Were “In the Penal Colony” to be written today, Kafka could only be speaking of an exercise machine. Instead of the sentence to be tattooed on its victims, the machine would inscribe lines of numbers. So many calories, so many miles, so many watts, so many laps.

Modern exercise makes you acknowledge the machine operating inside yourself. Nothing can make you believe we harbor nostalgia for factory work but a modern gym. The lever of the die press no longer commands us at work. But with the gym we import vestiges of the leftover equipment of industry into our leisure. We leave the office, and put the conveyor belt under our feet, and run as if chased by devils. We willingly submit our legs to the mangle, and put our stiffening arms to the press.

It is crucial that the machines are simple. The inclined planes, pins, levers, pulleys, locks, winches, racks, and belts of the Nautilus and aerobic machines put earlier stages of technical progress at our disposal in miniature. The elements are visible and intelligible for our use but not dangerous to us. Displaced, neutralized, they are traces of a necessity which no longer need be met with forethought or ingenuity. A farmer once used a pulley, cable, and bar to lift his roofbeam; you now use the same means to work your lats.

Today, when we assume our brains are computers, the image of a machine-man, whether Descartes’s or La Mettrie’s, has an old and venerable quality, like a yellowed poster on the infirmary wall. Blood pressure is hydraulics, strength is mechanics, nutrition is combustion, limbs are levers, joints are ball-in-socket. The exercise world does not make any notable conceptual declaration that we are mechanical men and women. We already were that, at least as far as our science is concerned. Rather it expresses a will, on the part of each and every individual, to discover and regulate the machine-like processes in his own body.

And we go to this hard labor with no immediate reward but our freedom to do it. Precisely this kind of freedom may be enough. Exercise machines offer you the superior mastery of subjecting your body to experimentation. We hide our reasons for undertaking this labor, and thoughtlessly substitute a new necessity. No one asks whether we want to drag our lives across a threshold into the kingdom of exercise.

Exercise is no choice. It comes to us as an emissary from the realm of biological processes. It falls under the jurisdiction of the obligations of life itself, which only the self-destructive neglect. Our controversial future is supposed to depend on engineered genes, brain scans, neuroscience, laser beams. About those things, we have loud, public, sterile debates—while the real historic changes are accomplished on a gym’s vinyl mats, to the sound of a flywheel and a ratcheted inclined plane.