Acute effects of passive stretching on skeletal muscle microvascular P <scp>o</scp> ₂ and HIF-1α expression: influence of stretch intensity and duration

This study investigated the acute effects of passive stretching on microvascular oxygen partial pressure ([Formula: see text]) and hypoxia-inducible factor-1α (HIF-1α) expression in rat skeletal muscle, focusing on stretch intensity and duration. Twenty male Wistar rats were assigned to either a stretch or sham group. In <i>study 1</i>, the soleus muscle was passively stretched at varying intensities by changing its length from the optimal length (L_O) by 2, 4, 6, 8, and 10 mm, whereas [Formula: see text] was simultaneously measured. In a separate experiment, the muscle was stretched from L_O to 8 mm and maintained in the stretched position for 2 h, whereas in the sham group it was kept at L_O throughout. After stretching, the muscle was rapidly frozen, and HIF-1α mRNA was quantified by real-time PCR. Passive stretching induced an acute, intensity-dependent decrease in [Formula: see text]. Values during high-intensity stretches (6-10 mm) were significantly lower than in the sham group (25 ± 9 vs. 39 ± 7 mmHg, 8 mm vs. L_O; <i>P</i> < 0.05). Sustained 8 mm stretching caused a rapid decline in [Formula: see text] within 40 s, followed by a stable low plateau for 2 h (time <i>F</i> = 11.2; group <i>F</i> = 17.9; interaction <i>F</i> = 2.10; <i>P</i> < 0.01). Interestingly, 2 h of stretching reduced HIF-1α mRNA expression. These findings demonstrate that passive stretching elicits an intensity-dependent and sustained reduction in microvascular Po₂, which may suppress HIF-1α mRNA expression in skeletal muscle.<b>NEW & NOTEWORTHY</b> The intramuscular oxygen dynamics during stretching remain unclear. Real-time changes in Po₂ during passive stretching were investigated using phosphorescence quenching. Microvascular Po₂ decreased in an intensity-dependent manner and remained low during prolonged stretching. The decline reached a plateau about 2 min after stretch onset. Moreover, 2 h of sustained stretching downregulated HIF-1α mRNA. Collectively, these findings indicate that passive stretching induces acute, intensity-dependent microvascular hypoxia that persists and alters hypoxia-related transcriptional responses in skeletal muscle.