Neuromuscular block is commonly monitored using the adductor pollicis (AP) because of its easy access. However, the hand may not always be accessible for neuromuscular monitoring during surgery. In that situation, monitoring of the flexor hallucis brevis (FHB) secondary to stimulation of the tibial nerve at the ankle joint may be used as an alternative.

During propofol and remifentanil anesthesia, acceleromyography of the thumb and big toe were recorded. Single twitch responses were measured simultaneously after cumulative administration of rocuronium from 80 to 200 µg/kg at intervals of 40 µg/kg. Furthermore, the amount of rocuronium required for 50% and 95% twitch height depression were calculated. Rocuronium was infused continuously to maintain 5% to 15% twitch responses. We also obtained the onset, duration of action, and antagonism effect of neostigmine from both muscles via neostigmine (20 µg/kg) administration.

ED₅₀ and ED₉₅ were significantly lower in the AP than in the FHB. The highest twitch response at peak and neostigmine antagonism were significantly higher in the FHB than in the AP. However, there was no significant difference in the onset time or duration of neostigmine between AP and FHB.

Due to its resistance to rocuronium, the onset of FHB is not a good indicator of optimal conditions for tracheal intubation. Also, because of its higher antagonism effect, there is potential risk of overlooking a residual block. Sufficient recovery of the block should be readjusted to estimate recovery in the FHB with the use of other reliable clinical tests.