Trends and Future Directions in the Sports Performance of Deaf and Hard-of-Hearing Athletes: A Systematic Review

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Hearing impairment is not a barrier in the development of an athlete’s physical fitness. Inclusion in sports participation and specific tools (i.e., communication aids) appear to be crucial factors in the performance enhancement of deaf and hard-of-hearing athletes.

Abstract

The aim of this systematic review was twofold: to identify the main trends and issues that are being addressed by researchers in the context of physical fitness and sports performance in deaf and hard-of-hearing (D/HH) athletes and to indicate the needs and future directions that should be implemented in the training process of athletes with hearing impairments. The methodology of this systematic review was planned according to PRISMA guidelines. A search of electronic databases (PubMed, EBSCO, Scopus) was conducted to identify all studies on physical fitness, sports performance and participation, and D/HH athletes from 2003 to 2024. In total, 87 full-text articles were assessed to determine eligibility, while 34 studies met the inclusion criteria and were subjected to detailed analysis and assessment of their methodological quality. The presented systematic review indicates evidence that D/HH athletes are characterized by a similar or higher level in selected motor abilities compared to hearing athletes. Moreover, it seems that hearing impairment is not a barrier in the development of an athlete’s physical fitness, including aerobic capacity, muscular strength and power or speed of reaction. Furthermore, inclusion in sports participation and specific tools (i.e., communication aids) appear to be crucial factors for performance enhancement.

1. Introduction

Beyond the different kinds of disabilities, there is a single variable that combines them all. Regardless of the form and severity of the impairment, people with disabilities are faced with various barriers, with social, physical and psychological issues seeming to be the most common [1,2]. However, there are some tools that can be used to overcome these barriers, among which sports are recognized as a universal platform whose importance cannot be overemphasized [3]. This is primarily because for the population of people with disabilities, sports have become a form of social inclusion and meaningful interaction through physical activity that can be undertaken together with both able-bodied individuals and those with various impairments [4]. Secondly, sports are known to enhance everyday life by impacting various motor abilities [5], which in turn contributes to improving self-reliance [6] in the population of people with disabilities/in those with disabilities. Lastly, sports are also a platform for self-actualization, especially in the form of training and competitions that allow athletes to strengthen self-character and to reinvent themselves [7].

As disability is a multidimensional concept [8], three main athletic movements can be indicated: (1) the Paralympic Games; (2) Deaflympics; and (3) the Special Olympics. Each of them is characterized by a long tradition; however, only deaf sports are confined to a small sample of the population of people with impairments [9]. Therefore, the uniqueness of sports for people with deafness gives them multiple opportunities, especially in the context of socializing with those who have similar social needs; nevertheless, at the same time, there are multiple challenges that should be taken into consideration, including communication [10].

Moreover, deaf and hard-of-hearing athletes seem to experience a different level of sports participation [11], which can be related mostly to the intrinsic differences in their morpho-functional development [12], which requires coaches to have special skills and the ability to adequately adapt methods and forms of training to the individual needs of the deaf athlete. One of the most challenging issues in the context of deaf sports is to deal with the effects of the intrinsic disturbances of the somatosensory and vestibular systems on athletic performance [13]. As was indicated by Assaiante et al. [14], the hearing and balance organs of the human body are closely related; thus, an impairment of the abovementioned systems impacts the muscular coordination mainly by reducing motor function and decreasing balance.

However, in the context of the Deaflympics, among the athletes that are eligible to compete in this event, there are no restrictions except for the loss of at least 55 dB in the better ear [15]. Compared to the Paralympic Games, there are not as many adaptations required for a single sports discipline as there are for multiple physical, visual or intellectual impairments [16]. This may affect the development of a unique sporting environment, in which only D/HH athletes can integrate and interact with the population of people with hearing impairments. It should also be admitted that deaf sports should be seen as a phenomenon that impacts both the social and physical aspects of people with deafness and the hard of hearing.

To date, several qualitative analyses concerning the population of people with deafness or hardness of hearing have been undertaken [17,18,19]; however, it is difficult to find a systematic analysis that has analyzed the issue of deaf sports. Simultaneously, to the best of the authors’ knowledge, no study has revealed the main research trends and needs of the above-mentioned population within the sports context. Given the above and the gap in the current scientific literature, it seems justified to perform additional research in order to assess the variables that are related to the physical fitness and sports performance of deaf (D) and hard-of-hearing (HH) athletes.

Therefore, the aim of this systematic review was twofold (1): to identify the main trends and issues that are being addressed by researchers in the context of physical fitness and sports performance in D/HH athletes; and (2) to indicate the needs and future directions that should be implemented in the training process of athletes with hearing impairments. This may enable the enhancement of the sports performance of D/HH athletes mainly by reducing the number of barriers induced by the acquired or congenital impairment. It may also contribute to implementing in the training process the unique requirements that are associated with the disability.

2. Materials and Methods

2.1. Study Design

The methodology of this systematic review was planned according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [20] (see Supplementary File S1).

2.2. Inclusion and Exclusion Criteria

In this systematic review, inclusion criteria for studies were as follows: (a) cross-sectional study or comparative study (in order to determine athlete’s profile and not the impact of an intervention); (b) inborn/acquired hearing impairment; (c) hearing loss of at least 55 decibels (dB) in the better ear; (d) no cochlear implantation; (e) well-trained (training at least 3 times/week) athlete; (f) male and/or female athletes with hearing impairment ≥ 16 years old participating in sport competition; (g) no health condition except the hearing impairment. The exclusion criteria were as follows: (a) article type other than cross-sectional or comparative study; (b) physical, intellectual or visual impairment; (c) hearing loss of less than 55 dB in the better ear; (d) poor methodological design; (e) well-trained/elite athletes were not the study group (opinion of parents/society); (f) no full-text available; (g) manuscript not in English language; (h) male and/or female athletes with hearing impairment under 16 years old.

