Experimental knee pain reduces muscle strength

Pain is the principal symptom in knee pathologies and reduced muscle strength is a common observation among knee patients. However, the relationship between knee joint pain and muscle strength remains to be clarified. This study aimed at investigating the changes in knee muscle strength following experimental knee pain in healthy volunteers, and if these changes were associated with the pain intensities. In a crossover study, 18 healthy subjects were tested on 2 different days. Using an isokinetic dynamometer, maximal muscle strength in knee extension and flexion was measured at angular velocities 0, 60, 120, and 180 degrees/second, before, during, and after experimental pain induced by injections of hypertonic saline into the infrapatellar fat pad. On a separate day, isotonic saline injections were used as control condition. The pain intensity was assessed on a 0- to 100-mm visual analogue scale. Knee pain reduced the muscle strength by 5 to 15% compared to the control conditions (P < .001) in both knee extension and flexion at all angular velocities. The reduction in muscle strength was positively correlated to the pain intensity. Experimental knee pain significantly reduced knee extension and flexion muscle strength indicating a generalized muscle inhibition augmented by higher pain intensities.

Perspective: This study showed that knee joint pain has a significant impact on muscle function. The findings provide evidence of a direct inhibition of muscle function by joint pain, implying that rehabilitative strengthening exercises may be antagonized by joint pain.