Introduction {#j_hukin-2020-0004_s_001}
============

Biofeedback (BFB) as a performance enhancement strategy in athletes has been receiving notable attention among sports practitioners ([@j_hukin-2020-0004_ref_005]; [@j_hukin-2020-0004_ref_023]; [@j_hukin-2020-0004_ref_038]).  BFB provides real-time understandable physiological information to an individual that enhances psychophysiological and affective indices ([@j_hukin-2020-0004_ref_015]; Wilson et al.,  2014).  Among various approaches to BFB there is a breathing strategy known as heart rate variability biofeedback (HRV BFB) ([@j_hukin-2020-0004_ref_027]).  HRV BFB is executed by paced breathing at a specific frequency, known as resonance frequency (RF), that elicits maximal heart rate oscillations.  RF usually ranges from 4 to 6.5 breaths/min ([@j_hukin-2020-0004_ref_027]).  In addition, RF exhibits a 0-degree phase shift between the heart rate and respiration as well as a 180-degree phase shift between the heart rate and blood pressure ([@j_hukin-2020-0004_ref_047]).  The physiological phenomena with HRV BFB are believed to improve autonomic function from baroreflex gain and vagal activation ([@j_hukin-2020-0004_ref_016]; [@j_hukin-2020-0004_ref_027]; [@j_hukin-2020-0004_ref_026]; [@j_hukin-2020-0004_ref_039]; [@j_hukin-2020-0004_ref_047]). 

In an athletic setting, the first documented HRV BFB intervention was administered on wrestlers who exhibited reduced muscle relaxation disorders and an improved rate of relaxation ([@j_hukin-2020-0004_ref_048]).  A recent review conducted by [@j_hukin-2020-0004_ref_021] synthesized the effect of HRV BFB on athletic performance and found that 86% (n = 6) of studies reviewed performance enhancement via improvement in psychophysiological variables. 

One notable strength of the previous study was the employment of systematic procedures.  On the other hand, the review included different quantitative study designs (two case reports, one quasi-experimental study, four experimental studies) which may have contributed to bias in findings.  Thus, the purpose of this study was to conduct a systematic review of studies only employing randomized trials on the effect of HRV BFB on psychophysiology and exercise performance of athletes. 

Methods {#j_hukin-2020-0004_s_002}
=======

Search Strategy {#j_hukin-2020-0004_s_002_s_001}
---------------

The search was conducted between July 1^st^ and August 31^st^ 2017 using the search term "heart rate variability biofeedback" and (athletes or athletic population or sport or performance or sport performance) in electronic databases: Springerlink, SportDiscus, Web of Science, PROQUEST Academic Research Library, Google Scholar, and ScienceDirect ([@j_hukin-2020-0004_ref_021]).  In addition, a manual reference search was administered on the records found.  PRISMA guidelines were used for reporting. 

Eligibility Criteria {#j_hukin-2020-0004_s_002_s_002}
--------------------

To be included in the systematic review, the studies had to meet the following criteria: 1.  experimental studies involving athletes randomly allocated among groups (randomized control trial); 2.  availability of HRV BFB as a treatment compared with a control condition (CON)/ a placebo (PLA)/other BFB; 3.  any performance-related variable as a dependent index; and, 4.  peer-reviewed articles written in English. 

Study Selection {#j_hukin-2020-0004_s_002_s_003}
---------------

Literature search and selection of studies were completed by a single investigator (JP).  All studies were coded and organized in an Excel spreadsheet.  The second investigator (YSC) evaluated data extraction.  For each article included in the systematic review, the following data were encoded: author/s and year of publication, sample size information, intervention, measured performance variable/s, and results.  These studies were also assessed for 'risk of bias' using an eight-point scale from Consolidated Standards of Reporting Trials (CONSORT) statement wherein each item is answerable by 0 (absently or inadequately described) or 1 (explicitly described and present).  A study with a score of 0-2 is regarded as having a high risk of bias, 3-5 with medium risk of bias, and 6-8 considered as having low risk of bias ([@j_hukin-2020-0004_ref_002]).  A consensus was reached between JP and YSC for any disagreement presented in data extraction and CONSORT output ([Table 1](#j_hukin-2020-0004_tab_001){ref-type="table"}). 

