One of the important challenges of developing athletic potential is to find nonchemical and economically viable ergogenic advantages for improving strength and power, as well as performance. It is interesting to note that during strength and power tasks, it is common for athletes to clench the jaw, develop muscular tension in the face and neck, and activate core muscles via a modified Valsalva maneuver, presumably to accrue an ergogenic advantage. The theoretical basis for this potential advantage has been indirectly formed, in part, via studies of the effects of mandibular orthopedic repositioning appliances (MORA), the Jendrassik maneuver (JM), remote voluntary contractions (RVC), and motor overflow. Collectively, this research suggests the possibility of an ergogenic effect of some of these practices, including jaw clenching. A recent review of the literature suggests that RVC may manifest as a potentiation phenomenon specifically termed concurrent activation potentiation (CAP), which may be useful to enhance training and, possibly, the performance of some acute strength and power tasks in sport (9).
Historically, investigators believed that oral devices such as the MORA, which aligned the temporal mandibular joint, as well as biting, had an effect on strength (10,19,27). On the other hand, a number of studies failed to find any ergogenic effect of using the MORA (1,20,28), and some have been critical of the experimental design of those studies that did (11). Thus, these studies offer little conclusive evidence that jaw clenching offers an ergogenic benefit, but they have raised questions about the possibility.
The potential increases in strength associated with jaw clenching may also be explained by the JM (29). During the JM, neurologically impaired patients clench their teeth, hook together their flexed fingers, and attempt to pull their hands apart. This action results in increased strength of reflexes, often measured clinically by assessing deep tendon reflexes (29). Several studies have assessed the effect of the JM, consistently demonstrating a positive relationship between the JM and the outcome variables assessed, including the H reflex, T reflex, motor evoked potentials, electromyography, force, rate of force development (RFD), and strength (3-6,12,21,22,25,29). Interestingly, even small muscle group contractions, such as those associated with mastication, increase the H reflex in the pretibial and soleus muscles, which is thought to be developmentally important for stabilization of postural stance. In addition to mastication, previous studies in which subjects performed jaw clenching demonstrated increased H reflex activity (22) as well as 170% greater motor evoked potentials during the performance of the JM compared with baseline (25). Ultimately, the motor functions of the jaw have been functionally related to lower-body muscle groups (21).
Remote voluntary contractions may be useful for increasing the H reflex. Acutely, the H reflex increases concurrently with RVC and has been demonstrated to increase by approximately 20% after eight sets of 10 repetitions of resistance training exercise. Consequently, the H reflex demonstrates both CAP and postactivation potentiation responses (26).
Another possible explanation for why RVCs potentiate the prime mover centers around the idea that intercorticol communication, or motor overflow, occurs between motor areas of the brain, and from one cerebral hemisphere to another. These cortical mechanisms are explained by theories including the cortical connection theory and the transcallosal facilitation hypothesis (9). In other words, when one part of the motor cortex is active, as would occur during an RVC, connections to other areas of the motor cortex are affected. Although there are many possible explanations for this occurrence, one is functional synergy, such as would be required between the upper and lower body during locomotion. Explanations may also be evolutionary or developmentally based, such as the need to eat during locomotion, or to grasp and bite at the same time. The M I motor area is responsible for control of muscle force and movement direction and contains functional subdivisions of the face, arms, and legs. These divisions are overlapping, interconnected, and plastic in their reorganizational response to motor learning (7).
Only one study specifically measured the effect of RVCs such as jaw clenching on RFD, using a test of grip strength. In this case, jaw clenching resulted in a 12.1% improvement in RFD compared with the non-jaw clench condition. Furthermore, maximum RFD with teeth clenched before, as well as before and during the test of grip strength, was 8.5 and 15.8% greater, respectively, compared with grip strength when the teeth were not clenched (15). No previous study has assessed the effect of RVCs such as jaw clenching on the performance of athletic tasks such as jumping. Therefore, the purpose of this study is to investigate the existence of the CAP phenomenon by assessing the effect of jaw clenching to determine whether this practice influences RFD, time to peak force (TTPF), and peak force (PF) during the countermovement jump.
Experimental Approach to the Problem
This study evaluates the hypothesis that maximal jaw clenching concurrent with the countermovement jump would have an ergogenic effect on the RFD, TTPF, and PF during the performance of this jump. Independent variables include the jaw clench (JAW) and non-jaw clench (NON-JAW) conditions. Dependent variables include the RFD, TTPF, and the PF during the countermovement jump.
Fourteen men's and women's (20.20 ± 2.10 years) NCAA Division II track and field athletes served as subjects. All subjects were familiar with plyometric training. The subjects were informed of the risks associated with the study and provided written consent. The study was approved by the institution's internal review board.
