Moreover, a piece of resin was made for each subject in a SAM articulator to increase the VDO. They were arbitrarily stuck on tooth 46 with the help of a flow composite, creating an artificial occlusal interference of 1 mm.
Isokinetic assessment was performed to evaluate quadriceps and hamstring peak torque of the dominant knee using a Cybex Norm dynamometer (Henley Healthcare, Sugar Land, TX, USA) Asking the subject to kick a ball identified leg dominance. The subject was seated on the dynamometer (with 105° of coxofemoral flexion). The knee to be tested was placed on the knee flexion extension plate of the isokinetic device, according to the manufacturer's instructions for isolating knee flexion and knee extension. The length of the dynamometer was adapted to the length of the leg of each subject. Several straps around the thigh, waist, and chest stabilized the body to avoid compensation (Figure 3). The range of knee motion was fixed at 100° of flexion from the active maximum extension. The gravitational factor of the dynamometer's lever arm and lower leg-segment ensemble was compensated for during the measurements. A 10-minute warm-up at a light intensity (75–100 W) on a cycle ergometer and stretching exercises of the hamstring and quadriceps muscles preceded all measurements. The modalities of the isokinetic protocol were inspired from a previous study, which assessed quadriceps and hamstrings performances (10). An adequate familiarization with the dynamometer was provided in the form of further warm-up isokinetic repetitions in the concentric mode at the angular speed of 120°·s−1. The protocol included concentric (angular speeds of 60 and 240°·s−1) and eccentric exertions (angular speeds of 30°·s−1) of both flexor and quadriceps muscles groups. Before assessment, 3 preliminary submaximal repetitions routinely preceded each test speed. All sets of testing were separated by 1 minute of rest. Subjects performed the tests under 3 conditions: in maximum intercuspidation without a splint, using the balanced splint, and using the piece of resin that created an imbalanced occlusion. The order of testing for these conditions was randomly determined, and all tests were performed on the same day (30 minutes between each test). The same experimenter performed all the tests while a second experimenter ensured that the mouth remained closed during the isokinetic assessment. Subjects were instructed to give 100% effort and received oral encouragement (but no visual feedback).
To compare the influence of dental occlusion on knee muscular performances, it was tempting to select the same isokinetic angular velocity for both modes of contraction. Unfortunately, at high eccentric velocities, the period of constant velocity, expressed as a percentage of the whole range of motion, appears drastically reduced (21). Moreover, the eccentric peak torque occurs at the final part of the movement, corresponding to a deceleration period. These limitations could have led to erroneous results. Therefore, we decided to select the 30°·s−1 velocity because low speed is frequently recommended for optimizing familiarization conditions with eccentric exercise. On the other hand, the isokinetic eccentric torque-velocity curve in humans appears to remain essentially constant (9).
Statistical analyses were carried out using Statistica 7.1 (StatSoft, Inc., Paris, France). Results are expressed as mean ± SD. A repeated-measures analysis of variance design was used to assess the statistical significance of differences between mean values of the different conditions. The Newman-Keuls post hoc test was used to compare the 3 experimental conditions. Differences were considered significant at p ≤ 0.05.
The results for all 3 strength measurement conditions are shown in Figure 4. Whatever the muscle group and jaw position, the highest peak torques were observed in eccentric mode (30°·s−1) and the lowest values at fast velocity (240°·s−1) in the concentric mode. For each isokinetic testing modality, the quadriceps torque development was higher in comparison with the hamstrings. Concentric performances did not show any significant difference between the 3 jaw conditions, irrespective of the tested muscle group (p > 0.05). In contrast, significant differences (p ≤ 0.05) were observed in the eccentric trials related to quadriceps performance between the resin condition and the 2 other modalities (without splint or with a balanced splint). The imbalanced occlusion created by the resin component corresponded to an average decrease of 9% in eccentric peak torque. Eight subjects of the 12 demonstrated this decrease in the resin modality compared with the balanced splint modality (Figure 5). The eccentric hamstrings peak torques also showed a significant difference (p ≤ 0.05) between measurements with balanced splint and with resin (7% decrease when occlusion was imbalanced). The decrease occurred for 9 of the 12 subjects (Figure 6).