###### 

CONSORT scores of articles included for systematic review

  References                     Item 1   Item 2   Item 3   Item 4   Item 5   Item 6   Item 7   Item 8   Total
  ------------------------------ -------- -------- -------- -------- -------- -------- -------- -------- -------
  [@j_hukin-2020-0004_ref_007]   1        0        0        0        0        1        1        0        3
  [@j_hukin-2020-0004_ref_017]   0        0        1        0        0        1        1        0        3
  [@j_hukin-2020-0004_ref_034]   1        0        1        0        1        1        1        0        5
  [@j_hukin-2020-0004_ref_034]   1        0        1        0        1        1        1        0        5
  [@j_hukin-2020-0004_ref_040]   1        0        1        0        0        1        1        0        4
  [@j_hukin-2020-0004_ref_041]   1        0        1        0        0        1        1        1        5

***Item 1*** *-- Were the groups comparable at baseline on key characteristics? ** Item 2** -- Did the study include a true control group (randomised participants -- not a comparison group)? ** Item 3** - Was the randomisation procedure adequately described and carried out? ** Item 4** -- Did the study report a power calculation and was the study adequately powered to detect intervention effects? ** Item 5** -- Were the assessors blinded to treatment allocation at baseline and posttest? ** Item 6** -- Did at least 80% of the participants complete follow-up assessments? ** Item 7** -- Did the study analyses account for potential differences at baseline? ** Item 8** -- Did the study compute effect sizes?* 

Statistical Analysis {#j_hukin-2020-0004_s_002_s_004}
--------------------

Independent T-tests using pre-post mean differences and standard deviation of variables from HRV BFB and groups' comparison were administered.  Then, corresponding Cohen's *d* as effect size (ES) with 95% confidence limits were calculated ([@j_hukin-2020-0004_ref_008]; [@j_hukin-2020-0004_ref_025]; [@j_hukin-2020-0004_ref_032]).  Missing pre-post mean differences and SD in studies were computed based on previous methods ([@j_hukin-2020-0004_ref_018]; Zu et al.,  2013).  ES was interpreted as small (*d* = 0.20), medium (*d* = 0.50), or large (*d* = 0.80) ([@j_hukin-2020-0004_ref_008]).  Statistical power calculation from post hoc was also conducted using G\*Power ver 3.1 ([@j_hukin-2020-0004_ref_013]).  Indices with alpha = 0.00 were set at alpha = 0.01. 

Results {#j_hukin-2020-0004_s_003}
=======

[Figure 1](#j_hukin-2020-0004_fig_001){ref-type="fig"} shows the flowchart and selection process of the studies.  The database search indicated 656 potential articles with an additional 4 identified articles from reference lists.  After removal of duplicates (n = 90), 570 articles underwent initial screening based on the article title/abstract.  This process led to excluding 557 articles after failing to meet all the items in the inclusion criteria.  Further assessment for eligibility of 13 full articles led to removal of seven studies leaving six articles included in the analysis. 

![ Flow diagram of the search process](hukin-73-103-g001){#j_hukin-2020-0004_fig_001}

Participants {#j_hukin-2020-0004_s_003_s_001}
------------

The six studies included a total of 187 (females: n = 89; males n = 98) athletes including: 60 (females: n = 27; males: n = 33) university, state, and national basketball athletes; 24 (females: n = 12; males: n = 12) university, state, and national standard long distance runners; 20 male professional soccer players; and, 84 (females: n = 51; males: n = 33) university student dancers. 

Experimental protocols {#j_hukin-2020-0004_s_003_s_002}
----------------------

Of the six studies included, all studies compared HRV BFB and a control condition (CON) wherein a CON involved regular dance/sport training.  Three studies also compared HRV BFB and a placebo (PLA) with two studies using motivational video ([@j_hukin-2020-0004_ref_034]; [@j_hukin-2020-0004_ref_034]) and one using choreology ([@j_hukin-2020-0004_ref_017]).  Two studies compared HRV BFB with the alternative BFB intervention neurofeedback (NFB) which utilized electronenchaphalogram (EEG) signals ([@j_hukin-2020-0004_ref_017]; [@j_hukin-2020-0004_ref_040]). 

