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Task-dependence of Jaw Elevator and Depressor Co-activationDepartment of Prosthodontics, University Dental Clinic, University of Erlangen-Nürnberg, Glückstrasse 11, D 91054 Erlangen, Germany; Correspondence: * corresponding author, peter.proeschel{at}rzmail.uni-erlangen.de
Elevator muscle activity per unit bite-force has been shown to be higher in chewing than in isometric biting. We tested the hypothesis that surplus elevator activity is evoked in response to a possible co-activation of jaw-opener muscles during the masticatory power stroke. In 32 subjects, digastric and bilateral masseter and temporalis activities were recorded during unilateral chewing of test foods, isometric biting on a force transducer, and during balancing of the jaw against maximum effort of depressor muscles. During elevator peak effort in chewing, the digastric activity was 113% higher than during peak effort in isometric biting. Comparison of balancing and chewing trials revealed that a 6% increase of elevator activity would suffice to compensate for this increased depressor action. Elevator activity in chewing, however, was up to 130% higher than in clenching. We conclude that depressor counteraction could have only a minor influence on the generation of surplus muscle activity in chewing.
Key Words: antagonistic co-activation • masticatory muscles • EMG • force-estimation
Estimates of chewing force from electromyograms (EMG) calibrated in isometric biting on force transducers (Hagberg, 1987; Slagter et al., 1993; Tate et al., 1994; Proeschel and Raum, 2001) proved to exceed actual masticatory loads (Schindler et al., 1998; Proeschel and Morneburg, 2002). This is due to a dependence of activity/bite-force relations on the conditions under which muscles contract. Recent studies found higher elevator activities when a bite force was generated in a dynamic contraction like chewing than when the same force was applied in isometric clenching on a bite-fork (Proeschel and Morneburg, 2002). The physiological cause of a higher activity per unit bite-force in mastication is obscure. Among other factors, a co-activation of depressor muscles could possibly influence activity/force characteristics of the elevators. If depressors would counteract the jaw-closing force during the masticatory power stroke, the central nervous system might increase elevator activity to maintain the net occlusal load (Pruim et al., 1978; Miles and Madigan, 1983). Co-contraction of antagonistic jaw muscles is supposed to support load reduction in case of unexpected occlusal disturbance (Grillner, 1972; Miles and Wilkinson, 1982; Gibbs et al., 1984) or to smoothe and stabilize chewing movements (Moyers, 1950; Carlsöö, 1956; Winnberg and Pancherz, 1983; Gibbs et al., 1984). Since isometric clenching on a bite-fork involves no jaw movements or unexpected disturbances, a possible depressor co-activation in this task might be different than in chewing. The aim of this study, therefore, was to compare elevator and depressor co-activation in natural chewing and in isometric biting on a transducer and to determine the amount of elevator muscle activity required to compensate for the action of jaw-opener muscles in both motor tasks.
Subjects Thirty-two dental students (mean age 25 ± 2 yrs; 22 males, ten females) with complete dentitions and no temporomandibular disorders volunteered for this study after having given informed consent. The experimental protocol was approved by the ethics committee of our medical faculty. Electric activities of right and left masseter and anterior temporal muscles were recorded by means of bipolar Ag/AgCl surface electrodes (Hellige, Freiburg, Germany) with 2-cm electrode distance. For the depressor activity, a smaller bipolar electrode with 6-mm electrode distance was attached to the anterior belly of the right digastric muscle in the anterior submental region. Prior to electrode attachment, the skin was degreased with alcohol and rubbed with grinding paper to reduce impedance.
Experimental Procedures
Evaluation of Data and Statistics
Elevator Muscle Activities In unilateral biting (Fig. 1  ), the masseter activity on the working side (113 ± 47 µV) was slightly lower (p < 0.01) than on the balancing side (130 ± 61 µV). In contrast, the temporalis activity on the working side (109 ± 37 µV) was higher (p < 0.01) than on the balancing side (70 ± 36 µV). The mean biting force amounted to 278 N ± 51 N. In the chewing of winegum (Fig. 1), all elevator activities were higher than in isometric biting. The working side masseter activity was 130% higher than in biting (p < 0.01), while the balancing side activity was enhanced by only about 30%. The temporalis activities were enhanced by 130% on the balancing side (p < 0.01) and by 70% on the working side (p < 0.01), respectively. In the chewing of bread, a similar combination of elevator activities was found; however, the levels were lower than with the winegum bolus. When the jaw was balanced against maximum depressor effort (Fig. 1), elevator activities were symmetric with respect to the sides. Masseter muscles thereby developed a mean activity of 94 ± 59 µV, while temporalis muscles produced 31 ± 32 µV. In 90% of all balancing contractions, jaw gapes did not subside below 0.3 mm, indicating that no occlusal contacts occurred.
Digastric Muscle Activities In the chewing and biting trials, digastric activities were low compared with elevator activities and ranged between about 9 and 20 µV (Fig. 1  ). In the balancing trial, the digastric muscle produced a considerably higher mean activity of 87 ± 36 µV. In unilateral biting, rise or decay of digastric activity occurred randomly and was not related to the loading or unloading phases of the biting cycles. The minimum EMG in the average biting cycle was 5.2 ± 2.2 µV (Fig. 2). During elevator peak effort, the digastric muscle reached 8.9 ± 3.1 µV. Maximum digastric activity occurred randomly before or after the peak force and amounted to 11.6 ± 3.6 µV. In chewing, digastric activity was significantly higher in all phases than in isometric biting (Fig. 2). With winegum, the minimum level was 8.4 ± 3 µV. At the instant of elevator peak effort, 18.7 ± 9.5 µV was reached. The maximum digastric activity amounted to 39.2 ± 11 µV (Fig. 2). With bread, quite similar levels (8.9, 19.8, and 40 µV) were obtained (Fig. 2). In contrast to isometric biting, the digastric activity in chewing was systematically related to the phases of the chewing cycles: The minimum activity always occurred during jaw closing, while the maximum EMG appeared during opening.
