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

Abstract

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.

Coordination amongst quadriceps muscles suggests neural regulation of internal joint stresses, not simplification of task performance

“…As described in ref. 28, we found that the ratio between VM and VL was altered systematically as the spring was attached and detached, consistent with regulation of the net mediolateral force on the patella (i.e., compensating for the lateral patellar load). Note that VI activity was not recorded in these animals.…”

“…7B). Previous results from our laboratory demonstrated that the CNS adjusts the balance between VM and VL activity to compensate for this lateral patellar force (28). If the CNS regulates joint stresses and strains, the covariation between the activity of VL and VM should be unaffected by this change in balance so that the net mediolateral force on the patella remains consistently low.…”

“…A differential control of these muscles in response to perturbations (similar to what we demonstrated previously in ref. 28) might be necessary to prevent patellar dislocations in more challenging contexts (e.g., walking on a rough terrain or during changes in running direction in rodent predator evasion or in human sport activities).…”

“…We also analyzed data from a separate experiment (28) examining the effects of attaching a spring that imposed a lateral load on the patella. Details of the experimental methods can be found elsewhere (28). EMGs in VM and VL were recorded before (no spring), during (attached), and after (detached) attaching the spring.…”

A Perspective on Muscle Synergies and Different Theories Related to Their Adaptation

“…Previous investigations using animal models presented evidence that the CNS uses muscle synergies (low dimensional modules) and combines them linearly to perform a motor task [1,2]. The central nervous system (CNS) chooses muscle activations to perform daily tasks and to prevent the stresses and strain that occur in the joints [3,4]. These muscle synergies encode information about muscle coactivation patterns within CNS for movement.…”