Interventions {#j_hukin-2020-0004_s_003_s_003}
-------------

The studies applied a variety of HRV BFB protocols.  Two studies utilized 10 consecutive days of HRV BFB administered at 20-min/day ([@j_hukin-2020-0004_ref_034]; [@j_hukin-2020-0004_ref_034]).  Other studies followed protocols for HRV BFB suggested by [@j_hukin-2020-0004_ref_027] using 10 formal sessions with two 20-min daily sessions at convenience (Choudhary, et al.,  2016; [@j_hukin-2020-0004_ref_017]; [@j_hukin-2020-0004_ref_040]).  Two studies following a protocol set by [@j_hukin-2020-0004_ref_027] lasted for 10 weeks, while the study of [@j_hukin-2020-0004_ref_040] was carried out for 4 weeks. 

[@j_hukin-2020-0004_ref_041] administered HRV BFB for fifteen 30-min sessions (twice a week).  The first three sessions followed the protocol set by [@j_hukin-2020-0004_ref_027].  Then sessions 4 -- 9 combined HRV BFB with other biofeedback schemes including a skin conductance level (SCL), electromyography (EMG) of masseter and a posterior cervical region and hand temperature.  The remaining sessions (10^th^ -- 15^th^) integrated HRV BFB with math tasks, hyperventilation, and winning and losing video games. 

Instruments {#j_hukin-2020-0004_s_003_s_004}
-----------

Different HRV BFB equipment was utilized for HRV BFB.  Two studies utilized freeze-framer from Boulder Creek, California, US (Gruzelier et al.,  2015; [@j_hukin-2020-0004_ref_040]).  Three studies used Biograph Pro Comp Infinity 5.0 from Thought Technology Ltd., Canada ([@j_hukin-2020-0004_ref_007]; [@j_hukin-2020-0004_ref_034]; [@j_hukin-2020-0004_ref_034]). [ @j_hukin-2020-0004_ref_041] used HRV BFB with Nexus 10 Mark II hardware and Biotrace1 commercial software from Mind Media, Herten, Netherlands. 

Performance Evaluation {#j_hukin-2020-0004_s_003_s_005}
----------------------

Studies included in the review assessed various performance related variables.  Changes in performance (%) and effect sizes with 95% confidence limits of HRV BFB and groups' comparison, as well as study power are displayed in [Table 2](#j_hukin-2020-0004_tab_002){ref-type="table"}. 

###### 

Effect Size with 95% Confidence Interval of Studies in HRV BFB vs Comparison Group

