<p>Physical activity (PA) engagement can improve bone mineral density (BMD), while how varying PA patterns affect BMD in adults and the potential mechanisms remain unclear. This study investigated the associations between PA trajectories and BMD and examined whether skeletal muscle and body fat partially explained these associations. In total, 393 participants from the Healthy Lifestyle Promotion Cohort completed three waves of surveys from 2018 to 2023. PA trajectories were identified using an optimal Latent Class Trajectory Modeling method. Areal BMD (aBMD) at the hip and volumetric BMD (vBMD) at the lumbar spine were derived from Quantitative Computed Tomography, and skeletal muscle index (SMI) and percent body fat (PBF) were measured using multi-frequency bioelectrical impedance analysis. The associations between PA trajectories and current BMD were estimated by regression models, and indirect associations through SMI and PBF were determined using a regression-based mediation analysis. Four PA trajectories were observed, including steady low-PA (44.53%), steady middle-PA (19.85%), rising high-PA (6.11%), and declining high-PA (29.52%) trajectories. Of the associations between PA trajectories and BMD, the rising high-PA trajectory showed the strongest association with higher hip aBMD (<i>β</i> [95% CI] = 0.088 [0.034, 0.143]), while the steady middle-PA trajectory (<i>β</i> [95% CI] = 38.108 [27.328, 48.889]) showed the strongest association with higher lumbar vBMD. The increased SMI and decreased PBF partially explained the associations between PA trajectories and BMD. Sustaining a moderate or an increasingly high level of PA over a long period was associated with higher BMD in adults. These beneficial effects may be partially explained through favorable changes in SMI and decreased PBF.</p>