Volume 112, Issue 4, Pages 805-812 (February 2017) In Vivo Sarcomere Length Measurement in Whole Muscles during Passive Stretch and Twitch Contractions  Kevin W. Young, Bill P.-P. Kuo, Shawn M. O’Connor, Stojan Radic, Richard L. Lieber  Biophysical Journal  Volume 112, Issue 4, Pages 805-812 (February 2017) DOI: 10.1016/j.bpj.2016.12.046 Copyright © 2017 Terms and Conditions

Figure 1 Minimally invasive sarcomere length profiler. A laser source continuously sweeps across wavelength and illuminates muscle through a 250-μm-diameter fiber optic probe. The same optical probe collects muscle resonant reflections that are combined with the reference arm and sent to a detector. Resonant reflection spectra are encoded into interferograms and used to calculate sarcomere length with nanometer resolution and ∼1 ms time resolution. To see this figure in color, go online. Biophysical Journal 2017 112, 805-812DOI: (10.1016/j.bpj.2016.12.046) Copyright © 2017 Terms and Conditions

Figure 2 Saggital plane ultrasound image of typical probe insertion into rabbit tibialis anterior muscle. Biophysical Journal 2017 112, 805-812DOI: (10.1016/j.bpj.2016.12.046) Copyright © 2017 Terms and Conditions

Figure 3 Three-dimensional finite difference time domain simulations of sarcomere geometry and beam propagation from an optical probe are used to estimate reflection collection efficiency. (A) Simulation geometry of 3 μm sarcomere length at 0° tilt. (B–E) Electric field propagation for 0°, 5°, 10°, and 15° tilts, respectively. (F–H) Collection of reflected spectra is dramatically reduced at >5° of tilt between the long axes of sarcomeres and the optical probe for idealized sarcomeres with 3 μm length. (F) Raw reflection spectra from sarcomeres integrated from cross sections. (G) Mode-overlap integral for coupling into the optical probe at varying degrees of tilt. (H) Reflection spectra measured by the optical probe. To see this figure in color, go online. Biophysical Journal 2017 112, 805-812DOI: (10.1016/j.bpj.2016.12.046) Copyright © 2017 Terms and Conditions

Figure 4 Simulations of misaligned and heterogeneous sarcomeres highlight the importance of collinear illumination of the long axes of sarcomeres. (A) Simulation geometry of 3 μm sarcomere length with 5% coefficient of variation at 0° tilt. (B) Raw reflection spectra from sarcomeres integrated from cross sections. Resonance is destroyed for 5, 10, and 15 degree tilts. (C) Mode-overlap integral for coupling into the optical probe at varying degrees of tilt. (D) Reflection spectra measured by the optical probe. To see this figure in color, go online. Biophysical Journal 2017 112, 805-812DOI: (10.1016/j.bpj.2016.12.046) Copyright © 2017 Terms and Conditions

Figure 5 Sarcomere length (SL) as a function of passive MTU strain (black). The main plot shows sarcomere length as the MTU was linearly strained, held, and then shortened. Sarcomere length changes show clear nonlinearities during each phase. See text for explanation Insets show the sample RRSs used to provide the sarcomere length values. All resonant reflections are shown on the same scale. To see this figure in color, go online. Biophysical Journal 2017 112, 805-812DOI: (10.1016/j.bpj.2016.12.046) Copyright © 2017 Terms and Conditions

Figure 6 Sample data from a dynamic isometric twitch. The main plot shows sarcomere length (SL) and force data (black). Sarcomere length changes are remarkably linear during force production. Insets show sample RRSs that provide sarcomere length estimates. All resonant reflections are shown on the same scale. To see this figure in color, go online. Biophysical Journal 2017 112, 805-812DOI: (10.1016/j.bpj.2016.12.046) Copyright © 2017 Terms and Conditions