  Study                          Parameter                              \% Change   p-      ES      95% Confidence Interval   Power           
  ------------------------------ -------------------------------------- ----------- ------- ------- ------------------------- ------- ------- ------
  HRV BFB vs CON                                                        HRV BFB     CON     value                             LL      UL      
  [@j_hukin-2020-0004_ref_007]   5-km running                           -13.0       -0.71   0.16    -0.60                     -0.84   0.22    0.51
  [@j_hukin-2020-0004_ref_034]   dribbling                              25.1        6.54    0.06    0.89                      0.60    1.18    0.50
                                 passing                                76.6        13.2    0.00    2.14                      1.79    2.49    0.96
                                 shooting                               104         9.43    0.00    2.00                      1.66    2.34    0.93
  [@j_hukin-2020-0004_ref_034]   shooting movement                      98.1        27.3    0.01    1.38                      1.07    2.36    0.59
                                 time                                   -17.1       0.00    0.05    -0.95                     -1.24   -0.02   0.52
                                 choice RT                              -21.3       0.00    0.01    -1.23                     -1.53   -0.27   0.47
  [@j_hukin-2020-0004_ref_040]   dance performance practice-corrected   15.1        8.46    0.40    0.55                      -0.06   1.77    0.56
                                 difference                             NA          NA      0.31    0.66                      0.04    1.89    0.53
  [@j_hukin-2020-0004_ref_041]   target absent RT                       -29.4       -1.58   0.00    -1.72                     -2.05   -0.70   0.82
                                 target present RT                      -22.6       -11.0   0.31    -0.47                     -0.72   0.42    0.53
                                 congruent task accuracy incongruent    10.5        0.57    0.00    2.64                      2.26    3.84    1.00
                                 task accuracy                          15.6        0.95    0.00    2.94                      2.54    4.20    1.00
                                 days present                           NA          NA      0.00    1.51                      1.20    2.50    0.69
                                 days absent                            NA          NA      0.00    -1.52                     -1.83   -0.52   0.69
                                 differential training                  NA          NA      0.18    -0.62                     -0.90   0.28    0.51
  HRV BFB vs PLA                                                        HRV BFB     PLA                                                       
  [@j_hukin-2020-0004_ref_034]   dribbling                              25.1        3.06    0.04    0.99                      0.70    1.92    0.51
                                 passing                                76.6        23.9    0.01    1.38                      1.07    2.36    0.59
                                 shooting                               104         28.8    0.00    1.63                      1.31    2.64    0.77
  [@j_hukin-2020-0004_ref_034]   shooting movement                      98.1        30.8    0.00    1.39                      1.08    2.37    0.60
                                 time                                   -17.1       0.00    0.42    -0.37                     -0.64   0.52    0.56
                                 choice RT                              -21.3       0.00    0.00    -1.51                     -1.82   -0.51   0.69
  HRV BFB vs NFB                                                        HRV BFB     NFB                                                       
  [@j_hukin-2020-0004_ref_040]   dance performance practice-corrected   15.1        12.2    0.86    0.12                      -0.51   1.39    0.86
                                 difference                             NA          NA      0.94    0.05                      -0.58   1.31    0.94

***ES*** *- effect size; **LL** - lower limit; **UL** - upper limit; **NA -** not applicable*

[@j_hukin-2020-0004_ref_007] determined 5-km performance of track athletes.  The study of [@j_hukin-2020-0004_ref_034] assessed performance using movement and choice reaction time (RT) along with a 3-min shooting score. [ @j_hukin-2020-0004_ref_034] measured weaving in and around cones whilst dribbling for 30 s, 30-s passing with specified targets on the wall, and 3-min shooting at marked perimeters. [ @j_hukin-2020-0004_ref_041] established visual tracking RT of professional soccer players with and without a target.  Accuracy under congruent and incongruent stimuli using a Stroop task was also determined.  Further, injury prevention was identified from attendance records (days present and absent or differential training days -- the number of days on which athletes followed personalized training due to recent injuries).  In the study of [@j_hukin-2020-0004_ref_017], four dance experts rated dance performance for artistry and technique. [ @j_hukin-2020-0004_ref_040] utilised two qualified dance assessors that evaluated dance performance considering a technicality, musicality, timing, partnering skill, performing flair, and overall execution from a customized scale with scoring of one to five.  In addition, rating scores were divided by the number of practice sessions for each dancer to derive "improvement per practice session".  Then, scores were averaged to get the group score from "practice-corrected average difference".  Characteristics of each study are presented in [Table 3](#j_hukin-2020-0004_tab_003){ref-type="table"}. 

