, Hunan Normal University, Changsha, China, 5Faculty
for Sport and Physical Education, University of Montenegro, Podgorica, Montenegro, 6Montenegrin Sports Academy (MSA),
Podgorica, Montenegro, 7Montenegrosport, Podgorica, Montenegro
There is evidence that functional training is beneficial for the overall physical fitness
of athletes. However, there is a lack of a systematic review focused on the effects of
functional training on athletes’ physical fitness. Thus, the aimed of the present review is to
clarify the effects of functional training on physical fitness among athletes. In accordance
with the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes (PRISMA)
Statement guidelines, the systematic search of PubMed, SCOPUS, EBSCOhost
(SPORTDiscus), and CINAHL Plus databases was undertaken on the 2nd November
2020 to identify the reported studies, using a combination of keywords related to
functional training, physical fitness, and athletes. From the 145 studies, only nine articles
met all eligibility criteria and were included in the systematic review. The assessment was
performed on the Pedro scale, and the quality of the study included in the nine studies
was fair (ranging from 3 to 4). The results showed that speed (n=6) was the aspect of
physical fitness studied in functional training interventions, followed by muscular strength
(n=5), power (n=4), balance (n=3), body composition (n=3), agility (n=3), flexibility (n
=1) and muscular endurance (n=1). Existing evidence concludes that functional training
significantly impacts speed, muscular strength, power, balance, and agility. Furthermore,
there are still limit numbers of evidence showing effect of functional training on flexibility
and muscular endurance. In contrast, no significant improvement was found in body
composition where functional training was conducted.
Systematic Review Registration: https://www.crd.york.ac.uk/prospero, identifier:
CRD4202123092.
Keywords: flexibility, muscular endurance, body composition, balance, speed
INTRODUCTION
Athletes’ successful performance is usually attributed to the unique combination of talent and
physical fitness, technical, tactical, and psychological qualities (Smith, 2003). Among those criteria,
physical fitness is considered the most critical quality to determine athletes’ competitive ability
(Gabbett et al., 2007). Excellent physical fitness is essential for improving the athletes’ technical
and tactical level and performance and is the basic requirement for competing athletes under high-
intensity training (Chunlei, 2016). The loss of an athlete’s physical fitness component can jeopardize

Xiao et al. Functional Training & Physical Fitness
this ability and lead to sports injuries (Dengguang and Yang,
2007a). For example, in tennis players, decreased muscle strength
and postural control limits the ability to start quickly and change
direction, which further hinders their ability to hit the ball
effectively and maintain a stable body, and also increased the
likelihood of sports injuries (Kovacs, 2006).
A substantial number of publications proved a significant
positive correlation between physical fitness components and
exercise training intervention. The American College of Sports
Medicine guidelines support the use of traditional resistance
training, traditional resistance training enhances physical fitness
performance by gradually increasing exercise load during the
training process (Feito et al., 2018). However, the training
specificity literature has shown that the benefits of traditional
resistance training for improving physical fitness is rarely
transferred to sports performance (Li et al., 2019; Li, 2021). Most of
the traditional resistance training methods are not multi-articular
and multiplanar; these aspects seem fundamental for eliciting
greater sports performance (Fernandez-Fernandez et al., 2016;
Santos-rosa et al., 2020). On the other hand, a new exercise
training method that has recently received much attention to
developing athletes’ physical fitness is functional training (Feito
et al., 2018). Several studies have confirmed that functional
training can enhance speed (Tomljanovi´
c et al., 2011; Sander
et al., 2013; Alonso-Fernández et al., 2017; Yildiz et al., 2019;
Baron et al., 2020; Keiner et al., 2020), muscular strength (Oliver
and Brezzo, 2009; Tomljanovi´
c et al., 2011; Elbadry, 2014;
Cherepov and Shaikhetdinov, 2016; Keiner et al., 2020), power
(Tomljanovi´
c et al., 2011; Alonso-Fernández et al., 2017; Yildiz
et al., 2019; Keiner et al., 2020), balance (Oliver and Brezzo,
2009; Elbadry, 2014; Yildiz et al., 2019), body composition (Oliver
and Brezzo, 2009; Tomljanovi´
c et al., 2011; Alonso-Fernández
et al., 2017), agility (Tomljanovi´
c et al., 2011; Cherepov and
Shaikhetdinov, 2016; Yildiz et al., 2019), flexibility (Yildiz et al.,
2019) and muscular endurance (Oliver and Brezzo, 2009).
Additionally, other research has discovered positive effects of
functional training on physical fitness in football players (Oliver
and Brezzo, 2009; Sander et al., 2013; Baron et al., 2020; Keiner
et al., 2020), handball players (Elbadry, 2014; Alonso-Fernández
et al., 2017), martial artists (Cherepov and Shaikhetdinov, 2016),
tennis players (Yildiz et al., 2019) and volleyball players (Oliver
and Brezzo, 2009). Despite the significance of functional training
for improving the physical fitness components among athletes,
there is no publication that summarized crucial information on
the impacts of functional training protocols on physical fitness
among athletes.
Conceptually, functional training refers to the training of
partial chains and connections in the human motion chain
that involves completing specific target actions, including
multi-dimensional motion trajectory acceleration, deceleration,
and stability training activities that meet the characteristics
of particular target actions (Cook, 2012). The action mode
of functional training involves acceleration, deceleration and
stability of multiple joints and planes. The action mode
determines the broad participation and effective pertinence
of functional training (National Academy of Sports Medicine,
2001). Moreover, Boyle believes that the essence of functional
training is purposeful training. It is a multi-plane exercise in
stable control and weight-bearing. It is a series of exercises that
involve balance and proprioception and are supported by body
parts (Boyle, 2016). Therefore, functional training differs from
traditional resistance training; it can be any exercise performed
to enhance a specific movement or activity (Pacheco et al., 2013).
With a definition this broad, the literature on functional training
has incorporated various exercise programs with varying designs
and focuses. The principle of functional training is the specificity
of training, which means that training in a specific activity is
the best way to maximize the performance in that particular
area (Hawley, 2008). In other words, the closer the training
is to the desired outcome (i.e., a specific task or performance
criterion), the better the result will be. For example, when the
functional training program includes the element of strength
training, the training improves the outcome of muscle strength
(Skelton et al., 1995; Alexander et al., 2001; Giné-Garriga et al.,
2010). The results presented by the different studies on functional
training effects on physical fitness components among athletes
are encouraging, but limited scientific information is available
to determine its possible benefits on the different physical fitness
components of performance. Therefore, this systemtic review
aimed of the present review is to clarify the effects of functional
training on physical fitness among athletes.
