Conditioned patellar tendon-tap reflexes in patients with ACL reconstruction.

The patellar tendon-tap stretch reflexes were examined in six neurologically healthy young subjects (mean age = 27.1 yrs) who had developed persistent quadriceps strength deficit due to ACL reconstruction. Each subject was tested on two separate days. A specially designed apparatus was used to examine the unilateral and conditioned patellar tendon-tap reflex response utilizing three different conditioning intervals: 25 ms, 75 ms, 150 ms, and a unilateral reflex (control). Peak isometric force and contraction time were measured by using a strain gauge. Also, peak-to-peak EMG was measured by using bipolar surface electrodes which were placed over the middle of the rectus femoris. All data were collected with a microcomputer (sample rate = 1 kHz). Due to the small sample, the Kruskal-Wallis nonparametric analysis of variance was performed. All subjects demonstrated quadriceps strength deficits in the ACL leg when compared with the contralateral leg. This analysis determined that for both the ACL leg and the Non ACL leg the size of stretch reflex was facilitated at the long-latency conditioning intervals (75 and 150 ms), whereas it was inhibited at the short-latency conditioning interval (25ms). However, the ANOVA model failed to reveal any differences in the conditioned stretch reflex between the ACL leg and the Non ACL leg. Also, no differences were observed at the unilateral condition. Taken together, these results indicate that ACL reconstruction results in significant strength deficits, but does not alter unilateral or conditioned reflex profiles.