###### 

HRV BFB and Athlete Performance

  References                     Participants      Training Modality       Conclusion/s                                                                                                        
  ------------------------------ ----------------- ----------------------- --------------------------------------------------------- --------------------------------------------------------- ------------------------------------------------------
  [@j_hukin-2020-0004_ref_007]   n = 24            regular sport           university, state, national level long distance runners   HRV BFB:                                                  5-km time:
                                 12M; 12F          training for HRV BFB/                                                             once a week formal HRV BFB training;                      HRV BFB ↔ CON
                                 22.5 ± 1.72 yrs   CON                                                                               2 x 20 min/day home practice                              
  [@j_hukin-2020-0004_ref_017]   n = 64            regular dance           university dancers                                        HRV BFB:                                                  Artistry:
                                 22M; 42F          training for                                                                      10 HRV BFB sessions                                       HRV BFB ↔ CON
                                                   HRV BFB/                                                                          at 20 min/session                                         HRV BFB ↔ PLA
                                 NR                CON/ NFB                                                                                                                                    HRV BFB ↔ NFB
                                                                                                                                     NFB: 10 alpha/theta training at 20 min/session            Technique: HRV BFB ↔ CON HRV BFB ↔ PLA HRV BFB ↔ NFB
  [@j_hukin-2020-0004_ref_034]   n = 30            regular sport           university, state                                         HRV BFB:                                                  Dribbling:
                                 17M; 13F          training for            and national                                              10 consecutive HRV                                        HRV BFB ↔ CON
                                                   HRV BFB/                basketball athletes                                       BFB at 20 min/session                                     HRV BFB „PLA
                                 21.1 ± 2.82 yrs   PLA/CON                                                                           PLA: motivational video clips for 10 days at 10 min/day   Passing: HRV BFB „ CON HRV BFB „ PLA
                                                                                                                                                                                               Shooting: HRV BFB „ CON HRV BFB „ PLA

***HRV BFB*** *- heart rate variability biofeedback; **PLA -** placebo; **CON -** control; **NFB -** neurofeedback; **NR -** not reported; **SCL -** skin conductance level; **EMG -** electromyography; **RT** - reaction time*

Performance Outcomes {#j_hukin-2020-0004_s_003_s_006}
--------------------

HRV BFB vs. CON

For gross motor skill executed in relatively short duration comparing HRV BFB and a a CON, [@j_hukin-2020-0004_ref_034] demonstrated non-significant difference in dribbling between the HRV BFB and the CON group at *p* = 0.06, ES\[95% CL\] = 0.89\[0.60, 1.18\].  On the other hand, the HRV BFB group presented significant enhancement in passing and shooting at *p* \< 0.001, 2.14\[1.79, 2.49\] and *p* \< 0.001, 2.00\[1.66, 2.34\], respectively.  In a similar study, [@j_hukin-2020-0004_ref_034] found significant improvement in 3-min shooting after HRV BFB compared to a CON at *p* = 0.01, 1.38\[1.07, 2.36\]. [ @j_hukin-2020-0004_ref_040] observed non-significant difference in performance scores of dancers at *p* = 0.40, 0.55\[-0.06, 1.77\].  No significant difference in practice-corrected difference scores at *p* = 0.31, 0.66\[0.04, 1.89\] was also found. 

###### 

HRV BFB and Athlete Performance

  References                     Participants             Training Modality                              Conclusion/s                                                                                                                 
  ------------------------------ ------------------------ ---------------------------------------------- ---------------------------------------------------- ----------------------------------------------------------------------- -----------------------------------------------------------------
  [@j_hukin-2020-0004_ref_034]   n = 30 16M; 14F          regular sport training for HRV BFB/            university, state and national basketball athletes   HRV BFB: 10 consecutive HRV BFB at 20 min/session                       Choice RT: HRV BFB " CON HRV BFB " PLA
                                 21.7 ± 2.71 yrs          PLA/CON                                                                                             PLA: motivational video clips for 10 days at 10 min/day                 Movement RT: HRV BFB ↔ CON HRV BFB ↔ PLA
                                                                                                                                                                                                                                      Shooting: HRV BFB „ CON HRV BFB „ PLA
  [@j_hukin-2020-0004_ref_040]   n = 18 9M; 9F 21.6 yrs   regular dance practice for HRV BFB/ NFB./CON   university dancers                                   HRV BFB: 10 formal HRV BFB training at 20 min/session in 4 weeks        Performance HRV BFB ↔ CON HRV BFB ↔ NFB
                                                                                                                                                              NFB = 10 alpha/theta training sessions at 20 min/session                Practice-Corrected Difference Score HRV BFB ↔ CON HRV BFB ↔ NFB
  [@j_hukin-2020-0004_ref_041]   n = 20 20M 30.4 ±        regular sport training for HRV BFB/ CON        professional football players                        HRV BFB: Fifteen 30-min biofeedback sessions at twice/week              Target-Absent Visual Task RT: HRV BFB " CON
                                 4.10 yrs                                                                                                                     4th-9th session: HRV BFB + SCL                                          Target-Present Visual-Task RT: HRV BFB ↔ CON
                                                                                                                                                              \+ EMG + hand temperature                                               Congruent Task: Accuracy: HRV BFB „ CON
                                                                                                                                                              10th -15th session: HRV BFB + math tasks + videos of matches won/lost   Incongruent Task Accuracy: HRV BFB „ CON
                                                                                                                                                                                                                                      Date Present: HRV BFB „ CON
                                                                                                                                                                                                                                      Days Absent: HRV BFB " CON
                                                                                                                                                                                                                                      Differential Training Days: HRV BFB ↔ CON

