Optimization of jaw muscle activity and fine motor control during repeated biting tasks

ObjectiveTo investigate if repeated holding and splitting of food morsel change the variability of force and jaw muscle activity in participants with natural dentition.MethodsTwenty healthy volunteers (mean age = 26.2 ± 3.9 years) participated in a single session divided into six series. Each series consisted of ten trials of a standardized behavioural task (total 60 trials) involving holding and splitting a flat-faced tablet (8 mm, 180 mg) placed on a bite force transducer with the anterior teeth. The hold and split forces along with the electromyographic (EMG) activity of the left and right masseter (MAL and MAR), left anterior temporalis (TAL) and digastric (DIG) muscles were recorded. A series (ten trials) of natural biting tasks was also performed before and after the six series of the behavioural task.ResultsThe mean hold force (P < 0.001) but not the mean split force (P = 0.590) showed significant effect of number of series. No significant effect of series was seen on the variability of hold and split force and the EMG activity except for the variability of EMG activity for MAL during the hold phase (P = 0.021) and DIG during the split phase (P < 0.001). The behavioural task had no effect on the EMG activity of the natural biting task.ConclusionThere was no evident optimization of jaw motor function in terms of reduction in the variability of bite force values and muscle activity, when this simple task was repeated up to sixty times in participants with normal intact periodontium.     

Motor control of jaw muscles in chewing and in isometric biting with graded narrowing of jaw gape

When a certain bite force is applied during unilateral chewing, the combination of jaw elevator muscle activities is different than when a comparable force is applied in unilateral isometric biting, e.g. on a force transducer. Masticatory peak force is generated in a nearly isometric phase of the chewing cycle, with a jaw gape of about 1 mm. In contrast, peak force in isometric biting on force measuring equipment usually induces jaw gapes of 6 mm or even more. Therefore, we tested the hypothesis that the jaw gape influences relative activation of elevator muscles in unilateral isometric biting. We further examined whether such influence could explain the different activity combinations of chewing and isometric biting. In thirty asymptomatic males, masseter and temporalis activities were recorded during intermittent isometric biting with jaw gapes of 6, 5, 3, 2 and 1 mm and during unilateral chewing. Activity combinations were described by working/balancing ratios and by temporalis/masseter ratios. With decreasing jaw gape the working/balancing ratio of the posterior temporalis decreased (P < 0.002) while that of the masseter increased (P < 0.001). Likewise, the temporalis/masseter ratio on the balancing side increased (P < 0.001). With decreasing jaw gape, activity ratios of isometric biting approached ratios of chewing. We conclude that: (i) relative jaw muscle activation in isometric biting depends on the jaw gape, (ii) relative muscle activation in chewing resembles relative activation of isometric biting with a small 'chewing-like' gape. This suggests that characteristic activity combinations in chewing are mainly a result of the approximately isometric contraction during the slow closing phase of the chewing cycle.     

Progressive Jaw Muscle Fatigue and Electromyogram Activity Produced by Isometric Unilateral Biting

The purpose of this article is to report on the duration of bite force required during unilateral biting to produce the onset of subjective pain-discomfort in 30 healthy adult female subjects. A custom-designed strain gauge scale, or bite force meter, was constructed for the measurement of bicuspid/molar bite-force.

There is an apparent association between clenching and bruxing to induce pain-discomfort in the masseter muscles. The authors found that the pain-discomfort occurred on the contralateral side during unilateral biting significantly more often (P <0.01) than on the ipsilateral side.     

The effect of stimulus intensity and gape on electrically-evoked jaw reflexes in man

The changes in masseter EMG and biting force evoked by electrical stimulation of the lip were measured in normal people. Stimulation at noxious intensities elicited a characteristic, biphasic inhibitory response in the masseter EMG and a transient reduction in the biting force. The pattern of masseter inhibition and force change varied both with stimulus intensity and with inter-incisal separation. In general, the decrease in biting force was greater at intermediate jaw positions than in the open and closed positions. The duration of the EMG inhibitory period increased as the stimulus intensity increased but was independent of the jaw separation. Reflex thresholds could not be correlated with pain thresholds because subjects found it difficult to decide at what level an electrical stimulus became painful. Nevertheless this stimulation of high-threshold afferents may be useful in estimating the rate of relaxation of tension in the jaw muscles.     

