The finding that asymptomatic women displayed steeper fatigue-related changes than women with myogenic temporomandibular disorder runs counter to a widespread clinical assumption, and understanding why requires a closer look at what the median frequency and root mean square parameters actually measure. Median frequency, derived from the power spectral density of the surface electromyographic signal, reflects the average firing behavior of the motor units contributing to the detected signal. As a sustained contraction progresses, the conduction velocity of action potentials traveling along muscle fiber membranes slows, largely because metabolic byproducts such as hydrogen ions and inorganic phosphate accumulate in the interstitial space and alter membrane excitability. This slowing compresses the frequency content of the signal toward lower values, which is why a negative slope of the median frequency regression line over time is interpreted as a hallmark of peripheral muscle fatigue. The root mean square, by contrast, quantifies the amplitude of the interference pattern and is sensitive to the number of active motor units and their firing rates. When a muscle must sustain a given force and its individual fibers begin to fatigue, the nervous system typically recruits additional motor units or increases firing rates to compensate, producing the characteristic amplitude rise captured by a positive RMS slope.

The choice of a 30-second sustained contraction at 50 percent of maximum bite force is methodologically significant. De Luca’s classic work on fatigue assessment suggested that contractions exceeding roughly 30 percent of maximal voluntary strength generate enough intramuscular pressure to occlude blood flow, restricting oxygen delivery and forcing the muscle toward anaerobic metabolism. This creates a relatively standardized metabolic environment in which fatigue develops predictably, while still allowing the task to be tolerable for participants. Shorter protocols of five to ten seconds at maximal effort, which have dominated much of the prior TMD literature, capture a different physiological regime: the signal is dominated by neural drive and motivation rather than by accumulating metabolic fatigue, and the brief duration limits the time over which spectral compression can develop. By using a longer submaximal task with real-time force feedback from a bilateral bite dynamometer, the present design ensured that participants in both groups worked at genuinely equivalent relative loads, reducing the possibility that differences in effort or force output would confound the comparison between groups.

Several physiological explanations could account for the paradoxical finding of greater fatigue signatures in the asymptomatic group. One possibility involves differences in motor unit recruitment strategies. Individuals with chronic myofascial pain often adopt altered motor strategies, including co-contraction patterns, reduced relaxation between contractions, or a shift toward greater reliance on slower, more fatigue-resistant motor units even during low-intensity tasks. If the TMD group habitually recruits a more homogeneous population of low-threshold motor units, the spectral shift during a sustained contraction may be less pronounced because the already slow-conducting fibers have less room to slow further. The asymptomatic group, by contrast, may recruit a broader spectrum of motor units, including faster, higher-threshold units whose conduction velocities decline more dramatically as fatigue develops, producing the steeper median frequency slopes observed in their masseter muscles.

Another consideration is the role of pain-related protective mechanisms. Persistent masticatory muscle pain is frequently accompanied by a phenomenon in which the central nervous system modulates activation to protect painful tissues, sometimes described as pain-adaptive motor behavior. This can include reduced maximal voluntary force, altered distribution of load across synergistic muscles, and periodic micro-pauses in activation. If participants in the TMD group distributed the bite task differently, for example by intermittently shifting load between the masseter and temporalis or between the two sides, the effective metabolic demand on any single muscle could have been lower, blunting the fatigue signature. The finding that the left anterior temporalis showed a steeper RMS increase in the control group while the masseter slopes differed on both sides is consistent with the idea that the two muscles contribute differently to sustained biting, and that this division of labor may be reorganized in the presence of chronic pain.

The asymmetry between the left and right sides in the results also deserves attention. The significant between-group differences appeared in the right and left masseter for median frequency but only in the left anterior temporalis for root mean square. Bilateral molar biting is not necessarily symmetric; handedness, chewing side preference, occlusal contact patterns, and subtle anatomical asymmetries of the mandible can all produce lateralized differences in muscle activation. In asymptomatic individuals, such asymmetries are usually small and functionally inconsequential, but in individuals with TMD, unilateral symptoms can amplify them. The fact that the TMD group did not show greater fatigue on either side suggests that whatever lateralization exists in their chewing behavior did not translate into a systematically faster fatigue development in this protocol, but the side-specific findings in the control group highlight that masticatory muscle fatigue is not a uniform, bilateral phenomenon even in healthy individuals.

The composition of the TMD group also matters for interpreting the results. Participants were required to have a primary myogenic diagnosis, either myalgia or myofascial pain, but could carry comorbid intra-articular diagnoses such as arthralgia or disc displacement with reduction. This heterogeneity reflects clinical reality, since pure muscle-only TMD is uncommon, but it introduces potential variability. Individuals with concurrent joint involvement may adopt different mandibular positioning or muscle activation patterns than those with isolated muscle pain, and these differences could increase within-group variability, making it harder to detect between-group differences in fatigue slopes. The relatively balanced distribution of myalgia and myofascial pain diagnoses within the group, with and without referral patterns, suggests the investigators aimed to capture the typical clinical presentation, but future studies stratifying by specific diagnosis combinations could clarify whether particular subgroups show distinct fatigue profiles.

