The goal of my lab is to develop in vitro and in vivo muscle physiology tools that record muscle contraction and limb movement in response to forces measured inside and outside of the body during swimming. Towards this goal, we ask the following questions:
Muscle contractile output (shortening and force) is governed by three parameters: 1. Intrinsic properties e.g. size, speed, fiber type. 2. The command signal from nerve impulses. 3.The mechanical response from the environment, independent of the nervous system – e.g. a muscle shortens more slowly if moving a bowling ball vs. a beach ball. Although much work has characterized 1 & 2, there are no experimental tools for measuring muscle’s interactions with the mechanical environment. A major goal of locomotor physiology is to understand how neural input links to muscle behavior (e.g. power output). Yet, without understanding muscle’s mechanical response to dynamic loads, we cannot decode neural input to predict muscle output. More simply, different mechanical environments (e.g. ground vs. water) cause different limb motions, even with identical neural input. To address this missing link between neural input and muscle output, our lab is developing tools for exploring muscle-environment interactions.
Theory predicts that isolated muscles can produce large amounts of power. Intrinsically, this power is limited by the activation kinetics, contractile speed and fiber architecture of muscle. However, muscles often operate far below their theoretical limits during locomotion. For example, muscles in swimming frogs generate ~75 W/kg of power in vivo vs. ~250 W/kg under ‘optimal’ in vitro conditions. Currently, we are developing models that predict how the coupled interactions of fluid dynamics, limb morphology and muscle force-velocity properties further limit in vivo muscle power. Future work aims to extend this theory to predict the limits of swimming speed and efficiency.
Many vertebrates and invertebrates use jointed limbs to power locomotion. The differential roles of joints have been well established in terrestrial locomotion. For example in some running animals, the hip generates most of the power, while the ankle enables tendons to stretch and recoil to save energy. Aquatic tetrapods (e.g. frogs, turtles, waterfowl) have inherited limb joints from their terrestrial ancestors, but how do these joints function in water? Does a jointed limb confer any performance advantages during swimming?
We are currently constructing a frog-inspired swimming robot powered by independently controlled joint actuators at the hip, knee and ankle to address this question. Further, this frog-inspired limb can be generalized to mimic the motions of vertebrate limbs to address broader questions in comparative physiology-biomechanics.