Lower body negative pressure (LBNP) enhances the oxygen supply of the knee extensor muscles during highly intensive resistive strength training in supine position

Kurzfassung

Due to its impact on the cardiovascular system, lower body negative pressure (LBNP) can be used to simulate orthostasis and replace the gravity dependent blood pressure component in the lower extremities during exercise in supine position. Highly intensive resistive exercise includes periods of intense muscle force impeding blood flow through the muscle that alternate with periods of lower force facilitating muscle perfusion. In the present study, we hypothesized that LBNP enhances the availability of oxygen in thigh muscles during intense resistive muscle contractions, with possible enhanced oxygen exploitation and consequential benefits for energy metabolism and maintenance of muscle cell activity. Series of 15 slow repetitions of concentric (4s) and eccentric (4s) contractions were performed on a robotically controlled, horizontal leg press by 18 healthy male subjects divided into control and LBNP intervention groups. At knee flexion, contractions started with a target force corresponding to 10% of the one-repetition maximum (1RM). Within first half of the range of motion, force increased to 60% 1RM and remained constant up to full extension. Force profile was consequently reversed during the eccentric phases of motion. Subjects from both groups performed the same training profile, in the control group without LBNP and in the LBNP group with -40mmHg negative pressure. Overall oxygen consumption was measured through spirometry and local total hemoglobin (tHb) content and tissue oxygen saturation index (TSI) of the Vastus lateralis muscle were analyzed with near infrared spectroscopy. Muscle biopsies were taken before and 10min, 30min and 60min after the intervention and the levels of total and phosphorylated form of the energy sensing protein AMPK determined through Western Blot. Overall oxygen consumption was elevated with LBNP. At high contraction forces, tHb content reached almost equal levels in both the control and LBNP intervention and was elevated in the LBNP group during the periods of low force with a concomitant reduction of TSI. Furthermore, phosphorylation of AMPK was enhanced with LBNP 10min and 30min post-exercise. Taken together, during particular highly intense resistive exercise on a leg press, simulated orthostasis enhanced the oxygen supply and utilization in thigh muscles, affecting energy sensing signaling pathways involved in the regulation of protein synthesis in muscle cells.