A prominent theory in #neuroscience hypothesizes that the #CNS generates muscle activity to simplify #motorcontrol, so that muscles with similar task actions are controlled as a single functional unit. Our results do not support this hypothesis! A thread. pnas.org/content/early/…
This theory predicts that correlations between muscles that have consistent task actions (e.g. the vastii muscles, VM, VL and VI) should be equally strong, and stronger than their correlations with task-inconsistent muscles (e.g. RF, that extends the knee and flexes the hip).
Alternatively correlations among quadriceps muscles may reflect the need to balance mediolateral (ml) forces on the patella, so that VM and VL (that have opposite ml action on the patella) are more correlated than any other muscle pair (RF and VI produce no ml patellar forces). 
We recorded activity in VM, VL, VI, and RF in rats during locomotion and found that activity of VM and VL (red and blue) were highly correlated for all speeds and inclines of locomotion, while activity of the other muscles (including VI, green) were more independent.
Accordingly, VM-VL correlation was significantly stronger than correlation between any other muscle pairs for all the behavioral conditions.
If VM-VL covariation aims limiting patellar displacement, this covariation may be especially important when the knee is subject to potentially unstabilizing ground reaction forces. Accordingly, VM-VL correlation was stronger during stance than during swing.
Finally, we applied a lateral force to the patella by means of a chronically implanted spring. The CNS compensated by changing the balance between VM and VL (Barroso et al., 2019) but kept their high covariation, hence maintaining the net ml force on the patella consistently low.
These results suggest that muscle covariation patterns reflect regulation of stresses and strains within joints better than simplification of task performance. In addition, these experiments highlight the critical role of the nervous system in regulating internal joint mechanics.
