Watch the Owning Off-Beats online guitar lesson by Chris Buono from 40 Day Rhythm SWAT Camp
A groove is like a safe you can't have it almost locked. You've either got it nailed at any tempo, or you need to spend some quality time with your metronome and maybe a few James Brown records.
The most important thing to recognize about playing rhythm is that you are as responsible for the notes you don't play as you are for the ones you do. If you want an exciting way to tune-up your rhythm chops, check out the lively guitar being played in the Caribbean and West Africa, where strummers keep things uniquely funky by landing chords everywhere except the downbeat.
But first, let's warm up on two simpler styles that weightlessly play against the strong beats. Ex. 1a is a typical reggae pattern. Keep beats one and three internalized while whipping the chords on beats two and four with a focused downstroke. Next, move on to Ex. 1b and make sure you can handle an all-off-beat, all-upstroke ska riff.
Now you're ready for some spirited grooves that deftly dance around the downbeats. In the Trinidad-flavored soca pattern in Ex. 2, the chords zoom by and seem to catch air off each downbeat without actually landing on any of them. (To stay anchored, keep those invisible downbeats strong by tapping your foot on each.)
Finally, try the lively lick in Ex. 3, and you'll be amazed by what Afro-Funk players can do with a James Brown-style strum. You may want to tap out this tricky rhythm a couple of times before you play it, because it lands many places, but never on the downbeat. Be sure to tap your foot on the quarter notes, or you may end up hearing this rhythm inside out. Finally, insert the chords.
The key to strumming these last two examples is keeping your right-hand going at all times. Strum all sixteen pulses of the measure, and mute the strings on the beats you lay out on. Most of all, be patient. You'll eventually master an unfamiliar rhythm if you first slow it down to a tempo you can handle.