METHODS
Protocol and Registration
The data selection, collection and analysis of this review
were performed following the Preferred Reporting Items for
Systematic Reviews and Meta-Analyses (PRISMA) guidelines
(Moher et al., 2009) and were prospectively registered on the
International Prospective Register of Systematic Reviews; https://
www.crd.york.ac.uk/prospero, CRD4202123092.
Search Strategy
The literature search was undertaken in four international
databases: the SCOUPS, PubMed, EBSCOhost (SPORTDiscus),
and CINAHL Plus. The search was conducted on the 2th
November, 2020. In each database, a search was conducted by
title, taking a predefined combination of keywords: (“functional
training” OR “functional exercise” OR “functional skill∗”
OR “functional task training” OR “therapeutic exercise”)
AND (“physical fitness” OR “physical endurance” OR
“cardiorespiratory fitness” OR “physical conditioning” OR
“skill-related fitness” OR “skill related fitness” OR “skill related
physical” OR “skill-related physical” OR “skill related physical
fitness” OR “skill-related physical fitness” OR “fitness, physical”
OR “speed” OR “power” OR “reaction time” OR “agility” OR
“balance” OR “coordination” OR “health related physical fitness”
OR “health related physical” OR “health related fitness” OR
“health-related physical” OR “health-related fitness” OR “health-
related physical fitness” OR “aerobic endurance” OR “muscular
strength” OR “muscular endurance” OR “body composition” OR
“flexibility”) AND (“player∗” OR “athlete∗” OR “sportsman∗”
OR “sportswoman∗” OR “sportsperson∗” OR “Jock∗”). We
also explored other relevant articles in the reference lists of the
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Xiao et al. Functional Training & Physical Fitness
TABLE 1 | Inclusion criteria according to the PICOS conditions.
Items Detailed inclusion criteria
Population Athletes (male/female)
Intervention Functional training
Comparison Two or more groups and single-group trials
Outcome Physical fitness (speed, power, reaction time, agility, balance,
coordination, aerobic endurance, muscular strength,
muscular endurance, body composition, flexibility)
Study designs RCT or Non-RCT
studies included in the review and examined the reference lists
of previous related reviews. All titles were manually searched for
potential inclusion. Reference lists of retrieved papers, authors’
names, and review articles were retrieved manually for additional
relevant citations.
Eligibility Criteria
We used the PICOS (population, intervention, comparison,
outcome, study designs) criteria as the inclusion criteria, is
presented in Table 1. Only records presenting functional training
on aspect of physical fitness of athletes were included. Thus,
studies were included if they met the following criteria: (1) A full
text, peer-reviewed study published in English, describing the use
of athletes (male and female) to explore the effects of functional
training interventions on physical fitness, randomized controlled
trial (RCT), non-randomized controlled trial (Non-RCT) with
two or more groups, and single-group trials with pretest
and post-test design; (2) In this study, only included studies
on planned and organized functional training intervention to
improve or maintain physical fitness. Notably, studies using
functional training or combinations of functional training and
other exercise training interventions (e.g., resistance training)
were also included from this review; (3) Investigate the effects
of functional training on physical fitness among athletes and
assess at least one physical fitness component outcome; (4) There
were no restrictions on the sample size, study location, and
intervention time for the included studies.
Studies were excluded if they met several exclusion criteria:
(1) Studies that combined functional training interventions
with additional non-exercise training (e.g., psychological
interventions) and interventions including unsupervised
training courses were not included in the study; (2) Studies
published articles, meeting abstracts, case reports, and short
communications in languages other than English were excluded;
and (3) Observational studies and interventions focusing
solely on counseling for functional training implementation
were excluded.
Study Selection
The retrieved studies were imported into Mendeley reference
management software to remove any duplicates. Firstly, the
search strategies were assisted by an experience librarian.
Secondly, two independent reviewers (Xiao, Bai) screened the
titles and abstracts of all the identified articles in the initial
screening phase to identify relevant studies. Irrelevant materials
were removed from the database before assessing all other titles
and abstracts using our predetermined inclusion and exclusion
criteria. Articles that remain at the end and enter a qualitative
synthesis must have the whole text, and the whole text must be
read. Items for which the full text is not available are dropped.
If there were any disagreements, a third reviewer (Soh) was
consulted until a consensus was achieved.
Data Extraction and Quality Assessment
After the data search was complete, data were obtained from
eligible studies in a predetermined extraction form [Including,
(1) Author, title, publication year; (2) Research design; (3) Sample
size, control group; (4) Participant characteristics (age, gender,
etc.); (5) Intervention features (type, length, and frequency);
(6) Measures index, and (7) Research outcomes]. One author
abstracted information into the standard form and the other
author checked it.
The PEDro scale (www.pedro.org.au) has been proven to be
a useful measure of the quality of experimental methodology
in developing a systematic review, and has good validity and
reliability (Lima et al., 2013). The PEDro scale is designed to
evaluate the four fundamental methodological aspects of a study,
such as random process, blind technique, group comparison and
data analysis. The assessment of the 11 items in the PEDro
scale was performed by two well-trained, independent raters
using a yes (1 point) or No (0 points) response rating scale,
and disagreements were resolved by a third rater. However, the
eligibility criteria were not considered in the total score since
this was related to external validity. The total PEDro score
ranges from 0 to 10 points, and higher scores reflect a better
methodological quality. The higher the PEDro score, the higher
the quality of the corresponding method. Studies scoring 8 to 10
were considered to be methodologically excellent in quality, those
ranging from 5 to 7 to be good in quality, while a score between
3 and 4 is fair in quality, and those scoring below 3 to be poor in
quality (Foley et al., 2003). The judgment of overall scientifific
evidence was based on number, methodological quality and
consistency of outcomes of the studies in three levels of evidence:
(1) strong evidence, provided by generally consistent findings in
multiple (≥2) number and results studies, (2) moderate evidence,
when only one study is available or findings are inconsistent in
multiple (≥2) studies, (5) no evidence, when no case-control
studies are found.
RESULTS
The search results were screened and read by formulating
literature inclusion and exclusion criteria. This systematic review
contains nine articles involving RCT and Non-RCT on the effects
of functional training on physical fitness among athletes. They
were published between the years of 2009–2020. In Table 2, the
studies’ characteristics are presented.