2.3. Literature Search Strategy and Study Selection Process

A search of electronic databases (PubMed, EBSCO, Scopus) was conducted by two authors (E.G., J.P.-T.) to identify all studies on physical fitness, sport performance and deaf/HH athletes from 2003 to 2024 in order to analyze the most current available scientific data. The following methods were used: (a) data mining; (b) data discovery and classification. As a prerequisite, all studies were performed in healthy populations including males and females (≥16 years). Search terms were combined by Boolean logic (AND/OR) in the PubMed, EBSCO and SCOPUS databases. The search was undertaken using the following seven keyword combinations in English with the assumed hierarchy of their importance: ‘deaf’; ‘sport’; ‘athlete’; ‘hearing impairment’; ‘inclusion’; ‘hearing loss’; ‘performance’. Furthermore, two researchers (E.G., J.P.-T.) with expertise in adapted physical activity and hearing impairment reviewed the reference lists of the included studies and screened Google Scholar to find additional research. Moreover, if a systematic review was identified, the aforementioned authors (E.G., J.P.-T.) screened the references list in each of the identified qualitative analysis in order to define the missing studies. If the crucial data were not included in the original articles, corresponding authors were contacted directly by one of the authors of this present study (E.G.).

2.4. Data Extraction and Data Items

In order to determine which variables should be extracted, two independent researchers (E.G., J.P.-T.) used the Joanna Briggs institute (JBI) Qualitative Data Extraction Tool for systematic reviews of qualitative evidence [21], which was consequently updated by them as part of an independent process. The above-mentioned form included the following domains: (a) basic data; (b) methodology; (c) methods; (d) phenomena of interest; (e) setting; (f) geography; (g) culture; (h) participants; (i) data analysis; (j) author’s conclusions; and (k) reviewers’ comments. A third independent author (J.S.-R.) also checked the data for accuracy and consistency.

2.5. Methodological Quality of Included Studies (Risk of Bias)

In order to evaluate the risk of bias of the included studies, the JBI Critical Appraisal Checklist [22] for analytical and comparative cross-sectional studies was used. According to the current scientific literature [22], the JBI checklist is believed to be the newest and simultaneously the most preferred tool that can be used in order to assess the methodological quality of the above-mentioned kind of research. The JBI checklist consists of 8 items, which were scored as follows: ‘Yes’, ‘No’, ‘Unsure’, ‘Not applicable’. If the manuscript criterion was fulfilled, a ‘Yes’ was assigned to the analyzed item, and it simultaneously received score of one. If a ‘No’, ‘Unsure’ or ‘Not applicable’ was assigned to the evaluated item, a zero score was yielded. If missing data were identified, a ‘No’ was assigned. Two independent investigators with expertise in deaf sport and adapted physical activity (J.P.-T., J.S.-R.) read and ranked each of the included articles. Furthermore, an independent co-author (E.G.) was designated to resolve all discrepancies that could occur among the investigators during the assessment. The methodological quality was indicated by the total score (out of a possible 8 points) with the higher values representing the better quality of the included publications.

2.6. Synthesis Methods

In order to fulfil the aims of this study, manuscripts were grouped in a table created by a single researcher (E.G.) according to the main issue that was being examined by the authors of the selected research. In addition, they were summarized according to the year of publication, study design, sample size, type of sport, the research tool/test used during the measurement, the main outcomes, and the main findings of the study, including statistical results (if applicable).

3. Results

Study Selection and Characteristics

Figure 1 presents the flow of the systematic review. A total of 87 full-text articles were assessed to determine eligibility (see

Table 1. The assessment of the methodological quality of the included studies (risk of bias) using the JBI method for cross-sectional study.

Number	Author	Q1	Q2	Q3	Q4	Q5	Q6	Q7	Q8	Sum
1.	Rochon et al. [23]	U	U	U	Y	Y	Y	U	Y	4/8
2.	Vujkov et al. [24]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
3.	Kurková et al. [25]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
4.	Güzel et al. [26]	N	Y	Y	Y	U	U	Y	Y	5/8
5.	Nemček and Mókušová [11]	N	Y	Y	Y	U	U	Y	Y	5/8
6.	Milašius et al. [27]	N	Y	Y	Y	U	N/A	Y	Y	5/8
7.	Soto-Rey et al. [28]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
8.	Karademir et al. [29]	U	Y	Y	Y	U	Y	Y	Y	6/8
9.	Brancaleone et al. [10]	U	Y	Y	Y	Y	Y	Y	Y	7/8
10.	Acak [30]	Y	U	Y	Y	Y	Y	Y	Y	7/8
11.	Uchida et al. [31]	U	U	Y	Y	Y	Y	Y	Y	6/8
12.	Okan [32]	U	Y	Y	N	Y	Y	U	Y	5/8
13.	Acak and Kaya [33]	Y	Y	Y	Y	U	U	Y	Y	6/8
14.	Ercan et al. [34]	U	Y	Y	Y	N	N	Y	Y	5/8
15.	Güzel et al. [35]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
16.	Szulc et al. [36]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
17.	Vuljanić et al. [37]	Y	Y	Y	Y	U	U	Y	Y	6/8
18.	Tatlici et al. [38]	U	Y	Y	Y	Y	Y	Y	Y	7/8
19.	Gümüs and Eler [39]	N	Y	Y	Y	Y	Y	Y	Y	7/8
20.	Neuls et al. [40]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
21.	Akınoğlu and Kocahan [41]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
22.	Klimešová et al. [42]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
23.	Nemček and Mókušová [4]	N	N	Y	Y	U	N	Y	Y	4/8
24.	Baranauskas et al. [43]	U	Y	Y	Y	Y	Y	Y	Y	7/8
25.	Buśko et al. [44]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
26.	Suner-Keklík et al. [45]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
27.	Cobanoglu et al. [46]	U	Y	Y	Y	Y	Y	Y	Y	7/8
28.	Makaraci et al. [47]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
29.	Makaraci et al. [48]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
30.	Güngör and Şahin [49]	U	Y	Y	Y	Y	Y	Y	Y	7/8
31.	Brancaleone et al. [50]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
32.	Soslu et al. [51]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
33.	Yapici et al. [52]	Y	Y	Y	Y	Y	Y	Y	Y	8/8
34.	Brancaleone et al. [53]	Y	Y	Y	Y	Y	Y	Y	Y	8/8