The restriction to women aged 18 to 40 with a body mass index below 25 was a deliberate design choice with important implications. Sex differences in masticatory muscle morphology, hormonal influences on pain sensitivity and muscle metabolism, and the higher prevalence of TMD among women all justify studying a single sex, but they also limit generalizability. Estrogen fluctuations across the menstrual cycle have been linked to altered pain perception and potentially to differences in muscle fatigue resistance, and although the study did not report cycle phase control, the homogeneous age range reduces some sources of hormonal variability. Similarly, excluding individuals with elevated body mass index avoids confounding from differences in craniofacial morphology and adipose tissue effects on surface EMG signal quality, since subcutaneous fat acts as a low-pass filter that attenuates high-frequency signal components and can distort median frequency estimates.

The use of the angulation coefficient, the slope of the linear regression of each parameter over the 30-second contraction, rather than simple pre-post differences, represents a methodological refinement worth emphasizing. A slope captures the rate of change continuously across the entire task and is less sensitive to noise at any single time point than a two-point comparison. It also allows detection of differences in the trajectory of fatigue even when the absolute starting and ending values are similar between groups. Two individuals could begin and end a contraction with comparable median frequencies yet differ substantially in how quickly that decline unfolded, and the regression slope distinguishes these patterns. This approach is particularly valuable when comparing clinical populations to controls, because baseline signal characteristics often differ between groups, and slope-based measures partially normalize for those baseline differences.

From a clinical standpoint, the results raise questions about how fatigue testing should be interpreted in TMD assessment. If individuals with chronic masticatory muscle pain do not exhibit exaggerated fatigue responses during standardized sustained tasks, then reduced fatigue capacity may not be a defining feature of myogenic TMD, contrary to models that emphasize muscle overuse and fatigue as primary pain drivers. Alternatively, the altered fatigue signature in the TMD group could reflect a genuinely different, perhaps more economical, motor strategy developed over time, or a protective downregulation of activation that limits metabolic stress. Distinguishing between these interpretations requires studies that combine surface EMG with measures of muscle oxygenation, such as near-infrared spectroscopy, or with intramuscular recordings that can resolve single motor unit behavior, since surface signals aggregate the activity of many units and cannot fully separate peripheral metabolic fatigue from central modulation of drive.

The cross-sectional design also constrains causal inference. The observed differences in fatigue slopes could be a consequence of living with chronic pain, a pre-existing trait that predisposed individuals to develop TMD, or an artifact of differences in task performance such as force fluctuation, jaw positioning, or motivation. Longitudinal designs following asymptomatic individuals through the onset of TMD symptoms, or intervention studies that reduce pain and then reassess fatigue parameters, would help determine whether the blunted fatigue response is a cause or consequence of the disorder. Additionally, the 30-second duration, while longer than many prior protocols, remains a relatively brief sustained contraction; longer tasks approaching true endurance limits might reveal group differences that a half-minute contraction does not, particularly if the TMD group’s altered recruitment patterns only become metabolically consequential over longer time scales.

Finally, the findings underscore the value of bilateral, force-controlled protocols in masticatory muscle research. Unilateral maximal bites and short isometric bursts, the dominant paradigms in earlier work, may not represent the sustained, submaximal loading that characterizes real-world masticatory function, where muscles repeatedly contract at moderate intensities over extended chewing periods. A sustained bilateral submaximal task with continuous force feedback more closely approximates this functional demand and, as the present results demonstrate, can produce group patterns that contradict those seen in maximal-effort tests. This discrepancy suggests that fatigue assessed under one set of task conditions may not generalize to others, and that a comprehensive characterization of masticatory muscle function in TMD will ultimately require protocols spanning the range from brief maximal efforts to prolonged submaximal endurance tasks, ideally combined with measures of pain, jaw function, and psychological factors that shape how individuals with chronic orofacial pain use their muscles in daily life.

Subject of Research: EMG fatigue signs during sustained bilateral submaximal biting were more pronounced in asymptomatic women than in women with myogenic TMD.

Article Title: Comparison of MF and RMS EMG parameters in women with TMD and asymptomatic women during sustained bilateral submaximal bite force protocol

Article References: Pires, P. F., Pelai, E. B., Galvani, S. C. F., Pulino, B., de Moraes, M., de Castro-Carletti, E. M., Foltran-Mescollott, F., Guerra, R. C., & Rodrigues-Bigaton, D. (2026). Comparison of MF and RMS EMG parameters in women with TMD and asymptomatic women during sustained bilateral submaximal bite force protocol. Frontiers in Pain Research, 7, Article 1962220. https://doi.org/10.3389/fpain.2026.1962220

Image Credits: AI Generated

DOI: 10.3389/fpain.2026.1962220

Keywords: EMG, TMD, muscle fatigue, masseter, temporalis, bite force, median frequency, RMS, orofacial pain, electromyography, Comparison, parameters