## 4 Discussion
### 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 1 st minute of recovery, which suggests a higher level of aerobic capacity [24; 34]. Moreover, the study conducted by [PERSON] 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 distinction 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 [PERSON] 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 [PERSON] et al. [38] and [PERSON] and [PERSON] [49] did not find any significant differences between D/HH athletes; however, the study performed by [PERSON] et al. [28] found that athletes with hearing impairment are characterized by ashorter 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].