Study Selection
Figure 1 shows the flow chart of records selection. A total of 143
potential articles were identified through the electronic database
search (36 from PubMed; 107 from SCOPUS; 0 from EBSCOhost
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Xiao et al. Functional Training & Physical Fitness
TABLE 2 | Characteristics of the studies examined in the present review.
Study Design Type of
athletes
Population
characteristics
Interventions Type of exercise
training
Measures index Outcomes
Oliver and
Brezzo (2009)
Pre-post
test
Collegiate
athletes
(volleyball and
soccer players)
Sex: F, TB: NR, EG1 =15,
Age: 19.9 ±1.8yr., WT:
71.8 ±8.5kg, ht.: 174.5 ±
11.9cm, BMI: NR, CG =11,
Age: 18.5 ±0.5yr., WT:
63.3 ±6.7kg, ht.: 166.4 ±
5.6cm, BMI: NR
Freq.: 4
times/week,
time: 10 min,
length: 13
weeks
Functional balance
training group
(EG1), control
group (CG)
Body composition (WT,
BF, BMI), strength
(quadra-ped: left, right;
single leg squat (left,
right), muscular
endurance (sit up),
balance (biodex
balance test: left, right)
Single leg squat (right,
left)↑, sit up↑, BMI↔,
WT↔, BF↔,
quadra-ped (left,
right)↔, Biodex (right,
left)↔
Tomljanovi ´
c
et al. (2011)
Pre-post
test
Moderately
trained athlete
Sex: M, TB≥146 months,
Age: 22–25yr., EG1 =11,
WT: 78.89 ±12.32, ht.:
179.69 ±6.39, CG =12,
WT: 82.42 ±12.92, ht.:
185.00 ±10.58, BMI: NR
Freq.: 3
times/week,
time: NR,
Length: 5
weeks
Functional training
(EG1), traditional
resistance training
(CG)
Agility (5-10-5 test,
HEX), power (CMJ: AT,
PEAKPWR, JH, GCT),
strength (SMB, LMB),
speed (10 m, 20 m,
10–20 m), body
composition (WT, ht.,
BF%, BF, LBM, H2o)
HEX↑, SMB↑, JH↑,
PEAKPWR↑, GCT↑,
LMB↔, AT↔,
5–10–5↔, 10m↔, 20
m↔, 10–20 m↔,
WT↔, ht.↔, BF%↔,
BF↔, LBM↔, H2o(I)↔
Sander et al.
(2013)
Pre-post
test
Elite youth
soccer players
EG1 =65, EG2 =56, Sex:
NR, TB≥146 months, mean
age: 15.1yr., Mean ht.:
170.9, Mean WT: 62.3, BMI:
NR
Freq.: NR,
time: NR,
Length: 8
days
Completed the
NWP first and the
WPS 4 days later
(EG1), completed
the WPS first and
the NWP 4 days
later (EG2)
Speed (linear sprint:
5 m, 10 m, 15 m, 20 m,
25 m, 30 m, and
CDS:5 m left and right,
10m left and right)
5 m↑, 10 m↑, 15 m↑,
20 m↑, 25 m↑, 30 m↑,
5 m left↔, 5 m right↑,
10 m left↑, 5 m right↑
Elbadry
(2014)
Pre-post
test
Young handball
players
EG1: n =10, Sex: F, TB: 3
±0.7yr., Age: 13 ±1.5yr,
WT: 44 ±2.7, ht.: 147 ±
2.95, BMI: NR; CG: n =10,
Sex: F, TB: 3 ±0.8yr., Age:
14 ±1.8yr, WT: 42 ±3.4,
ht.: 148 ±3.11, BMI: NR
Freq.: 3
times/week,
time: 60 min,
Length:
10weeks
Functional
strength training
group (EG1),
control group (CG)
Balance (SST, dynamic
balance), strength
(handgrip, LS, BS)
SST↑, Dynamic
balance↑, Handgrip
Strength↔, LS↔, BS↑
Alonso-
Fernández
et al. (2017)
Pre-post
test
Handball
players
Sex: F, TB≥60 months.,
Age: 15.2 ±0.6yr., EG1 =
7, WT: 63.17 ±9.44, ht.:
164 ±5, BMI: 23.83 ±
3.46, CG =7, WT:67.29 ±
0.03, ht.: 166 ±9.24, BMI:
24.61 ±3.93,
Freq.: 2
times/week,
time: 10 min,
Length: 8
weeks
Combining
strength,
coordination and
plyometric
exercises (EG1),
Control group (CG)
Body composition (WT,
BMI, BF%,), VO2max),
speed (RSA), power
(CMJ: AT, JH, GCT,
PEAKPWR)
WT↔, BMI↔, BF%↑,
AT↔, JH↔, GCT↔,
PEAKPWR↔, RSA↑,
Yildiz et al.
(2019)
Pre-mid-
post
test
Prepubertal
tennis players
EG1 =10, EG2 =10, CG =
8, Sex: NR, TB: 3.1 ±
1.1yr., Age: 9.6 ±0.7yr.,
WT: 31.3 ±4.1, ht.: 134.1
±6.8, BMI: NR
Freq.: 3
times/week,
time:
65-70 min,
Length: 8
weeks
Functional training
group (EG1),
Traditional training
group (EG2),
Control group (CG)
Flexibility (Sit and
reach), power (CMJ),
speed (10 m test),
agility (T-test), balance
(RDB, LDB, SB)
Sit and reach↑, CMJ↑,
10 m test↑, T-test↑,
RDB↑, LDB↑, SB↑
Baron et al.
(2020)
Pre-post
test
Young football
players
EG1: n =20, Sex: NR, TB:
NR, Age: 16.8 ±0.6 yr., WT:
66.5 ±7.4 kg, ht.: 175.7 ±
6.4 cm, BMI: 21.5 ±1.8
Freq.: NR,
time:
70-90 min,
Length: 12
weeks
Functional training
(EG1)
Speed and acceleration
(0-5 m, 5– 10 m,
10–30 m, 30 m)
0-5 m ↔, 5–10 m↑,
10–30 m↑, 30 m↑
Keiner et al.