Deep and superficial masseter muscle blood flow in women

Purpose: Although changes in blood perfusion have been described as being associated with temporomandibular disorder (TMD) myofascial pain, very little is known about blood flow levels in the deep and superficial masseter muscle. This study investigated blood flow in deep and superficial sites of six healthy female particpants at baseline and during intermittent and continuous biting exercises and recovery. Materials and Methods: Blood flow was monitored unilaterally using a single-fiber probe laser Doppler flowmeter. The blood flow was continuously monitored at baseline and during two biting exercises: (a) intermittent at 25%, 50%, and 100% maximum voluntary bite force for 30 seconds each followed by 90 seconds rest between each biting level and (b) continuous biting at similar maximum voluntary bite force levels followed by 90 seconds rest. Results: There was significantly higher blood flow in the deep sites compared to the superficial sites (p < 0.001) and a significant increase in blood flow during biting compared to baseline (p < 0.001). There were no significant changes in blood flow among the three levels of biting, between the intermittent and continuous exercises, or from baseline blood flow compared to recovery. Conclusions: This study showed regional differences in masseter muscle blood flow, perhaps related to differences in muscle fiber type and pattern of muscle fiber recruitment.     

EMG-force relationships are influenced by experimental jaw-muscle pain

The bite force at different levels and the corresponding electromyographic (EMG) activity of the masseter and anterior temporalis muscles were recorded in 12 healthy subjects in order to evaluate the modulation of EMG-force curves by a standardized painful stimulus. Hypertonic saline (5%) was infused into the right masseter muscle for up to 15 min to induce pain. The pain intensity was scored continuously by the subjects on a 10-cm visual analogue scale (VAS). Subjects were asked to bite on a force transducer at the maximum voluntary bite force (MVBF). They were then asked to bite at submaximum levels of 12, 25, 37, 50, 67, 75 and 87% of MVBF. The biting was performed in three different positions (right first molar, left first molar and incisor) before, during and after infusion. Hypertonic saline caused moderate pain during infusion (mean VAS +/- s.e.m. = 6.5+/-0.5 cm). Both the MVBF and the maximum EMG activity in the right masseter and the left anterior temporalis muscles were significantly decreased during muscle pain when the subjects bit on the painful side. The EMG-force curves could be fit by linear relationships. The slope of the curve became less steep in the right masseter muscle during and after painful biting in every position. The results suggest that tonic saline-induced jaw-muscle pain is able to modulate the motor unit recruitment pattern of the jaw-closing muscles on the painful side. The main effect of pain in this experiment was an inhibition of static EMG activity.     

Effect of biting speed and jaw separation on force used to incise food

Biting food too quickly might affect the control of jaw-closing muscles and the estimation of bite force. The objectives of this study were to compare the incisal bite forces used to cut food and the activity of masseter (MA) and anterior temporalis (AT) muscles between slow, habitual and fast biting speeds and also between small and large jaw openings. Twenty subjects were asked to use their incisors to cut through a 5 mm thick of chewing gum. In the first experiment, subjects bit at 10-mm incisal separation with slow, habitual and fast biting speeds, and in the second experiment, subjects bit with their habitual speed at 10- and 30-mm incisal separations. The activities in the MA and AT muscles were assessed with surface electromyography, and the bite force was recorded by a force sensor placed beneath the chewing gum. Peak bite forces and associated MA amplitudes were increased significantly as biting speed was increased (P's < .05). Anterior temporalis amplitude was significantly increased during fast biting compared to slow and habitual biting (P's < .001). At 30-mm incisal separation, both peak bite force and AT amplitude were significantly increased, whereas MA amplitude was significantly decreased, compared to those at 10-mm separation (P's < .05). Biting off food quickly with incisor teeth results in larger activities in both MA and AT muscles. In addition, biting a large piece of food resulted in increased activity of AT muscle. Both conditions could be injury stimulator for jaw muscles.     

