Two studies were undertaken to evaluate the use of EMG biofeedback to encourage hamstring relaxation during KT-1000 measurement of anterior tibial displacement. In study 1, 60 ACL-deficient patients were studied in three groups using 15 lb and 20 lb in each group: in group 1 the patients were simply retested 15 min after the initial test sequence, in group 2 they were initially retested with EMG biofeedback and then again without, and in group 3 they were retested twice with EMG biofeedback. No significant difference in mean anterior tibial displacement was seen between the initial measurements and retest measurements when no EMG biofeedback was used. A significant increase in mean anterior tibial displacement was seen when the retesting was performed with EMG biofeedback. No further increase was seen with repeated retesting with EMG biofeedback. In study 2, 40 patients were evaluated 4-12 months following ACL reconstruction. KT-1000 measurements of anterior tibial displacement of both the operated and non-operated knees were made at 15 lb and 30 lb with and without the use of EMG biofeedback. EMG biofeedback was associated with a significant increase in unilateral measurement of anterior tibial displacement. When side-to-side differences were compared, there was a small but statistically significant increase in anterior tibial displacement with the use of EMG biofeedback. Although the use of EMG biofeedback to encourage hamstring relaxation does increase unilateral measurements of anterior tibial displacement, it does not appear to have a clinically significant effect on measurement of side-to-side difference. It may have a role in testing patients who have difficulty achieving hamstring relaxation or in aiding inexperienced examiners.