(2020)
Pre-post
test
Elite adolescent
soccer players
EG1: n =11, EG2: n =11,
EG3: n =14. CG: n =12
Sex: NR, TB: NR, Age:
17.45 ±0.52yr, WT: 73.0 ±
7.0 kg, ht.: 1.78 ±0.06 m,
BMI: NR
Freq.: 2
times/week,
time: 60 min,
Length: 10
months
Traditional strength
training (EG1),
plyometrics and
sprint training
(EG2), functional
training group
(EG3), control
group (CG)
Speed (20 m, CDS:
CODSR, CODSL),
power (SJ: AT, JH),
strength(1RM)
20 m↑, CDS (CODSR,
CODSL)↑, SJ↑, 1RM↑
↑, significant within-group improvement from pretest to post-test; ↔, non-significant within-group change from pretest to post-test; WT, weight; ht, height; M, Male; yr, year; NR, not
reported; Freq., frequency; CG, control group; EG, experimental group; HEX, hexagon test; 5–10–5 test, 5–10–5 meter shuttle run test; SMB, standing overarm medicine ball throw;
LMB, lying medicine ball throw; SST,Static strength test; OMBT, overhead medicine ball throw; CMJ, vertical counter movement jump; AT, air time; JH, jump height; GCT: ground contact
time; PEAKPWR, power peak; NWP, regular soccer warm-up; WPS, warm-up program supplemented with functional exercises; CDS, change of direction sprint; HIIT, high-intensity
interval training; RSA, repeated sprint ability; BM, body mass; BF%, body fat mass percentage; BF, body fat; LBM, lean body mass; H2o(I), total body water; RDB, right dynamic balance;
LDB, left dynamic balance; SB, static balance; FJ, ten-fold jump; SQ, Squat for 30s; PU, Push-ups, TLPP, Torso lifting from a prone position for 30s, SJ, Jump performance; 1RM, 1
repetition maximum (squat maximum strength performance); SHR, Shuttle run; LS, leg strength; BS, back strength.
Frontiers in Physiology | www.frontiersin.org 4September 2021 | Volume 12 | Article 738878

Xiao et al. Functional Training & Physical Fitness
FIGURE 1 | PRISMA flow chart of the study selection process.
(SPORTDiscus); 0 from CINAHL Plus), and additional relevant
articles in screening the reference lists of studies that were
included in the review and reference lists of previous related
reviews (n=1), and Google Scholar (n=1). After exclusion
of the duplicates (15), the title and abstract of 130 were assessed
for eligibility. After elimination at the title and abstract level 48
articles, the remaining 82 articles were subsequently read. After
reading, another 73 articles were eliminated, leaving nine relevant
articles that satisfied the inclusion criteria and were included in
the qualitative synthesis.
Study Quality Assessment
An assessment of the study quality, according to the PEDro
list, is presented in Table 3. The mean PEDro score of the
included studies was 3.44 (range 3–4), which indicates that
the included studies were of fair quality, and none of the
studies met all the PEDro list quality criteria. All studies
specified their eligibility criteria, similar baseline group, between-
group comparisons, point measure and variability. None of the
studies reported on allocation concealment, blind subject, blind
therapist, blind assessor, or intention to treat analysis, except for
three studies which described random allocation (Tomljanovi´
c
et al., 2011; Alonso-Fernández et al., 2017; Yildiz et al., 2019), and
only one study reported follow-ups (Oliver and Brezzo, 2009).
Nevertheless, it is challenging to include blind subjects, blind
therapists, and blind assessors as participants and assessors, since
the included studies were exercise training interventions. This
situation calls for higher quality and better evidence level studies
to be conducted in the future.
Population Characteristics
The nine included studies’ population characteristics were
reported based on the following aspects: (1) Athlete classification.
In the included literature, only one article did not report the
athlete classification (Tomljanovi´
c et al., 2011) but only reported
moderately trained athletes, and eight articles reported the type
of athlete, including football players (Oliver and Brezzo, 2009;
Sander et al., 2013; Baron et al., 2020; Keiner et al., 2020), martial
artists (Cherepov and Shaikhetdinov, 2016), handball players
(Elbadry, 2014; Alonso-Fernández et al., 2017), tennis players
(Yildiz et al., 2019) and volleyball players (Oliver and Brezzo,
2009); (2) Sample size. In total, the nine studies consisted of
330 subjects, ranging from 14 (Alonso-Fernández et al., 2017)
to 121 (Sander et al., 2013) participants, with a median of 26
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Xiao et al. Functional Training & Physical Fitness
TABLE 3 | Summary of methodological quality assessment scores.
References Oliver and
Brezzo
(2009)
Tomljanovi ´
c
et al. (2011)
Sander et al.
(2013)
Elbadry
(2014)
Cherepov and
Shaikhetdinov
(2016)
Alonso-
Fernández
et al. (2017)
Yildiz et al.
(2019)
Baron et al.
(2020)
Keiner et al.
(2020)
Eligibility criteria 1 1 1 1 1 1 1 1 1
Random allocation 0 1 0 0 0 1 1 0 0
Allocation concealment 0 0 0 0 0 0 0 0 0
Group similar at baseline 1 1 1 1 1 1 1 1 1
Blind therapist 0 0 0 0 0 0 0 0 0
Blind assessor 0 0 0 0 0 0 0 0 0
Follow-up 1 0 0 0 0 0 0 0 0
Intention to treat analysis 0 0 0 0 0 0 0 0 0
Between group comparisons 1 1 1 1 1 1 1 1 1
Point measure and variability 1 1 1 1 1 1 1 1 1
PEDro score 4 4 3 3 3 4 4 3 3
(Oliver and Brezzo, 2009) and mean of 36.7; (3) Gender. All
nine studies focused on athletes, three studies focused on females
(Oliver and Brezzo, 2009; Elbadry, 2014; Alonso-Fernández et al.,
2017), one study focused on male (Tomljanovi´
c et al., 2011),
and the remaining five studies did not report gender (Sander
et al., 2013; Cherepov and Shaikhetdinov, 2016; Yildiz et al.,
2019; Baron et al., 2020; Keiner et al., 2020); (4) Age. Most
studies report the subjects’ age, except for one (Cherepov and
Shaikhetdinov, 2016), and only one study reported the age range
of the subjects (Tomljanovi´
c et al., 2011). An analysis of age
reports in seven studies found that the age range of the subjects
ranged from 9.6 years to 25 years (Oliver and Brezzo, 2009;
Sander et al., 2013; Elbadry, 2014; Alonso-Fernández et al., 2017;
Yildiz et al., 2019; Baron et al., 2020; Keiner et al., 2020); (5)
Body Mass Index. Most studies reported the height and weight
of the subjects (Oliver and Brezzo, 2009; Tomljanovi´
c et al., 2011;
Sander et al., 2013; Elbadry, 2014; Alonso-Fernández et al., 2017;
Yildiz et al., 2019; Keiner et al., 2020), only two studies reported
the BMI of the subjects (Alonso-Fernández et al., 2017; Baron
et al., 2020), and only one study did not state the weight, height,
BMI of the subjects (Cherepov and Shaikhetdinov, 2016). For the
consistency of literature analysis, the following formula was used
to calculate the BMI of the subjects in the relevant studies:
BMI =weight (kg)/height2(m). The BMI of the participants
in the study ranged from 17.26 to 24.4 kg/m2; (6) Training
background. Among the nine studies, five studies reported the
training background of athletes (Tomljanovi´
c et al., 2011; Sander
et al., 2013; Elbadry, 2014; Alonso-Fernández et al., 2017; Yildiz
et al., 2019) while the other four studies did not describe the
training background (Oliver and Brezzo, 2009; Cherepov and
Shaikhetdinov, 2016; Baron et al., 2020; Keiner et al., 2020).
