Which weights activate more muscle when you lift to failure?

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A few months ago, I wrote about the growing body of research suggesting that it doesn’t really matter how much weight you lift, as long as you lift to failure. A series of studies, mostly by McMaster University researcher Stuart Phillips’ group, has compared the effects of doing three sets to failure with a heavy weight (around 80 per cent of max, which you can typically lift 8 to 12 times) with three sets of a lighter weight (around 30 per cent of max, or 20 to 25 reps). The results suggest that both techniques produce a similar level of muscle growth.

How is this possible? It’s clear that when you lift a heavy weight, you’re activating more muscle than when you lift a lighter weight. But the theory is that if you lift a light weight to failure, the muscle fibres you recruit initially will gradually fatigue, so that by the time you reach the end of the set - and failure - you’re recruiting as much muscle as you can, just as you would with a heavier weight.

A new study from Joel Cramer’s group at the University of Nebraska-Lincoln, published in the European Journal of Applied Physiology, puts this theory to the test. They had volunteers lift three sets to failure at either 80 per cent or 30 per cent of max, and measured a wide variety of responses, including EMG signals, which provide an estimate of how many muscle fibres are being recruited at any given time.

As expected, muscle recruitment rises toward the end of the set. But it’s clear that the heavy weight requires more muscle recruitment throughout the set. In fact, the last rep with the heavy weight demands more than 100 per cent of what the subjects were able to recruit during a single all-out contraction against immovable resistance. The light weight doesn’t get close to that.

So if light weights don’t require maximum muscle recruitment, even at failure, why do they stimulate muscle and strength gains? The researchers suggest different mechanisms may be at work in the two protocols. Several different pieces of evidence suggest that the subjects endured greater “metabolic stress” during the low-weight workout. 

That’s a pretty substantial edge for the low-load group, which also puts the muscle under stress for a greater amount of time. After the workout, the low-load group had a greater increase in muscle cross-sectional area; this swelling may be related to the accumulation of metabolic byproducts. There was also a greater change in the frequency of the EMG signal in the low-load group, which is another indicator of metabolic byproducts such as increased inorganic phosphate and decreased pH. Collectively, this metabolic stress may be a trigger for muscle growth.

What does this all mean? I think the “first-order” advice remains the same: if your focus is health and fitness, lift to failure and don’t worry too much about the weight. But once you start looking for more advanced “second-order” effects, for example to maximise athletic performance, then different workouts have subtly different effects. I don’t think there’s any universal consensus on what type of strength training endurance athletes should do, but understanding how these different protocols work might help you decide what fits best into your training plan.