In vivo Ankle Plantarflexor Torque Measurement

In vivo peak isometric torque of the ankle plantarflexors was assessed as previously described [26 (link)]. Briefly, anesthesia was induced using an induction chamber and 5% isoflurane in oxygen. Anesthesia was maintained using 1.5% isoflurane at an oxygen flow rate of 0.4L/min. The left hindlimb was depilated and aseptically prepared and the foot placed in a foot-plate attached to a servomotor (Model 300C-LR; Aurora Scientific, Aurora, Ontario, Canada). The left peroneal nerve was severed and platinum-iridium needle electrodes (Model E2-12; Grass Technologies, West Warwick, RI) were placed on either side of the sciatic nerve to elicit contraction of the plantarflexor muscles [27 (link)]. Peak isometric torque was defined as the greatest torque measured during a 200-ms stimulation using 1-ms square-wave pulses at 300 Hz and increasing amperage 0.6 to 2.0 mA (models S48 and SIU5; Grass Technologies). Fatigability of the plantarflexor muscles was assessed using a protocol that has been shown to induce ~50% torque loss over the course of 120 contractions by using physiological stimulation frequencies of the hind limb, which range from 45–60 Hz [28 (link), 29 (link)]. Briefly, for this study, the mice performed 120 submaximal isometric contractions for 2 min using 330 ms stimulations at 50 Hz.