For the consistency of literature analysis, the training background
of the athletes was recorded in months. The training background
of the subjects ranged from 36 months to 146 months.
Interventions Characteristics
The nine included studies’ intervention characteristics were
reported based on the following aspects: (1) Training length. The
shortest intervention length is 8 days (Sander et al., 2013) and
the longest being 10 months (Keiner et al., 2020); (2) Duration
of each training session. Most studies reported the duration of
each training session (Oliver and Brezzo, 2009; Elbadry, 2014;
Alonso-Fernández et al., 2017; Yildiz et al., 2019; Baron et al.,
2020; Keiner et al., 2020), only three studies did not state the
duration (Tomljanovi´
c et al., 2011; Sander et al., 2013; Cherepov
and Shaikhetdinov, 2016). The duration of each training session
analysis of 6 research reports found that they ranged from 10 min
(Oliver and Brezzo, 2009; Alonso-Fernández et al., 2017) to
90 min (Baron et al., 2020); (3) Training frequency. Among the
nine studies included, six studies reported frequency of training
(Oliver and Brezzo, 2009; Tomljanovi´
c et al., 2011; Elbadry, 2014;
Alonso-Fernández et al., 2017; Yildiz et al., 2019; Keiner et al.,
2020) while the other three studies did not (Sander et al., 2013;
Cherepov and Shaikhetdinov, 2016; Baron et al., 2020). The
frequency analysis of 6 research reports found that the frequency
ranged from 2 times/week to 4 times/week (Oliver and Brezzo,
2009; Tomljanovi´
c et al., 2011; Elbadry, 2014; Alonso-Fernández
et al., 2017; Yildiz et al., 2019; Keiner et al., 2020).
Outcome and Measures
The outcomes for the present study were grouped according
to the effects of functional training on different physical
fitness components among athletes. All authors of this study
independently classified the papers according to other research
topics (components). Disagreements were resolved through
discussion among all authors until a consensus was reached.
Effect of Functional Training on Speed
Six of the nine studies included in this systematic review
presented inferences about the effect of functional training on
speed performance (Tomljanovi´
c et al., 2011; Sander et al., 2013;
Alonso-Fernández et al., 2017; Yildiz et al., 2019; Baron et al.,
2020; Keiner et al., 2020). The speed tests used in these studies
included linear sprint test of 5 m (Sander et al., 2013), 10 m
(Tomljanovi´
c et al., 2011; Sander et al., 2013; Yildiz et al., 2019;
Baron et al., 2020), 15 m (Sander et al., 2013), 20 m (Tomljanovi´
c
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Xiao et al. Functional Training & Physical Fitness
et al., 2011; Sander et al., 2013; Keiner et al., 2020), 25 m (Sander
et al., 2013) and 30 m (Sander et al., 2013). They also included
change of direction sprint test (5 m left and right test, 10 m
left and right test) (Sander et al., 2013; Keiner et al., 2020),
repeated sprint ability test (Alonso-Fernández et al., 2017), and
acceleration test (0–5 m, 5–10 m, 10–20 m, 10–30 m, 0–30 m)
(Tomljanovi´
c et al., 2011; Baron et al., 2020). The subjects
included young football players (Sander et al., 2013; Baron
et al., 2020; Keiner et al., 2020), moderately trained athletes
(Tomljanovi´
c et al., 2011), handball female players (Alonso-
Fernández et al., 2017) and prepubertal tennis players (Yildiz
et al., 2019). Four studies showed improvement in linear sprint
test (Sander et al., 2013; Yildiz et al., 2019; Keiner et al., 2020),
change of direction sprint test (Sander et al., 2013; Keiner et al.,
2020) and repeated sprint ability test (Alonso-Fernández et al.,
2017). Additionally, Baron et al. (2020) observed a significant
improvement for 5–10 m test and 10–30 m test, contrary to the 0–
5 m test and 0–30 m test. However, only one study did not observe
any significant change in 10, 20, and 10–20 m tests (Tomljanovi´
c
et al., 2011).
Effect of Functional Training on Muscular
Strength
Muscular strength was only evaluated in five of the studies
that were included in this review (Oliver and Brezzo,
2009; Tomljanovi´
c et al., 2011; Elbadry, 2014; Cherepov
and Shaikhetdinov, 2016; Keiner et al., 2020). The aspects
valued and assessment tools used were pull up (Cherepov and
Shaikhetdinov, 2016), medicine ball throwing (Tomljanovi´
c
et al., 2011; Cherepov and Shaikhetdinov, 2016), pronequadra-
ped core test, single-leg squat test (Oliver and Brezzo, 2009),
1 repetition maximum (Keiner et al., 2020), handgrip strength
test and static strength test (Elbadry, 2014). The subjects include
martial artists (Cherepov and Shaikhetdinov, 2016), moderately
trained athletes (Tomljanovi´
c et al., 2011), collegiate women
athletes (volleyball and soccer players) (Oliver and Brezzo,
2009), elite adolescent soccer players (Keiner et al., 2020) and
young handball players (Elbadry, 2014). Studies conducted by
Cherepov and Shaikhetdinov (2016) and Keiner et al. (2020)
revealed a significant increase in muscular strength after the
functional training intervention. Nonetheless, three studies
observed a significant improvement on standing overarm
medicine ball throw (Tomljanovi´
c et al., 2011), leg squat (right
and left) (Oliver and Brezzo, 2009), back strength test (Elbadry,
2014), but no significant change on lying medicine ball throw
(Tomljanovi´
c et al., 2011), quadra-ped left and right test (Oliver
and Brezzo, 2009), leg strength test and handgrip strength
test (Elbadry, 2014).
