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Muscle transplants are performed to strengthen knee extension in polio patients. The time of contraction of muscles in relationship to gait is under investigation. Relative activity is evaluated by electromyographic recording of muscle action potentials. Voltages are recorded from three pairs of flexible wire electrodes implanted in the working muscles. The electromyograph incorporates three commercial preamplifiers, a three-channel electronic switch, a dc amplifier, and a long persistent cathode-ray tube with 3-second sweep. An image of the cathode-ray screen is projected optically via a half-silvered mirror into the lens of a 16-mm movie camera. The camera shoots through the mirror to photograph the walking patient on the same frame with the three muscle traces. Recording speed is 64 frames per second. Frame-by-frame analysis is accomplished by means of a hand-crank projector. This system of recording thus provides simultaneous visualization of muscle electrical activity and the mechanical aspects of gait.