We performed X-ray diffraction analyses on rat plantaris muscle to determine if there are strain-specific structural changes at the molecular level after eccentric contraction (ECC). ECC was elicited in situ by supramaximal electrical stimulation through the tibial nerve. One hour after a series of ECC sessions, the structural changes that remained in the sarcomere were evaluated using X-ray diffraction. Proteins involved in cell signaling pathways in the muscle were also examined. ECC elicited by 100, 75, and 50 Hz stimulation respectively developed peak tension of 1.34, 1.12 and 0.79 times the isometric maximal tetanus tension. The series of ECC sessions phosphorylated the forkhead box O proteins (FoxO) in a tension-time integral-dependent manner, as well as phosphorylated the mitogen-activated protein kinases (MAPK) and a protein in the mammalian target of rapamycin (mTOR) pathway in a maximal tension dependent manner. Compared to isometric contractions, ECC was more efficient in phosphorylating the signaling proteins. X-ray diffraction revealed that the myofilament lattice was preserved even after intense ECC stimulation at 100 Hz. Additionally, ECC < 75 Hz preserved the molecular alignment of myoproteins along the myofilaments, while 75-Hz stimulation induced a slight but significant decrease in the intensity of meridional troponin reflection at 1/38 nm⁻¹, and of myosin reflection at 1/14.4 nm⁻¹. These two reflections demonstrated no appreciable decrease with triple repetitions of the standard series of ECC sessions at 50 Hz, suggesting that the intensity decrease depended on the instantaneous maximal tension development rather than the total load of contraction, and was more likely linked with the phosphorylation of MAPK and mTOR signaling proteins.

To explore the role of polyamine metabolism in muscle-type-specific hypertrophy, we analyzed changes in the expression profiles of polyamine metabolic enzymes and key enzymes for muscle metabolism induced by voluntary wheel running exercise in different muscle types. Effect of the polyamine precursor, putrescine, which was reported to have potential to regulate muscle volume, was also tested. Polyamine synthetic enzymes were upregulated in hypertrophic soleus muscle whereas polyamine catabolic enzymes were upregulated in non-hypertrophic plantar muscle by exercise in correlation with increased mitochondria-related protein expression. The increased catabolic enzymes of polyamines in the plantar muscle were hypothesized to be possibly involved in restriction of hypertrophy in fast-type skeletal muscles correlating with increased aerobic metabolism. Putrescine administration minimally affected polyamine metabolism and muscle volume indicating that it did not effectively regulate muscle hypertrophy. Polyamine oxidase localized in the perinuclear and inter-myofibrillar region suggesting a correlation between aerobic metabolism and polyamine catabolism.

We investigated the propagation of extending waves along twitching muscle fibers triggered by the internal shortening of early local contraction in the end-plate region. Bullfrog sartorius muscles were minimally stimulated, and the displacement of carbon particles attached to the muscle surface was captured using a high-speed camera. We found an extending wave along the fiber at a velocity of 5.35 ± 0.33 m·s⁻¹, faster than the conduction of action potentials at 3.04 ± 0.31 m·s⁻¹. Local compression of the muscle surface blocked the propagation of the extending wave, indicating its mechanical nature. Muscle stretching increased the extending wave velocity. These findings provide direct evidence that mechanically transmitted extending waves originate from early local contractions in the end-plate region and propagate along muscle fibers ahead of the contraction wave.

Background: At present, the rotational function of the hip adductor muscle group remains unclear. This study aimed to clarify the rotational function and stabilizing role of the pectineus, adductor longus, and adductor brevis (adductor muscle group) based on anatomical findings and T₂ values (ms) obtained from magnetic resonance imaging (MRI). T₂ values are prolonged in tissues with higher water content, and in skeletal muscle, it has been demonstrated that T₂ values increase in proportion to exercise intensity. Methods: Using fixed specimens (n = 6, aged 61–96 years), we observed the three-dimensional arrangement of muscles in the neutral position of the hip joint and observed the extension or shortening of muscles associated with passive maximum internal and external rotation of the hip joint. In addition, we evaluated the activity of the adductor muscle group by T₂ values (ms) from MRI pre- and post-internal rotation (forward step with the left leg) and pre- and post-external rotation (backward step with the left leg) movements of the right hip joint in a standing position (n = 8, healthy adult subjects, mean age 29.1 ± 5.3 years). Results: Regarding functional anatomy, the arrangement of the gluteus minimus and adductor muscle groups was almost parallel across the femoral neck. In the evaluation of adductor muscle group activity using MRI, the percent change in T₂ values (%) of the pectineus was 6.38 ± 1.35 pre- and post-internal rotation and 1.35 ± 0.71 pre- and post-external rotation, whereas that of the adductor longus and brevis was 4.84 ± 1.31 pre- and post-internal rotation and 1.31 ± 0.68 pre- and post-external rotation. The percent change in T₂ values pre- and post-internal rotation exercise was significantly greater than that pre- and post-external rotation exercise in the pectineus, adductor longus, and brevis muscles (p < 0.05). Conclusions: The adductor muscle groups are suggested to contribute to joint stability in the coronal plane and provide joint internal rotation in the standing position.