The Beastie Boys @ The U.S. Cellular Arena

I envisioned a world without The Beastie Boys Sunday night, and it was a sad one. Though they haven't been the most prolific presence in my life-they can go five years between studio albums without flinching, and they only tour through cities the size of Milwaukee once or twice a decade these days-they've been a constant presence, and a welcome one. Perhaps more than any other act, they captured both the spirit and the scope of '90s music. They were the one group at the intersection of both the alternative nation and the '90s rap renaissance.

The impetus for my morose-and thankfully misguided-fears of a world sans Beasties was Ad Rock. The chattering class has speculated for over a decade about how a group like the Beastie Boys can continue into old age, and has noted Mike D's deep-set wrinkles and MCA's grayed head. But on stage Sunday Mike D and MCA looked as limber and youthful as ever. It was Ad Rock, long the most punk-rock and vital of the trio, who looked like hell. His gait was awkward. He limped, and his muscles seemed to contort involuntarily, as if he'd been inflicted by a serious central nervous disorder. The group put on a great, albeit brief, show for this stop on their get out the vote tour, but I wasn't the only one in the crowd distracted-saddened-by Ad Rock's apparent ill health. If his rickety appearance had been the cause of some sort of affliction, it would be unlikely that someone so infirm could continue to tour for much longer.

After much post-show Internet research, though, I concluded that Ad Rock's instability Sunday probably resulted from a confluence of not-particularly-serious factors:

1. He recently suffered a leg injury recently, which explains his awkward movements.

2. He was probably the slightest bit drunk, having alluded to drinking a Bloody Mary pre-show. The might have explained some of erratic gestures.

3. His ill-fitting baggy clothes sometimes created the illusion that he was moving unnaturally and that he lacked muscular control.

So I feel a little silly getting so worked up about the possibility that Ad Rock's condition might spell the end of The Beastie Boys, but in my defense, Sunday's concert was a fine reminder of what was at stake. After all these years, the group still knows how to whip the crowd into euphoria with their peppy, filler-free rap show. Much of the credit goes to their DJ, Mix Master Mike, who keeps the rappers as on their toes as the audience. Drawing from his crates of records, he filled just about every song with unexpected breaks, turning out impromptu remixes of fan favorites. It's difficult for a rap group to phone in a concert when they aren't quite sure what their DJ might play next.

A too-short encore placed the Beasties in their live-band set up, as Tenacious D joined them for "Time For Living" and Ad Rock screamed out the closer, "Sabotage." That his legs seemed to give out from under him as he thrashed away on his guitar was apparently a sign of showmanship, not ailment.