Q1—Were the criteria for inclusion in the sample clearly defined?; Q2—Were the study subjects and the setting described in detail?; Q3—Was the exposure measured in a valid and reliable way?; Q4—Were objective, standard criteria used for measurement of the condition?; Q5—Were confounding factors identified?; Q6—Were strategies to deal with confounding factors stated?; Q7—Were the outcomes measured in a valid and reliable way?; Q8—Was appropriate statistical analysis used?; Y—yes; N—no; U—unsure; NA—not applicable.

), while 34 studies met the inclusion criteria and were subjected to detailed analysis and assessment of their methodological quality (see

Table 2. The summary of the studies from 2003 to 2024 evaluating deaf sport, i.e., (I) sport performance; (II)—participation in deaf sport; (III) athlete–coach communication, (IV)—athlete’s quality of life and self-esteem.

I—Sport Performance of Athletes in Deaf Sport
Aerobic Capacity of Athletes with Hearing Impairment
Author	Participants Characteristics	Sport	Research Issue	Research Tool	Research Test	Main Findings
Vujkov et al. [24]	nP = 28 SG1 (deaf international handball players); nP = 13/age = 30.4 ± 9.1 SG2 (healthy national handball players); nP = 15/age = 26.2 ± 3.6	Handball	Comparison of the aerobic capacity between elite deaf and non-deaf players	Treadmill	VO2max test	No significant differences were noted in HRmax, HR at speed 12 km/h and VO2max between deaf and non-deaf players Lower values of HR (1st minute of recovery), oxygen pulse, absolute VO2max, pulmonary ventilation, tidal volume (p < 0.05) and maximal speed and speed at VO2max, (p < 0.01) were noted in deaf players
Güzel et al. [26]	nP = 59 SG1 (deaf 3rd division players); nP = 18/age = 23.2 ± 3.6 SG2 (deaf 1st division players); nP = 18/age = 24.6 ± 2.1 SG3 (deaf hearing handicapped players); nP = 23/age = 26.99 ± 4.88	Football	Comparison of VO2max between deaf players of different sport levels	Tanita HD 358 body composition (Tokyo, Japan) Portable telemetric heart pulse rate monitor (Polar RS 400 multi, Tempere, Finland)	Shuttle run test (VO2max)	Maximum running pace and VO2max differed between SG2 and SG3 (p < 0.01) SG1 was characterized by the highest heart beat rates (p < 0.01)
Milašius et al. [27]	nP = 36 SG1 (deaf basketball players in 2005); nP = 12/age = 28.2 ± 5.5 SG2 (deaf basketball players in 2009); nP = 12/age = 24.9 ± 4.8 SG3 (deaf basketball players in 2013); nP = 12/age = 28.0 ± 5.0	Basketball	Establishment and evaluation of body’s capacity and functional capacity of elite deaf basketball players from Deaflympic Games in 2005, 2009 and 2013	Stadiometer (Seca 225, GmbH & Co, Hamburg, Germany) Tanita BC418 MA (Tokyo, Japan) Electric reactiometer (Baltec Sport, Kaunas, Lithuania) Ergometer “Monark 894 E” (Vansbro, Sweden) Official FIBA game registration protocols	Step ergometry test Repeated anaerobic alactic working test Hexagon test	3.5 greater muscle mass was noted in SG3 (2013) compared to SG1 (2005) (p < 0.05) Deaf players (2013) were characterized by higher physical capacities compared to SG1 (2005) that was related with improvement of the quality of the game 49.2% improvement of goals precision was observed between deaf players (2005 vs. 2013) Higher number of defensive rebounds, fewer turnovers were found between 2005 and 2013 deaf players
Ercan et al. [34]	nP = 55 males SG1 (deaf players); nM = 30/age = 24.1 ± 3.21 CG (hearing players); nM = 22/ahe = 25.5 ± 3.03	Football	Determination of the morphologic findings and cardiac status of deaf players compared to hearing players	Tanita BC 418 MA III (Tokyo, Japan) Symptom limited treadmill (Tepa-TM-Pro 2000 Model, Tepa, Turkey) General Electric, Vivid 7 (New York, USA)	VO2max test Echocardiographic test	No significant differences were noted in HRmax, (1st minute of recovery) between the groups. Derived myocardial performance index was significantly decreased in deaf players (p < 0.05)
Neuls et al. [40]	nP = 74 males, ≥ 17 years SG1 (deaf soccer players); nM = 31 SG2 (non-deaf soccer players); nM = 43	Soccer	Comparison of performance-associated differences between deaf and non-deaf players	Soehnle 7307 (Backnang, Germany) Tanita BC-418 MA, Tanita, (Tokyo, Japan) Force platforms (AMTI OR6-7-1000, (Montreal, Canada) Treadmill (Lode Valiant, Groningen, the Netherlands), Oxygen mask Resting10-lead electrocardiogram (ECG) assessment (Delta 60D, Cardioline, Cavareno, Italy)	CMJ test Incremental running test	Deaf players were significantly shorter (p = 0.012), had higher BFP (p < 0.001), Ln rMSSD/RR (p = 0.006) and HRrest (p = 0.001) and lower FFM (p = 0.003), VO2max, (p < 0.001), and maximal power output (p < 0.001) compared to hearing players
Baranauskas et al. [43]	nP = 14 female/26.4 ± 4.5	Basketball	Evaluation of the body composition, physical working capacity, nutrition intake and blood parameters (iron, vitamin D)	BIA tetra-polar electrodes (Jinryang Industrial Complex, Kyungsan City, Korea) 7-day food recall survey questionnaire Cycle ergometer (Ergoline-Select 200, Bitz, Germany)	VO2max test PWC170 test	No differences were found for VO2peak and the low VO2peak, while PWC170 indicated on average aerobic fitness level Fat intake was found to exceed the maximum recommended content for athletes (p = 0.012), which was not found for carbohydrate and protein (minimum levels) Deaf players were characterized by deficits of vitamin D and iron in blood serum
Speed of Response in Athletes with Hearing Impairment
Soto-Rey et al. [28]	nP = 123; nF = 33, nM = 90 SG1 (hearing impairment athletes) nP = 44; nF = 14, nM = 25/age = 25.6 ± 5.0 SG2 (athletes without hearing impairment) nP = 79; nF = 19, nM = 57/age = 22.6 ± 3.7	N/A—team/individual	Differences in manual time of the speed of response to visual stimuli between hearing impairment and non-hearing impairment athletes	Record sheet SuperLab Pro software (version 2.0) Laptop ASUS Eee PC 1005 (10.1 inch display, Taipéi, Taiwán)	N/A	Hearing impairment athletes were characterized by a significantly shorter time of the speed of response compared to hearing athletes, with significant gender (shorter reaction time in male hearing impairment athletes) (p < 0.003) type of sport (individual) (p < 0.04) difference In each of the studied groups, faster RT was noted both in female and male athletes who competed compared to female and male athletes that did not participate in competition
Tatlici et al. [38]	nP = 18 SG1 (deaf athletes)/age = 22.55 ± 1.81 SG2 (hearing athletes)/age = 22.77 ± 1.85	Wrestling	Comparison of the speed of response time between deaf and non-deaf athletes	Electronic reaction time meter (Newtest 1000, Bayburt, Turkey)	Visual hand reaction test	No significant relationships were found between intra-group right- and left-hand speed of response levels (p > 0.05)
Güngör and Şahin [49]	nP = 42; nF = 20/age = 17.85 ± 1.38; nM = 22/age = 18 ± 1.19 SG1 (deaf athletes); nP = 22; nF = 11, nM = 11 SG (deaf non-athletes); nP = 20; nF = 9, nM = 11	Basketball	Determination of the mental rotation and speed of response performance in deaf athletes and deaf non-athletes based on sport and gender	Computer software MP36 (Biopac System, CA, USA)	Mental rotation test Reaction time test	No significant difference of mental rotation and the time of the speed of response variables were noted between deaf and non-deaf athletes Deaf male athletes were found to be better in the correct number (p < 0.03) of mental rotation and simple visual reaction variables (p < 0.03, p < 0.04) compared to deaf female non-athletes
Postural Control in Athletes with Hearing Impairment
