Effect of Localized Exercise on Blood Volume and Orthostatic Tolerance

Abstract

Previous studies have documented connections between orthostatic intolerance and changes in whole-body blood volume after exercise. However, few studies have looked at whether localized exercise has a direct influence on blood volume in the exercised limb and how that might influence orthostatic tolerance. PURPOSE: To determine the effects of localized exercise on blood volume within the gastrocnemius muscle and orthostatic tolerance. METHODS: Nine subjects (Age = 20.2 ± 1.3 yrs, Weight = 75.3 ± 21.1 kgs) participated in three graded LBNP (Lower Body Negative Pressure) tests to pre-syncope while forearm blood flow (FBF), heart rate (HR), blood pressure (BP), and leg blood volume were measured. Near-Infrared Spectroscopy (Portamon, Artinis, Inc.) in the right calf muscle was used to determine lower leg blood volume. Near-Infrared Spectroscopy measures continuous changes in blood volume by measuring the change in oxygenated (O2Hb) and deoxygenated (Hhb) hemoglobin concentration to determine total hemoglobin (tHb) concentrations Each subject was tested in a control trial (with no exercise), after exercise in the left leg (contralateral) of the exercise, and in the right leg (ipsilateral) of the exercise in a randomized order. Exercise trials were performed using an isokinetic dynamometer (Cybex Norm, Cybex Inc.) immediately before the LBNP testing. Subjects performed three sets of 30 plantar flexions with a 30 second rest period between sets. Cumulative stress index (CSI) was subsequently calculated (minutes completed x box pressure) to assess changes in orthostatic tolerance between the control and exercise conditions. RESULTS: CSI significantly decreased (p<0.05) after exercise in both the right and left leg trials (Right = 452.9 ± 51.8 mmHg.min; Left = 556.2±58.3mmHg.min) compared to control values (Control = 640.0 ± 70.3 mmHg.min) suggesting reduced orthostatic tolerance with leg exercise. Furthermore, during LBNP blood volume in the right leg (measured) was similar (P<0.05) after right and left leg exercise ( Right 0 = -4.2 ± 2.2, -70 mmHg = -16.0 ± 3.4; Left 0 = -2.7 ± 1.0, -70 mmHg 24.4 ± 4.0) CONCLUSION: Localized exercise in the lower leg significantly reduced orthostatic tolerance. However, it appears as if blood volume in the exercised leg is similar to blood volume in the non-exercised leg, suggesting no direct influence of localized exercise on venous compliance and LBNP tolerance.