The Mechanics of the Medial Gastrocnemius Muscle in the Freely Hopping Wallaby (Thylogale Billardierii)

Muscle force, electromyogram and length were monitored in the medial head of the gastrocnemius (MG) muscle in freely hopping wallabies (Thylogale billardierii Desmarest). During take-off hops from rest, MG muscle developed force with an isometric contraction. For constant-speed hops, force was produced in MG muscle during rapid stretch. The muscle resisted this stretch with a constant impedance that was independent of hopping speed. The rate of stretch of the muscle during high-speed hopping was as high as 1 ms−1 (5–6 muscle lengths per second) at the onset of stretch and slowed to no stretch at the peak of force. Since the mechanical impedance was constant while the stretch velocity changed, there was no significant viscosity present in the muscle. The tendon stretched by 3.2% at 7kmh−1 hopping and by 4.4% at 18kmh−1 hopping. Elastic energy storage in the tendons increased with hopping speed but the percentage of total work done by elastic recoil of the whole muscle did not increase at higher hopping speeds. The significance of the muscle stretch is in producing high forces rapidly and, in addition, there is considerable energy storage in the tend