Philip N. Ainslie

Estimating human energy expenditure: A review of techniques with particular reference to doubly labelled water

This review includes an historical overview of the techniques for measuring energy expenditure (EE). Following this overview, the ‘gold standard’ method of measuring EE, the doubly labelled water (DLW) method, is emphasised. Other methods, such as direct calorimetry, indirect calorimetry systems, heart rate and EE relationships, questionnaires and activity recall, motion sensors, combined heart rate and motion sensors for the estimation of EE are then highlighted in relation to their validation against the DLW method. The major advantages and disadvantages for each method are then considered. The preferred method to determine EE is likely to depend principally on factors such as the number of study participants to be monitored, the time period of measurements and the finances available. Small study participant numbers over a short period may be measured accurately by means of indirect calorimetric methods (stationary and portable systems). For periods over 3–4 days, EE should ideally be measured using the DLW method. However, the use of motion sensors is very promising in the measurement of EE, and has a number of advantages over the DLW method. Furthermore, if used correctly, both heart rate and questionnaire methods may provide valuable estimates of EE. Additional studies are needed to examine the possibility of improving the accuracy of measurement by combining two or more techniques. Such information, if confirmed by scientific rigour, may lead to an improvement in the estimation of EE and population-based physical activity levels. The accurate measurement of physical activity and EE is critical from both a research and health prospective. A consideration of the relevant techniques used for the estimation of EE may also help improve the quality of these frequently reported measurements.

Differential responses to CO2 and sympathetic stimulation in the cerebral and femoral circulations in humans

The relative importance of CO2 and sympathetic stimulation in the regulation of cerebral and peripheral vasculatures has not been previously studied in humans. We investigated the effect of sympathetic activation, produced by isometric handgrip (HG) exercise, on cerebral and femoral vasculatures during periods of isocapnia and hypercapnia. In 14 healthy males (28.1 ± 3.7 (mean ±s.d.) years), we measured flow velocity (; transcranial Doppler ultrasound) in the middle cerebral artery during euoxic isocapnia (ISO, +1 mmHg above rest) and two levels of euoxic hypercapnia (HC5, end‐tidal PCO2, P u2009ET,COu20092 , =+5 mmHg above ISO; HC10, P u2009ET,COu20092 =+10 above ISO). Each PET,CO2 level was maintained for 10 min using the dynamic end‐tidal forcing technique, during which increases in sympathetic activity were elicited by a 2‐min HG at 30% of maximal voluntary contraction. Femoral blood flow (FBF; Doppler ultrasound), muscle sympathetic nerve activity (MSNA; microneurography) and mean arterial pressure (MAP; Portapres) were also measured. Hypercapnia increased and FBF by 5.0 and 0.6% mmHg−1, respectively, and MSNA by 20–220%. Isometric HG increased MSNA by 50% and MAP by 20%, with no differences between ISO, HC5 and HC10. During the ISO HG there was an increase in cerebral vascular resistance (CVR; 20 ± 11%), while remained unchanged. During HC5 and HC10 HG, increased (13% and 14%, respectively), but CVR was unchanged. In contrast, HG‐induced sympathetic stimulation increased femoral vascular resistance (FVR) during ISO, HC5 and HC10 (17–41%), while there was a general decrease in FBF below ISO. The HG‐induced increases in MSNA were associated with increases in FVR in all conditions (r= 0.76–0.87), whereas increases in MSNA were associated with increases in CVR only during ISO (r= 0.91). In summary, in the absence of hypercapnia, HG exercise caused cerebral vasoconstriction, myogenically and/or neurally, which was reflected by increases in CVR and a maintained . In contrast, HG increased FVR during conditions of ISO, HC5 and HC10. Therefore, the cerebral circulation is more responsive to alterations in PCO2, and less responsive to sympathetic stimulation than the femoral circulation.

Multivariate System Identification for Cerebral Autoregulation

The effect of spontaneous beat-to-beat mean arterial blood pressure (ABP) fluctuations and breath-to-breath end-tidal carbon dioxide

Evidence against redox regulation of energy homoeostasis in humans at high altitude

(P_{ETCO_2})

Effects of five nights of normobaric hypoxia on the ventilatory responses to acute hypoxia and hypercapnia

and end-tidal oxygen

Cerebral oxidative metabolism is decreased with extreme apnea in humans; impact of acidosis

(P_{ETO_2})