Fig. 1. Electrode implantation and force transducer calibration. (A) Anatomy of the plantaris longus in the X. leavis hindlimb showing implantation of electrodes. Muscle activity and changes in muscle fascicle length were measured by bi-polar EMG electrodes and sonomicrometry crystals, respectively. Plantaris longus force was measured by a strain gauge force transducer (inset) tied to the inner surface of the Achilles tendon (see text for further details). (B) Representative tendon force transducer calibration (frog 5). The foot was removed from the animal and the plantaris muscle was mechanically isolated from proximal tissues (see text) allowing the muscle–tendon unit to be mounted in-line with a calibrated load cell in a simple jig. (C) The data record shows force from the calibrated load cell (broken line) and the voltage output from the force transducer (solid line) for a series of loading cycles. (D) Load cell force is plotted against force transducer output to show the linear response of the force transducer over the cycles shown in C.