Rigid control of motor unit firing rates in the human tibialis anterior muscle persists during neurofeedback

The conventional framework of motor unit (MU) control assumes that MUs in a MU pool are constrained by a fixed recruitment order and a common input. This rigid-control framework has been challenged by recent findings suggesting that MU activity could be flexibly modulated, potentially mediated by descending cortical inputs. In this study, rather than evaluating flexibility from the perspective of recruitment thresholds, we investigated control flexibility by assessing if human participants can voluntarily modulate MU firing rates beyond rigid-control constraints. Specifically, we examined whether participants could voluntarily modulate the firing rates of a pair of MUs from the tibialis anterior muscle during real-time feedback. Two tasks involving target-reach with different visual feedback derived from the MUs firing rates were conducted. In both tasks, there was no evidence that participants were able to change MU firing rates in a way that would violate rigid control robustly. Our findings demonstrate limited flexibility in MU control in human tibialis anterior muscle within single-session training, even when real-time MU activity feedback was provided. The results suggest that MU flexibility is not inherently present in the human lower limb.