Biting intentions modulate digastric reflex responses to sudden unloading of the jaw

The mouth-opening muscular performance in adults and elderlies with and without dysphagia: A systematic review and meta-analysis

OBJECTIVES

To characterize mouth-opening muscular performance (MOMP) in adults and elderly individuals with dysphagia and healthy controls.

METHODS

We searched the PubMed, EMBASE, CINAHL, Cochrane, Scopus, and Web of Science databases from inception to Jan. 26, 2023. Two independent researchers considered the titles, abstracts, and full texts of potentially eligible papers from 1451 search results. Twenty-five studies that evaluated mouth-opening maximal strength (MOMS) in healthy adults, elderly individuals, and patients with dysphagia met the inclusion criteria.

RESULTS

We found comparable, reliable values with significant sex differences in maximal mouth opening strength (MMOS) in the meta-analysis for healthy elderly patients (females 5.31 ± 0.47 kg vs. males 7.04 ± 0.70 kg; mean difference of 0.84 kg). Age has also emerged as an essential factor in reducing strength. There was a significant reduction in the MMOS score in the only study that compared dysphagic individuals to healthy elderly individuals. In another study, the MMOS score was comparable to the meta-analysis of healthy elderly individuals.

CONCLUSIONS

Both sex and age play significant roles in the MMOS. There is no reliable data on the normal mouth-opening strength and endurance of healthy adults, patients with dysphagia, or individuals with other relevant clinical problems.

The mouth-opening muscular performance in adults with and without temporomandibular disorders: A systematic review

Background

The mouth‐opening muscular performance in patients with temporomandibular disorders (TMDs) is unclear. Understanding the impairments of this muscle group within specific TMDs is important to develop proper management strategies.

Objective

To characterise the mouth‐opening muscular performance in adults with and without TMDs.

Methods

PubMed, EMBASE, CINAHL, Scopus, Web of Science and Cochrane databases were searched from inception to 12 November 2020. Bibliographies were searched for additional articles, including grey literature. Case‐control, cross‐sectional and interventional studies reporting mouth‐opening muscular strength and/or endurance were included. Risk of bias was assessed by the SIGN checklist for case‐control studies and by the NIH quality assessment tool for cross‐sectional studies. Results were pooled with a random‐effects model. Confidence in cumulative evidence was determined by means of the GRADE guidelines.

Results

Fourteen studies were included; most were rated as having a moderate risk of bias. Only three studies assessed patients with TMDs and the other 11 assessed healthy adults. Significant sex differences in muscular performance were found for healthy adults in the review (strength deficit for females versus males). There was a significant reduction in maximal mouth opening performance (strength and endurance) in the three studies that assessed patients with temporomandibular disorders.

Conclusion

Sex plays a significant role in maximal mouth opening strength. There is a lack of reliable data on the normal mouth‐opening strength and endurance of healthy adults as well as for patients with TMDs.

Implications

Lack of reliable TMDs patient data and comparable healthy adult data highlight future direction for research.

Maintaining arm control during self-triggered and unpredictable unloading perturbations

We often perform actions where we must break through some resistive force, but want to remain in control during this unpredictable transition; for example, when an object we are pushing on transitions from static to dynamic friction and begins to move. We designed a laboratory task to replicate this situation in which participants actively pushed against a robotic manipulandum until they exceeded an unpredictable threshold, at which point the manipulandum moved freely. Human participants were instructed to either stop the movement of the handle following this unloading perturbation, or to continue pushing. We found that participants were able to modulate their reflexes in response to this unpredictable and self-triggered unloading perturbation according to the instruction they were following, and that this reflex modulation could not be explained by pre-perturbation muscle state. However, in a second task, where participants reactively produced force during the pre-unloading phase in response to the robotic manipulandum to maintain a set hand position, they were unable to modulate their reflexes in the same task-dependent way. This occurred even though the forces they produced were matched to the first task and they had more time to prepare for the unloading event. We suggest this disparity occurs because of different neural circuits involved in posture and movement, meaning that participants in the first task did not require additional time to switch from postural to movement control.

The slack test does not assess maximal shortening velocity of muscle fascicles in humans

The application of a series of extremely high accelerative motor-driven quick releases while muscles contract isometrically (i.e. slack test) has been proposed to assess unloaded velocity in human muscle. This study aimed to measure gastrocnemius medialis fascicle shortening velocity (V_F) and tendinous tissue shortening velocity during motor-driven quick releases performed at various activation levels to assess the applicability of the slack test in humans. Gastrocnemius medialis peak V_F and joint velocity recorded from 25 participants using high frame rate ultrasound during quick releases (at activation levels from 0% to 60% of maximal voluntary isometric torque) and during fast contractions without external load (ballistic condition) were compared. Unloaded joint velocity calculated using the slack test method increased whereas V_F decreased with muscle activation level (P≤0.03). Passive and low-level quick releases elicited higher V_F values (≥41.8±10.7 cm s^-1) compared with the ballistic condition (36.3±8.7 cm s^-1), while quick releases applied at 60% of maximal voluntary isometric torque produced the lowest V_F These findings suggest that initial fascicle length, complex fascicle-tendon interactions, unloading reflex and motor-driven movement pattern strongly influence and limit the shortening velocity achieved during the slack test. Furthermore, V_F elicited by quick releases is likely to reflect substantial contributions of passive processes. Therefore, the slack test is not appropriate to assess maximal muscle shortening velocity in vivo.

Goal-dependent modulation of the long-latency stretch response at the shoulder, elbow, and wrist

Many studies have demonstrated that muscle activity 50-100 ms after a mechanical perturbation (i.e., the long-latency stretch response) can be modulated in a manner that reflects voluntary motor control. These previous studies typically assessed modulation of the long-latency stretch response from individual muscles rather than how this response is concurrently modulated across multiple muscles. Here we investigated such concurrent modulation by having participants execute goal-directed reaches to visual targets after mechanical perturbations of the shoulder, elbow, or wrist while measuring activity from six muscles that articulate these joints. We found that shoulder, elbow, and wrist muscles displayed goal-dependent modulation of the long-latency stretch response, that the relative magnitude of participants' goal-dependent activity was similar across muscles, that the temporal onset of goal-dependent muscle activity was not reliably different across the three joints, and that shoulder muscles displayed goal-dependent activity appropriate for counteracting intersegmental dynamics. We also observed that the long-latency stretch response of wrist muscles displayed goal-dependent modulation after elbow perturbations and that the long-latency stretch response of elbow muscles displayed goal-dependent modulation after wrist perturbations. This pattern likely arises because motion at either joint could bring the hand to the visual target and suggests that the nervous system rapidly exploits such simple kinematic redundancy when processing sensory feedback to support goal-directed actions.