Corresponding author: D. F. B. Haeufle E-mail: daniel.haeufle@inspo.uni-stuttgart.de Journal of Bionic Engineering 9 (2012) 211–223 Can Quick Release Experiments Reveal the Muscle Structure? A Bionic Approach D. F. B. Haeufle 1,2 , M. Günther 1,2 , R. Blickhan 3 , S. Schmitt 1,2 1. Institute of Sports and Exercise-Science, University of Stuttgart, Allmandring 28, D-70569 Stuttgart, Germany 2. Stuttgart Research Centre for Simulation Technology (SRC SimTech), University of Stuttgart, Pfaffenwaldring 7a, D-70569 Stuttgart, Germany 3. Institute of Motion Science, Friedrich-Schiller-University, Seidelstraße 20, D-07749 Jena, Germany Abstract The goal of this study was to understand the macroscopic mechanical structure and function of biological muscle with respect to its dynamic role in the contraction. A recently published muscle model, deriving the hyperbolic force-velocity relation from first-order mechanical principles, predicts different force-velocity operating points for different load situations. With a new approach, this model could be simplified and thus, transferred into a numerical simulation and a hardware experiment. Two types of quick release experiments were performed in simulation and with the hardware setup, which represent two extreme cases of the contraction dynamics: against a constant force (isotonic) and against an inertial mass. Both experiments revealed hyperbolic or hyperbolic-like force-velocity relations. Interestingly, the analytical model not only predicts these extreme cases, but also additionally all contraction states in between. It was possible to validate these predictions with the numerical model and the hardware experiment. These results prove that the origin of the hyperbolic force-velocity relation can be mechanically explained on a macroscopic level by the dynamical interaction of three mechanical elements. The implications for the inter- pretation of biological muscle experiments and the realization of muscle-like bionic actuators are discussed. Keywords: force-velocity relation, isotonic, quick release, proof of concept, artificial muscle Copyright © 2012, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(11)60115-7 1 Introduction Work has been ongoing for decades to reveal the functional principles of biological muscles. The basis of this work is the knowledge from a variety of well de- fined muscle experiments performed with isolated muscles [1,2] or muscle fibers [3] . Typical muscle experi- ments are isometric, isokinetic, and quick release ex- periments. Isometric contractions are used to reveal the force-length dependency in muscle contraction [4,5] . Isokinetic contractions [2,6–9] , isotonic quick release ex- periments [2,10–12] and quick release contractions against an inertial mass [13,14] are used to determine the dynamic properties of muscles, namely, the force-velocity de- pendency. Each of these experiments is repeated several times, the isotonic experiments at several lengths, and the dynamic contractions for varying external loads and/or velocities. The force-length and force-velocity relations can only be determined from such a set of ex- periments, as each experimental condition retrieves one operating point of the muscle. To interpret the experimental results quantitatively and find the underlying mechanisms, many muscle models were introduced. Microscopic Huxley-type muscle models allow to account for the sub-cellular and molecular origin of global muscle contraction dynam- ics [15–23] . With at least 30 parameters in the more recent approaches and several coupled differential and rate equations, such models can predict the force-velocity characteristics of a half sarcomere (smallest structural unit in the muscle). Macroscopic Hill-type muscle models on the other hand can predict the overall force production of a muscle or a muscle tendon com- plex [1,8,14,24–26] . Here, the force-length and force-velocity characteristics as known from the experiments are im- plemented as phenomenological relations in a so-called