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Thanks to Dr. Herman Pontzer (Duke University) for his valuable feedback. We also thank the following agencies for their funding: Fondo Nacional de Desarrollo Científico y Tecnológico (11180361 to R.F.-V.); Spanish Ministry of Education, Culture and Sport (FPU15/04059 to J.M.A.A.; FPU15/02645 to J.H.M.; FPU14/04172 to F.J.A.-G.); University of Granada (Plan Propio de Investigación 2019 [Programa Contratos-Puente] to F.J.A.-G.; Plan Propio de Investigación 2016 [Excellence actions: Unit of Excellence on Exercise and Health UCEES]); Spanish Ministry of Economy and Competitiveness (ACTIBATE study; ACTIVEBRAINS study); Fondo de Investigación Sanitaria del Instituto de Salud Carlos III (PI13/01393 to ACTIBATE study); Junta de Andalucía, Consejería de Conocimiento, Investigación y Universidades and European Regional Development Funds (FEDER: ref. SOMM17/6107/UGR to ACTIBATE study); Redes Temáticas de Investigación Cooperativa RETIC (Red SAMID RD16/0022 to ACTIBATE study); EXERNET Research Network on Exercise and Health in Special Populations (DEP2005-00046/ACTI); Fundación Iberoamericana de Nutrición (ACTIBATE study); AstraZeneca HealthCare Foundation (ACTIBATE study); PTA 12264-I to FIT-AGEING study., Supplementary Information The online version contains supplementary material available at https://doi. org/10.1038/s41598-021-91750-x., The constrained total energy expenditure (TEE) model posits that progressive increases in physical activity (PA) lead to increases in TEE; but after certain PA threshold, TEE plateaus. Then, a compensatory reduction in the expenditure of non-essential activities constrains the TEE. We hypothesized that high PA levels as locomotion associate with a compensatory attenuation in arm movements. We included 209 adults (64% females, mean [SD] age 32.1 [15.0] years) and 105 children (40% females, age 10.0 [1.1] years). Subjects wore, simultaneously, one accelerometer in the non-dominant wrist and another in the hip for ≥ 4 days. We analyzed the association between wrist-measured (arm movements plus locomotion) and hip-measured PA (locomotion). We also analyzed how the capacity to dissociate arm movements from locomotion influences total PA. In adults, the association between wrist-measured and hip-measured PA was better described by a quadratic than a linear model (Quadratic-R2 = 0.54 vs. Linear-R2 = 0.52; P = 0.003). Above the 80th percentile of hip-measured PA, wrist-measured PA plateaued. In children, there was no evidence that a quadratic model fitted the association between wrist-measured and hip-measured PA better than a linear model (R2 = 0.58 in both models, P = 0.25). In adults and children, those with the highest capacity to dissociate arm movements from locomotion—i.e. higher arm movements for a given locomotion—reached the highest total PA. We conclude that, in adults, elevated locomotion associates with a compensatory reduction in arm movements (probably non-essential fidgeting) that partially explains the constrained TEE model. Subjects with the lowest arm compensation reach the highest total PA., AstraZeneca HealthCare Foundation PTA 12264-I, EXERNET Research Network on Exercise and Health in Special Populations DEP2005-00046/ACTI, Fundación Iberoamericana de Nutrición, Fondo Nacional de Desarrollo Científico y Tecnológico 11180361 FONDECYT, Ministerio de Economía y Competitividad MINECO, Instituto de Salud Carlos III PI13/01393 ISCIII, Universidad de Granada UGR, European Regional Development Fund RD16/0022,SOMM17/6107/UGR ERDF, Consejería de Conocimiento, Investigación y Universidad, Junta de Andalucía, Vicerrectorado de Investigación y Transferencia, Universidad de Granada Health [UCEES]),Plan Propio de Investigación 2016 (Excellence actions: Unit of Excellence on Exercise,Plan Propio de Investigación 2019 (Programa Contratos-Puente)

A.P.-F. and J.M.A.: conception and design, statistical analysis, interpretation of data, writing paper. F.J.A.G. and F.M.A.: conducted experiments, analyzed and interpretation of data, feedback on the paper. J.M.G.: feedback on the paper. J.H.M., J.S., and F.B.O.: interpretation of data, feedback on the paper. All authors read and approved the final manuscript., Heart rate variability (HRV) is a valid and non-invasive indicator of cardiac autonomic nervous system functioning. Short-term HRV recordings (e.g., 10 min long) produce data that usually is manually processed. Researcher subjective decision-making on data processing could produce inter- or intraresearcher differences whose magnitude has not been previously quantified in three independent human cohorts. This study examines the inter- and intra-researcher reproducibility of HRV parameters (i.e., the influence of R–R interval selection by different researchers and by the same researcher in different moments on the quantification of HRV parameters, respectively) derived from short-term recordings in a cohort of children with overweight/obesity, young adults and middle-age adults. Participants were recruited from 3 different studies: 107 children (10.03 ± 1.13 years, 58% male), 132 young adults (22.22 ± 2.20 years, 33% males) and 73 middle-aged adults (53.62 ± 5.18 years, 48% males). HRV was measured using a Polar RS800CX heart rate monitor. The intraclass correlation coefficient (ICC) ranged from 0.703 to 0.989 and from 0.950 to 0.998 for inter-and intra-researcher reproducibility, respectively. Limits of agreement for HRV parameters were higher for the interresearcher processing compared with the intra-researcher processing. On average, the intraresearcher differences were 31%, 62%, and 80% smaller than the inter-researchers differences based on Coefficient of Variation in children, young and middle-aged adults, respectively. Our study provides the quantification of the inter-researcher and intra-researcher differences in three independent human cohorts, which could elicit some clinical relevant differences for HRV parameters. Based on our findings, we recommend the HRV data signal processing to be performed always by the same trained researcher and we postulate a development of algorithms for an automatic ECG selection., Spanish Ministry of Economy and Competitiveness DEP2013-47540 DEP2016-79512-R PTA 12264-I, Spanish Ministry of Education, Culture and Sport FPU 16/02760 FPU15/02645 FPU15/04059 FPU14/06837 FPU14/04172, Unit of Excellence on Exercise and Health (UCEES), EXERNET Research Network on Exercise and Health in Special Populations DEP2005-00046/ACTI, Unit of Excellence in Sport and Health (UCEES) - University of Granada, Junta de Andalucía, European Union (EU) SOMM17/6107/UGR

Cold exposure is necessary to activate human brown adipose tissue (BAT), resulting in heat production. Skin temperature is an indirect measure to monitor the body’s reaction to cold. The aim of this research was to study whether the most used equations to estimate parameters of skin temperature in BAT-human studies measure the same values of temperature in young lean men (n = 11: 23.4 ± 0.5 years, fat mass: 19.9 ± 1.2%). Skin temperature was measured with 26 ibuttons at 1-minute intervals in warm and cold room conditions. We used 12 equations to estimate parameters of mean, proximal, and distal skin temperature as well as skin temperature gradients. Data were analysed with Temperatus software. Significant differences were found across equations to measure the same parameters of skin temperature in warm and cold room conditions, hampering comparison across studies. Based on these findings, we suggest to use a set of 14 ibuttons at anatomical positions reported by ISO STANDARD 9886:2004 plus five ibuttons placed on the right supraclavicular fossa, right middle clavicular bone, right middle upper forearm, right top of forefinger, and right upper chest.