Ever feel like your lifting routine has too much downtime? It does. A study from the American Council on Exercise finds that a 20-minute high-intensity interval-style strength workout (HIIT) done three times a week builds the same amount of muscle as a slower, 45-minute session. If you do multi-joint exercises, which work several muscle groups at once (like chest presses and leg extensions) you’ll see gains after three weeks, says Cedric X. Bryant, chief science officer at ACE.
That’s significant, since three weeks out is often when exercisers get discouraged and quit a new routine. To judge strength improvements, researchers tracked participants’ one- and five-rep max for eight exercises—something you can do outside the lab. How do you know when you’ve surpassed your one-rep max? If you can do two reps at that weight in two consecutive workouts, add additional weight plates.