[@j_hukin-2020-0004_ref_017] found that university dancers exhibited non-significant outcomes in artistry and technique between HRV BFB and CON groups. 

In exercise of longer duration, [@j_hukin-2020-0004_ref_007] reported no significant improvement in gross motor skill movement via 5-km performance between the HRV BFB group and a CON at *p* = 0.16, ES\[95% CL\] = -0.60\[-0.84, 0.22\]. 

In fine motor ability, [@j_hukin-2020-0004_ref_034] found no significant reduction in movement time seen after HRV BFB and a CON at *p* = 0.05, -0.95\[1.24, -0.02\].  On the other hand, there was significantly shorter choice reaction time found after HRV BFB compared to a CON at *p* = 0.01, - 1.23\[-1.53, -0.27\]. [ @j_hukin-2020-0004_ref_041] detected significant improvement in visual tracking scores with a target absent stimulus among soccer athletes after HRV BFB compared to a CON at *p* = 0.00, -1.72\[-2.05, -0.70\].  However, visual tracking with a target present stimulus did not significantly improve after HRV BFB compared to a CON at *p* = 0.31; ES = -0.47\[-0.75, 0.42\].  Furthermore, soccer players after HRV BFB significantly increased accuracy under congruent stimuli at *p* = 0.00, 2.64\[2.26, 3.84\] compared to a CON.  Similarly, the HRV BFB group showed significantly higher accuracy following incongruent stimuli compared to a CON at *p* = 0.00, 2.94\[2.54, 4.20\]. 

[@j_hukin-2020-0004_ref_041] found significantly reduced absence from sport training sessions compared with a CON at *p* = 0.00, 1.51\[1.20, 2.50\].  The number of absences from training in the entire sport season that followed treatment from injury was also significantly lower in the HRV BFB group than a CON at *p* = 0.00, -1.52\[-1.83, - 0.52\].  Differential training between the HRV BFB group and a CON was not significantly different, *p* = 0.18, -0.62\[-0.90, 0.28\]. 

HRV BFB vs. PLA {#j_hukin-2020-0004_s_003_s_007}
---------------

[@j_hukin-2020-0004_ref_034] discovered a significantly higher dribbling score in the HRV BFB group than a PLA at *p* = 0.04, 0.99\[0.70, 1.92\].  Passing also significantly improved in the HRV BFB group compared with a PLA at *p* = 0.01, 1.38\[1.07, 2.36\].  The HRV BFB group significantly increased shooting performance compared to a PLA, *p* = 0.00, 1.63\[1.31, 2.64\].  In the study of [@j_hukin-2020-0004_ref_034], athletes after HRV BFB significantly increased shooting scores at *p* = 0.01, 1.39\[1.08, 2.37\].  In dance, artistry and technique were similar in both the HRV BFB and the PLA group ([@j_hukin-2020-0004_ref_017]). 

For fine motor task ability examining HRV BFB and a PLA, non-significant improvement in movement time was identified between the HRV BFB group and a PLA at *p* = 0.42, -0.37\[-0.64, 0.52\].  However, the HRV BFB group presented significantly lower choice reaction time than the PLA at *p* = 0.00, -1.51\[-1.82, - 0.51\]. 