Güzel et al. [35]	nP = 38 SG (deaf players)/age = 27.17 ± 4.64 CG1 (deaf controls)/age = 17.9 ± 0.88 CG2 (healthy controls)/age = 21.2 ± 2.86	Football	Determination of the postural control of deaf players and comparison with deaf sedentary control and normal hearing players	Biodex-Balance System (Biodex Medical Systems, Inc., Shirley, NY, USA)	Postural stability test (bilateral/lateral stance with eyes open)—static surface	Significant differences were found for MLI score of dominant and non-dominant legs (p = 0.027, p = 0.045) and for limits of stability testing in overall (p = 0.008) and right direction (p = 0.017) in all three groups Deaf athletes and deaf controls were characterized by a significant difference in MLN score of non-dominant leg (p = 0.012) and in right direction of limits of stability test (p = 0.016)
Cobanoglu et al. [46]	nP = 25 SG1 (deaf basketball players); nP = 12/age = 22.5 SG2 (hearing individuals); nP = 13/24	Basketball	Comparison of the static balance and knee proprioception between deaf elite players and hearing individuals	Biodex-BioSway Balance System (SD 950–340, Biodex Medical Systems, Inc., Shirley, NY, USA) Dualer IQ Digital Inclinometer (J-Tech Medical, Midvale, UT, USA)	Postural stability test Knee proprioception test	Postural stability was better in hearing individuals than in elite deaf players (p < 0.05) In deaf players, postural stability was different in terms of dominant (MLSL) and non- dominant side ( OSI, APSI, MLSL) (p < 0.05)
Makaraci et al. [47]	nP = 23 males SG1 (deaf basketball players), nM = 11/age = 25.18 ± 4.57 SG2 (deaf volleyball players), nM = 12/24.42 ± 4.42	Basketball, Volleyball	Assessment of the postural sway values in parallel and single-leg stance and indication of the differences between sport specializations	A force plate (Kistler, type 5691A, Winterthur, Switzerland)	Postural sway test (feet apart/single-leg stance—dominant/non-dominant leg) with open/closed eyes	Better results in parallel stance and dominant leg postural sway values were found in deaf volleyball players (p ≤ 0.05) No difference was identified between branches in non-dominant leg postural sway values Better values of postural sway were noted both for parallel- and single-leg stance with eyes opened in both groups compared to stances with eyes closed
Brancaleone et al. [50]	nP = 155 males, females/age = 18–30 SG1 (D/HH athletes); nP = 55; nF = 19, nM = 36/age = 20.62 ± 1.8 SG2 (hearing athletes); nP = 100; nF = 32, nM = 68/age = 20.11 ± 1.59	N/A	Determination of the effect of hearing status and stance condition on static postural control between D/HH and hearing athletes	A tri-axial force plate (Bertec FP4060, Bertec Corp., 110 Columbus, OH, USA)	mCTSIB/firm/foam surface—eyes open, firm/foam surface—eyes closed	Larger center of pressure velocity, medio-lateral amplitude of root-mean square and sway area were noted in D/HH athletes (p < 0.01)
Strength and Power in Athletes with Hearing Impairment
Szulc et al. [36]	nP = 55 females SG1 (deaf soccer players), nF = 20/age = 23.7 ± 5.0 SG2 (non-deaf soccer players); nF = 25/age = 20.3 ± 3.8	Soccer	Differences in body composition, strength and power of the lower limbs and height of jump between deaf and non-deaf players	Siber Hegen anthropometry (Switzerland) Gulick tape Tanita BC-418 MA (Tokyo, Japan) Biodex dynamometer (Biodex S4 Pro, BiodexMedical Systems, Inc., Shirley, New York, USA)	ACMJ test CMJ test SPJ test	No differences in body build and composition were found between the groups, except for WC, CC and WHR (p < 0.01, ES 0.24–0.79) Non-deaf soccer players generated greater MVC in knee joint flexion, torque and power for 300 degrees for both lower limbs (p < 0.01, ES 0.19–0.48) and in left-leg flexion for 180 degrees (p = 0.02, ES 0.13) Hearing impairment did not limit the opportunities for development of physical fitness in deaf women (statistical insignificance in ACMJ, CMJ)
Akınoğlu and Kocahan [41]	nP = 137/age = 23.99 ± 6.56 SG1—deaf female athletes; nF = 52 SG2—deaf male athletes/nM = 85	Karate, taekwondo, free style/Greco-Roman wrestling, running, basketball, judo, swimming	Determination of the health-related physical fitness parameters of deaf athletes	IsoMed 2000 (D. and R. Ferstl GmbH, Hemau, Germany) Human Body Equilibrium 360 device (HUBER360) Jamar hand dynamometer Tanita Body Composition Analyzer (TBF 300 M, Tokyo, Japan)	Static trunk flexion ET, flexion/extension ET, dynamic ET, lateral bridge test	Isokinetic muscle strength ratio was found to be 53–54% with 5% less for dominant/non-dominant ratio for all athletes In deaf male athletes anterior and posterior endurance of core muscles were lower than in deaf female athletes All athletes were characterized by a low level of grip strength and functional reach back/forward and a higher level of balance control with eyes opened than with eyes closed. A similar level of BMF% in able-bodied sedentary people was found in female athletes, while a slightly lower level of BMF% was noted in deaf male athletes
Buśko et al. [44]	nP = 29 females SG1 (deaf soccer players); nF = 13/age = 23.6 ± 4.3 SG2 (deaf non-players); nF = 16/age = 17.9 ± 2.8	Soccer	Difference in the physical fitness between deaf players and deaf non-players	Force plate (JBA Zb, Staniak, Poland)	EUROFIT test	Deaf players had significantly higher scores in jumping performance in ACMJ, CMJ, SPJ and higher lower limbs power in CMJ than deaf non-players (p < 0.05), except the relative power in ACMJ, SPJ and maximal power output in SPJ (no statistical significance). Deaf players were more physically fit (p < 0.001) than deaf non-players, except for bent-arm hang test
Suner-Keklík et al. [45]	nP = 26 males SG1 (deaf players)/age = 24 ± 3 SG2 (hearing players)/age = 22 ± 2	Basketball	Evaluation of shoulder muscle strength and endurance of deaf players and comparison with hearing players	Isokinetic dynamometer (Cybex NORM®, Humac, CA, USA)	Isokinetic test (internal and external rotators muscle strength)	No difference between internal and external rotation (Nm/kg) were observed in both groups (p > 0.05) Hearing players had higher external/internal rotation ratio (p = 0.017) In both groups, strength parameters of dominant extremities were higher than non-dominant extremity
Makaraci et al. [48]	nP = 26 males SG1 (deaf volleyball players), nM = 12/age = 24.25 ± 4.31 SG2 (non-deaf volleyball players)/age = 22.07 ± 3.2	Volleyball	Comparison of jumping performance with different jumping protocols (CMJ, SJ, DJ) between deaf and non-deaf players	A force plate (Kistler, type 5691A, Winterthur, Switzerland)/sampling frequency of 1000 Hz	CMJ test SJ test DJ test	Non-deaf players were characterized by better jumping performance in CMJ, SJ and DJ protocols than deaf athletes (p ≤ 0.05)
Soslu et al. [51]	nP = 51 males SG1 (deaf basketball players), nM = 11/age = 26.18 ± 4.79 SG2 (deaf volleyball players), nM = 12/age = 26.33 ± 4.27 SG3 (non-deaf basketball players), nM = 14/age = 26.93 ± 4.87 SG4 (non-deaf volleyball players), nM = 14/age = 24.93 ± 5.1	Basketball, volleyball	Comparison of the CJM performance between deaf and non-deaf players	A three-dimensional Kistler force plate (Type 5691A; Kistler, Winterthur, Switzerland)	Repetitive CMJ test	A decrease in the number of jumps, jump heights, the produced force, the acceleration at the time of jump and jump velocity were found in all athletes, however deaf athletes were more affected (p < 0.05)