This study compared jaw-opener activation during elevator peak effort in chewing of natural foods and in isometric biting on a bite-fork. These tasks were examined because they are used for estimation of chewing force from masticatory EMGs calibrated in isometric biting (Hagberg, 1987; Slagter et al., 1993; Pröschel et al., 1994; Tate et al., 1994). The digastric muscle was selected to represent the depressor group, since its activity was supposed to be proportional to the jaw-opening force (Ahlgren et al., 1978; Winnberg and Pancherz, 1983; Pancherz et al., 1986). Even though several studies were concerned with digastric EMGs, the respective results could not easily be applied to answer the present problem. Most investigations examined either chewing (Winnberg and Pancherz, 1983) or isometric biting (Pruim et al., 1978; Gibbs et al., 1984; Pancherz et al., 1986; Van Willigen et al., 1993). When both tasks were performed in one investigation, biting was done in modes different from those used in our study (Carlsöö, 1956; Munro, 1974; Koole et al., 1991). In isometric transducer biting, the digastric signal was randomly fluctuating. There was no significant digastric co-contraction at the instant of elevator peak effort. This is in contrast to other studies, however, in which biting was performed with the expectation of a sudden yield of resistance (Miles and Madigan, 1983; Van Willigen et al., 1993). Depressor activation in chewing exceeded activation in clenching in all phases of the contraction cycle. In particular, during elevator peak effort, the digastric activity in chewing was 113% higher than in isometric biting. Since this was accompanied by elevator activity enhancements of up to 130%, one might conclude that the latter were in fact evoked to compensate for depressor co-activation, as was hypothesized. However, similar activity enhancements of agonistic and antagonistic muscles do not automatically imply similar force enhancements in both muscle groups. From the information gathered in the various motor tasks, an attempt is made in the following to estimate the underlying load conditions: In jaw balancing, elevator activities of 93 µV (masseter) and 32 µV (temporalis) compensated for the maximum digastric activity of 84 µV. In chewing, the digastric EMG during elevator peak effort reached just about 20% of that maximum possible value, while, in transducer clenching, 13% was attained. Hence, the digastric action in chewing or in clenching could, in principle, be compensated for by 20% or 13% of the activity generated by the elevator muscles in jaw balancing. For the masseter, this would correspond to 19 µV in mastication (20% of 93 µV) or 12 µV in isometric biting (13% of 93 µV), and for the temporalis, 6.4 µV (20% of 32 µV) or 4 µV (13% of 32 µV), respectively. Thus, an increase of masseter activity by 7 µV (= 19 µV – 12 µV) and of temporalis activity by 2.4 µV would suffice to compensate for the 113% increase of digastric activity in chewing. Related to the activities of isometric biting (masseter, 113 µV; temporalis, 109 µV), this corresponds to an increase of masseter activity by 6% and of temporalis activity by 2.2%. Despite its approximate character, this estimation indicates that depressor co-contraction could have only a minor influence on the production of additional elevator activity in chewing. When a 6% enhancement of elevator activity could compensate for the higher depressor counteraction in chewing, the observed enhancements of up to 130% must have other causes. The higher masticatory activity could suggest a higher bite force in chewing. A proportional extrapolation based on the enhanced activity, however, yielded chewing forces with unrealistic magnitudes (Proeschel and Raum, 2001). Moreover, activity combinations very similar to those of the present study were found to be accompanied by about equal forces in chewing and isometric biting (Proeschel and Morneburg, 2002). The same investigation finally demonstrated quite different activity/force relations in both motor tasks. The present study shows that depressor co-activation makes no significant contribution to this phenomenon. Therefore, other features—like different jaw gapes (Manns and Spreng, 1977) and contraction dynamics (Abbink et al., 1999) or the overcoming of jaw inertia in mastication—had to be examined. Even though the digastric co-activation during elevator peak effort was significantly higher in chewing than in isometric biting, it was still small compared with the maximum possible level of digastric activity. One may speculate about the physiological meaning of a small digastric co-activation during chewing. Some authors have argued that even low activities could stiffen the muscles (Miles and Wilkinson, 1982; Miles and Madigan, 1983), providing a mechanism for immediate response to sudden disturbances in forceful biting (Grillner, 1972). This conception is based on experiments in which isometric biting was performed with considerable jaw gapes (Pruim et al., 1978; Miles and Madigan, 1983; Van Willigen et al., 1993). In chewing, however, the jaw gape during elevator peak effort is just about 0.5 mm (Proeschel and Raum, 2001), which is too small for the assumed protecting mechanism to become effective (Van Willigen et al., 1997). So if the observed increase of digastric activity would contribute to muscle stiffening at all, this may have a stabilizing rather than a protective function (Carlsöö, 1956). Targeted arm movements, for instance, were shown to be stabilized against disturbance by control of impedance achieved by co-activation of antagonistic muscle groups (Burdet et al., 2001). The situation in chewing is similar, since the teeth had to be guided into a stable intercuspal position despite possible aberration by the bolus. The increasing digastric activity during elevator peak effort might therefore reflect increased impedance of the elevator/depressor group, in contrast to isometric biting, where no motion of the mandible had to be stabilized.
The investigation was supported by the Wilhelm-Sander-Foundation, Germany, Grant No. 86.027.1. The authors thank Mrs. Jean Rickwood for editing the manuscript in English. Received for publication November 18, 2002. Revision received March 27, 2003. Accepted for publication May 23, 2003.
Journal of Dental Research, Vol. 82, No. 8,
617-620 (2003)
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