Low levels of anterior tibial loading enhance knee extensor reflex response characteristics

We examined whether neuromuscular reflexes were altered with anterior loads applied to the tibio-femoral joint. A ligament testing device was modified by attaching a reflex hammer to a steel mounted frame to illicit a patellar tendon tap, while anterior directed loads displaced the tibia on the femur. Five trials were acquired while anterior-directed loads (20, 50, 100 N; counterbalanced) were applied to the posterior tibia between 20 N pre (20 NPre) and post (20 NPost) baseline conditions on two different days. Surface electromyography (sEMG) recorded mean quadriceps (Q) and hamstring (H) reflex time (RTime = ms) and reflex amplitude (RAmp = %MVIC). A load cell on the anterior tibia measured the timing (KETime = ms) and amplitude (KEAmp = N) of the knee extension force, and was used to calculate electromechanical delay (EMD = ms) and peak knee extension moment (KEMom = N m/kg). Data from 19 recreationally active subjects revealed good to excellent response consistency between test days and between baseline conditions for RTime, RAmp, KETime and KEAmp. With anterior tibial loading, RTime was faster at 50 N vs. 20 NPost, and RAmp was greater at 20 NPre vs. 20 NPost (Q and H) and at 50 N vs. 100 N (Q only). KEMom was greater at 20 NPre and 50 N vs. 20 NPost, and EMD was shorter at 50 N vs. 20 N, 20 NPre and 20 NPost. These results suggest that knee extensor reflex responses are enhanced with low (50 N) but not moderate (100 N) anterior loading of the knee.