Instrumentation to Record Evoked Ankle Movements in Anesthetized Rats

When limb movements are evoked by electrical stimulation, muscle contraction (e.g. 15-20 ms) and relaxation are fast and would be poorly sampled by video-capture with average frame rates of 60 Hz. Our aim was to design a device capable of monitoring ankle joint excursion evoked by nerve stimulation in anesthetized rats. A custom footplate was coupled to the wiper of a low-torque rotary style potentiometer so output voltage was proportional to ankle dorsiflexion and plantarflexion. Anesthetized rats lay prone with the hip and knee clamped at 90o. The foot was taped to the footplate so resting ankle angle was 90°. In eight healthy rats, pulses were delivered to the tibial and common peroneal nerves (10-400 μs, 1-200 Hz, 1-150 V). Average (±SD) minimum excursions in dorsiflexion and plantarflexion were 0.34° ± 0.34° and 0.46° ± 0.39°, respectively. Maximum dorsiflexion was reached at 56° ± 13°, whereas plantarflexion was attained at 178° ± 11° (180° = full plantarflexion). Changes in pulse amplitude, duration, and frequency provided intermediate dorsiflexion and plantarflexion. This device is able to monitor gradations of evoked limb movements throughout the entire joint range and will be useful to evaluate whether interventions improve function in weak or paralyzed muscles.