Electromyographic analysis of quadriceps fatigue after anterior cruciate ligament reconstruction

STUDY DESIGN: Prospective, observational study. OBJECTIVES: To document changes in surface electromyographic activity during sustained maximum quadriceps contractions in patients before and 5 weeks after anterior cruciate ligament (ACL) reconstruction. BACKGROUND: Quadriceps weakness after injury and reconstruction of the ACL is well documented. The effect of weakness on muscle fatigue, however, is not well understood. METHODS AND MEASURES: Electromyographic signals were recorded from the vastus lateralis, vastus medialis, and rectus femoris muscles during 30-second maximum isometric contractions at 30 degrees, in 42 patients preoperatively and 5 weeks postoperatively. Signal amplitude was quantified by integrating the rectified signal (iEMG) for the initial and final 5 seconds and comparing the involved and uninvolved sides. Median frequency (MF) was computed from 4,096 point fast Fourier Transforms performed at the beginning and end of the 30-second contractions. RESULTS: Patients had moderate preoperative quadriceps weakness (16% deficit) and gross postoperative weakness (41% deficit). Weakness was associated with deficits in both MF and iEMG (r = 0.69-0.67). During the preoperative fatigue test, torque declined similarly on the involved and uninvolved sides (significant fatigue effect). During the postoperative fatigue tests, however, torque increased on the involved side and declined on the uninvolved side (significant side by fatigue interaction). For the initial 5 seconds, MF was lower on the involved than the uninvolved side but subsequently showed a smaller decline over 30 seconds preoperatively and postoperatively (significant side by fatigue interactions). IEMG was lower on the involved side preoperatively and postoperatively. During the fatigue tests, iEMG increased similarly in the involved and uninvolved sides both preoperatively and postoperatively. CONCLUSION: Quadriceps endurance exercises are not indicated after ACL reconstruction. Quadriceps weakness after ACL reconstruction was associated with fatigue resistance. Lower initial MF and smaller decline in MF during sustained contraction is consistent with fast-twitch fiber atrophy and explains fatigue resistance.