"Scientists race to find evolutionary ‘missing links’ between major vertebrate lineages to explain the origin of new groups. In this effort, biomechanics is crucial since major transitions can spring from mechanical innovations (e.g. amphibian limbs or bird wings). Yet, biomechanics is limited due to its own missing link: we poorly understand interactions among muscle dynamics, skeletal structure and external forces of limbs on the ground. Aiming to bridge gaps in both evolution and biomechanics, PIPA offers three approaches to investigate a long-standing mystery: how did musculoskeletal transformations drive the origin and radiation of frogs? 1) PIPA combines external force measurements with in vivo and in vitro muscle physiology for insights inaccessible in other systems during locomotion. We will determine whether muscles operate near their mechanical limits. Also, we will reveal how well muscles meet the demands of various tasks (walking vs. swimming vs. jumping) to settle debates on whether muscles are ‘tuned’ for specific tasks. 2) Beyond muscle physiology, PIPA will computationally simulate the evolution of limbs in response to hypothetical selection pressures. Such models will test whether muscle dynamics (AIM 1) evolved for locomotor specialization (jumping) versus generalization for multiple tasks. Specifically, we will evaluate whether the muscular complexity of derived frogs evolved such that they can both generate extreme power and execute fine control. (3) To directly test conclusions from AIMS 1&2, PIPA uses innovative muscle-controlled paleo-robots to hypothetically ‘replay’ the anatomical and physiological transformations of frog evolution. Such integrative techniques will clarify our understanding of limb evolution, resolve long-standing evolutionary questions and discover general principles that will ultimately advance limbed robotics and prosthetics engineering."