Physical fitness showed stronger associations with body composition than self-reported physical activity, with muscular strength remaining the most important predictor of body composition, and these associations were consistent across two independent samples collected in 2021 and 2025.
Key Findings
Results
Physical fitness showed stronger associations with body composition than self-reported physical activity in adults.
Cross-sectional data from 320 adults aged 34-82 years collected in 2025 were analyzed using sex-specific linear regression models.
Physical activity was assessed using a validated questionnaire, while physical fitness was measured through a standardized performance test battery.
Body composition was obtained via bioelectrical impedance analysis.
This pattern replicated findings from a previous 2021 dataset, indicating robustness across different societal conditions.
Results
Muscular strength was the most important predictor of body composition and showed the strongest association with phase angle.
For males, muscular strength was associated with phase angle at β = 0.40, p < .001.
For females, muscular strength was associated with phase angle at β = 0.31, p = .002.
Phase angle, a measure derived from bioelectrical impedance analysis, served as a key body composition outcome.
This association was consistent across both the 2021 and 2025 samples.
Results
No significant differences in body composition were found between the 2021 and 2025 samples.
The comparison provided post-COVID body composition statistics for adults living in Germany.
The 2025 sample consisted of 320 adults aged 34-82 years.
Despite no change in body composition, differences in other variables were observed between the two time points.
Results
Participants in 2025 had higher physical activity and muscular strength but lower coordination compared to participants in 2021.
Physical activity levels were higher in the 2025 sample than in the 2021 sample.
Muscular strength was higher in the 2025 sample compared to 2021.
Coordination scores were lower in 2025 compared to 2021.
These differences occurred despite no significant changes in body composition between the two time points.
Discussion
The consistency of associations between physical fitness and body composition across two independent samples indicates a robust pattern under different societal conditions.
Data were collected in two distinct periods: 2021 and 2025, which span pre- and post-COVID societal conditions.
The study was designed as a replication study to evaluate the reproducibility of previously observed associations.
The authors conclude this consistency 'highlights the importance of PF for supporting healthy aging with regard to BC.'
Sex-specific linear regression models were used in both datasets to ensure comparability.
What This Means
This research suggests that how physically fit a person is — particularly how strong their muscles are — is more closely linked to healthy body composition (the balance of fat, muscle, and other tissues in the body) than simply how much physical activity they report doing. The study analyzed data from 320 German adults between the ages of 34 and 82, collected in 2025, and compared these results to a similar dataset from 2021. The researchers found that muscular strength was consistently the strongest predictor of body composition measures, especially a marker called 'phase angle' which reflects cellular health and is measured using a technique called bioelectrical impedance analysis.
The study also found that, despite living through different societal conditions including the COVID-19 pandemic, the overall body composition of the adults in the 2025 sample did not differ significantly from those measured in 2021. However, people in 2025 reported being more physically active and had greater muscular strength, though their coordination scores were lower. These changes in fitness did not translate into measurable differences in body composition between the two groups.
This research suggests that the relationship between physical fitness — especially strength — and body composition is a robust and reproducible pattern, not just a one-time finding. This may be relevant for understanding how to support healthy aging, as maintaining muscle strength appears to be particularly important for body composition outcomes regardless of broader social or environmental changes.
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Schilling R, Fiedler J, Krell-Roesch J, Woll A, Schmidt S. (2026). Associations between physical activity, physical fitness, and body composition in adults living in Germany: a cross-sectional replication study.. Scientific reports. https://doi.org/10.1038/s41598-026-61611-6