Sarcomeres, the functional units of contraction in striated muscle, are composed of an array of interdigitating protein filaments. Direct interaction between overlapping filaments generates muscular force, which produces animal movement. When filament length is known, sarcomere length successfully predicts potential force, even in whole muscles that contain billions of sarcomere units. Inability to perform in vivo sarcomere measurements with submicrometer resolution is a long-standing challenge in the muscle physiology field and has hampered studies of normal muscle function, adaptation, injury, aging, and disease, particularly in humans. Here, we develop theory and demonstrate the feasibility of to our knowledge a new technique that measures sarcomere length with submicrometer resolution. In this believed novel approach, we examine sarcomere structure by measuring the multiple resonant reflections that are uniquely defined by Fourier decomposition of the sarcomere protein spatial framework. Using a new supercontinuum spectroscopic system, we show close agreement between sarcomere lengths measured by resonant reflection spectroscopy and laser diffraction in an ensemble of 10 distinct muscles.
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