Effect of Functional Training on Power
Among the nine studies included in this review, four studies
reported on power (Tomljanovi´
c et al., 2011; Alonso-Fernández
et al., 2017; Yildiz et al., 2019; Keiner et al., 2020), and five
studies did not report on it (Oliver and Brezzo, 2009; Sander
et al., 2013; Elbadry, 2014; Cherepov and Shaikhetdinov, 2016;
Baron et al., 2020). The aspects valued and assessment tools
used involved vertical countermovement jump test (jump height,
air time, ground contact time, power peak) (Tomljanovi´
c et al.,
2011; Alonso-Fernández et al., 2017; Yildiz et al., 2019; Keiner
et al., 2020). The subjects include moderately trained athletes
(Tomljanovi´
c et al., 2011), female handball players (Alonso-
Fernández et al., 2017), prepubertal tennis players (Yildiz et al.,
2019) and elite adolescent soccer players (Keiner et al., 2020).
One study had an 8-week intervention period (Yildiz et al.,
2019), while the other study had an intervention period of 10
months (Keiner et al., 2020). The results of these studies reveal
that functional training can improve power (Yildiz et al., 2019;
Keiner et al., 2020). On the other hand, Tomljanovi´
c et al. (2011)
observed a significant improvement in the countermovement
jump test (jump height, ground contact time, power peak) but
not for the vertical counter movement jump test (air time)
(Tomljanovi´
c et al., 2011). However, only one study reported
that the 8-week functional training did not yield significant
results in the vertical counter movement jump test (flight time,
jump height, ground contact time, and power peak) (Alonso-
Fernández et al., 2017).
Effect of Functional Training on Balance
Balance (static and dynamic balance) was valued only in three
of the nine studies included in this review. The measurement
tools were the dynamic balance (right and left), static balance
(Yildiz et al., 2019), biodex balance test (left and right) (Oliver
and Brezzo, 2009) and standing stork test (Elbadry, 2014). The
subjects include prepubertal tennis players (Yildiz et al., 2019),
collegiate women athletes (volleyball and soccer players) (Oliver
and Brezzo, 2009), and young handball players (Elbadry, 2014).
One study reported an 8-week intervention period (Yildiz et al.,
2019), while the other studies had an intervention period of 10
weeks (Elbadry, 2014). The results of these studies reveal that
functional training can improve balance (Elbadry, 2014; Yildiz
et al., 2019). Nevertheless, one study reported that 13 weeks of
functional training did not significantly improve balance (Oliver
and Brezzo, 2009).
Effect of Functional Training on Body
Composition
The body composition aspect appeared to be undervalued by the
studies included in this review. Only three of the studies assessed
this aspect by using different measurement tools like body weight
(Oliver and Brezzo, 2009; Tomljanovi´
c et al., 2011; Alonso-
Fernández et al., 2017), body height (Tomljanovi´
c et al., 2011),
body fat mass percentage (Tomljanovi´
c et al., 2011), lean body
mass (Tomljanovi´
c et al., 2011), total body water (Tomljanovi´
c
et al., 2011), body mass index (Oliver and Brezzo, 2009; Alonso-
Fernández et al., 2017) and body fat (Oliver and Brezzo, 2009;
Alonso-Fernández et al., 2017). The subjects include moderately
trained athletes (Tomljanovi´
c et al., 2011), handball players
(Alonso-Fernández et al., 2017) and collegiate women athletes
(volleyball and soccer players) (Oliver and Brezzo, 2009). Two
studies observed no significant effect of functional training on
body composition (Oliver and Brezzo, 2009; Tomljanovi´
c et al.,
2011). However, Alonso-Fernández et al. (2017) observed a
significant improvement in body fat, but not for body weight and
body mass index (Alonso-Fernández et al., 2017).
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Xiao et al. Functional Training & Physical Fitness
Effect of Functional on Agility
Agility was not the main aspect in many of the studies included in
this review. Only three studies valued this criterion through four
exercises: shuttle run 3 ×10 m (Cherepov and Shaikhetdinov,
2016), agility 5–10–5 test, hexagon test (Tomljanovi´
c et al., 2011)
and T-test (Yildiz et al., 2019). The subjects include martial artists
(Cherepov and Shaikhetdinov, 2016), moderately trained athletes
(Tomljanovi´
c et al., 2011) and prepubertal tennis players (Yildiz
et al., 2019). Studies conducted by Cherepov and Shaikhetdinov
(2016) and Yildiz et al. (2019) revealed significant increases in
agility after the functional training intervention. In contrast,
Tomljanovi´
c et al. (2011) observed a significant improvement
in the hexagon test but no significant change on the agility
5–10–5 test.
Effect of Functional Training on Flexibility
Only one study included in this systematic review presented
inferences about the effect of functional training on flexibility.
The flexibility was measured based on the sit and reach test,
commonly used in health-related and physical fitness test
batteries to evaluate the hamstring and lower back flexibility
(Hui and Yuen, 2000). The participants of this study were pre-
pubertal tennis players. This study revealed a highly significant
improvement in flexibility after 8 weeks of functional training
(Yildiz et al., 2019).
Effect of Functional Training on Muscular
Endurance
Muscular endurance was assessed in one out of the nine studies
included in this review (Oliver and Brezzo, 2009). This study uses
the one-minute sit up test to evaluate muscular endurance (Oliver
and Brezzo, 2009). The study subjects included female collegiate
volleyball and soccer players. Oliver and Brezzo (2009) reported
positive results in this aspect after the intervention.
DISCUSSION
This systematic review provides a comprehensive overview
of the impact of functional training physical fitness among
athletes and their bey relevant knowledge for athletes to improve
their physical fitness. This revision is intended to be different
from other published studies on using the functional training
intervention among athletes. The main findings indicated
that functional training could increase physical fitness (speed,
strength, power, flexibility, agility, balance, aerobic, and muscular
endurance) among athletes. However, no data was found in
reaction time and coordination reporting. The reviewed papers
varied significantly regarding the participants (type of athletes,
age, and gender) and the physical fitness components studied.
