Underweight children are agile but lack power

Given the knowledge gap in literature on the impact of undernutrition on muscular power and agility in school-aged children, the aim of this study was to compare physical fitness in such underweight- and normal weight children. In this cross-sectional study, 853 children were included (459 boys; mean age: 9.2 (1.8) years). The children were grouped according to their BMI-for-age-and-sex: normal weight (− 1 ≤ z-score < 2) and underweight (z-score < − 1). Within the underweight group, three thinness subgroups were composed: grade 1 (− 2 ≤ z-score < − 1), grade 2 (− 3 ≤ z-score < − 2) and grade 3 (z-score < − 3). Their agility, muscular endurance and power were assessed with the Performance and Fitness test battery (PERF-FIT). Regardless the country they lived in, the underweight children showed better agility (p = 0.012) and muscular endurance (p = 0.004) than those with normal weight. They presented with lower muscular power than the normal weight group, shown by significantly shorter overhead throwing distances (p = 0.017) and less standing long jump peak power (p < 0.001). The standing long jump peak power decreased further with increasing thinness grade (p = 0.027). Conclusion: Underweight children are more agile, but have lower muscular power compared to their normal weight peers. Its relationship with motor competence and physical activity, necessitates attention for tackling muscular strength deficiencies in these children, enabling them to meet the basic requirements for a healthy lifestyle later in life.