GO LONG TO GET BIG: The words “stress” and “benefit” aren’t often mentioned in the same sentence. Neither are “scrawny” and “jacked” – but the progression from one to the other depends, at least in part, on generating some metabolic stress in your strength workouts, says Brad Schoenfeld.

His recent study in the journal Sports Medicine reveals how sets that torture your muscles can also help them grow. The key is to begin lifting – and then forget to stop. “Sets lasting longer than 20 to 30 seconds substantially increase metabolic stress,” says Schoenfeld. That burning sensation you feel is your muscles filling with hydrogen ions, he explains. These lower your muscles’ pH, making them more acidic. That seems to make them bigger by stimulating the production of proteins and hormones that act as growth factors for muscle tissue. The other benefit of extended dance-remix sets: they recruit more muscle fibres, including your fast-twitch fibres, which typically don’t report for duty unless you’re lifting 80% or more of your one-rep max. Longer-lasting sets with lighter weights call them into action by thoroughly exhausting your lower-threshold (i.e. slow-twitch) fibres.

Don’t push yourself like this on every set, says Schoenfeld. A few of these sets at the end of a traditional workout should be plenty. Or you can dedicate one workout a week to timed sets lasting at least 20 seconds. Either way, just make sure you hit that sour spot, when your arms or legs are shaking. If your goal is to stretch the limits of your T-shirt then it pays to push your muscles.