Induced Anterior Knee Pain Immediately Reduces Involuntary and Voluntary Quadriceps Activation

Objective: To examine the immediate effects of experimentally induced anterior knee pain (AKP) on involuntary and voluntary quadriceps strength and activation. Design: Crossover 3 x 3 randomized controlled laboratory study with repeated measures. Setting: Human Performance Research Center, Brigham Young University. Participants: Thirteen neurologically sound volunteers (age, 21.9 +/- 3.2 years). Interventions: Subjects underwent 3 different conditions (pain, sham, and control). To induce AKP and sham condition, 5% sodium chloride and 0.9% sodium chloride (total volume of 1.0 mL for each condition), respectively, were injected into the infrapatellar fat pad on the dominant leg. No injection was performed for the control condition. Main Outcome Measures: The vastus medialis peak Hoffmann reflex normalized by the peak motor response (H: M ratio) was used to measure involuntary quadriceps activation. Quadriceps central activation ratio (CAR) using maximal isometric knee extension torque (N.m) was calculated to assess voluntary quadriceps activation. The visual analog scale was used to measure pain perception. Results: Our pain model increased perceived pain immediately after the 5% hypertonic saline injection and pain lasted for 12 minutes on average (F-40,F-743 = 16.85, P < 0.001). During the pain condition, subjects showed a 12% decrease in H: M ratio (F-2,F-59 = 8.64, P < 0.001), a 34% decrease in maximal isometric knee extension torque (F-2,F-59 = 5.89, P < 0.01), and a 5% decrease in CAR (F-2,F-59 = 3.83, P = 0.03). Conclusions: Our data showed that joint pain may be an independent factor to alter function of the muscles surrounding the painful joint. Both involuntary and voluntary inhibitory pathways may play a role in an immediate reduction of muscle activation.