Yapici et al. [52]	nP = 29 SG1 (hearing-impairment national team soccer players), nP = 13/age = 23.5 ± 3.1 SG2 (without hearing-impairment elite soccer players), nP = 16/age = 20.6 ± 1.4	Soccer	Comparison of agility, speed, jump and balance performance between deaf and non-deaf elite soccer players	Tanita Body Composition Analyzer (BC 418, Tokyo, Japan) Y-Balance test Brower Timing System (USA)—10/20/30 m Test	Y-balance test 10, 20, 30 m sprint test Illinois test 505 agility test Zigzag test CMJ test Agility and shooting test	Hearing impairment players were characterized by lower BFP (which was related with balance scores) and higher speed, agility skills and CMJ performance Balance skills were lower in hearing-impairment players compared to without hearing-impairment players Hearing impairment players have the potential to perform a similar/better level of sport performance
Body Composition, Nutrition and Concussion in Athletes with Hearing Impairment
Gümüs and Eler [39]	nP = 20 males/age = 27 ± 6.26	Handball	Evaluation of the physical and physiological characteristics of elite deaf players	Seca brand digital height gauge (Hamburg, Germany) Tanita Analyzer (BC-418, Tokyo, Japan) Force dynamometer (Takei grip-D, Tokyo, Japan) Newtest 2000 instrument Traffic obstacles	Handgrip strength test Visual reaction time test Vertical jump test Agility test	Deaf players were characterized by parallel body height, body mass, BMI, reaction speed, agility and body fat Flexibility (11.9 ± 5.22 cm) and jumping (30.1 ± 3.62–36.5 ± 4.45 cm) values were found to be lower than in hearing players
Klimesova et al. [42]	nP = 62 males/25.8 ± 8.3	Football, hockey, volleyball, futsal, running	Evaluation of the hydration status, the beverage intake and the differences between perceived adequate fluid intake and laboratory indicator of hydration status	Soehnle 7307 (Leifheit, Nassau, Germany) Tanita BC-418 MA, Tanita (Tokyo, Japan) USG A handheld refractometer (ATAGOSUR-NE, Tokyo, Japan) A self-report fluid consumption questionnaire	N/A	54% of deaf athletes—inadequate hydration 6% of deaf athletes—poor beverage consumption 2.2 ± 0.9 L (everyday life) 1.0 ± 0.5 L (training practice)– common fluid beverage intake
Brancaleone et al. [53]	nP = 162 deaf and hard-of-hearing athletes and able-bodied athletes	N/A	To examine the differences in knowledge of and attitudes toward concussion between athletes who are deaf and hard of hearing	The Rosenbaum Concussion Knowledge and Attitudes Survey	N/A	Athletes who are deaf/hard of hearing have poorer level of knowledge about concussion compared to hearing athletes
II—Participation in Deaf Sport
Kurková et al. [25]	nP = 53; nF = 21, nM = 32/age = 27.7	Alpine skiing, badminton, cycling, orienteering, swimming, tennis, table tennis	Participation in segregated or integrated sports	Self-made survey questionnaire	N/A	Elite-level deaf athletes would take the opportunity to compete with hearing athletes. Inclusive competing of hearing and deaf athletes plays an important role in social inclusion
Karademir [29]	nP = 135; nF = 32, nM = 103/age = 24.9 ± 4.43	Futsal	Social anxiety—fear of negative evaluation	FNE Questionnaire	N/A	Sport experience does not impact the level of fear of social negative evaluation of deaf athletes, which is higher in female deaf athletes
Vuljanić et al. [37]	nP = 31/nF = 11, nM = 20/age = 34.6	Athletics, bowling, chess, curling, handball, shooting, table tennis	Preferences of participation of deaf athletes in integrated/segregated competition	Survey questionnaire from Kurková et al. (2011)	N/A	29% of deaf athletes—preference for inclusive competition system 25% of deaf athletes—preference for separate competition system 19% of deaf athletes—do not have a preference
III—Athlete—Coach Communication in Deaf Sport
Rochon et al. [23]	nP = 95; n = 73 (hearing coaches), n = 22 (deaf athletes)	Football, hockey, water polo, basketball, field hockey, lacrosse, soccer, volleyball, baseball, softball, track, swimming, diving, bowling, wrestling, golf, tennis, badminton	Communication during training/competition between athlete and coach with ASM	Self-made survey questionnaire	N/A	Direct communication with ASM lessened the barriers between hearing coach/team mates and D/HH athletes and enhanced D/HH athlete’s sport performance (in athletes and coaches opinion)
Brancaleone et al. [50]	nP = 130; nF = 57, nM = 73/age = 18 < 25	N/A	Analysis of the communication patterns between D/HH athletes and trainers	Adapted 30-item questionnaire from Rochon et al. [23]	N/A	Deaf athletes were significantly likely to use ASL to communicate with trainer and during game play HH athletes were more likely to use oral communication with trainer and during game-play The subjective rate of communication between deaf athlete and trainer was assessed as ‘good’ The subjective rate of communication between HH athlete and trainer was assessed as ‘excellent’
Acak [30]	nP = 253; nF = 52, nM = 201/age = 18 to >30	Football, volleyball, basketball, handball	Evaluation of coaches’ professional skills in deaf athletes’ opinion	Coach Assessment Scale for Deaf Athletes	N/A	Female deaf athletes were more satisfied with their coaches compared to male athletes Level of education of deaf athletes impacted the evaluation of coaches’ skills Hearing coach or hearing coach with sign language skills were found to be the most preferred coach model in the opinion of deaf athletes
Okan [32]	nP = 125; nF = 16, nM = 106/age = 16 to >31	Football	Evaluation of the differences of deaf players’ perception of coaches with and without hearing impairment	Trainer Evaluation Scale for Deaf Athletes	N/A	Female deaf players had more positive evaluation of coaches’ skills compared to male deaf players Level of education of deaf athletes impacted on the evaluation of coaches’ technical sub-dimension skills (p < 0.05) Coaches with hearing aids were evaluated more negatively compared to hearing coaches (p < 0.05) Coaches who could communicate only in sign language were evaluated more negatively compared to oral communicative coaches (p < 0.05)
IV—Quality of Life and Self-Esteem of Athletes in Deaf Sport
Nemček and Mókušová [11]	nP = 152/nF = 48.7%, nM = 51.3% SG1 (competitive athletes)—18.4% SG2 (recreational sport participation)—9.9% SG3 (non-athletes—sedentary lifestyle)—71.7%	N/A	The impact of the level of sport participation on life satisfaction of athletes with hearing impairment	SQUALA Questionnaire	N/A	Athletes with hearing impairment showed higher subjective life satisfaction than non-athletes with hearing impairment
Uchida et al. [31]	nP = 112; nF = 46, nM = 66/age = 26.9 ± 7.42	N/A—Deaflympic and World Deaf Athletic Championships athletes	Intrapersonal and interpersonal aspects of deaf athletes’ self-esteem based on sport participation level	SPS Questionnaire, IDES Questionnaire	N/A	Elite-level deaf athletes had fewer negative relationships with others Dramatic sports experiences can improve the level of self-esteem in elite deaf athletes
Acak and Kaya [33]	nP = 95, males/age = 17 > 29	Football	Self-esteem of hearing impairment athletes in relation to adapted (deaf) football training	CSEI Questionnaire	N/A	The level of self-esteem differed in terms of impairment status No significant differences were noted between self-esteem status and age, sportive age, educational status and living-place variables
Nemček and Mókušová [4]	nP = 164 SG1(elite competitive-level athletes with hearing impairment) nP = 30/age = 22.2 ± 1.8 SG2—(recreational athletes), nP = 22/age = 25.2 ± 2.0 CG—(non- athletes), nP = 112/age = 26.8 ± 1.9	N/A—Deaflympic level athletes	The impact of the level of sport participation on life satisfaction of deaf athletes	SQUALA and WHOQOL Questionnaires	N/A	Sport participation followed by social relationships were found to be the predominant domains of the sense of quality of life

