Abstract
Muscle is a complex, hierarchically organized, soft contractile engine. To understand the limits on the rate of contraction and muscle energetics, we construct a coarse-grained multiscale model that describes muscle as an active sponge. Our analysis of existing experiments across species and muscle types highlights the importance of spatially heterogeneous strains and local volumetric deformations. Our minimal theoretical model shows how contractions induce intracellular fluid flow and power active hydraulic oscillations, yielding the limits of ultrafast muscular contractions. We further demonstrate that the viscoelastic response of muscle is naturally non-reciprocal—or odd—owing to its active and anisotropic nature. This enables an alternate mode of muscular power generation from periodic cycles in spatial strain alone, contrasting with previous descriptions based on temporal cycles. Our work suggests a revised view of muscle dynamics that emphasizes the multiscale spatiotemporal origins of soft hydraulic power, with potential implications for physiology, biomechanics and locomotion.
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All original data supporting the findings of this work were obtained from published literature as indicated in Supplementary Section V. Source data are provided with this paper.
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Acknowledgements
S.S. acknowledges support from the Harvard Society of Fellows and L.M. acknowledges partial support from the NSF-Simons Center for Mathematical and Statistical Analysis of Biology under grant number 1764269, the Simons Foundation and the Henri Seydoux Fund. We thank S. Srinivasan for useful discussions.
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L.M. conceived the research topic and approach. S.S. and L.M. formulated the theoretical model. S.S. performed the analytical calculations, and compiled and analysed the data. S.S. and L.M. wrote the paper.
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Supplementary Figs, 1–3, Tables 1 and 2 and Discussion.
Source data
Source Data Fig. 2
Data for strain in oscillating muscle extracted from references cited in the main text.
Source Data Fig. 3
Parameters for muscle dynamics estimated from references provided in Supplementary Table 1.
Source Data Fig. 5
Muscle rheology data extracted from references cited in the main text.
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Shankar, S., Mahadevan, L. Active hydraulics and odd elasticity of muscle fibres. Nat. Phys. 20, 1501–1508 (2024). https://doi.org/10.1038/s41567-024-02540-x
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DOI: https://doi.org/10.1038/s41567-024-02540-x
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