The effects of muscle fatigue on neuromuscular function and anterior tibial translation in healthy knees.

We investigated the effect of quadriceps and hamstring muscle fatigue on anterior tibial translation and muscle reaction time in 10 healthy subjects. The six men and four women had an average age of 21.3 years and had no known pathologic knee conditions. Each patient underwent a knee examination, arthrometer measurements of tibial translation, subjective functional assessment, and an anterior tibial translation stress test before and after quadriceps and hamstring muscle-fatiguing exercise. The recruitment order of the lower extremity muscles in response to anterior tibial translation did not change with muscle fatigue. However, the results showed an average increase of 32.5% in anterior tibial translation (range, 11.4% to 85.2%) after fatigue. Muscle responses in the gastrocnemius, hamstring, and quadriceps originating at the spinal cord and cortical level showed significant slowing and, in some cases, an absence of activity after the quadriceps and hamstring muscles were fatigued. The increases in displacement after fatigue strongly correlated (0.62 to 0.96) with a delay in cortical-level activity (intermediate and voluntary). Muscle fatigue, which appears to affect the dynamic stability of the knee, alters the neuromuscular response to anterior tibial translation. Therefore, fatigue may play an important role in the pathomechanics of knee injuries in physically demanding sports.