Nonetheless, functional training may be an effective physical
fitness intervention among athletes based on positive findings in
these studies. Following the framework in the “Results” section,
the physical fitness components of the studies were analyzed
in detail.
Effect of Functional Training on Speed
Most sports experts agree that speed, an elementary motor skill,
is vital to succeeding in many sports disciplines (Karaleji´
c et al.,
2014). Six studies evaluated this component in their research
by using the linear sprint test (Sander et al., 2013; Yildiz et al.,
2019; Keiner et al., 2020), change of direction sprint test (Sander
et al., 2013; Keiner et al., 2020), and repeated sprint ability
test (Alonso-Fernández et al., 2017), which yielded significantly
positive results. However, one study reported that some of
the measures of repeated sprint ability test (5–10 m, 10–30 m)
exhibited a significant increase, but there was no significant
change on repeated sprint ability test (0–5 m, 0–30 m) after
functional training (Baron et al., 2020). Furthermore, only one
study did not observe a significant effect of functional training on
linear sprint test (10, 20, 10–20 m test) (Tomljanovi´
c et al., 2011).
This finding may be a result of improvements in functional status
and increased speed. Campa et al. (2019) also demonstrated that
better movement patterns might improve speed performance.
However, wrong movement patterns will negatively impact
the ability to perform fundamental movement patterns with
precision and appropriate efficiency, besides increasing the risk
of athletic injuries (Kollock et al., 2019).
Effect of Functional Training on Muscular
Strength
Strength has a great influence on physical fitness components.
Athletes must intensify strength training to improve their skills
and maintain a good competitive state in their respective fields
(Dengguang and Yang, 2007b). Meanwhile, muscle strength can
be divided into upper limb muscle strength study (Tomljanovi´
c
et al., 2011; Elbadry, 2014; Cherepov and Shaikhetdinov, 2016)
and lower limb muscle strength study (Oliver and Brezzo, 2009;
Elbadry, 2014; Keiner et al., 2020). Three studies reported on
upper limb strength; one study reported significant improvement
in muscular strength (pull up, 3 kg stuffed ball overhead
throwing) (Cherepov and Shaikhetdinov, 2016) while the other
study reported no significant increases in static strength test
(handgrip strength and back strength test) (Elbadry, 2014).
However, only one study reported that some of the measures
of upper limb muscular strength have significant increases
(standing overarm medicine ball throw), but there was no
significant improvement in the muscular strength test (lying
medicine ball throw) (Tomljanovi´
c et al., 2011).
Regarding those studies that assessed muscle strength of lower
limbs, one study reported significant improvement in muscular
strength (1 RM) (Keiner et al., 2020). On the contrary, no
significant improvement in muscular strength (static strength
test: leg strength test) was found in another study (Elbadry, 2014).
Furthermore, one study reported that some of the measures of
lower limb muscular strength (left leg squat and right leg squat)
showed significant improvements, but the findings were not
statistically significant (quadra-ped left and right test) (Oliver and
Brezzo, 2009). In terms of upper limb muscular strength, better
results are observed in studies with long-term interventions
(Cherepov and Shaikhetdinov, 2016; Keiner et al., 2020). In
addition, only one of the five included studies on muscle strength
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Xiao et al. Functional Training & Physical Fitness
reported on upper limb muscular strength and lower limb
muscular strength (Elbadry, 2014). Therefore, more research
should analyze strength.
Effect of Functional Training on Power
Based on the findings of four studies that analyze the benefits
of functional training on power, it was not possible to draw a
definite conclusion on this aspect. Two studies confirmed that
functional training programs increase athletes’ performances in
the vertical countermovement jump test (Yildiz et al., 2019;
Keiner et al., 2020). However, these two studies alone are not
enough to support the idea that functional training is beneficial
in improving athletes’ power, and it is worth noting that this
hypothesis has been supported by literature from non-athlete
participants (Miszko et al., 2003; Liu et al., 2014).
In addition, Tomljanovi´
c et al. (2011) reported that some
of the measures of vertical countermovement jump test (jump
height, ground contact time, power peak) were statistically
significant, but there was no significant increase in the vertical
countermovement jump test (air time) (Tomljanovi´
c et al., 2011).
The functional training program consisted of mostly upper body
exercises, which did not test the performance of the lower body.
This improvement may be mainly related to neural coordination
of movement (Tomljanovi´
c et al., 2011). However, another study
reported that high-intensity interval training protocols based
on functional exercises program had no significant increases in
power (flight time, jump height, jump speed) (Alonso-Fernández
et al., 2017). This finding is consistent with the studies by
Buchheit et al. (2009) and Rey et al. (Viaño-Santasmarinas et al.,
2018). However, these data opposed the reported results by
Dello Iacono et al. (2016) involving handball players because the
functional training program might not be enough to stimulate
the neuromuscular system related to power (Luo et al., 2005).
Power in the upper/lower extremities is necessary to produce
explosive actions among athletes (Girard and Millet, 2009; Chelly
et al., 2010). However, the included study only reported the
effect of functional training on lower body power but ignored
the effect of functional training on upper body power, which
was an important gap in the existing literature. Therefore, it
is necessary to consider and correct the research on power in
functional training.
Effect of Functional Training on Balance
The static and dynamic balance were tested in three of the nine
studies included in this review. Two studies confirmed that a
functional training program increases static balance and dynamic
balance (Elbadry, 2014; Yildiz et al., 2019). This finding may be
explained by the adaptations that occurred in all the sensory
systems assisting postural control, such as the vestibular, visual,
and the somatosensory and motor systems controlling muscular
output (Taube et al., 2008; Latorre Román et al., 2015).
However, only one study conducted was observed that
performance in the biodex balance test (left, right) was
not significantly improved (Oliver and Brezzo, 2009). This
observation may result from all the subjects being in-season, not
only in practice and competition, but also in a regimen strength
and conditioning program (Oliver and Brezzo, 2009). Therefore,
the interference of other factors (e.g., exercise training factors)
should be avoided in future research.