Small vertical changes in jaw relation affect motor unit recruitment in the masseter

Strategies for recruitment of masseter muscle motor units (MUs), provoked by constant bite force, for different vertical jaw relations have not previously been investigated. The objective of this study was to analyse the effect of small changes in vertical jaw relation on MU recruitment behaviour in different regions of the masseter during feedback‐controlled submaximum biting tasks. Twenty healthy subjects (mean age: 24·6 ± 2·4 years) were involved in the investigation. Intra‐muscular electromyographic (EMG) activity of the right masseter was recorded in different regions of the muscle. MUs were identified by the use of decomposition software, and root‐mean‐square (RMS) values were calculated for each experimental condition. Six hundred and eleven decomposed MUs with significantly (P < 0·001) different jaw relation‐specific recruitment behaviour were organised into localised MU task groups. MUs with different task specificity in seven examined tasks were observed. The RMS EMG values obtained from the different recording sites were also significantly (P < 0·01) different between tasks. Overall MU recruitment was significantly (P < 0·05) greater in the deep masseter than in the superficial muscle. The number of recruited MUs and the RMS EMG values decreased significantly (P < 0·01) with increasing jaw separation. This investigation revealed differential MU recruitment behaviour in discrete subvolumes of the masseter in response to small changes in vertical jaw relations. These fine‐motor skills might be responsible for its excellent functional adaptability and might also explain the successful management of temporomandibular disorder patients by somatic intervention, in particular by the use of oral splints.     

Effects of interocclusal distance on bite force and masseter EMG in healthy participants

The aim of this study was to investigate effects of interocclusal distance (IOD) on bite force and masseter electromyographic (EMG) activity during different isometric contraction tasks. Thirty‐one healthy participants (14 women and 17 men, 21·2 ± 1·8 years) were recruited. Maximal Voluntary Occlusal Bite Force (MVOBF) between the first molars and masseter EMG activity during all the isometric‐biting tasks were measured. The participants were asked to bite at submaximal levels of 20%, 40%, 60% and 80% MVOBF with the use of visual feedback. The thickness of the force transducer was set at 8, 12, 16 and 20 mm (= IOD), and sides were tested in random sequence. MVOBF was significantly higher at 8 mm compared with all other IODs (P < 0·001). Only in women, IOD always had significant influence on the corresponding root‐mean‐square (RMS) value of EMG (P < 0·011). When biting was performed on the ipsilateral side to the dominant hand, the working side consistently showed higher masseter EMG activity compared with the balancing side (P < 0·020). On the contralateral side, there was no difference between the masseter EMG at any IODs. The results replicated the finding that higher occlusal forces can be generated between the first molars at shorter IODs. The new finding in this study was that an effect of hand dominance could be found on masseter muscle activity during isometric biting. This may suggest that there can be a general dominant side effect on human jaw muscles possibly reflecting differences in motor unit recruitment strategies.     

Association of oral parafunctional habits with anxiety and the Big-Five Personality Traits in the Saudi adult population

BackgroundOral parafunctional habits are related to any abnormal hyperactivity of the oromandibular system. They are prevalent in all societies in varying intensity and have potential physical and psychological implications. The aim of this study was twofold: (1) to determine the prevalence of various types of oral parafunctional habits in the Saudi adult population, and (2) to examine their association with and the level of anxiety and personality factors.MethodsThis cross-sectional study was conducted with an electronic data collection form distributed to the public through social media. The questionnaire comprised of the following: (a) demographic information, (b) the Hamilton Anxiety Rating Scale (HAM-A), (c) the Ten-Item of Big-Five Personality Traits, (d) a list of oral parafunctional habits. Data were analysed using a Pearson’s Chi square and binary logistic regression.ResultsThe participants reported several parafunctional habits including daily gum chewing (86%), lip/object biting (59%), clenching (45%), nail biting (36%) and grinding (32%). Males were 3 [2.3–3.7] times and younger age groups were 1.5 [1.1–2.0] times more likely to be associated with nail biting compared to their counter groups (adj.P < 0.001 and adj.P = 0.007). Lip/object biting was significantly 1.3 [1.1–1.7] times more prevalent in males (adj.P = 0.015). Participants who reported being extroverts were more likely to be associated with clenching (46.4%) (P = 0.024). An emotionally stable person was significantly less likely to be associated with nail biting (28.4%), grinding (24.9%), clenching (35.8%), and lip/object biting (48.4%) (P < 0.001each). Participants who reported conscientiousness and emotional stability were significantly less associated with TMD (P = 0.007, P < 0.001).ConclusionOral parafunctional habits are highly prevalent in the Saudi adult population in varying degrees. Possible risk factors include males, younger age groups, single people, and being financially constrained.     

