The Supreme Court ruled on Monday that crowding at California prisons constitutes cruel and unusual punishment, and ordered the state to reduce the number of inmates by more than 30,000. An outraged Justice Scalia dissented, noting that many of the released prisoners “will undoubtedly be fine physical specimens who have developed intimidating muscles pumping iron in the prison gym.” Do people really get super-buff in the slammer?

Not anymore. It’s true that most state and federal prisons had extensive collections of free weights and weight machines through the 1980s, and that inmates could spend significant portions of their days bulking up. But that all changed around 20 years ago. As stories about prison gyms spread in popular culture, they became an increasing source of public concern. Some shared Scalia’s worry that muscle-bound ex-cons would be even more dangerous after their release, and legislators across the country responded. In 1996, an amendment to an appropriations bill expressly prohibited the federal Bureau of Prisons from purchasing “training equipment for boxing, wrestling, judo, karate, or other martial art, or any bodybuilding or weightlifting equipment of any sort.” Many states, including California, made the same decision, either by statute or policy. These days, whatever free weights you’d still find in U.S. prisons are decades old. (These are often chained to the walls, to deter thieves and prevent inmates from beating each other with barbells.)

Prisoners get most of their exercise in the yard, a large outdoor space with a track and basketball court. In warm-weather states like California, there may be some exercise equipment in a covered gym area adjacent to the yard. There are some complicated rules governing this equipment in federal prisons: Roman chairs are acceptable, because they work the nonthreatening abdominals, while back- and bicep-building chin-up bars are verboten. Many federal prison administrators avoid confusion by limiting inmates to cardio machines like stationary bikes and treadmills. State prisons, which typically have tighter budgets, sometimes have no exercise equipment at all. Most facilities do, however, stock a selection of yoga and meditation videos.

The lack of free weights and weight machines hasn’t completely prevented prisoners from packing on “intimidating muscles,” as Justice Scalia calls them. In the absence of iron to pump, they turn to whatever heavy items they can find. Resourceful prisoners check out books from the library for the sole purposes of lifting them. Those who work in the mess hall can hoist large pots full of water. They do pull-ups from the rails of their bunk beds, which are sometimes stacked three- or even four-high in the crowded California penitentiaries. And there may be serious competition for access to the gym area, where younger, stronger inmates exclude their weaker colleagues. In several prisons, inmates have developed their own rules for using and maintaining decrepit exercise gear.

Despite popular approval, sociologists and many prison officials have criticized the prohibition on weights in correctional facilities. Some research suggests that weight lifting decreases aggression among inmates. Wardens have noted that idleness is the biggest threat to order in a prison, and weight lifting gives the convicts something to do.

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Explainer thanks John Amtmann of Montana Tech, Wendy Feldman of CustodialCoaching.com, Michael Frantz, author of Jail Time: What You Need to Know…Before You Go to Federal Prison!, Katharine Pawelko of Western Illinois University, Ed Ross of the Federal Bureau of Prisons, and Robert Weisberg of Stanford Law School.