Hemodynamic measurements
CONTRACTION
Using a Philips CX50 (2013) Doppler ultrasound machine (Philips Medical Systems, Andover, MA, USA), the widest accessible part of the popliteal and femoral veins was located and visualized in a longitudinal plane before beginning the hemodynamic measurements. The diameters of the veins were calculated at baseline. The diameter of the popliteal vein was 0.90 ± 0.29 cm and of the femoral vein 1.05 ± 0.26 cm. For the two electrode setups (TTE & MPE), PVV was measured in the popliteal and femoral veins at baseline (i.e., electrodes attached but no NMES administered), at ML I, and at ML II. For each subject in the study, all hemodynamic measurements were performed by the same ultrasonographist. During three consecutive NMES-stimulation cycles venous measurements were recorded, and subsequently the peak venous velocity (PVV) was assessed. The measuring tool on the ultrasound machine provided, when using doppler, the ability to save the recordings of blood flow in cm/s and measure the peak venous velocity after the stimulation was complete. The diameter of the vein during the stimulation was not calculated. PVV measurements during three NMES-stimulation cycles were performed and the mean of the three was used for statistical analysis. After analyzing the refill time of the veins, an eight second OFF-time was decided to be used between ON-times for the veins to be adequately refilled with blood before the next upcoming muscle contraction and PVV measurement. To quantify the potential benefit of NMES versus the baseline resting state, for each subject and setting, the percentual increase in PVV at ML I and ML II