Physiotherapists routinely prescribe upper limb exercises for patients who have undergone a median sternotomy during cardiac surgery. It is not currently known whether upper limb exercises should be unilateral or bilateral and conducted with or without additional loading to minimise pain and further sternal separation in patients with sternal instability. Eight patients who had chronic sternal instability after cardiac surgery were included in this study. During a selected regimen of upper limb exercises, the amount of sternal separation at different vertical points on the sternum was measured by ultrasound. The amount of sternal separation was not related to type of upper limb activity, but both unilateral and unilateral loaded positions were found to be significantly associated with sternal pain (p = 0.009). In this group of patients with chronic sternal instability, bilateral upper limb movements were significantly less associated with sternal pain than unilateral movements.