ABQ—Athlete Burnout Questionnaire; ACMJ—akimbo countermovement jump; APSI: Anteroposterior Stability Index; ASM—American Sign Language, BC—body circumference; BFM%—percentage of body fat mass; BFP—body fat percentage; BH—body height; BIA—bioelectrical impedance analysis; BMI—Body Mass Index; CC—calf circumference; CG—control group; CJM—countermovement jump; CSEI—Coopersmith Self-Esteem Inventory; D/HH—deaf/hard of hearing; DJ—drop jump; ES—effect size; ET—endurance test; FFM—fat-free mass; SPJ—spike jump; FNE—the Fear of Negative Evaluation Scale; HR—heart rate; IDES—Inventory of Dramatic Experience for Sport; mCTSIB—Modified Clinical Test of Sensory Interaction and Balance; MLI—medial–lateral index; MLSN—Mediolateral Stability Index; MPS—Multidimensional Perfectionism Scale; MVC—maximal voluntary contraction; N/A—not applicable; nF—number of females; nM- number of males; nP—number of participants; OSI—Overall Stability Index; PANAS—Positive and Negative After Schedule; RT—reaction time; SEBT—Star Excursion Balance Test; SES—Self-Esteem Scale; SG—study group; SJ—squat jump; SPS—Self-Positiveness Scale; SQUALA—Subjective Quality of Life Analysis; WC—waist circumference; WHOQOL—World Health Organization Quality of Life Questionnaire; WHR—waist-to-hip ratio.