Impact of sleep bruxism on training-induced cortical plasticity

PurposeTo investigate if sleep bruxism (SB) influences training-induced cortical plasticity and performance in terms of accuracy and precision of a tooth-clenching task (TCT).MethodsThirty-eight participants were allocated into SB group (N = 19) and control group (N = 19) according to presence of SB based on a 2-week screening. The participants were instructed to perform a standardized TCT for 58 min at three different force levels (10%, 20% and 40% of maximum voluntary contraction; MVC) in three series (first and third without visual-feedback and second with visual-feedback). Accuracy and precision of the TCT were calculated from actual bite force values. Transcranial magnetic stimulation was applied to elicit motor evoked potentials (MEPs) from the masseter and first dorsal interosseous muscle (FDI) before the TCT (pre-TCT-session) and 5-min after the TCT (post-TCT-session).ResultsAccuracy was significantly dependent on the series and target force level (P < 0.001), however, there was a significant decrease only in the control group at 10% MVC from first to third session (P < 0.001). No significant differences between groups were observed for the precision of the TCT. Masseter MEPs in the SB group in the pre-TCT-session at 120% and 160% motor threshold were significantly lower than in the control group (P < 0.05). Masseter MEPs of the control group in the post-TCT-session were significantly higher than the pre-TCT-session (P < 0.05) but not SB. FDI MEPs were only dependent on stimulus intensity (P < 0.001).ConclusionsSB is associated with significant changes not only in excitability of corticomotor control but also motor learning of jaw movements and force control.     

牙齿病理性磨损充填治疗的临床效果研究

目的观察尚未出现牙髓症状的牙齿病理性磨损充填治疗的临床效果。方法选择牙列完整、后牙骀面有病理性磨损且具有充填空间的患者10例，患牙48颗，均无牙髓症状。使用后牙树脂充填后，比较治疗前、治疗后3个月和1年时，患者主观感觉和咀嚼功能（咬合力、咀嚼效率和咬肌肌电活动）的变化，观察树脂充填的临床效果（美国公共健康部评价系统）。结果患牙经充填治疗后患者临床症状消失，咬合力增加（P〈0．05），咀嚼效率明显提高（P〈0．01），治疗前后下颌息止位、大力咬合时、咀嚼运动时，颞肌前束和咬肌的活动强度、活动对称性及用力方式差异均无统计学意义（P〉0．05）；治疗后3个月和1年时复查，48颗后牙树脂充填体各项指标A级率为100％。结论对后牙骀面尚未出现牙髓症状的病理性磨损用树脂进行充填是一种有效的治疗方法，可以解除患者的临床症状，减缓磨损的发展，增加咀嚼效率及患牙咬合力。     

SEMG activity of jaw‐closing muscles during biting with different unilateral occlusal supports

Summary The aim of this study was to test the hypothesis that experimental and reversible changes of occlusion affect the levels of surface electromyographic (SEMG) activity in the anterior temporalis and masseter areas during unilateral maximal voluntary biting (MVB) in centric and eccentric position. Changes were achieved by letting 21 healthy subjects bite with and without a cotton roll between the teeth. The placement alternated between sides and between premolar and molar areas. The SEMG activity level was lower when biting in eccentric position without than with a cotton roll between teeth (P < 0·043). It was always lower with premolar than with molar support when biting with a cotton roll (P < 0·013). In the anterior temporalis areas, the SEMG activity was always lower on the balancing than on the working side (P < 0·001). Such a difference was also found in the masseter areas but only during molar‐supported centric biting (P = 0·024). No differences were found when comparing the SEMG levels in masseter areas between centric and eccentric biting (P > 0·05). In the anterior temporalis area, the balancing side SEMG activity was lower in eccentric than in centric but only in molar‐supported biting (P = 0·026). These results support that the masseter and anterior temporalis muscles have different roles in keeping the mandible in balance during unilateral supported MVB. Changes in occlusal stability achieved by biting with versus without a cotton roll were found to affect the SEMG activity levels.     

