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Accurate measurement and analysis of the mechanical events in active muscle requires the use of high-speed equipment. A hydraulic servo-valve, controlled by analog units (integrators, adders, inverters), can be used to control the speed of shortening of muscle at rates as high as 1 mm per millisecond. The apparatus can be used for isometric, isotonic, and controlled release experiments. Both release and stretch, at high or low speeds, can be produced during a single contraction cycle. Force is measured by an unbonded strain gauge of high natural frequency and low compliance. To maintain constant force on the muscle, a signal proportional to measured force is fed into an error detector, whose output controls the servovalve piston. The instrumentation described can provide the necessary and sufficient information to specify completely both transient and steady-state mechanical properties of muscle.