ABSTRACT
The purpose of this study was to determine the effect of baroreceptor unloading on the sensitivity of the cardiovagal and sympathetic arms of the baroreflex during upright posture. Beat-by-beat R-R interval, arterial blood pressure and cardiac output (Doppler ultrasound), as well as muscle sympathetic nerve activity (MSNA) were recorded during periods in supine (Supine) and 60 deg head-up tilt (HUT) positions (n = 8 volunteers). Cardiovagal baroreflex sensitivity (BRS) was measured by the spontaneous sequence analysis method using systolic blood pressure and R-R interval, while sympathetic BRS was determined using the slope of the linear relationship between decreasing segments of diastolic blood pressure (DBP) and corresponding increases in MSNA. On changing to HUT, mean R-R interval and cardiac output decreased, while mean measures of MSNA, DBP and total peripheral resistance increased (P < 0.05). Cardiovagal BRS decreased from Supine to 60 deg HUT (19 +/- 2 ms mmHg(-1) versus 7.6 +/- 1.2 ms mmHg(-1); P < 0.01). In contrast, sympathetic BRS increased from -6.1 +/- 1.4 a.u. mmHg(-1) in Supine to -14 +/- 2 a.u. mmHg(-1) in HUT (P < 0.01). Thus, HUT produced differential effects on cardiac versus sympathetic BRS. The data suggest that dynamic baroreflex-mediated cardiovascular control is dominated by sympathetic control during baroreceptor unloading.
Subject(s)
Baroreflex/physiology , Head/physiology , Heart/innervation , Heart/physiology , Peroneal Nerve/physiology , Posture/physiology , Vagus Nerve/physiology , Adult , Blood Pressure/physiology , Female , Heart Rate/physiology , Hemostasis/physiology , Humans , Male , Reproducibility of Results , Sensitivity and Specificity , Statistics as Topic , Supine Position/physiology , Sympathetic Nervous System/physiologyABSTRACT
Daily activities involve dynamic muscle contractions that yield nonstationary myoelectric signals (MESs). The purpose of this work was to determine the individual effects of four time-varying factors (the number and firing rate of active motor units, muscle force and joint angle) on the mean frequency of a MES. Previous theoretical and experimental work revealed that although changes in the number and firing rate of active motor units contribute to the nonstationarities of the signal, they do not significantly affect the mean frequency. In the experimental work, 12 subjects performed 25 static contractions, one for each force (20, 30, 40, 50 and 60% of maximum voluntary contraction) and elbow joint angle (50, 70, 90, 110 and 130 degrees extension) combination. A MES was recorded from the surface of the biceps brachii during each contraction. The results indicated that muscle force only weakly affects the mean frequency. Also shown was that alteration in muscle geometry resulting from changes in elbow joint angle does significantly affect the mean frequency. Knowing this is important for the assessment of muscle fatigue during dynamic contractions.
