• human wrist biomechanics;
  • in vivo two photon microscopy;
  • muscle architecture;
  • muscle fiber length;
  • sarcomere operating length;
  • second-harmonic generation microendoscopy;
  • sarcomere inhomogeneity;
  • sarcomere length variability;
  • human microendoscopy;
  • clinical microendoscopy;
  • multiphoton imaging;
  • multiphoton microendoscopy;
  • dye-free in vivo imaging;
  • skeletal muscle



Second-harmonic generation microendoscopy is a minimally invasive technique to image sarcomeres and measure their lengths in humans, but motion artifact and low signal have limited the use of this novel technique.


We discovered that an excitation wavelength of 960 nm maximized image signal; this enabled an image acquisition rate of 3 frames/s, which decreased motion artifact. We then used microendoscopy to measure sarcomere lengths in the human extensor carpi radialis brevis with the wrist at 45° extension and 45° flexion in 7 subjects. We also measured the variability in sarcomere lengths within single fibers.


Average sarcomere lengths in 45° extension were 2.93±0.29 μm (±SD) and increased to 3.58±0.19 μm in 45° flexion. Within single fibers the standard deviation of sarcomere lengths in series was 0.20 μm.


Microendoscopy can be used to measure sarcomere lengths at different body postures. Lengths of sarcomeres in series within a fiber vary substantially. Muscle Nerve, 48: 286–292, 2013