Before performing the experimental measurements, subjects were familiarized with the test procedures during a pretest orientation session, including jumping while clenching the jaw maximally on a mouthguard as well as jumping with an open mouth. Subjects refrained from participating in resistance and plyometric training for at least 48 hours before participation in the study. During the test session, subjects warmed up for 5 minutes on a bicycle ergometer, and they performed dynamic stretching lasting approximately 12 seconds for each major muscle group. Ten warm-up jumps were conducted, followed by 5 minutes of rest before the test countermovement jumps. This warm-up was consistent with previously published warm-up protocols (17). During the test, subjects performed the JAW and NON-JAW maximal countermovement jumps on a force plate in a counterbalanced order. During the JAW condition, subjects were instructed to attain their maximum voluntary isometric contraction of their jaw muscles by biting down maximally on a mouthguard during the concentric phase of their countermovement jump. During the NON-JAW condition, subjects were instructed to jump with their mouth open, to avoid jaw clenching. In both test conditions, subjects were instructed to jump as high as they could and were encouraged to reach overhead to raise the vertical position of the center of mass as high as possible at takeoff, because this technique is thought to enhance jump height (23). Additionally, subjects were instructed to reach overhead because it is more specific to a variety of sports than standardizing hand placement, such as on the hips. Other aspects of jump technique, such as the depth and duration of the countermovement, as well as the quickness of the concentric phase of the jump, were not standardized, because this study sought to determine whether jaw clenching affected the RFD, TTPF, and PF.
Instrumentation and Data Processing
The countermovement jumps were performed by taking off from and landing on a 50.8 × 46.4-cm force plate (OR6-5, Advanced Mechanical Technologies, Inc., Watertown, Mass). Kinetic data were collected at 1000 Hz, real-time displayed, and saved with the use of computer software (NetForce 2.0, Advanced Mechanical Technologies, Inc.) for later analysis. Ground reaction force data were collected for the sample period and were used to calculate variables from the vertical force components (Fz). The force plate was calibrated with known loads to the voltage recorded before the testing session. Ground reaction force data were collected and used to calculate the RFD from the vertical force components of the force-time record during each countermovement jump. The force plate was zeroed before subjects stepped on. Rate of force development for the countermovement jump was derived as the difference in the force-time record from the point of peak concentric force minus the onset of the concentric phase, divided by time and expressed as newtons per second. Concentric force production was deemed to begin when the vertical ground reaction force exceeded body mass as measured by the force plate. Time to peak force was determined by subtracting the time of the beginning of the concentric muscle action from the point at which the subjects reached their peak concentric force, using the force-time records. Peak force was measured as the maximal value attained during the concentric portion of the countermovement jump, based on the force-time record. This analysis of data was consistent with previously published methods (2,17).
Data are presented as mean values ± SD and were evaluated with SPSS 15.0 using paired-sample t-tests to compare differences between the JAW and NON-JAW conditions. An a priori alpha level was set at p ≤ 0.05.
The RFD was 3362.31 ± 1449.59 and 2706.66 ± 1476 N·s−1 for the JAW and NON-JAW conditions, respectively, representing a statistically significant difference between groups (p = 0.049). Time to peak force was also significantly different between the JAW and NON-JAW conditions (p = 0.016). The TTPF was 308.07 ± 123.5 and 385.79 ± 166.6 milliseconds for the JAW and NON-JAW conditions, respectively. Figures 1 and 2 demonstrate examples of force-time records and the differences in RFD and TTPF during the NON-JAW and JAW conditions, respectively. No significant difference (p = 0.60) in PF was found between the JAW (884.17 ± 268.89 N) and NON-JAW (869.96 ± 242.44 N) conditions.
This is the first study to investigate the effect of RVCs such as jaw clenching on the performance of an athletic task such as jumping. Results indicate that jaw clenching produces an ergogenic effect with respect to jumping ability, as evidenced by enhanced RFD and TTPF. These findings are consistent with previously published recommendations describing the potential CAP phenomenon associated with jaw clenching (9). Only one previous study has investigated the effect of jaw clenching on RFD, specifically examining its effect on grip strength in a nonathletic population; that study found that jaw clenching increased grip strength by approximately 15.8% (15). In the present study, jaw clenching improved the RFD during the countermovement jump by an average of approximately 19.5%. Previous reports indicate that jaw clenching before, as well as during grip strength tests, resulted in acute strength improvements (15). In fact, some evidence suggests that jaw clenching alone may be ideal, because it facilitated more soleus H reflex than the combination jaw clenching and RVCs of other parts of the body, such as hand clenching (21). Soleus H reflex activity seems to be related to jaw clenching force level, possibly because of afferent input from oral motor structures and descending influence from the higher brain (21). Thus, subjects in the present study were encouraged to clench their jaws maximally in the JAW condition. Results of this study also suggest that lower-body muscle groups in addition to the soleus are influenced by jaw clenching.
