Teeth clenching reduces arm abduction force

Occlusal support and postural stability in children: An observational study

BACKGROUND

The relationship between uneven occlusal contact and fall risk has been described in elderly participants; however, multiple confounding factors, such as systemic disease or deterioration in vestibular function, have not been well considered. This study aimed to investigate this relationship in pediatric participants at the age of approximately 9-12, where these confounding factors are absent and the replacement of primary molars by permanent ones generally occurs and transiently results in uneven occlusal contact.

METHODS

Sixty-five children (36 boys and 29 girls, mean age: 9.5 ± 1.5 years) were enrolled in this study. Participants were asked whether they had experienced at least two falls within the past 1 year. The occlusal contact area ratio was calculated by dividing the greater half (right or left dental arch) by the smaller half (the other half) with U-shaped pressure sensor sheet. The total length and rectangular area of the center of pressure while standing upright for 10 s were measured with a stabilometer and used as postural sway parameters.

RESULTS

The occlusal contact area ratio significantly correlated with the postural sway parameters. Comparison between those who had experienced the fall incidences and those who had not after propensity score matching that adjusts their overall physical function revealed that there were no significant differences in the postural sway parameters and occlusal contact area ratio regardless of the history of fall incidence.

CONCLUSIONS

Uneven occlusal contact leads to an increase in postural sway in children; however, this may not be associated with fall incidence.

Medium latency excitatory reflex of soleus re-examined

We aimed to study the receptor origin and postsynaptic potential profile of the medium latency reflex (MLR) response that develops in the soleus muscle when common peroneal nerve of antagonist tibialis anterior (TA) muscle is electrically stimulated. To achieve this aim, we electrically stimulated common peroneal nerve and recorded surface electromyography (SEMG) responses of soleus and TA muscles of informed volunteers. Additionally, we recorded intramuscular EMG from the soleus muscle. Stimulation of common peroneal nerve induced a direct motor response (M-response) in the TA and MLR in SEMG of the soleus. Using voluntarily-activated single motor units (SMUs) from the soleus muscle we noted that there were two distinct responses following the stimulus. The first response was a reciprocal inhibitory reflex probably originating from the antagonist muscle spindle primary (Ia) afferents. This was followed by an indirect reflex response activated by the contraction of the TA muscle during the M-response. This contraction generated a rapid acceleration in the direction of dorsiflexion hence inducing a stretch stimulus on soleus muscle. The response of soleus to this stimulus was a stretch reflex. We suggest that this stretch reflex is the main contributor to the so-called soleus MLR in the literature. This study illustrated the importance of using SMUs and also using discharge-rate based analysis for closely examining previously 'established' reflexes.

CORP: Measurement of upper and lower limb muscle strength and voluntary activation

Muscle strength, the maximal force-generating capacity of a muscle or group of muscles, is regularly assessed in physiological experiments and clinical trials. An understanding of the expected variation in strength and the factors that contribute to this variation is important when designing experiments, describing methodologies, interpreting results, and attempting to replicate methods of others and reproduce their findings. In this review (Cores of Reproducibility in Physiology), we report on the intra- and inter-rater reliability of tests of upper and lower limb muscle strength and voluntary activation in humans. Isometric, isokinetic, and isoinertial strength exhibit good intra-rater reliability in most samples (correlation coefficients ≥0.90). However, some tests of isoinertial strength exhibit systematic bias that is not resolved by familiarization. With the exception of grip strength, few attempts have been made to examine inter-rater reliability of tests of muscle strength. The acute factors most likely to affect muscle strength and serve as a source of its variation from trial-to-trial or day-to-day include attentional focus, breathing technique, remote muscle contractions, rest periods, temperature (core, muscle), time of day, visual feedback, body and limb posture, body stabilization, acute caffeine consumption, dehydration, pain, fatigue from preceding exercise, and static stretching >60 s. Voluntary activation, the nervous system’s ability to drive a muscle to create its maximal force, exhibits good intra-rater reliability when examined with twitch interpolation (correlation coefficients >0.80). However, inter-rater reliability has not been formally examined. The methodological factors most likely to influence voluntary activation are myograph compliance and sensitivity; stimulation location, intensity, and inadvertent stimulation of antagonists; joint angle (muscle length); and the resting twitch.

