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Physical inactivity and sedentary behaviors are associated with adverse health outcomes in both adults and children. The purpose of this study was to investigate the effects of a 9 weeks program using a Cycle Desk during school time in primary school French children from high or low socio economic status (SES) on body composition, physical fitness (PF) and executive functions. Methods: Seventy-five children completed a test battery before (T0) and after 9 weeks of use of Cycle Desk (T1) evaluating their anthropometric characteristics, body composition, PF and executive functions. Results: Body mass index increased significantly, while body fat decreased after the use of Cycle Desks. Specifically, lean mass increased in the high-SES group while it decreased in the low-SES group. Performances on PF and executive functions tests increased significantly after 9 weeks of use of Cycle Desk. More specifically, the low-SES group had a greater improvement in PF between T0 and T1, compared to the high-SES group. Regarding executive functions, the high-SES group had a greater improvement in performance at T1 compared to the low-SES group. Conclusion: The promotion of low-intensity PA with the use of Cycle Desk at school, seems to be an interesting strategy in the fight against adverse effects of sedentary behaviour among children. Moreover, this strategy appears to be particularly effective in children of low SES.

Scepticism or disinterest: constitutional thinness remains little explored today, with only about forty studies published to date. The literature appears not only equivocal regarding the physiological characterisation of constitutional thinness, but also with regard to its definition and diagnosis. The first axis aimed to define the diagnosis and characterise the physiology of constitutional thinness, and the second objective was to explore the muscle tissue of this population. The present thesis work allowed to propose a decision tree to help in the diagnosis of constitutional thinness, based on a systematic review of the literature. Moreover, the meta-analytical approach showed that the constitutionally thin individuals present a specific physiology regarding body composition, energy metabolism and hormonal regulation; which is however very different from that of patients with anorexia nervosa. Preliminarily to muscle cell exploration, this doctoral work validated new methods of immunohistochemical muscle staining. Finally, the exploration of the skeletal muscle of constitutionally thin subjects revealed a strong muscular hypotrophy, a low capillarisation, an alteration of some enzymatic activities but also a low storage of muscle triglycerides and glycogen. These results have brought new insights in the understanding of this atypical physiology which characterizes constitutional thinness.; Scepticisme ou désintérêt : la maigreur constitutionnelle reste aujourd’hui très peu explorée, avec seulement une quarantaine d’études publiées à ce jour. La littérature apparaît non seulement équivoque vis-à-vis de la caractérisation physiologique de la maigreur constitutionnelle, mais également à propos de sa définition et de son diagnostic. Le premier axe avait pour objectif de définir le diagnostic et de caractériser la physiologie de la maigreur constitutionnelle et le second objectif visait à explorer le tissu musculaire de cette population. Les travaux de thèse ont permis de proposer un arbre décisionnel d’aide au diagnostic de la maigreur constitutionnelle basé sur une revue systématique de la littérature. Par ailleurs, l’approche méta-analytique a montré que l’individu maigre constitutionnel présente une physiologie particulière en termes de composition corporelle, métabolisme énergétique et régulation hormonale ; néanmoins très différente de celle du patient présentant une anorexie mentale. De façon préliminaire à l’exploration cellulaire du muscle, ce travail de doctorat a permis de valider de nouvelles méthodes de colorations immunohistochimiques musculaires. Finalement, l’exploration du muscle squelettique du sujet maigre constitutionnel a mis en évidence une forte hypotrophie musculaire, une faible capillarisation, une altération de certaines activités enzymatiques mais également un faible stockage en triglycérides et en glycogène musculaires. Ces résultats ont ainsi permis d’apporter de nouvelles perspectives dans la compréhension de cette physiologie atypique qui caractérise la maigreur constitutionnelle.

Constitutional thinness (CT) is a nonpathological state of underweight. The current study aimed to explore skeletal muscle energy storage in individuals with CT and to further characterize muscle phenotype at baseline and in response to overfeeding. Thirty subjects with CT (15 females, 15 males) and 31 normal-weight control subjects (16 females, 15 males) participated in the study. Histological and enzymological analyses were performed on muscle biopsy specimens before and after overfeeding. In the skeletal muscle of CT participants compared with controls, we observed a lower content of intramuscular triglycerides for type I (−17%, p < 0.01) and type IIA (−14%, p < 0.05) muscle fibers, a lower glycogen content for type I (−6%, p < 0.01) and type IIA (−5%, p < 0.05) muscle fibers, a specific fiber-type distribution, a marked muscle hypotrophy (−20%, p < 0.001), a low capillary-to-fiber ratio (−19%, p < 0.001), and low citrate synthase activity (−18%, p < 0.05). In response to overfeeding, CT participants increased their intramuscular triglycerides content in type I (+10%, p < 0.01) and type IIA (+9%, p < 0.01) muscle fibers. CT individuals seem to present an unusual muscle phenotype and different adaptations to overfeeding compared with normal-weight individuals, suggesting a specific energy metabolism and muscle adaptations. ClinicalTrials.gov registration no. NCT02004821. Novelty Low intramuscular triglycerides and glycogen content in skeletal muscle of constitutionally thin individuals. Low oxidative capacity, low capillary supply, and fiber hypotrophy in skeletal muscle of constitutionally thin individuals. Increase in intramuscular triglycerides in constitutional thinness in response to overfeeding.