Bone Tissue Responsiveness To Mechanical Loading-Possible Long-Term Implications of Swimming on Bone Health and Bone Development

Swimming induces bone loss via regulating mechanical sensing pathways in bone marrow

Bone is an organ capable of perceiving external mechanical stress in real time and responding dynamically via mechanosensing proteins such as Piezo1 and YAP/TAZ. Upon sensing the mechano-signals, cells within the bone matrix collaborate to coordinate bone formation and resorption, while bone marrow cells are also stimulated and mobilized. High-load exercise stimulates osteoblast differentiation and bone formation. However, the mechanism through which the low-load exercises affect bone homeostasis is still unclear. In this work, we established a long-term swimming training model to unload the mechanical stress in mice. Throughout the training model, we observed a significant loss in trabecular bone mass, as evidenced by microCT scanning and histological staining. Single-cell sequencing of the tibial bone marrow tissue revealed a significant increase in the percentage of bone marrow neutrophils, along with alterations in Integrins and the ERK1/2 signaling pathway. Notably, the changes in both Integrins and the ERK1/2 signaling pathway in macrophages were more pronounced than in other cell types, which suggests a mechanical adaptive response in these cells. Moreover, the involvement of Integrins is also critical for the crosstalk between monocyte precusors and macrophages during swimming. Together, this study provides a resource of the alterations of bone marrow cell gene expression profile after swimming and highlights the importance of Integrins and the ERK1/2 signaling pathway in the bone marrow microenvironment after swimming.

Does Swimming Exercise Impair Bone Health? A Systematic Review and Meta-Analysis Comparing the Evidence in Humans and Rodent Models

BACKGROUND

The effect of swimming on bone health remains unclear, namely due to discrepant findings between studies in humans and animal models.

OBJECTIVE

The aim of this systematic review and meta-analysis is to identify the available evidence on the effects of swimming on bone mass, geometry and microarchitecture at the lumbar spine, femur and tibia in both humans and rodent animal models.

METHODS

The study followed PRISMA guidelines and was registered at PROSPERO (CRD4202236347 and CRD42022363714 for human and animal studies). Two different systematic literature searches were conducted in PubMed, Scopus and Web of Science, retrieving 36 and 16 reports for humans and animal models, respectively.

RESULTS

In humans, areal bone mineral density (aBMD) was similar between swimmers and non-athletic controls at the lumbar spine, hip and femoral neck. Swimmers' tibia diaphysis showed a higher cross-sectional area but lower cortical thickness. Inconsistent findings at the femoral neck cortical thickness were found. Due to the small number of studies, trabecular microarchitecture in human swimmers was not assessed. In rodent models, aBMD was found to be lower at the tibia, but similar at the femur. Inconsistent findings in femur diaphysis cross-sectional area were observed. No differences in femur and tibia trabecular microarchitecture were found.

CONCLUSION

Swimming seems to affect bone health differently according to anatomical region. Studies in both humans and rodent models suggest that tibia cortical bone is negatively affected by swimming. There was no evidence of a negative effect of swimming on other bone regions, both in humans and animal models.

Impact of Long-Term Swimming Exercise on Rat Femur Bone Quality

Considering the conflicting evidence regarding the potential long-term detrimental effect of swimming during growth on femur quality and fracture risk, our aim was to investigate the effect of eight months of swimming on femur quality. Twenty male eight-week-old Wistar rats were assigned into a swimming (SW; n = 10; 2 h/day, 5 days/week) or active control group (CG; n = 10, housed with running wheel) for eight months. Plasma osteocalcin and C-terminal telopeptide of type I collagen concentrations (ELISA) were assessed at baseline, four, and eight months of protocol. Femur structure (micro-computed tomography), biomechanical properties (three-point bending), and cellular density (histology) were determined after the protocol. SW displayed a lower uncoupling index, suggesting higher bone resorption, lower empty lacunae density, cortical and trabecular femur mass, femur length and cortical thickness, and higher cortical porosity than CG (p < 0.05). Although both biomarkers' concentrations decreased in both groups throughout the experiment (p < 0.001), there were no significant differences between groups (p > 0.05). No differences were also found regarding biomechanical properties, bone marrow adiposity, and osteocyte and osteoclast densities (p > 0.05). Long-term swimming was associated with unbalanced bone turnover and compromised femur growth, lower femur mass, and deteriorated cortical bone microarchitecture. However, femur trabecular microarchitecture and biomechanical properties were not affected by swimming.