Electromyographic analysis of muscle fatigue in anterior cruciate ligament deficient knees

The aim of this study was to detect possible differences in muscle fatigue and recovery of knee flexor and extensor muscles in patients with a deficient anterior cruciate ligament compared with patients with a normal anterior cruciate ligament. Surface electromyography of 15 patients with anterior cruciate ligament deficiency was performed while the muscles were under 80% of maximum isometric contraction, and after 1, 2, 3, and 5 minutes of rest. During the first 60 seconds of contraction, all muscles recorded significantly decreased mean power frequency and increased amplitude. The rate of decrease of mean power frequency was significantly greater in the injured quadriceps and normal hamstrings. All muscles except two recovered to the initial mean power frequency level after 1 minute of rest. All but two muscles in the injured and normal limb recorded an overshoot of mean power frequency during the recovery phase. This overshoot phenomenon also was seen for some muscles in the amplitude analysis. The findings confirm the fatigue state in all the muscles, suggest recruitment of more Type II fibers as the muscle fatigues, and show the physiologic adaptation of the quadriceps and hamstrings to anterior cruciate ligament insufficiency. The current study indirectly shows a dissociation between low intramuscular pH and mean power frequency during the recovery phase. It also indirectly suggests that the atrophied thigh muscles have fiber type composition similar to that of the normal side.