The muscles in the body have big jobs to do, but like most big jobs, they’re broken down into manageable units. In striated muscle — the kind found in biceps, for instance — the basic units are sarcomeres, just a few microns long, which expand and contract by the millions.

Scientists want to know more about the functioning of sarcomeres, but viewing them usually requires removing live muscle tissue. Now, Scott L. Delp, Mark J. Schnitzer and others at Stanford University have devised a far less invasive way to look at sarcomeres in action. They use laser light and a microendoscope, a needle-shaped lens just 350 microns in diameter that can be inserted into muscle tissue without causing damage.

Dr. Schnitzer said the technique took advantage of the fact that when single-frequency light from a laser illuminates a material that is highly structured and directional, the material emits a harmonic, at twice the original frequency. “In the case of muscles, we have a highly ordered arrangement with directionality,” he said. So when a sarcomere is illuminated by flashes of laser light through the endoscope, the harmonics travel back through it to a photodetector. The data is digitally assembled to create an image.

The researchers, who describe the technique in Nature, first inserted a relatively large hypodermic needle into the forearm, inserted the endoscope through the needle tube and then removed the needle. Dr. Schnitzer said subjects experienced only mild discomfort.

The imaging technique should help in understanding why muscles weaken in certain disorders like cerebral palsy. The initial task, however, will be studying how healthy sarcomeres perform. “We need to define what’s normal first,” Dr. Schnitzer said.