HRV BFB vs. other BFB intervention {#j_hukin-2020-0004_s_003_s_008}
----------------------------------

[@j_hukin-2020-0004_ref_040] presented similar both performance and practice-corrected performance after HRV BFB and NFB at *p* = 0.86, 0.12\[-0.51, 1.31\] and *p* = 0.94, 0.05\[-0.58, 1.31\], respectively. [ @j_hukin-2020-0004_ref_017] recorded non-significant difference in artistry and technique between HRV BFB and NFB. 

Discussion {#j_hukin-2020-0004_s_004}
==========

The purpose of this systematic review was to examine the effect of HRV BFB on performance indices of athletes.  When compared to a control condition (CON) that involved regular training only, HRV BFB displayed contrasting effects on the gross motor skills during short duration performance.  In addition, the effect of HRV BFB on gross motor function during exercise of longer duration is unclear.  Confounding results in fine motor skills after HRV BFB were also identified in comparison with a CON. 

In this study, short duration (" 10 minutes) gross motor tasks after HRV BFB delivered conflicting results.  For example, [@j_hukin-2020-0004_ref_034] showed improved shooting and passing performance in basketball players after HRV BFB compared to a CON.  However, non-significant difference in dribbling was seen after HRV BFB and a CON.  In a similar study, [@j_hukin-2020-0004_ref_034] recorded increased shooting following HRV BFB compared to a CON.  Raymond et al. ( 2015) presented no significant difference in dance performance and practice-corrected difference in HRV BFB and a CON.  The conflicting results in short duration gross motor ability can be attributed to underpowered trials ([Table 3](#j_hukin-2020-0004_tab_003){ref-type="table"}).  Future research in HRV BFB and gross motor abilities should employ adequate sample size to facilitate enough power to detect meaningful difference ([@j_hukin-2020-0004_ref_004]). 

The influence of HRV BFB on longer duration („ 10 min) gross motor performance in comparison with a CON is unclear due to limited literature.  In the study of [@j_hukin-2020-0004_ref_007], 5-km performance of athletes in the HRV BFB and the CON group was not significantly different.  However, a notable trend in running enhancement of 13.0% (ES = -0.60) was demonstrated by athletes from the HRV BFB group.  On the other hand, the CON group displayed a 0.71% change in performance.  Better performance exhibited by athletes following HRV BFB suggests the positive influence of HRV BFB on longer duration gross motor ability.  HRV BFB may have reduced physiological stress of athletes from increased blood flow to internal organs, elevated minimum left ventricular elastance, baroreflex sensitivity gain, and improved pulmonary function ([@j_hukin-2020-0004_ref_014]; [@j_hukin-2020-0004_ref_028]; McEwen and Seewan, 2003).  More studies are needed to establish the effect of HRV BFB on gross motor ability. 

In another light, the fine motor ability of athletes after HRV BFB and a CON displayed conflicting results. [ @j_hukin-2020-0004_ref_034] observed improved choice reaction time after HRV BFB.  Conversely, no significant enhancement in movement reaction time existed between the HRV BFB and the CON group. [ @j_hukin-2020-0004_ref_041] found enhancement in reaction time with a target absent stimulus after HRV BFB.  However, there was no significant change in the target present stimulus in both groups.  Non-significant findings in fine motor ability between HRV BFB and CON can also be due to insufficient sample size leading to low statistical power.  In the study of [@j_hukin-2020-0004_ref_034], employment of unequal sample size of males and females between groups may have influenced the non-significant results ([@j_hukin-2020-0004_ref_042]).  Males and females utilize different processing strategies in reaction time wherein males demonstrate faster reaction time than females ([@j_hukin-2020-0004_ref_001]; [@j_hukin-2020-0004_ref_009]).  This can be supported by the greater vagal activity observed in females due to the presence of more oestrogen than in males ([@j_hukin-2020-0004_ref_010]; [@j_hukin-2020-0004_ref_011]; [@j_hukin-2020-0004_ref_024]).  Oestrogen improves the activity of choline uptake and synthesis of acetylcholine, thereby increasing vagal function ([@j_hukin-2020-0004_ref_010]).  In addition to unequal distribution of males and females between groups, inclusion of athletes from various competitive levels in the study by [@j_hukin-2020-0004_ref_034] may have increased the variability in fine motor skill execution ([@j_hukin-2020-0004_ref_006]; [@j_hukin-2020-0004_ref_033]).  Indeed, previous studies have shown better reaction times in elite athletes compared to non-elite athletes ([@j_hukin-2020-0004_ref_029]; [@j_hukin-2020-0004_ref_046]; [@j_hukin-2020-0004_ref_049]).  Addressing the aforementioned shortcomings when examining fine motor skill with HRV BFB training in future studies should be warranted. 

