Spinal root magnetic stimulation: evaluating the limitations of a less invasive method to assess voluntary activation of the respiratory muscles

Spinal root magnetic stimulation can be used to assess diaphragm voluntary activation (VA), but its validity during inspiratory efforts remains unclear. Additionally, its use to evaluate abdominal muscle VA during expiratory efforts has not been explored. This study investigates spinal root magnetic stimulation as a method to assess VA of inspiratory and expiratory muscles. In thirteen healthy participants (8 female, 18-40 years), magnetic stimulation was delivered over the cervical spine (C6) and lower thoracic spine (T10), to activate the diaphragm (n=11) during inspiratory efforts and the abdominal muscles (n=11) during expiratory efforts, respectively. A nasogastric catheter recorded inspiratory transdiaphragmatic and expiratory gastric pressures. During voluntary 25%/50%/75%/100% maximal inspiratory and expiratory efforts, stimuli were delivered at 100% stimulator output to produce a superimposed twitch and, ~5 seconds later, a resting twitch, to calculate VA. Good utility of the technique was assessed per individual as r 2 ≥0.7 and 95% prediction interval (95%PI) limits within 25% of the predicted VA. For inspiratory muscles, VA had poor correspondence and predictability [r 2 =(0.26-0.85); 95%PI limits: (27.1-74.5%)] with contraction intensity in 8/11 participants and good correspondence and predictability [r 2 =(0.83-0.90); 95%PI limits: (22.7-23.8%)] in 3/11 participants. For expiratory muscles, VA had good correspondence and predictability [r 2 =(0.73-0.97); 95%PI limits: (10.4-20.8%)] with contraction intensity in 7/11 participants and poor correspondence and predictability [r 2 =(0.13-0.72); 95%PI limits: (19.0-40.0%)] in 4/11 participants. Spinal root magnetic stimulation can be used with caveats to assess expiratory muscle VA only, requiring individualised testing across multiple contraction intensities and a comparison of estimated vs. measured resting twitches.