As you probably know, you have two kinds of voluntary muscles: slow twitch and fast twitch. The slows are what allow you sustained force, and the fast provide instant response to your brain's instruction; the catch is that the fasts cannot sustain their force. A boxer's punch or typing would be a likely example of fast-twitch muscles in action; a bench-press may use more of the slow-twitch.

It has occurred to me recently that musical creation has its parallels to these two kinds of muscles, and different musicians, like different athletes, may have different degrees in strength and skill in using these “muscles”.

For better or worse, I almost never play the same song the same way twice. I'm hard-wired to be a little inconsistent, and I revel in the opportunity to improvise. I equate this spontaneous approach to playing with the fast-twitch voluntary muscles – but more like a fast-twitch mental process. You snap an idea out of the air rather than the sustained effort of recollection. On the other hand, for a song to be cohesive among band members, there has to be a core of consistency. One can't just add an extra beat to every other bar or (significantly) change the chord changes on the fly. I equate this adherence to structure to slow-twitch: execution of music from the memory stores, rather than something off the cuff.

There are some musical genres that focus primarily on one or the other. Classical music, as it is performed today, provides virtually no room to improvise. If you deviate from Bach's printed notes, you're worse than an heretic – you presume to improve upon perfection. Ironically, Bach himself was known as a master improviser, and a performance of his music in his day would likely have seemed a lot less lofty and stuffy than a typical classical performance is today; you'd be welcome to applaud between movements, or chat and have a beer as the musicians played. Even musical duels were not uncommon (Bach's formidable improvisational skill tended to scare off his competition in advance of the contests...).

Jazz, on the other hand, is generally regarded as a vehicle for improvisation. No matter how closely and authentically you can emulate Charlie Parker's solos, you have zero credibility in the jazz world for being able to do so note-for-note, despite the skill required (Jaco Pastorius playing Parker's “Donna Lee” on electric bass notwithstanding – Jaco proved his original voice in other pieces). Technical skill, in fact, matters less than musical statement: Miles Davis was the antithesis of a “shredder”, although likely more by choice than ability – but he became a god in the jazz world for creatively re-inventing himself and leading musical trends every few years. He also prided himself on having a barely-functional memory.

As for what we'd generally call pop music these days, there is no consistency of standard where improvised parts are concerned. Bands like the Grateful Dead, Phish or Oysterhead prided themselves on rolling the dice with every performance, improvising the bulk of the night's music. Some performances produced brilliant music, others not so much. Bands that make use of onstage dancers or pre-recorded/sequenced music tracks have no choice but to hold to a rigid form, if not note-for-note. Alex Lifeson of Rush claimed he always felt cheated if live bands deviated from the recorded versions of their songs and solos (personally, I'm the opposite – I love to hear how an artists evolves the tune...within reason, at least).

That being said, it's often the mark of inexperienced or uncaring acts to leave too much to chance, and I'd count myself among such players for a few years in my career. One of the things I loved about working with the original Victoria Secret band (the prototype Mike Luno Band) was the pride we took in polishing the crucial parts of tunes to a shine (top, tail, tricky bits) and leaving the rest to a certain amount of creativity. There's a thrill in feeling those 'fast-twitch' mental muscles tone up, and sensing the synergy that comes from all band members contributing to the spontaneous creative stew – yet nailing the demanding time signature changes, unison lines and tricky vocal harmonies. Sort of like the “I've been rich and I've been poor” statement – I've been weak and I've been strong. Strong is better, even if it's not easy getting there.
With that in mind, if you happen to catch Mike Luno Band live anytime soon (such as at the Cobalt this June 6), you can expect a touch of the unexpected – Curtis continues to evolve his parts, I rarely play the recorded guitar solos or even rhythm parts by rote, melodies and harmonies are usually treated to a touch of variation, and we'll have Kirby on bass at the Cobalt– which means her distinctly musical style will inform the bass lines and backing vocals that night. All of the above, I consider to be good things. If we varied purely because none of us could remember how the original tunes went, that might be quite a different story – but we take the more respectable approach: we just pretend we forgot.