Heavy metal, as it was defined in the 1970s, was a celebration of hedonism, excess and overkill – the blues all revved up with everywhere to go. But these days, metal – or, to be precise, the '90s post-punk equivalent – is no longer the province of the flamboyant but of the deadly earnest. Hard rock is no longer a highway to hell littered with beer cans, coke spoons and fishnet stockings but a shredding soundtrack for doing push-ups, spitting up bile and keeling over from exhaustion. Once a labor of lust, metal now simply labors.

Helmet's Aftertaste and the Rollins Band's Come In and Burn are prime examples of the '90s brand of paramilitary headbanging. With their drillsergeant demeanors and drill-press riffs, the Rollins Band and Helmet typify this Spartan approach to hard rock: Singer Henry Rollins helped shape the sound with Black Flag, a loud-fast Southern California punk band that had mutated into a metal juggernaut by the time of its demise, in 1986. On Come In and Burn, its seventh album, Rollins' namesake band continues to build on the Flag's foundation with an elastic, almost jazzlike approach to metallic blues.

In contrast, Helmet emerged out of New York in the late '80s with a tight, taut sound that bridged the gap between thrash metal and the art-damaged vocabulary of Lower East Side guitar innovators such as Glenn Branca and the Band of Susans, both of whom employed Helmet founding member Page Hamilton. On Aftertaste, their fourth album, Helmet return to those take-no-prisoners roots after dabbling in some bluesy detours on their 1994 release, Betty, and experimenting with hip-hoppers House of Pain on the Judgment Night soundtrack.

Though neither the Rollins Band nor Helmet will likely be sitting in with Ornette Coleman any time soon, they bring a jazz ensemble's sense of interplay to the heavy-lifting routines of metal. They also feature guitarists who, like Rage Against the Machine's Tom Morello and the Jesus Lizard's Duane Denison, are demonstrating that the instrument still belongs on the cutting edge in these trip-hop-happy times.

On Come In and Burn, Chris Haskett's guitar engages in an agitated dialogue with Melvin Gibbs' bass and Sim Cain's drums, and the loud-to-quiet dynamics and roller-coaster tempos suggest a trio whose members are responding to one another on the fly rather than adhering to a carefully structured arrangement. Gibbs frequently drives the melodies with bulldozer riffs, notably on "During a City" and "Thursday Afternoon," freeing Haskett to create his own vocabulary of guitar sounds: an ambulance siren, a circling vulture, a damaged garbage disposal.

Texture – not the number of notes played – is what counts; one of Come In and Burn's most exciting moments is the guitar break on "Spilling Over the Side," in which Haskett shapes a funnel cloud of white noise with a handful of chords. Another highlight is the barreling "On My Way to the Cage," in which the guitar is silent for long stretches or is simply elbowed into the background by Gibbs' predatory bass.

Helmet's Hamilton, who plays all the guitar parts on Aftertaste, has also redefined his instrument. He's a master of the staccato chug, the slice-and-dice fill and the buzz-saw charge, but he rarely indulges in the kind of flash that would make anyone mistake him for Jeff Beck. Instead, Hamilton's style is all about an economic restraint that verges on repression. His guitar lines stretch like high-tension wires across a grid of cross-rhythms laid down by bassist Henry Bogdan and drummer John Stanier. When the wire snaps – with a gloriously cleansing scream in "Pure" or the drum volleys of "Diet Aftertaste" – it's cathartic.

Both Helmet and the Rollins Band have created a hard-rock language based on limits, on an interplay in which individual displays of ego-massaging virtuosity are suppressed in favor of the songs. It's a world in which rock, and not the rock star, is king, but there's also a sense that these two groups have gone about as far as they can with their current approaches. Both albums refine rather than significantly broaden styles established on earlier releases.

So even as Helmet develop a more pronounced sense of melody on such songs as "Like I Care" and "It's Easy to Get Bored," Hamilton remains merely a functional, essentially tuneless singer. His lyrics, largely of the common-sense, get-a-grip-on-yourself variety, aren't embarrassing, but they risk little and reveal even less. "Appearances are overrated," Hamilton sings in "Diet Aftertaste," but sometimes one wishes this self-effacing band would – just once – let it all hang out.

Come In and Burn also reaffirms that Rollins is one of rock's all-time great overachievers: He continues to make the most of a sing-speak drawl and an obsession with personal inadequacy. In recent years, he has become something of a pop celebrity by doing everything from playing bit roles in Hollywood movies to horsing around with MTV VJs, but you'll hear none of that playfulness on the new album. Instead he remains a solemn advocate of no-pain, no-gain purification. "I'm hurting like hell, but I'm learning," he declares on "Rejection." When, on "Starve," he sings, "I have all I need.... I keep my existence lean," he embodies the nofrills, nose-to-the-grindstone attitude of his ballistic blues.

Both Rollins and Hamilton could stand a few less grindstones and a few more frills. Otherwise, the monumental and often thrilling roar of their respective bands threatens to turn them into one-dimensional caricatures. Sometimes, workmanlike rock can sound too much like work. (RS 755)

GREG KOT