Although inconsistent results were observed in HRV BFB and a CON, there is a favourable trend in performance enhancement towards HRV BFB in overall motor function.  Possible physiological mechanisms of HRV BFB can be explained by the neurovisceral integration model ([@j_hukin-2020-0004_ref_037]; [@j_hukin-2020-0004_ref_045]).  HRV BFB increases activation of the vagal nerve ([@j_hukin-2020-0004_ref_016]; [@j_hukin-2020-0004_ref_026]).  The vagal nerve is connected to the anterior cingulate cortex, the brain region that plays a crucial role for multi-component behaviour (Duncan, 2010; [@j_hukin-2020-0004_ref_030]; [@j_hukin-2020-0004_ref_033]).  HRV BFB may have facilitated the production of neurotransmitters responsible for improving fine and gross motor function ([@j_hukin-2020-0004_ref_003]; [@j_hukin-2020-0004_ref_019]; [@j_hukin-2020-0004_ref_022]; [@j_hukin-2020-0004_ref_043]; [@j_hukin-2020-0004_ref_044]; [@j_hukin-2020-0004_ref_051]).  However, the exact mechanism affecting performance from improvement in vagal function is unknown.  Future studies should be sufficiently powered, and include biochemical markers to elucidate the mechanistic properties of HRV BFB. 

An interesting finding in this review is the increased attendance of athletes in training with HRV BFB. [ @j_hukin-2020-0004_ref_041] recorded an \~86% presence in training and \~4% absent rate out of 240 training sessions in soccer players in HRV BFB training.  For athletes from a CON group, the rates of attendance and absence were \~73% and \~14%, respectively.  Although differential training was not significantly different between groups, athletes following HRV BFB presented lower differential training of \~9% compared to a CON (\~13%).  The increased attendance, reduced absences, and a lower trend in differential training among athletes under HRV BFB may be related to improved physiological adaptations leading to resilience to stressors ([@j_hukin-2020-0004_ref_020]; [@j_hukin-2020-0004_ref_036], 2009; [@j_hukin-2020-0004_ref_045]).  Thus, HRV BFB can also be a promising intervention in increasing athlete's attendance in training by reduction of the risk of injuries. 

Aside from comparing HRV BFB and a CON, the researchers also found improvement in short-duration gross motor performance with HRV BFB compared to a PLA. [ @j_hukin-2020-0004_ref_034] displayed increased dribbling, shooting, and passing after HRV BFB compared to a PLA.  Similarly, [@j_hukin-2020-0004_ref_034] showed improved shooting after HRV BFB compared to a PLA.  Implications from these results point to HRV BFB as a superior alternative compared with a PLA in improving gross motor function of short duration. 

The influence of HRV BFB on fine motor skill differentiated with a PLA is vague due to the scarcity of literature. [ @j_hukin-2020-0004_ref_035] recorded non-significant difference in movement time between HRV BFB and a PLA.  On the other hand, HRV BFB presented enhanced choice reaction time in HRV BFB compared to a CON.  Therefore, additional HRV BFB studies including a PLA are needed. 

It is necessary to acknowledge that the outcomes of this study are limited to the type of athletes included in the analyses.  Generalization of results should be avoided.  In addition, this review evaluated performance indices to provide practitioners with a simple and direct link on HRV BFB and performance. 

Conclusion {#j_hukin-2020-0004_s_005}
==========

Findings of this study indicate the potential ergogenic ability of HRV BFB in improving fine and gross motor skills of athletes.