Background/Aims: There is limited evidence to support the routine prescription of sternal precautions following cardiac surgery, and the effects of dynamic tasks on sternal healing remain unclear. The aims of this study were to determine the reliability of ultrasound as a measure of sternal micromotion and to report the minimal detectable change in the acute cardiac surgery population. Methods: Twenty participants, 3–7 days following cardiac surgery, were recruited. Ultrasound imaging of the sternum was undertaken at rest and during five dynamic tasks, including deep inspiration, cough, upper limb elevation (unilateral and bilateral), and sit to stand. Tasks were repeated three times, and the order randomised. Ultrasound images were captured with a SonoSite M-Turbo device, and two blinded observers used MicroDicom software to measure sternal micromotion. The intra- and inter-observer reliability of the mean ultrasound measures were calculated using intraclass correlation coefficients; precision with the standard error of measurement and calculation of the minimal detectable change. Findings: The intra-observer reliability of the mean ultrasound measures for all tasks ranged from intraclass correlation (3,1) 0.990 to 0.997. The inter-observer reliability ranged from intraclass correlation coefficients (2,1) 0.994 to 0.998. At rest, the standard error of measurement was 0.05 mm and the minimal detectable change was 0.14 mm for both the lateral and antero–posterior motion of the sternal edges. Conclusions: Ultrasound is a reliable and precise measure of sternal micromotion in the acute cardiac surgery population. The results of this study suggest that future research exploring the effects of exercise and functional tasks on the healing sternum using ultrasound is warranted to better inform post-operative management and intervention for this patient population.