(a) The two muscles used in this study are the anconeus muscle, a primary elbow extensor, and the plantaris muscle, a primary ankle extensor. (b) Each muscle was instrumented with a pair of sonomicrometry transducers, which measured fascicle lengths, and two electromyography electrodes, which measured muscle activity patterns during hopping. The schematic of the muscle is representative of the anconeus muscle. (Online version in colour.)