Measuring Plantar Flexor Muscle Force

Maximal force production of the plantar flexor muscle group was measured in vivo with a 305C muscle lever system (Aurora Scientific Inc., Aurora, Canada), as previously described (Khairallah et al, 2012 (link); Call & Lowe, 2016 (link)). Animals were anesthetized via inhalation (~2% isoflurane, SomnoSuite, Kent Scientific), placed on a thermostatically controlled table with anesthesia maintained via nose‐cone (~2% isoflurane), the knee fixed with a pin pressed against the tibial head, and the foot firmly fixed to a footplate on the motor shaft. Contractions were elicited by percutaneous electrical stimulation of the tibial nerve (0.2 ms pulse, 500 ms train duration) at increasing frequencies. Following assessment of isometric torque, susceptibility to injury was assayed with 25 eccentric contractions as previously described (Khairallah et al, 2012 (link); Call & Lowe, 2016 (link)) at maximal isometric torque (150 mS duration, 0.2‐mS pulse train at 150 Hz). Eccentric contractions were achieved by translating the footplate 38° backward at a velocity of 800°/S after the first 100 mS of the isometric contraction. The decrease in the peak isometric force before the eccentric phase was taken as an indication of muscle damage.