Abstract
PurposeDuring 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.MethodsIn 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.ResultsDuring 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.ConclusionsOur 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.
Highlights
In upright body posture, perfusion of working leg muscles profits from the muscle pump, which increases the arterio-venous difference of blood pressure
lower body negative pressure (LBNP) led to a moderate orthostatic reaction characterised by an increase in heart rate (HR) from around 70 to 80/min (p < 0.05) and a decrease in stroke volume (SV) from approximately 110 to 80 ml (Fig. 2; p < 0.01)
We introduce a novel combination of a robotically controlled leg press within an LBNP chamber as an experimental model with promising results for simulating gravity like blood shifts in exercising leg muscles
Summary
Perfusion of working leg muscles profits from the muscle pump, which increases the arterio-venous difference of blood pressure. The muscle pump enhances central venous pressure and thereby increases the stroke volume of the heart (Leyk et al 1994; Laughlin and Schrage 1999; Rowland 2001). In an actual change in body posture from horizontal to upright, gravity mechanically affects muscle and alters neuromuscular control. Identical exercise performed at horizontal or upright posture may not result in identical muscle action, which complicates the comparison of gravity and posture dependent perfusion effects on muscle. We developed a simplified model by combining an electrically driven and robotically controlled leg press with a lower body negative pressure (LBNP) chamber for studying effects exclusively evoked by blood shifts to the exercising leg muscle without any further effects linked with an actual change in posture
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