Sarcomere lengths in human extensor carpi radialis brevis measured by microendoscopy

Introduction: 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. Methods: 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 degrees extension and 45 degrees flexion in 7 subjects. We also measured the variability in sarcomere lengths within single fibers. Results: Average sarcomere lengths in 45 degrees extension were 2.93 +/- 0.29 m (+/- SD) and increased to 3.58 +/- 0.19 m in 45 degrees flexion. Within single fibers the standard deviation of sarcomere lengths in series was 0.20 m. Conclusions: Microendoscopy can be used to measure sarcomere lengths at different body postures. Lengths of sarcomeres in series within a fiber vary substantially.