Estimating Energy Expenditure During Physical Activity In Simulated Military Operations

Abstract

Background Inadequate energy intake during simulated military operations (SMO) can lead to an unsustainable rate of loss in body mass (BM) often ascribed to prolonged physical activity (PA). Successful remedial strategies require an accurate assessment of energy expenditure (EE). Accelerometry (Ac) is often used to assess PA but can it accurately quantify EE? Purpose To assess the use of uniaxial Ac to estimate EE of physical activity (EEA) during SMOs. Methods 2 SMOs were used: (a) Defence 2.5 days. 6 men, mean (1 SD) age 25.5 (4.3) years; BM 76.7 (6.2) kg; fat-free mass (FFM) 62.9 (1.4) kg; VO2max (multistage fitness test) 3.8 (0.4) L·min−1; (b) Offence 6.5 days. 5 men, age 23.2 (3.7) years; BM 87.4 (6.3) kg; FFM 71.6 (2.7) kg; VO2max 4.1 (0.2) L·min−1. In Defence subjects patrolled an airfield. Offence involved long distance marches with heavy loads, followed by short sprints. EE was determined by doubly-labelled water (DLW). EEA was calculated as 0.9EE - BMR (basal metabolic rate). Other measurements were: right ankle (Ak) and wrist (Wr) z-axis Ac every 2 s; duration (time (min)) of PA (assumed when Ac exceeded 20 counts·min−1); and heart rate ((HR) during Defence only) every 5 s, from which relative heart rate reserve (%HRR) was calculated. RESULTS During Defence EEA(DLW) was 6.9 (6.8) MJ·day−1; Ak 196883 (189849) counts; Wr 99585 (87738) counts. During Offence, EEA(DLW) was 7.5 (5.8) MJ·day−1; Ak 223805 (176148) counts; Wr 388408 (328985) counts. Stepwise multiple linear regression analysis (EEA(DLW)) was conducted and 95% limits of agreement (LOA) were calculated for 4 equations (E1 to E4) (significance was set at α=0.05): Defence E1* 0.37 age −0.00001 Wr −0.0000 Ak + 0.0083 time + 0.46 FFM −35.77 (r2 = 0.09; LOA −4.26 to 7.81) MJ·day−1 E2* 0.000001 Wr −0.000001 Ak + 0.19 BM + 0.33 age −0.073 %HRR + 19.8 BMR −0.0087 time −0.64 FFM −101.97 (r2 = 0.64; LOA −4.31 to 1.83) MJ·day−1 E3 0.182 age + 37.9 BMR −0.38 %HRR −2.58 FFM −0.58 Body Mass Index + 6.6 VO2max + 0.062 HR −94.47 (r2 = 0.85; LOA −1.54 to 1.88) MJ·day−1 Offence E4* 0.00002 Ak + 4.9 VO2max + 0.22 BM + 0.00001 Wr −37.63 (r2 = 0.70; LOA −8.27 to 5.34) MJ·day−1 EEA based on Ak and Wr (E1, E2 and E4) differed (*p<0.05) from EEA(DLW) for both SMOs. Differences with EEA(DLW) during Defence were not significant when Ac was excluded (E3) in preference to %HRR and HR. Conclusion Uniaxial Ac alone (at the ankle and wrist) does not afford accurate estimates of EEA during SMOs. Cardiovascular strain (%HRR and HR) during physical activity should be considered when estimating EEA for Defence. Funded by the Human Sciences Domain, of the UK Ministry of Defence Scientific Research Programme.