).

Among 34 reports that had been assessed for their methodological quality, 16 were considered to score 8/8 points of eligibility to be included in the qualitative analysis. Seven articles were found to have 7/8 points of eligibility, four found were to have scored 6/8 points of eligibility, and five were assessed as scoring 5/8 points of eligibility. Moreover, two research studies were assessed as scoring 4/8 points of eligibility, which was the minimum score to be included into further analyses. The initial agreement of the two independent investigators (J.P-T., J.S.R.) was 90%. All discrepancies among the investigators were resolved by expert evaluation by an independent co-author (E.G.). Finally, 34 full-text articles were included in the systematic review (see Table 2).

4. Discussion

A careful examination of the current scientific data from the last two decades on the main trends and issues according to deaf sport and performance enhancement in D/HH athletes has indicated four main areas that are addressed by the researchers, i.e., (I) sport performance; (II) participation in deaf sport; (III) athlete–coach communication; and (IV) athlete’s quality of life and self-esteem. Moreover, this qualitative analysis found that the physical and physiological determinants of sport performance is a frequent topic analyzed in the scientific research. Interestingly, researchers focused not only on assessing the level of the cited variables in athletes with hearing impairments but also on the comparison of the analyzed variables with non-deaf athletes.

4.1. Sport Performance

Seven major variables can be identified according to the sport performance of D/HH athletes, namely (a) aerobic capacity, (b) reaction time, (c) postural control, (d) strength and power, (e) body composition, (f) nutrition and (g) concussion. The above-mentioned areas seem to be integrated; they can all impact on the athlete’s performance.

Among several factors that are listed in the domain of athletic performance, aerobic capacity assessed by VO2_max test on a treadmill was the most frequent (see Table 2). Interestingly, out of three studies that included both D/HH athletes and non-deaf athletes [24,34,40], only a single study indicated greater values of the VO2_max in the able-bodied group [40]. On the contrary, neither of the other two cited researches reported significant differences in the values of the VO2_max, while at the same time indicating even lower results in the HR at 1st minute of recovery, which suggests a higher level of aerobic capacity [24,34]. Moreover, the study conducted by Milašius et al. [27] reported a longitudinal change in the level of aerobic performance in deaf athletes. As was found in the cited study, deaf players over years were characterized by improvement in the aerobic capacity, which simultaneously impacted the results of their sport performance. In light of the similarities in the aerobic capacity between deaf and non-deaf athletes, it would seem logical that vestibular disfunction does not define the athletic performance in this field.

Another important motor ability that is strictly related to the athletic performance is strength and power. The analyzed studies indicated across a wide range of methods of assessment the strength and power performance; however, a force plate was the most common (see Table 2). Also interesting and partly surprising was that several authors [36,44,48] found better strength and/or power level in D/HH athletes compared to non-deaf athletes. On the contrary, only a single study conducted by Soslu et al. [51] found decreased power performance in deaf athletes compared to the non-deaf group. Thus, it seems justified to conclude that the level of strength and power is related primarily to the training volume, its intensity and the training methods used, while its effectiveness seems not to be related to the disability of the vestibular system.

