Muscles

Background: Fatigue in high-intensity exercise is often attributed to a defended ceiling of peripheral fatigue; evidence is mixed. Objectives: We aimed to assess whether the end-exercise potentiated-twitch decrement behaves as a regulated variable across candidate mechanisms. Methods: PubMed/MEDLINE, Scopus, and Web of Science were searched to 12 August 2026. Evidence came from controlled studies in adults, with permeabilized-fiber, rodent, and isolated-actomyosin experiments only where human measurement is impossible. Results: Nine mechanisms met the criteria. Inorganic phosphate and hydrogen ions impair the cross-bridge cycle; reduced sarcolemmal excitability (K⁺) and reduced sarcoplasmic reticulum Ca²⁺ release reverse over minutes and days, respectively. Only the cross-bridge effects are driven by metabolites (protons, ATP, lactate) that are agonists of group III/IV afferents; the afferents do not sense contractile impairment itself. Oxygen delivery, respiratory-muscle work, substrate availability, and perceptual load altered fatigue rate more than the end-exercise value, which varied with duration, prior exercise, muscle mass, and mode. Conclusions: Evidence fits a threshold in the phosphate–force relationship, though this does not demonstrate a defended limit. The decrement is not constant across tasks; we propose, as a testable hypothesis, that the regulated quantity is the summed afferent and effort signal. Selection was purposive, without formal risk-of-bias appraisal.

The tongue is a highly important, multifunctional, and complex organ in human physiology. It is critical for digestion, speech, immune surveillance, and sensory perception, playing a dynamic role in daily life. Abnormalities in the innervation and/or myofiber integrity of the tongue can have severe pathophysiological effects on swallowing, speaking, chewing and bodily balance. The age-related decrease in skeletal muscle mass and contractile strength, a recognized risk factor for sarcopenia and frailty, also impacts the tongue muscles. This report has examined the tongue proteome and evaluated changes in protein abundance associated with the aging phenotype. Interestingly, the proteomic cataloging of the contractile apparatus of tongue muscle identified myosin heavy chain isoform MyHC-13, which is the superfast type of myosin that is restricted to specialized skeletal muscles such as extraocular and laryngeal muscles. Proteomics revealed that tongue aging is associated with considerable changes in the protein expression pattern. Decreases were identified in membrane repair-associated annexin A isoforms, luminal calcium-binding proteins, contractile proteins, and cytoskeletal elements. Lower levels of fast myosin isoform MyHC-2B suggests potential age-associated fiber type shifting towards slower twitch properties. Elevated levels of the fibrosis marker collagen COL-VI, which increased deposition in aged tongue muscle was verified by immunofluorescence microscopy, agrees with the idea of a higher degree of susceptibility to reactive myofibrosis in sarcopenia of old age.

Performance fatigability is characterized by a decline in the capacity to maintain force or power during sustained or repeated activity. This exploratory study quantified knee-extensor mechanical performance and rectus femoris surface electromyographic (sEMG) changes during repeated maximal isokinetic knee extensions and explored sex-related differences. Twenty-nine healthy adults (20 men, 9 women; 20–31 years) performed 30 maximal concentric knee extensions at 180°·s⁻¹. Mechanical outcomes were analyzed in all 29 participants, whereas rectus femoris sEMG outcomes were analyzed in 23 participants. Peak torque (PT) decreased by 32.75 ± 11.66% (adjusted p = 6.08 × 10⁻¹¹), work per repetition by 33.35 ± 11.97% (adjusted p = 2.12 × 10⁻¹¹), and mean frequency (MNF) by 23.44 ± 8.33% (adjusted p = 6.08 × 10⁻¹¹). RMS increased significantly from Start to End (adjusted p = 0.000639), with a mean FI-RMS of −17.12 ± 23.11%. FI-PT and FI-work were very strongly associated (ρ = 0.938, adjusted p = 3.71 × 10⁻¹³), whereas no association involving rectus femoris MNF or RMS remained statistically significant after Holm–Bonferroni correction. No statistically significant sex-related differences were detected. Repeated maximal knee extensions reduced mechanical performance and rectus femoris MNF while increasing RMS. FI summarizes Start-to-End change across the measured outcomes; its reliability and validity require further investigation.