You ideally want to reach muscle failure within a certain timeframe, to exhaust all the motor units (different types of muscle fibers). Usually that’s about 60 seconds to failure.
Here’s an excellent post from Conditioning Research that does a great job of explaining the importance of this concept. (The bolding is mine.)
“…The aim is to progress through all 3 motor unit types quickly enough to recruit them all…..but not so quickly that only the fast twitch fibres get the bulk of the stimulation……and not so slowly that the slow / intermediate twitch units can recover and recycle back into the effort, so the fast ones are never called on.
So you do not want steady state easy cardio – that only hits the slow twitch units. Even when some get tired they recover and come back into play and you never tap into the intermediate or fast units.
And you do not want a really heavy weight where you fail immediately or after one or two reps. Then you need to call on all of the units – slow, intermediate and fast – to fire in tandem to shift it and as soon as the fast units are exhausted you are finished without really working the others.
You need the “goldilocks” load – enough to involve the slow units …. and then as they drop out to call in the intermediate ones and then as they fall off the fast ones come in until they are exhausted…”