ELECTROMYOGRAPHIC ANALYSIS OF QUADRICEPS FATIGUE FOLLOWING ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

Abstract

1650 Fatigue in normal muscle is associated with a shift in the EMG frequency spectrum toward lower frequencies and has been related to muscle fiber type proportion. During sustained isometric contractions median frequency (MF) is initially higher in muscles with a high proportion of fast-twitch fibers but shows a greater decline over time. MF decline primarily reflects reduced muscle fiber conduction velocity. In the present study EMG activity was recorded from the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles during 30s maximum isometric contractions at 30° in 32 patients prior to, and 5 weeks following anterior cruciate ligament reconstruction. Torque, integrated EMG (iEMG) and MF were computed for the initial and final 5s in both the involved and uninvolved quadriceps. Involved quadriceps were weaker both pre-op and post-op (p<0.001). Torque decline during the post-op fatigue test was less on the involved side (p=0.008) consistent with previous observations in this population. Initial MF on the involved side was lower and showed a smaller decline during the pre-op (p=0.018) and post-op (p<0.001) fatigue tests. IEMG on the involved side was lower pre-op (p<0.01) and post-op (p<0.001). A combination of lower iEMG and MF was related to pre-op (R=0.71) and post-op (R=0.62) weakness (p<0.001). Lower iEMG is thought to reflect quadriceps inhibition while lower MF may reflect fiber atrophy. Minimal decline in MF of involved vastii muscles suggests that conduction velocity was unaffected and is consistent with a loss of gylcolytic activity.