Characterizing motor unit firing rates in the human erector spinae

Muscles of the erector spinae (ES) are vital for maintaining posture, yet their motor unit firing rates (MUFRs) are not well described compared with many limb muscles. Therefore, this study aimed to characterize ES MUFRs across a full range of contractile intensities, including at maximum voluntary contraction (MVC). Intramuscular electromyography was used to record motor unit activity from the ES muscles (left and right sides) in 10 healthy participants. Data were collected during isometric trunk extension at intensities from 10% to 100% MVC. Voluntary activation was assessed using transcranial magnetic stimulation, and mean values were >88%. Torque-firing rate relationships were analyzed to identify rate-coding patterns and potential side-to-side differences at each intensity. Contrary to our hypothesis, the ES exhibited rate-coding patterns similar to limb muscles reported previously. A linear relationship was observed between torque and MUFR, with mean rates increasing from ∼10 Hz at 10% MVC to ∼34 Hz at 100% MVC. A significant side-related effect was found at higher intensities, in which the nondominant side demonstrated higher MUFRs compared with the dominant side. These findings indicate that whereas the ES serves a unique postural role, rate-coding strategies are comparable to those of the appendicular musculature. The observed side-to-side differences at high intensities may reflect functional asymmetries in trunk stabilization.NEW & NOTEWORTHY This study provides a comprehensive characterization of the human erector spinae motor unit firing rates across a full range of intensities, including maximal effort with assessment of voluntary activation during isometric back extensions. Contrary to neural circuitry differences and postural function, the erector spinae rate codes similar to limb muscles. Furthermore, we identified significant side-to-side differences in firing rates at higher intensities, indicating a neural compensation to maintain spinal stability.