Though the mechanisms by which the RFD and TTPF were enhanced though jaw clenching were not the focus of the present study, a number of explanations are possible. The possibilities include enhanced H reflex activity, excitatory cortical projections and motor overflow, increases in the activity of the alpha motor neuron activity, gamma loop, muscle spindle, and descending cortical input, as well as stimulus-invoked afferent input resulting in inhibition of the presynaptic inhibition, and postsynaptic changes in membrane potential. Some evidence indicates that the JM does not operate by direct facilitation of the alpha motor neuron, as demonstrated in a study of ischemic muscle that resulted in the blocking of the alpha motor neuron, despite the fact that the H reflex was still facilitated (13). Similarly, the gamma loop may have a limited role in the JM as well, as reported by Bussel et al. (4), who determined that the facilitation effect of the JM remains even if Ia afferent nerve fibers are blocked by ischemia and that spindle activation cannot affect the alpha motor neurons, thereby concluding that the JM is not routed via the gamma loop. Presynaptic modulation of the H reflex has been confirmed (29), and Hortobagyi et al. (16) report that the muscle spindle and cutaneous afferents, are not the likely contributors to the JM-induced increase in H reflex, maintaining that there is likely a cortical and segmental component. Presynaptic (inhibition of presynaptic inhibition) and postsynaptic changes in motor neuron membranes potential are possible explanations (8). According to Dowman and Wolpaw (8), H reflex amplitude facilitation increased in the absence of an effect on soleus EMG, and thus the H reflex change was not caused by an increase in excitatory input into the soleus motorneuron pool. These researchers suggest a change in a stimulus-evoked afferent input, at some point proximal to the muscle spindle such as presynaptic inhibition or change in motor neuron pool resistance.
Previous studies have examined the TTPF of trained subjects during the performance of Olympic weightlifting exercises, with mean values ranging from 291 ± 25.7 milliseconds (14) to 397.1 ± 42.5 milliseconds (18). In fact, previous unpublished data from the authors' laboratory indicate that for NCAA Division I athletes, TTPF was attained in 303 ± 49.5 milliseconds. In the present study, the TTPF during the NON-JAW condition of 385.79 ± 166.56 milliseconds was at the less optimal end of the range of previously reported TTPF values (18). On the other hand, in the present study, the TTPF during the JAW condition (308.07 ± 123.52 milliseconds) was similar to the more optimal values previously presented in the literature evaluating the time course of RFD. Thus, with respect to TTPF, jaw clenching improved the ability of this group of NCAA Division II track athletes to perform more similarly to NCAA Division I athletes.
At present, the evidence suggests that the potentiation effect of the JM is not a result of changes in alpha motor neuron, gamma loop, or muscle spindle activation. The JM effect is most likely attributable to a combination of cortical influence was well as inhibition of the presynaptic inhibition, mediated by stimulus-evoked afferent input, as well as changes in postsynaptic membrane potential that seem to be manifested in the RFD without affecting PF. Future research in this area should also examine the mechanisms underlying the CAP phenomenon. Subjects in this study clenched their jaws maximally on the mouth guard, which was consistent with previous recommendations (9). However, future studies should attempt to quantify the magnitude of the jaw clenching and control the degree of jaw activation in non-jaw clench conditions.
Finally, it is not known whether frequent forceful muscle activations of the jaw muscles and their effects on the temporomandibular joints and oral structures such as teeth and gums will result in potential injury to those structures. However, the etiology of jaw problems such as temporomandibular joint trauma includes acute injury as well as chronic psychodynamic stress resulting in chronic neuromuscular tension (24). It is unlikely that the brief jaw clenching associated with the concentric phase of the jumping will have a negative effect on the development of chronic neuromuscular tension. Questions remain regarding the practice of jaw clenching and its potential for causing acute trauma to oral structures, or whether maximal isometric contractions of these muscles and their effects on the oral structures are well tolerated and lead to adaptations, as other muscles, joints, and bones adapt to mechanical stress.
Clenching the jaw enhances the RFD and TTPF during the countermovement jump. Therefore, during jump training and the performance of athletic events that involve countermovement jumps, athletes may obtain an ergogenic advantage from clenching the jaw maximally during the concentric portion of the jump. This practice may involve the use of a mouthguard, as performed in this study. Coaches are encouraged to instruct their athletes to perform this type of RVC and assess the effect. It is possible that jaw clenching provides an ergogenic effect during other high-muscular power activities such as throwing events in track and field.
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Keywords:© 2008 National Strength and Conditioning Association
Jendrassik maneuver; remote voluntary contractions; motor control; plyometrics