A unique finding of the present study was that artificial imbalanced occlusion induced immediate alteration of eccentric muscle performances of both quadriceps and hamstring muscle groups. To our best knowledge, no previous studies have investigated the relationship between dental occlusion and eccentric analytical performances of the lower limbs. A secondary finding was that the balanced splint did not improve strength either for the concentric or the eccentric modalities.
Mouthguards are essential for preventing dental and maxillofacial injuries while participating in contact sports. Interestingly, some athletes wear occlusal splints or mouthguards to optimize their athletic performance irrespective of their role in preventing injury.
Accordingly, to determine whether there is a relationship between dental occlusion and the performance of the lower limbs, it seemed necessary to include objective measures of muscle strength. Isokinetic exercise represents a match between mechanically imposed velocity and the subject's movement (e.g., knee extension). The reliability of isokinetic testing has been measured repeatedly and found to be high (6,22). This tool is frequently considered a gold standard for analytical muscle strength measurements (14). Moreover, the isokinetic dynamometer made it possible to assess concentric and eccentric peak torque at different velocities (14).
The results of the present study failed to demonstrate improved strength of quadriceps and hamstring as a result of wearing a balanced splint. This is in complete accordance with previously reported research (7,28,36). Williams et al. (36) found no statistical difference between 3 mandibular positions on the isokinetic performance of leg extensors and flexors in 23 athletes. Schubert et al. (28) failed to find statistically significant differences in shoulder abduction, shoulder adduction, knee flexion, and knee extension strength whether athletes were wearing a MORA or not. In taekwondo athletes, Cetin et al. (7) reported that hamstring isokinetic peak torque significantly increased as a result of wearing custom-made mouthguards. However, no significant differences were found for quadriceps strength. Schubert et al. (28) suggested that the increase of concentric performance of subjects wearing a balanced splint reported by other authors may be the result of the placebo effect or dubious research design. In a recent balanced, randomized, placebo-controlled, within-group crossover investigation, Dunn-Lewis et al. (13) showed that a power balance performance upper teeth mouthguard improves performance of upper-body loaded power exercises in both men and women athletes. However, women athletes did not improve lower-body power performance by using mouthguards (power balance performance upper teeth mouthguard and off the shelf boil-and-bite upper teeth mouthguard).
Interestingly, the present study showed that artificial imbalanced occlusion induced immediate alteration of eccentric muscle performances of quadriceps and hamstring muscle groups. These eccentric contractions are involved in many athletic movements in various sports, sometimes in high intensity. This contraction mode is unique because muscles are capable of achieving higher absolute peak torque when contracting eccentrically as compared with contracting concentrically or isometrically (16,33). In the present study, the imbalanced occlusion created by the resin component correlated with an average decrease of 8 and 10%, respectively, in eccentric quadriceps peak torque compared with balanced splint or no splint conditions. An average decrease of 7% was reported in eccentric hamstring peak torques when occlusion was imbalanced compared with the balanced splint condition. However, it is important to note that there was some individual variability, and not every subject showed a decrease in strength when the occlusion was imbalanced. For hamstrings, 75% of the subjects showed a decrease in strength when the occlusion was imbalanced. The decreases in eccentric quadriceps peak torque concerned, respectively, 60% (imbalanced occlusion vs. no splint condition) and 67% (imbalanced occlusion vs. balanced condition) of our subjects.
