In a prospective, randomized, crossover design, we investigated the effects on craniofacial muscle activation during maximum-bite measurements under resting conditions, as well as muscle activation under dynamic and static requirements, while a customized mouthguard (CMG) was worn compared to the procedure with no mouthguard (Co). Muscular stimulation, the activation distribution, and motor performance were assessed.
Our study was divided into two aspects (a rest examination and stress examination). The rest examination (RE) took place on two different days. At their first examination appointment (RE1- Serves only for measurement preparation), subjects were instructed about the study protocol and informed consent was obtained. A dental impression and bite registration was taken from the subjects to prepare the CMG.
The second examination appointment (RE2) consisted of a maximum bite measurement via the SINFOMED K7 system (SinfoMed, Frechen, Germany). In RE2, we measured the neuromuscular activities of the masseter and temporalis muscles (sEMG) during a block randomized maximum bite measurement with and without a mouthguard. No familiarization time for the mouthguards was provided for the examinations.
During the following exercise tests (SE), the single-leg stand (SLS), countermovement jump with arms (CMVJa), and kettlebell swing (KBS) were quasi-randomized for the block randomized conditions (CMG and without CMG). The test participants are professional athletes, and all the performed exercises are part of their training. These two test days (SE1/SE2) were exactly 24 h apart for each subject. Figure 1 shows the study design of the examinations.
Maximum Bite Measurement
The myofunctional examination was randomly done with the SinfoMed K7 system (SinfoMed, Frechen, Germany). It measures neuromuscular activity using bipolar surface electrodes. For all tests, subjects were placed in a chair and required to assume a natural, upright, and relaxed position without head or neck support. The sEMG activities of the temporalis and masseter muscles were recorded bilaterally. The sEMG activity was recorded under the condition of maximum voluntary clenching (MVC) in intercuspal position. The subjects had to bite explosive and as hard as they could for two seconds. A 30-s break between the 2-s maximum pressing phases was maintained between clenches.
Exercise Measurement
At the start of each measurement appointment (SE1/SE2), the athletes were instructed in a standardized process, and electrode marks were placed at anatomically defined fixed points [23]. The EMG electrodes were affixed on the person and fixed with cohesive conforming bandages (Peha-haft, PAUL HARTMANN AG, Germany). The EMG signals were recorded from the masseter, sternocleidomastoid, erector spinae lumbar (L4), and rectus femoris muscle groups. Electrode placement is based on recommendations for surface EMG to assess a non-invasive assessment of muscles tone (SENIAM) [23]. Study participants were instructed not to remove the markings that had been made with a skin marker. Figure 2 shows our subjects’ EMG preparation. They all signed a consent form that permits us to publish the photographic material in a journal.
One-Legged Balance Test
Each subject completed two tests. They did the one-legged balance test on their spurring leg. Between tests, subjects rested for two minutes. At the start of the test, they had to firmly grasp their hips with their hands (Fig. 3). The one-stand leg had to be flexed at a 45° knee angle throughout the test. The angle was determined with a goniometer and marked with a skin marker in the back of the knee. The joint gap was palpated for the horizontal marker position, and the center of the horizontal marker was selected for the vertical marker. Figure 3 shows that the marker was used to check the execution standard with a cross-line laser (Bosch cross-line laser Quigo, Robert Bosch GmbH, Germany). The cross-line laser was located behind the person and was used for correction when the person left the defined position. Our balance measurement data were assessed with the Posturomed and the corresponding software (BIOSWING MircoSwing V.5.0, HAIDER BIOSWING GmbH, Germany). To determine balance ability, a device-automated score was displayed between 0 and 1000 points. 1000 points represent the highest possible score in the posturocybernetics test. The score is determined by the PC software based on the distance covered by the platform. Figure 3 illustrates standardized measurements on a Posturomed.
Kettlebell Swing Test
When performing the kettlebell swing, subjects did two rounds of 15 repetitions each. They were instructed to perform the exercise at maximum speed and with a clean performance quality. They were given a three-minute rest between the two test runs. A fixed turning point in the exercise was defined by reaching shoulder height and swinging through the legs. A 16-kg cast iron kettlebell (Color Kettlebells—Vinyl, Gorilla Sports, Germany) was used. To measure the maximum and average force in watts (W), an accelerometer (Beast Sensor, Beast Technologies S.r.l., Italy) was magnetically attached to the kettlebell and fixed with tape. All data were analyzed automatically. Only repetitions 6 to 10 from each trial were included in our data analysis. Figure 4 shows the execution of the kettlebell swing.
Counter Movement Jump
Jump height (cm) during the countermovement jump was measured via a high-speed force transducer (Achillex Jumpn'run, Xybermind GmbH, Germany). Subjects completed a total of three jumps with the intention of a maximum jump in vertical direction. In doing so, they were to actively use their arms as swinging elements. During the initial swing movement, subjects were instructed to avoid a long reversal phase to ensure fluid movement execution. The jump height was measured in centimeters. These data were evaluated using the appropriate software (Humotion Software, Xybermind GmbH, Germany). The test with the highest jump height was included in our calculation. Figure 5 shows the execution of a countermovement jump.
EMG
The muscular activation force and activation distribution were measured electromyographically (Ultium EMG System, Noraxon, USA). Eight wireless Ultium EMG sensors (EMG sensor, Noraxon, USA) use a 24 bit and a sampling rate of up to 4000 Hz. The signal was sent directly from the point of origin to the Ultium EMG receiver via direct-function wireless technology. Noraxon Dual EMG electrodes (EMG electrodes, Noraxon, USA) with a 10 mm diameter at a 20 mm distance between the electrodes was used to record muscle activity. The experiments were filmed with a time-synchronized USB camera (CX405 Handycam EXMOR CMOS SensorThus, SONY, Japan) so that the EMG signal could be allocated precisely to the motion execution.
Data Processing and Statistical Analysis
Raw data were processed using Noraxon MyoMuscle software (myoRESEARCH, Noraxon, USA). For the measurement with the EMG from Noraxon, rough values were processed using a high-pass filter with a high-pass frequency of 15 Hz. Subsequently, all signals were rectified and the curves digitally smoothened. The root mean square algorithm was used for 50 ms for this purpose. Amplitudes were then normalized to the mean. All data are expressed as mean value and standard deviation (SD). Data were tested for normal distribution using the Kolmogorov–Smirnov test.
The Wilcoxon rank test was used to compare group differences with the CMG and application without it. A p value < 0.05 was considered significant. All values are presented as means with standard deviation. GraphPad Prism 8 (GraphPad Software Inc., California, USA) was used for statistical analysis.
To evaluate muscular activation characteristics, the bilateral mean values of the respective muscles were recorded and presented as the sum mean value (Σ). Muscles were recorded bilaterally, and differences between right and left muscles were designated as activation symmetry (Δ).