Electromyographic analysis of quadriceps fatigue after anterior cruciate ligament reconstruction

Study Design: Prospective, observational study. Objectives: To document changes in surface electromyographic activity durin g 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 n ot well understood. Methods and Measures: Electromyographic signals were recorded from the vast us 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 integratin g the rectified signal (iEMG) for the initial and final 5 seconds and compa ring the involved and uninvolved sides. Median frequency IMF) was computed from 4096 point fast Fourier Transforms performed at the beginning and end of the 30-second contractions. Results: Patients had moderate preoperative quadriceps weakness (16% defici t) 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 side s (significant fatigue effect). During the postoperative fatigue tests, how ever, 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 show ed a smaller decline over 30 seconds preoperatively and postoperatively (si gnificant side by fatigue interactions). IEMG was lower on the involved sid e preoperatively and postoperatively During the fatigue tests, iEMG increas ed similarly in the involved and uninvolved sides both preoperatively and p ostoperatively. Conclusion: Quadriceps endurance exercises are not indicated after ACF reco nstruction. Quadriceps weakness after ACL reconstruction was associated wit h fatigue resistance. Lower initial MF and smaller decline in MF during sus tained contraction is consistent with fast-twitch fiber atrophy and explain s fatigue resistance.