Muscle power amplification may enhance sprint performance and broaden the thermal performance of the lizard Sceloporus occidentalis . II: Muscle power and series elastic element properties

ABSTRACT

Skeletal muscle is reliant on the myosin ATPase enzyme to power locomotion, making muscle and locomotor performance thermally sensitive. However, muscles performance is often enhanced by elastic tendons, and the power amplification they enable is thought to increase sprint performance and its thermal robustness. The previous study suggests in vivo ankle muscle powers that exceed predicted muscle powers and may contribute to the high thermal robustness of sprint performance in S. occidentalis. Here, we directly measure gastrocnemius muscle power and its thermal sensitivity, compare this to in vivo ankle muscle power to quantify power amplification, and determine the potential for the gastrocnemius muscle–tendon unit (MTU) to store the required elastic strain energy. Peak gastrocnemius muscle power was ∼83, 156 and 371 W kg−1 at 15, 25 and 35°C with Q10 values of ∼1.9–2.4. Peak in vivo muscle power at the ankle was ∼180, 622 and 881 W kg−1, indicating a ∼2.2-, 4.1- and 2.4-fold power amplification, at 15, 25 and 35°C, respectively. The fixed-end compliance of the gastrocnemius was ∼17%, series elastic elements experienced an ∼12% strain, and the muscle fibers stored ∼17 J kg−1. Hence, this species clearly uses power amplification in sprinting at all temperatures, and the high degree of power amplification at 25°C probably increases thermal robustness of sprint performance at 25–35°C. The degree of elastic strain energy storage observed in this MTU is comparable with avian, mammalian and anuran MTUs known to store and utilize elastic strain energy and modestly amplify muscle power.