In a study reported in The Journal of Experimental Biology in 1992, Dr. Richard Lieber and Dr. V. Reggie Edgerton from UCLA used laser diffraction to noninvasively measure sarcomere length changes of the glass catfish (Kryptopterus bicirri) during swimming. The idea first came to them upon observing that their bodies emitted rainbow like colors as they swam, most likely due to the diffraction of white light passing through their muscles.
A chamber was constructed so that the fish could swim against the flow of water entering the chamber at a constant velocity (about 3.2 body lengths per second), which would maintain the fish at a relatively stationary position in the chamber while sarcomere measurements were taken by transillumination with a helium-neon laser beam near the axis of the vertebral column. The resultant diffraction pattern was imaged onto a translucent screen and videotaped for subsequent analysis.
By tracking the movement of the first order diffraction line versus the 0th order, sarcomere lengths could be determined by using the grating equation. Interestingly, despite the laser transilluminating through the musculature on both sides of the fish, the diffraction pattern is only representative of the side closest to the screen, the last sarcomeres it passes through. This was discovered by fixing fish muscle at two different sarcomere lengths, one long (2.1 µm) and one short (1.9 µm), embedding them in plastic and then transillumintating both blocks. When the order of the blocks that the beam passed through was interchanged, the measured sarcomere length was that of the block closest to the screen.