On the other hand, postural control seems to be highly decreased in athletes with hearing impairment compared to those who are able-bodied (see Table 2). All of the analyzed studies indicated that the above-mentioned kind of disability impacted on the variables that are related to postural control [35,46,47,50]. This phenomenon can also be linked to the intrinsic differences that occur as a result of activation different models of sensorial compensatory mechanisms due to the vestibular impairment [5]. Nevertheless, future studies should investigate the direct effect of different visual and hearing perceptions in order to enable generalization.

Another motor ability that is crucial for high sport performance is the speed of response. Unfortunately, to the best of the authors’ knowledge, there are only three research studies that have undertaken this aspect in the scientific analyses (see Table 2). Moreover, the present systematic review performed here has yielded a partially inconsistent findings: two research studies conducted by Tatlici et al. [38] and Güngör and Şahin [49] did not find any significant differences between D/HH athletes; however, the study performed by Soto-Rey et al. [28] found that athletes with hearing impairment are characterized by a shorter speed of response compared to those who are non-deaf. Based on the cited research, this phenomenon can be explained mainly by the type of sport practiced (team/individual). Moreover, male athletes seem to have a shorter speed of response compared to female athletes [28].

4.2. Participation in Deaf Sport

To the best of the authors’ knowledge, the current available scientific studies in the context of participation in deaf sport are limited to three research studies (see Table 2); therefore, it is difficult to confirm the holistic perspective of strategies that are undertaken according to sport participation in the case of D/HH athletes. At the same time, it is difficult to confirm definitively the connection between the ways of participating and their impact on an athlete’s general fitness. Nevertheless, social inclusion seems to be the most important aspect that is related to the sport participation in this population [25,37]. For instance, the research conducted by Kurková et al. [25] pointed to the importance of inclusive competition between D/HH athletes and hearing athletes. This may be seen as a factor that can be related to the improvement of the sport performance in the population of athletes with hearing impairment due to enhancement of the individual level of sport-specific skills and motor abilities as a consequence of competing with non-impairment athletes. Secondly, this aspect may also be seen by D/HH athletes as a motivation for self-improvement of their physical fitness and athletic skill in order to be able to win against those who are not impaired. Partially similar findings were observed in the research by Vuljanic et al. [37], which were indicated on the inclusive system of competing in two aspects: (a) between deaf and hearing athletes, and (b) between deaf athletes seen as a separate group that is competing in an environment in which everyone is equal. However, further studies are needed to extend the perception of inclusion in the population of D/HH athletes.

4.3. Athlete Coach Communication

Undoubtedly, adequate communication between athlete and coach is a pillar for efficient cooperation. This issue is even more important in the case of D/HH athletes, in whom vestibular dysfunction is related to an intrinsic barrier to direct communication. All of the analyzed studies that are related to the above-mentioned issue (see Table 2) were consistent with regard to good communication with the coach being necessary in order to achieve high athletic performance [10,23,30,32]; however, some specific variables should be taken into consideration, including (a) the type of coach, (b) the gender of the coach/athlete and (c) the type of communication between the athlete and coach.

Based on the cited studies, it seems that hearing coaches (especially those with sign language skills) are more preferred by D/HH athletes compared to those who have hearing impairment [30,32]. Surprisingly, female athletes with disability of the vestibular system seem to have better communication compared to males [30,32]. Moreover, both sign language and oral communication seem to be received positively by the analyzed groups of athletes [10,23]; however, oral communication is likely more preferable for those who are HH [10].

4.4. Quality of Life and Self-Esteem

Another domain that is explored by multiple researchers within the context of sport performance of D/HH athletes is the quality of life and self-esteem. At the same time, as in the previously described areas of research, this domain is also limited to a small number of studies (see Table 2). Nevertheless, the researchers reached similar conclusions with regard to the relationship between sport performance and the athlete’s self-esteem and quality of life in that they seem to be intrinsically related.

With that in mind, this systematic review found that to be identified as a representative of athletes with hearing impairment is the predominant variable that decides on the quality of everyday life [4] and on the improvement in everyday satisfaction [11]. Moreover, the need to overcome obstacles, self-limits and barriers during competitions and training seems to impact on the level of self-esteem of D/HH individuals [31]. Interestingly, age and sport-specific experience seem not to be related to the level of self-esteem [33], although it must be noted that the main limitation of this topic is the limited number of research studies; thus, there is a need for the use of different surveys and protocols that should be implemented in order to provide wider scientific evidence.

4.5. Limitations

The presented qualitative analysis has several limitations that must be addressed. Even though the systematic review included two decades of available scientific research, there remain few data regarding the analyzed areas of research. On the other hand, this review simultaneously indicated the current trends in deaf sport, which enables the indication to be made of the directions that should be implemented in the future. Secondly, the analyzed data were related to various kinds of sports in which the studies’ protocols used different research tools and methods of assessment, which could be related to the different conclusions and inconsistency in the findings. Lastly, we have assessed only cross-sectional and comparative studies; thus, there may also be some experimental research that could contribute to the issue of sport performance in the context of deaf sport. However, in order to fully investigate this issue, further studies are needed.

5. Conclusions

The presented systematic review of the results of published scientific literature, on evidence, suggests that D and HH athletes tend to be similarly physically fit as hearing athletes. Moreover, it seems that hearing impairment is not a barrier in the development of selected groups of motor abilities, including aerobic capacity, muscular strength and power or speed of reaction. At the same time, multiple social and psychological factors seem to be associated with the sport performance of D and HH athletes, among which social inclusion and quality of life and self-esteem are the most common.

This systematic review also found a shortage of references in the sport sciences literature; thus, the population of deaf athletes presents an opportunity for novelty and originality in the field. Furthermore, inclusion in sport participation and specific tools (i.e., communication aids) appear to be crucial factors in their performance enhancement. In addition, specific features of this population and the activities influenced by their impairment when practicing sport appear as important areas to develop in the future (i.e., reaction time, postural control).