**Background:** Physical activity (PA) is associated with numerous health benefits, yet most children and adolescents worldwide are insufficiently active, with prevalence in Peru increasing from 82.6% in 2001 to 84.7% in 2016. PA is influenced by both genetic and environmental factors, and siblings offer a unique opportunity to study these influences because they share genes and environments but differ in age, sex, and lifestyle choices. Peru's distinct geographical diversity—spanning sea-level, Amazon, and high-altitude regions—provides a natural laboratory to examine environmental effects on PA. This study aimed to (1) estimate sibling resemblance in two PA phenotypes (total steps/day and minutes of moderate steps/day) and (2) investigate joint associations of individual characteristics (age, BMI) and geographical area of residence.
**Methods:** Data came from The Peruvian Sibling Study on Growth and Health, comprising 247 biological siblings (147 females, 100 males) from 110 nuclear families (67.2% two siblings, 32.8% three siblings) aged 6–17 years. Participants were recruited from three regions: sea-level (Barranco, 58 m), Amazon region (La Merced and San Ramon, 751 m), and high-altitude (Junín, 4107 m). PA was objectively measured using Omron Walking Style II pedometers over five consecutive days (three weekdays, two weekend days), recording total steps∙day⁻¹ and minutes of moderate steps∙day⁻¹ (≥3.0 METs). Height and weight were measured using standardized protocols, and BMI was calculated. Multilevel modeling was used to estimate intraclass correlation coefficients (ρ) for each PA phenotype, first with equal ρ across sib-types (Null model), then allowing different ρ (Model 1), then adjusting for age and BMI (Model 2), and finally adding geographical area (Model 3).
**Key Results:** No statistically significant differences were found among sib-ship pairs from the three regions for age or height. Siblings in the Amazon region were heavier (F=5.55, p<0.05), had higher BMI (F=13.55, p<0.05), and took more steps∙day⁻¹ (F=21.52, p<0.05) compared with other regions, but spent fewer minutes in moderate steps than those at sea-level (F=3.53, p<0.05). For both PA phenotypes, Model 1 (different ρ by sib-type) did not significantly improve fit over the Null model (equal ρ). From the Null model, the intraclass correlation for total steps∙day⁻¹ was ρ=0.44 (95%CI: 0.31–0.58) and for moderate steps was ρ=0.35 (95%CI: 0.22–0.51). Adjusting for individual characteristics (Model 2) and geographical area (Model 3) did not significantly change these correlations. In the full model for steps∙day⁻¹, sister-sister pairs took significantly fewer steps than brother-brother pairs (β=−2908.75±954.31), while brother-sister pairs did not differ significantly from brother-brother pairs. Older age was associated with fewer steps (β=−81.26±19.83 per year, p<0.05), while BMI was not significantly associated. Siblings at high-altitude (β=2508.92±737.94, p<0.05) and in the Amazon region (β=2213.11±776.63, p<0.05) took significantly more steps than those at sea-level.
**Clinical Implications:** The moderate sibling correlations for PA (ρ=0.35–0.44) that remained stable after adjusting for age, BMI, and geographical region suggest that genetic factors may be the primary driver of sibling resemblance in PA, rather than shared environmental factors. However, the significant regional differences in absolute PA levels indicate that environmental context still matters for overall activity. The finding that PA declines with age (81 fewer steps per year) reinforces the need for early interventions to maintain activity through adolescence. The lower PA levels at sea-level compared with high-altitude and Amazon regions suggest that urban environmental constraints (e.g., traffic, safety concerns, limited recreational space) may be important barriers. Clinicians and public health practitioners should consider both familial/genetic risk for inactivity and environmental opportunities when designing PA interventions for children and adolescents.