Does a core stabilization exercise program have a role on shoulder rehabilitation? A comparative study in young females

OBJECTIVES

This study aims to evaluate the effect of core stabilization exercises and to explore the immediate effect of core muscles-activated posture on shoulder maximal voluntary isometric contraction (MVIC) strength.

PATIENTS AND METHODS

Between November 2016 and January 2015, a total of 75 healthy female volunteers (mean age 25.36 years; range, 18 to 30 years) were included. Of these, 42 consecutive volunteers were assigned as the study group, while the remaining 33 volunteers were assigned as the control group. The study group completed a six-week core stabilization home-based exercise program. Two measurements were performed with six-week interval. A set of three repetitions for each shoulder side was performed by an electronic dynamometer under two conditions: core muscles relaxed and activated. Measurements were monitored real-time with a connected computer and recorded in Newton.

RESULTS

The activation of core muscles during the measurement significantly decreased the MVIC values in both groups (p<0.05). The MVIC values significantly increased after home-based exercise program in both conditions (p<0.05).

CONCLUSION

Our study showed that six-week core stabilization exercise program had a significant positive effect on the shoulder MVIC strength. This result may support the use of core stabilization exercises in the early periods of shoulder rehabilitation when the shoulder muscle strengthening exercises are painful.

Standardization of the Jendrassik maneuver in Achilles tendon tap reflex

Objective

For many decades, the Jendrassik maneuver (JM) has been used as a reinforcement for stretch reflexes, although the underlying mechanism of this reinforcement is still not fully understood. Moreover, the term JM has been used for many different muscle contraction strategies as there is no fixed movement for the maneuver in the literature. In this study, we aimed to investigate the effects of clenched hand pull, teeth clenching, and their combined effects to reach standardization.

Methods

Achilles tendon tap reflex responses in the soleus were recorded during rest (R), hand pull (HP), teeth clench (TC), and HP + TC combined, hereafter referred to as the JM.

Results

Reflex response amplitudes significantly increased during JM, HP, and TC in the soleus. HP and JM significantly changed the background activity in the soleus, but TC alone did not.

Conclusion

These results suggest that dominantly presynaptic disinhibitory mechanisms may be responsible for the increase in the tendon tap reflex during HP, TC, and JM.

Significance

Because the findings indicate that HP increases the background activity of the soleus, we suggest that researchers should use only TC during the Jendrassik maneuver to avoid any confounding background activity change.

Effects of Jaw Clenching While Wearing a Customized Bite-Aligning Mouthpiece on Strength in Healthy Young Men

Recent studies have investigated the ergogenic effects of different types of mouthguards. There is evidence of the benefits of clenching customized mouthpieces on force and power development. The phenomenon called concurrent activation potentiation (CAP) promoted in jaw clenching seems to enhance the muscular strength and power in maximal isometric contractions and powerful actions in sport. Moreover, a bite-aligning mouthpiece may promote a more aligned and powerful clench. The objective of the present study was to investigate the effects of jaw clenching while wearing a customized bite-aligning mouthpiece (MP) on jump ability and isometric maximal strength tests in contrast to 2 other conditions: nonjaw clenching (NON-JAW) and jaw clenching without the mouthpiece (JAW). A within-subjects design was used to assess the jump performance and the maximal isometric force in 28 physically active male subjects. Statistical analyses revealed significantly higher performance in JAW and NON-JAW conditions for handgrip force (HG-peakforce) and all variables of the back-row exercise (BRW) and countermovement vertical jump (CMVJ) (p ≤ 0.05). Significant differences between NON-JAW and JAW were found for HG-peakforce (p ≤ 0.05). These findings suggest that it is advisable to use a customized bite-aligning mouthpiece to improve strength and power performance.