Lower body negative pressure enhances oxygen availability in the knee extensor muscles during intense resistive exercise in supine position

Abstract

Purpose: During exercise in supine posture or under microgravity in space, the gravity-dependent component of local blood pressure in leg muscles at upright posture can be simulated by lower body negative pressure (LBNP). We hypothesized that during resistive exercise LBNP favors oxygen availability in lower extremities, benefiting energy levels and performance of working muscles.

Methods: In permutated crossover design, nine subjects performed a series of fifteen slow-paced concentric (4 s) and eccentric contractions (4 s) without or with 40 mmHg LBNP and 4 s pause between repetitions. The force at knee flexion was 6% of the one repetition maximum (1-RM) and gradually increased to 60% 1RM in the first half of the individual range of motion, subsequently remaining constant until full extension.

Results: During the low force periods of continuous exercise, LBNP enhanced the refill of capillary blood measured by near infrared spectroscopy, amplifying the increase of total haemoglobin by about 20 µmol/l (p < 0.01) and oxyhaemoglobin by about 10 µmol/l (p < 0.01). During continuous exercise, LBNP induced a trend towards a lower EMG increment. This LBNP effect was not found when the periods of low forces at knee flexion were extended by 4 s pauses. Increased respiratory oxygen uptake (+ 0.1 l/min, p < 0.05) indicated overall enhanced muscle energy turn-over.

Conclusions: Our results suggest stimulation of oxidative metabolism through LBNP enables working muscles to meet the energy demands of intense exercise. Further research is needed on the consequences for energy metabolism and the molecular control of growth and differentiation.

Keywords: Leg press; Lower body negative pressure; Muscle perfusion; Muscle pump; Simulated orthostasis; Slow concentric–eccentric resistance exercise.

Conflict of interest statement

Conflict of interest

No conflicts of interest, financial or otherwise, are declared by the authors.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Figures

Fig. 1

Robotically controlled leg press within the LBNP chamber (Institute of Aerospace Medicine, German Aerospace Center, Cologne)

Fig. 2

Mean values (± SEM, n = 9) of heart rate (a), stroke volume (b), cardiac output (c) and total peripheral resistance (TPR, d) at baseline before LBNP onset (BL1, LBNP experiments only), baseline before the first warm-up (BL2), fifteen repetitions of contractions during the main exercise set (C1–C15), after 3 min recovery (R1, when applicable under LBNP) and after 18 min recovery (R2). Exercise sessions were performed under control (open circle) and LNBP (filled circle) conditions, without or with 4 s pause in between repetitions. Statistical significance (p < 0.05 or p < 0.01) is shown for effects of LBNP during exercise with/without pause (asterisk) and the effects of added pause during exercise under LBNP (hash) or ambient pressure (plus)

Fig. 3

Changes in the levels of total haemoglobin (ΔtHb, a), tissue oxygen saturation (ΔTSI%, b), oxyhaemoglobin (ΔO2Hb, c) and deoxyhaemoglobin (ΔHHb, d). Time points and statistical significance are labeled as in Fig. 2

Fig. 4

Respiratory oxygen uptake (ΔV′O2, a means ± SEM, n = 9, deltas from baseline 1), carbon dioxide release (ΔV′CO2, b), and the respiratory exchange ratio (RER, c) during exercise sessions (E) and subsequent recovery phases (R1, R2) under control (gray bars) and LBNP conditions (black bars). Statistical significance is labeled as in Fig. 2

Fig. 5

Respiratory oxygen uptake (V′O2), carbon dioxide release (V′CO2) and ventilation during the recovery stages R1 (with LBNP) and R2 (ambient pressure) following the main exercise set

Fig. 6

Lactate levels (means ± SEM, n = 9, deltas from baseline 1) after the main exercise set, measured in capillary blood obtained from the ear lobe. Statistical significance is labeled as in Fig. 2

Fig. 7

EMG amplitude and median frequency across the fifteen contractions of the main exercise set relative to the first contraction (means ± SEM, n = 9), measured bilaterally on the vastus lateralis muscle. Statistical significance is labeled as in Fig. 2