Effect of Functional Training on
Body Composition
Three studies showed no significant effect of functional training
on body weight (Oliver and Brezzo, 2009; Tomljanovi´
c et al.,
2011; Alonso-Fernández et al., 2017), body height (Tomljanovi´
c
et al., 2011), body mass index (Oliver and Brezzo, 2009;
Tomljanovi´
c et al., 2011; Alonso-Fernández et al., 2017), lean
body mass (Tomljanovi´
c et al., 2011), and total body water
(Tomljanovi´
c et al., 2011). The study by Alonso-Fernández
et al. (2017) reported statistically significant improvement in
body fat, whereas two other studies showed no statistical
significance in terms of body composition (Oliver and Brezzo,
2009; Tomljanovi´
c et al., 2011). These results are in line with
those obtained by Camacho-cardenosa et al. (2016) who found
no statistically significant reduction in body fat. Considering that
calorie intake and food monitoring have a statistically significant
impact on this variable, it is safe to assume that athletes with more
regular and stable eating habits may enhance body composition
quality (Mettler et al., 2010).
Effect of Functional Training on Agility
Agility is an essential component in most field and team sports.
Traditional definitions of agility have simply identified speed
in directional changes as the defining component (Draper and
Lancaster, 1985). Out of the three studies that investigated agility
(Tomljanovi´
c et al., 2011; Cherepov and Shaikhetdinov, 2016;
Yildiz et al., 2019), only one of them reported that the agility 5-10-
5 test did not demonstrate significant improvement (Tomljanovi´
c
et al., 2011). The explanation of these results may be the improved
power qualities and enhanced postural control of the subjects
(Markovi´
c et al., 2007). In contrast, the impact of power and
explosive strength is lower in speed-led agility tests (e.g., agility
5–10–5) (Tomljanovi´
c et al., 2011).
Furthermore, in a study that compared the functional
movement screen training and traditional training on agility in
62 elite male high school baseball players, the training program
included static stretching, and it was showed that functional
movement screen program improves flexibility (trunk flexion
forward, trunk extension backward, the splits) (Song et al., 2014).
Despite the findings reported in previous studies, functional
training may be an effective way to increase agility. Future
research should consider exercise items in functional training
and only determine the effect of functional training on speed-led
agility tests.
Effect of Functional Training on Flexibility
The sit and reach is a field test used to assess hamstring and
lower back flexibility (Baltaci et al., 2003). This study found that
the functional training group showed significant improvement
at sit and reach, whereas no significant improvements were
observed in the traditional training and control groups (Yildiz
et al., 2019). Similarly, Weiss et al. reported that the 7
weeks functional training program intervention resulted in
significant improvements in the flexibility of college students
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Xiao et al. Functional Training & Physical Fitness
(non-athletes) (Weiss et al., 2010), which is different from the
participants included in the study. However, the functional
training intervention can significantly improve the flexibility of
the participants. Therefore, these results can only be regarded as
weak evidence at present, and they need to be compared to more
exercise training interventions.
Effect of Functional Training on Muscular
Endurance
Muscular endurance was measured with a one-minute sit up
test (Pritchard et al., 2001). In the studies, the no intervention
group also showed significant improvement on the one-minute
sit up test. The intervention group’s significant improvement and
the non-intervention group may be due to the routine training
program during the season (Oliver and Brezzo, 2009). However,
the sit up test measures rectus abdominal endurance and not
deep core musculature (Diener, 1995), which may be why the
intervention group and non-intervention group did not show
significant improvement.
LIMITATIONS
Overall, this review provides substantial evidence of fair
quality and the beneficial effects of different functional training
programs on physical fitness among athletes. However, there
are several limitations to this review. Firstly, only four studies
reported the gender of athletes (Oliver and Brezzo, 2009;
Tomljanovi´
c et al., 2011; Elbadry, 2014; Alonso-Fernández et al.,
2017). If present, it could be important, as there are differences
in assessing physical fitness components based on gender. This
may impact the final research results. Secondly, none of the
studies included in this review stated the sample size calculation
method. Determining the sample size is influenced by several
factors, including the purpose of the study, population size, the
risk of selecting a “bad” sample, and the allowable sampling
error (MacCallum et al., 1999). Thus, inappropriate, inadequate,
or excessive sample sizes can influence quality and accuracy
(Rodríguez del Águila and González-Ramírez, 2014). If the
sample size calculation method in the included research is wrong,
it may influence the outcome of the study. Thirdly, most studies
did not document or control exercises that were performed
by participants outside of the study setting. Additionally, most
studies did not consider the influence of temperature, time, and
other factors on physical fitness among athletes. Finally, the
studies did not have any short-term or long-term follow-up,
making it difficult to predict the long-term impact of functional
training on physical fitness among athletes.
CONCLUSION
The present analysis of this systematic review provides strong
evidence that functional training improved physical fitness in
terms of speed, muscular strength, power, balance, and agility,
while there is moderate evidence of the effect on flexibility and
muscular endurance. No significant improvement was found in
body composition. The results support the principle of specificity
in training, where the best gains in performance are achieved
when the training closely mimics the performance (Hawley, 2008;
Reilly et al., 2009). Furthermore, functional training is a relatively
new training modality, but it recently has gained momentum
among physical fitness training and has been identified as
a “Top 10 Fitness Trend” in 2018 (Thompson, 2017), with
four of the nine studies being published in the past 3 years.
Moreover, review trials show that functional training was most
common in resistance and strength training. Nevertheless, it
is necessary to be cautious about the results in view of the
limitations outlined in the present study. To better understand
the effectiveness of functional training in improving athletes’
physical fitness, additional research to examine the effect of
functional training on physical fitness components according
to the difference in the type of athletes is encouraged. It will
help verify the effectiveness of functional training to improve the
physical fitness components among different types of athletes and
promote functional training in the field of modern sports science
(Osipov et al., 2017).
DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included
in the article/supplementary material„ further inquiries can be
directed to the corresponding author.
AUTHOR CONTRIBUTIONS
The literature search and selection of studies was performed
by authors WX and KS. Following an initial screen of titles
and abstracts WX, full scrutiny of potentially eligible studies
was independently screened by WX and KS using the specific
inclusion criteria. OT arbitrated any disagreements in study
inclusion. Study quality assessment was performed by WX. All
authors contributed to manuscript revision, read, and approved
the submitted version.
FUNDING
The study was supported by the Non-profit Central Research
Institute Fund of Chinese Academy of Medical Science, grant
numbers: 2020-JKCS-022; and scientific research fund project
of Tianjin Education Commission of China phased research
results of the research project on the current situation and
mode of integration of Tianjin sports and medicine, grant
numbers: 2018SK145.
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