The purpose of this study was to identify differences in 3D kinematics, kinetics, and ankle joint muscle activity in subjects with functional instability (FI) of the ankle joint during a drop jump. Twenty-four subjects with the subjective complaint of FI of the ankle joint and 24 noninjured control subjects performed 10 single leg drop jumps onto a force-plate. Timing and magnitude of kinetic data, timing of kinematic data, and integrated EMG (IEMG) activity of the rectus femoris, peroneus longus, tibialis anterior, and soleus muscles during two 200-ms time periods either side of initial contact (IC) with the ground were analyzed and compared between groups. Subjects with FI demonstrated a significant decrease in pre-IC peroneus longus IEMG activity, which was accompanied by a change in frontal plane movement at the ankle joint during the same time period. Following IC, FI subjects were less efficient than control group subjects in reaching the closed packed position of the ankle joint. Significant differences were seen between the groups' time-averaged and peak vertical and sagittal components of ground reaction force. The altered pre-IC peroneus longus IEMG and increased inversion of the ankle joint observed in FI subjects could help to explain why subjects with FI may suffer from inversion injury to their ankle joint when subjected to an unanticipated ground contact. The kinematic and kinetic differences observed in subjects with FI may lead to repeated injury and damage to the supporting structures of the ankle joint.