Mouthguards (MG) have possible effects on performance and breathing. Due to the use of MG some studies reported increases in aerobic capacity, or decreases in cardio-respiratory parameters (Bailey et al., 2015; Garner, 2015; Piero et al., 2015). Other studies found no relevant effects (Bourdin et al., 2006; Collares et al., 2014; Golem et al., 2017). However, previous studies are inconsistent with respect to their designs and with regard to the special requirements of team ball sports. For these reasons, we would like to use this letter to the editor to report new results investigating cardiorespiratory parameters without and with a mouthguard measured in a highly specific basketball course.

We recruited ten professional male first league basketball players (age: 25.1 ± 4.6 years, height: 1.99 ± 0.1 m, weight: 99.6 ± 14.4 kg) who were free of injuries, and any orthopedic, metabolic, or cardiorespiratory diseases. The subjects completed a basketball specific course (BSC) at medium and high load with and without a "vented" boil-and-bite “Nike adult max intake” mouthguard (Nike, Beaverton, OR, USA; VentMG) in a randomized order. The tests were performed on one day. The highly specific basketball course included all relevant elements of a game based on the statistics of the National Basketball League (NBA, season 2013/2014; Schulze et al., 2017). The running distance between the course elements was set according to the average mileage of the players during a game. An audio signal paced the duration of rounds and breaks. The target speed was set by marks on the ground, which had to be reached in the moment of the signal. According to pre-tests, the measuring error was less than 1 second per round. The mean test duration was 14 minutes. The medium intensity load (Test 1) corresponded to the average of the NBA games. For the high intensity level (Test 2), the intensities were increased by 20%, and breaks reduced by 24%. Each test consisted of four runs (run 1-4 and run 5-7), and each run consisted of two rounds (Table 1). Heart rate (HR), oxygen uptake (VO₂), ventilation (VE), tidal volume (VT), breathing rate (BR), carbon dioxide output (VCO₂) were all recorded continuously. The spiroergometric data (K4b², Cosmed, Italy) were measured breath-by-breath. The VO₂ and VCO₂ values were calculated from the end-expiratory gas concentrations and ventilation. The ventilation values were calculated as the product of BR and VT. The "medium" and "high" values were calculated from the load times of the tests. All “maximum” values were calculated for the last 10 seconds of Test 2. The blood sampling for lactate measurements (20 μl) were done prior to the warm-up, immediately after each test, and during each break.

Analysis of the results (Table 2) by paired t-tests revealed that the VO₂ values were significantly higher by approximately 6% in high and maximum load with the VentMG. In contrast, VE was 5% to 7% lower in high and maximum load. Corresponding to the lower ventilation, the end-expiratory O₂ and CO₂ difference was increased. All these results correspond to the longer inspiration and expiration times. The share of anaerobic energy expenditure was significantly reduced because Lac and RQ were both lower. The lower maximum lactate values corresponded to the increase in VO₂ (Table 2). The metabolic rate corresponding to the VO₂ values during maximum load was 30.8 ± 5.7 kcal (NoMG) and 32.6 ± 5.5 kcal (VentMG) (p<0.002). Total (aerobic and anaerobic) metabolic rate was 32.3 ± 5.2 kcal and 33.1 ± 6.5 kcal, indicating that VentMG caused no relevant additional energy consumption. The peak exercise values could not be reached since the exercise intensity was limited by the course protocol. However, the VO₂ and VE values clearly show that the energy conversion efficiency was not affected by the VentMG, and that the breathing economy was even improved. The exercise ventilation is a marker of the muscle excitation and the resulting respiratory drive via C3/4 afferent nerves. Therefore, the VentMG may have induced decreased muscle strain and muscle reflexes to the respiratory center (Busse et al., 1991). A further explanation could be the better stabilization of the jaws by the VentMG, causing an improved movement economy. Thereby jaw clenching may support the whole-body stabilization in addition to the core muscles. A resulting lower local muscle activity would also be consistent with the lower lactate values and decreased RQ.

In conclusion the maximum load was not affected by the mouthguard, investigated in a basketball-specific exercise course in first league athletes. The lower ventilation for given VO₂ values with mouthguard use may be an effect of the improved biomechanics. However, further studies are needed to evaluate the cause and effect relationship, e.g. the effects of concurrent potentiation activation due to modified remote voluntary contractions by different modes of jaw clenching.