Objective <p>This prospective, within-patient controlled study aimed to evaluate the effects of continuous passive motion therapy on ankle mobility and muscle morphology in mechanically ventilated ICU patients—a population vulnerable to ICU-acquired weakness and joint contractures.</p> Results <p>Twelve ventilated patients completed the protocol. The ankle treated with continuous passive motion therapy showed a significant improvement in dorsiflexion passive range of motion, increasing from 9.90° [IQR: 6.20–13.15°] to 12.99° [IQR: 9.07–20.20°] (<i>p</i> = 0.004), while the control ankle showed no significant change (<i>p</i> = 0.388). The increase in passive range of motion was significantly greater in the continuous passive motion treated ankle compared with the control (4.18° vs. 0.42°, <i>p</i> = 0.028). No significant between-limb differences were observed in tibialis anterior muscle thickness, cross-sectional area, or pennation angle. Echointensity increased in both limbs without group differences. These findings suggest that continuous passive motion can be applied as an early intervention to preserve ankle mobility in ICU patients. Although short-term muscle morphology changes were not significant, passive range of motion gains suggest continuous passive motion may help prevent contractures and support early rehabilitation during critical illness.</p> <p><i>Trial registration</i>: ClinicalTrials.gov NCT06944431, registered on April 25, 2025. Retrospectively registered.</p>