Oral Fine Motor Control of Teeth Treated with Endodontic Microsurgery: A Single-Blinded Case-control Study

IntroductionPeriodontal mechanoreceptors (PMRs) are refined neural receptors present in abundance at the root apex and have a pivotal role in oral fine motor control. This case-control study aimed to evaluate the oral fine motor control of teeth treated with endodontic microsurgery (EMS) in comparison with the control teeth using a standardized behavioral biting task.MethodsFourteen eligible participants performed 5 trials of an oral fine motor control task that involved holding and splitting half of a peanut positioned on a force transducer with their EMS treated tooth and its contralateral control incisor tooth (28 teeth in total). The outcome variables were the mean food holding force, intra- and intertrial variability of the holding force, food splitting force, splitting duration, and the frequency of the stepwise splitting phase. The data were analyzed with parametric and nonparametric tests.ResultsThe results showed no statistically significant differences in the holding force, inter- and intratrial variability of the holding force, splitting force, or splitting duration between the teeth treated with EMS and the control (P > .05). However, there was a significantly higher frequency of stepwise ramp increase during the splitting phase with EMS treated teeth compared with the control (48% and 37%, respectively; P < .05).ConclusionsEMS treated teeth showed similar force regulation and oral fine motor control as the contralateral control. The findings of this study suggest that EMS treatment does not perturb the sensory information of PMRs and maintains the force regulation and oral fine motor control of the teeth.     

Stable tooth contacts in intercuspal occlusion makes for utilities of the jaw elevators during maximal voluntary clenching

Data are inconsistent concerning whether the level of the surface electromyographic (SEMG) activity of jaw‐closing muscles increases when biting forces elevated during maximal voluntary clenching (MVC). In this study, T‐Scan III system and BioEMG III system were used to record bite force, occlusal contacts and SEMG activity of the anterior temporalis (TA) and of the masseter muscles (MM) simultaneously. Recordings were obtained from 16 healthy young adult males during different conditions: (i) a fast MVC from resting position to intercuspal position (ICP); (ii) mandibular movements from ICP to protrusive or lateral edge‐to‐edge positions with teeth in contact with biting; (iii) a fast MVC in protrusive and lateral edge‐to‐edge positions. A higher level of SEMG activity was associated with a higher bite force during occluding movements (P < 0·05). However, during fast MVC from rest to ICP, the largest number of occlusal contacts was achieved and distributed more symmetrically, the highest level of biting force was obtained, but the SEMG activity of the jaw elevator muscles was reduced compared with its maximum level (P < 0·05). This phenomenon was not observed during the fast MVC in protrusive or lateral edge‐to‐edge positions. The present results that a lower SEMG activity was associated with the largest number of occlusal contacts and the highest level of bite force during centric MVC demonstrated a complex integration of jaw‐closing muscles when a stable occlusion is present.     

Masticatory muscles. Part IV. The masticatory muscles do not work homogeneously

The temporalis and masseter muscles have a complex architectural design with large attachment areas. As a consequence, each of these muscles is capable of producing a large number of mechanical effects. In addition, within each muscle the muscle fibres and sarcomeres undergo different excursions during jaw movements. This leads to intramuscular differences in the possible excursion range and the capacity to produce force. Depending on the desired motor task, the nervous system is able to vary both the magnitude and direction of muscle force by selective activation of specific muscle portions. The anterior temporalis and the superficial and deep masseter are capable of generating large forces, e.g. during biting and chewing. The posterior temporalis and the anterior and posterior deep masseter are suited to establish a precise adjustment of forces and movements during laterotrusion and protrusion/retrusion.     

