Adaptation of muscle activation after patellar loading demonstrates neural control of joint variables.

We evaluated whether the central nervous system (CNS) chooses muscle activations not only to achieve behavioral goals but also to minimize stresses and strains within joints. We analyzed the coordination between quadriceps muscles during locomotion in rats before and after imposing a lateral force on the patella. Vastus lateralis (VL) and vastus medialis (VM) in the rat produce identical knee torques but opposing mediolateral patellar forces. If the CNS regulates internal joint stresses, we predicted that after imposing a lateral patellar load by attaching a spring between the patella and lateral femur, the CNS would reduce the ratio between VL and VM activation to minimize net mediolateral patellar forces. Our results confirmed this prediction, showing that VL activation was reduced after attaching the spring whereas VM and rectus femoris (RF) activations were not significantly changed. This adaptation was reversed after the spring was detached. These changes were not observed immediately after attaching the spring but only developed after 3-5 days, suggesting that they reflected gradual processes rather than immediate compensatory reflexes. Overall, these results support the hypothesis that the CNS chooses muscle activations to regulate internal joint variables.