Researchers out of Japan and Australia recruited 36 healthy young adults for this study. They were curious to see how doing the same workout—an arm resistance exercise; lowering a heavy dumbbell in a bicep curl—at different intensities and frequencies affected fitness results. One group did a set of six contractions one day a week (low intensity, low frequency), another did a set of 30 contractions one day a week (high intensity, low frequency), and the last did a set of six contractions five days a week (low intensity, high frequency).
Everyone kept up with this routine for four weeks before getting their muscle strength and muscle thickness measured. After the month was up, those in the group that did a set of six contractions five times a week were the only ones who saw significant gains in both muscle strength and thickness. The group that did 30 contractions in one day saw an increase in thickness but not strength, while the group that did six contractions in one day did not increase muscle size or strength.
This study demonstrates that if muscle strength and thickness are your goals (and they should be, as they've been associated with things like weight management, heart health, and blood sugar balance), you'll want to do lighter, shorter workouts—but do them more frequently.