Indications for jaw gape‐related control of relative muscle activation in sequent chewing strokes

Summary Jaw muscle activity ratios in unilateral isometric biting differ from ratios of unilateral chewing but approach the latter if the jaw gape in biting is made as small as the minimum interocclusal distance (MID) of chewing. Especially, the masseter working/balancing side ratio (W/B‐ratio) becomes as asymmetric as in chewing, because of reduction in balancing side (BS) masseter activity. This behaviour of ratios might reflect a ‘chewing‐specific’ motor strategy induced when isometric biting is performed with a ‘chewing‐like’ gape. If this hypothesis applies, activity ratios should be associated with MIDs of sequent chewing strokes in a similar manner as with incremented jaw gapes in isometric biting. To test this prediction, bilateral surface electromyograms of masseter and anterior temporalis muscles and incisor movements were recorded during unilateral chewing in 52 subjects. W/B‐ratios of masseter and temporalis activities and temporalis/masseter‐ratios on both sides were calculated. The ratios were related to MIDs of consecutive chewing cycles. Three of the four ratios were associated with masticatory MID in the same manner as with jaw gape in isometric biting. In particular with decreasing MID, the masseter W/B‐ratio increased from 1·5 to 2·2 (P < 0·01). This increase in asymmetry was attributed to a stronger decrease in masseter activity on the BS than on the working side. We conclude that relative jaw muscle activation is associated with interocclusal distance in a similar way in isometric biting and in chewing. This analogy supports the idea of a common jaw gape‐related neuromuscular strategy facilitated by afferent signalling of interocclusal distance.     

Relation between integrated electromyographic activity and biting force during voluntary isometric contraction in human masticatory muscles.

The relation between integrated electromyographic activity and computed biting force during voluntary isometric contraction was evaluated in the masticatory muscles of healthy subjects. The slopes of the curves relating integrated electromyographic activity to computed biting force in masseter muscles were steeper on the non-preferred chewing side than on the preferred chewing side, and they progressively became steeper during the course of continuous isometric contraction of a given biting force.     

口腔护牙托对上臂三角肌肌力作用的研究

目的：探讨口腔护牙托对上臂三角肌肌力的作用,考察其促进提高运动成绩的可能性。方法：以软质树脂材料,采用直接法分别为8名受试者制作定制护牙托,测试戴托受试者平卧推举5kg重物紧咬时的三角肌前束部和嚼肌的肌电值。将该值经系列转换器输入计算机,进行信息数值化处理并输出肌电积分数值。结果：受试者评价该种护牙托的固位,舒适性和保护性均佳,戴护牙托举重5kg紧咬时的上臂三角肌肌电测试值为1．1600,不戴护托时的测试值为0．6125。前者比后者有明显提高,差异有显著性（P＜0．01）,戴护牙托与不戴紧咬时嚼肌的肌电测试值分别烟0．3825和0．2138,前者比后者亦有明显提高（P＜0．01）。结论：戴口腔护牙托紧咬提高嚼肌的肌力,进而反射性作用于上臂三角肌,使该肌的肌力亦得以增强。     

Changes in the masticatory function of complete denture wearers after relining the mandibular denture with a soft denture liner

PURPOSE: The purpose of this study was to examine changes in the masticatory function of complete denture wearers after relining the mandibular denture with a soft liner. MATERIALS AND METHODS: Conventional complete dentures were fabricated for 6 completely edentulous patients. One month after completing adjustments of the dentures, maximum biting force, masticatory performance, and electromyography of the masseter muscle during mastication were recorded. Chewing strokes, chewing time, muscular activity, and masticatory rhythm were calculated using the recorded electromyography. The mandibular dentures were then relined with a soft liner. One month after finishing adjustments of the relined dentures, the same tests were performed. These values were compared using a paired t test (alpha = 0.05). RESULTS: When using the soft liner, masticatory performance and maximum biting force were significantly greater (P = 0.019 and P = 0.023, respectively). In addition, the number of chewing strokes was significantly lower (P = 0.020), and chewing time was reduced (P = 0.010). A more stable masticatory rhythm was also found in the initial chewing stage. The muscular activity tended to decrease after the insertion of the lined denture, but no significant difference was found between before and after relining. CONCLUSION: It was shown that applying a soft lining material to the mandibular dentures of 6 edentulous patients improved masticatory function with no adverse effect on the muscular task.