Though the mechanisms involved in the alteration of eccentric performance were not the focus of the present study, some theories about these mechanisms may help explain our results. One explanation could arise from the specific patterns and mechanisms involved in this specific contraction mode. Eccentric contraction involves the development of tension while the muscle is being lengthened. Several neurophysiological and performance differences have been reported between concentric and eccentric contractions (11,16). Force generated during muscle lengthening should exceed maximum isometric or concentric force (34). However, during in vivo maximal voluntary activation of human skeletal muscle, maximal eccentric moment remains occasionally equal to or less than isometric (3,33). Moreover, the expected increase in eccentric torque with increasing velocity is frequently absent (9,16). To explain some discrepancies between in vivo and in vitro force-velocity eccentric relationships, some authors have suggested that central and peripheral neural factors intervene, causing the inhibition of force development (33,34). The greater moment exerted during eccentric (compared with concentric) activations when the muscle is electrically stimulated supports this hypothesis (3,35). But the exact mechanisms responsible for this inhibition remain indeterminate. Motoneuron activation is modulated by inhibitory Ib afferent feedback from Golgi organs and excitatory Ia and group II afferent feedback from muscle spindles (18). Therefore, some authors have suggested that the Ib afferents from the Golgi organs are responsible for the decrease of the α-motoneuron excitability during eccentric contraction (1,35). For Duclay and Martin (12), depressed Hoffmann reflexes (H-reflex), present during lengthening actions, are mainly attributable to presynaptic inhibition of Ia afferents. However, it is reasonable to consider whether other mechanisms or the effectiveness of the peripheral and central motor drive could act to reduce the voluntary activation level during maximal eccentric contractions.
Several authors have investigated the effects of motor activity in the jaw on general motor behavior (5,15,24,31). Miyahara et al. (24) demonstrated that the human soleus H-reflex is markedly facilitated when associated with voluntary teeth clenching in proportion to biting force. Takada et al. (31) showed that voluntary teeth clenching evokes a nonreciprocal facilitation of ankle extensor and flexor muscles and attenuated reciprocal Ia inhibition from the pretibial muscles to the soleus muscle. Moreover, Ebben et al. (15) suggested that, in addition to the soleus, lower-body muscle groups are influenced by jaw clenching.
On the basis of the studies that showed modulation of somatic motor function in association with teeth clenching, we postulate that an inadequate occlusal relationship may modify information coming from the orofacial region and neural centers and lead to perturbations of motor units recruitment strategies during maximal eccentric efforts.
Regarding the clinical significance of these findings, the decrease in strength with imbalanced occlusion compared with the other modalities remained relatively minor (decrease of 9 and 7% in quadriceps and hamstring eccentric peak torque, respectively) and concerned only maximal eccentric performances. Nonetheless, this difference could be important in competitive sports, where final rankings are determined by small margins. Findings in the present study corresponded to analytical muscle performances while sports activity requires whole body movements. Results could be different during functional dynamic tests involving several groups of muscles working across multiple joints. Further studies would clarify the implications of our findings in sports training and performance, particularly for athletes wearing protective mouthguards.
Although it seems illusory to revolutionize the quality of muscle strength by using an intraoral appliance, the findings of this study have practical applications. The present study is the first to provide evidence suggesting that an artificial imbalanced occlusion induced immediate and significant alteration of eccentric analytical muscle performances of the lower limb (decrease of 9 and 7% in quadriceps and hamstring eccentric peak torque, respectively).
Therefore, it is recommended that high-level athletes, especially athletes involved in a sport requiring maximal eccentric performance, have an occlusion examination on a regular and frequent basis. Moreover, the examination should be repeated every time an odontology therapy has been carried out.
Regarding athletes wearing dental protection, this study provides support for the evidence that a correctly balanced appliance has no negative effects on strength performance. Nevertheless, if the mouthguard integrates a new dental occlusion, it is recommended that the athlete has an individual assessment using appropriate clinical and instrumental methods. Strength tests should be planned to assess muscular performance with and without the dental protection. Moreover, it is recommended that the athlete wears the mouthguard before competition to allow time for muscular accommodations.
The authors would like to thank the participants who took part in the study. The authors also thank Mrs. A. Depaifve for her kind technical assistance. The authors disclose no conflicts of interest. This research received no specific funding.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
occlusal appliance; imbalanced occlusion; eccentric performance; isokinetic assessment; quadriceps; hamstring