I have a PhD in strength training – stop using the wrong weights for your workouts with this simple test

As a fitness writer, one of the most common fitness queries I’m subject to might surprise you: “How do I choose the correct dumbbell weight for my workout?” It’s a valid question, because if you fail at this formative task, you risk scuppering the effectiveness of your session. Choose a weight that’s too heavy and your form will break down; choose a weight that’s too light and you won’t hit a sufficient stimulus for building strength and muscle, among many other benefits.

To help you avoid this fate and optimise your time in the gym, I enlisted the help of Dr Milo Wolf – a fitness coach specialising in strength training, with an impressive CV comprising a PhD in sport science and the founding of a training app (MyoAdapt).

Below, he details a system that will help you choose effective weights for your workouts, whether you’re using dumbbells, barbells, machines or otherwise. He also delves into the science behind another reason why your progress in the gym might slow over time.

I’m using the humble dumbbell to introduce this question, but Wolf’s solution applies to any type of strength training equipment – from barbells to kettlebells to resistance machines. His answer also hinges on muscle growth as the primary goal. However, for beginners in particular, his guidance will lead to improvements in strength and further perks such as boosted bone density and heart health, too.

Now that we’ve given a nod to the nuance of the situation, the short answer to this question is: choose a weight light enough that it allows you to perform an exercise with good form, but heavy enough that it presents a challenge to the working muscles for the prescribed number of repetitions. Or, in Wolf’s words: “You should always aim to train hard, especially for muscle growth.”

This relates to the SAID principle, which stands for specific adaptations to imposed demands. In simple terms: the body adapts to handle the tasks we regularly ask it to do. If we want to trigger adaptations, we need to consistently ask it to do something beyond its usual capabilities or comfort zone – in the case of strength training, this means increasing variables such as the number of reps or sets and the weight we are lifting.

Wolf has a handy method for identifying weights capable of this when you start a new exercise plan. He recommends taking the last set of each exercise to failure – the point of muscular fatigue where you are unable to complete another rep with good form.

For example, you might be prescribed three sets of 10 repetitions on the dumbbell deadlift. If you used a pair of 15kg dumbbells for 10 repetitions on the first and second set, then completed a significantly higher number of reps (ie, 15) on the final set, this is a surefire sign that the weight you are using is too light.

“This shows you weren’t as close to failure on your earlier sets as you first thought,” says Wolf. “Through that feedback loop, you can learn if you need to add more weight to the exercise during your next workout. Within two or three workouts, that should have you using the right amount of weight for you.”

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Recent research published in the Sports (Basel) journal shows you can build muscle with sets of anywhere between three and 50 repetitions, Wolf says. This can be achieved while lifting anything from 30 per cent to 90 per cent of your one-rep max – the maximum weight you can lift for one repetition of an exercise – with the caveat that your set takes you within close proximity to muscular failure.

Training to, or close to, muscular failure is the common denominator behind successful strength training for muscle growth. This is the point where you are unable to complete another rep with good form. For example, doing a set of biceps curls and being unable to curl the dumbbells upward on the tenth rep because the working muscles have become too fatigued to do so.

If you want to choose an effective weight for each exercise during a workout, you need to pick one that takes your muscles to this point, or just shy of it, within the prescribed number of repetitions.

However, Wolf worked on a 2022 study – featured in the Sports Medicine journal – which found that people often struggle to determine when they have reached muscular failure.

“We saw that, for rep ranges below 12, people only underestimate this value by about a rep. That discrepancy isn't really big enough to worry about – whether you're training to actual failure or one rep away from failure, your results are actually going to be pretty similar,” he says.

“However, the higher the reps went above 12, the less accurate people became. People started underestimating themselves by several reps – sometimes five or more. And when you're talking about a discrepancy that large, that is when your muscle growth could really be taking a hit.”

This is why his guidance on choosing the correct weight for each exercise is so important. Choose wrong and the benefits you experience from strength training could be considerably dampened.

“Beginners might also be a bit less accurate at gauging how close they are to failure compared to more advanced trainees,” Wolf continues.

For this reason, he recommends sets of five to 12 repetitions for most people looking to build muscle.

“Make sure your sets take at least 20 seconds, but no longer than about a minute,” he adds. “If you're going over a minute for each set, you’re probably giving up because you're out of breath rather than reaching muscular failure.”

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Newcomers to lifting weights are likely to experience significant health and fitness benefits from strength training. It is only marginally hyperbolic to say this demographic can see results from simply walking in the proximity of a dumbbell. Advanced gym-goers, however, will likely need to fine-tune their training and fight harder to see progress.

The mechanisms behind this phenomenon – colloquially termed “newbie gains” – are uncertain, but Wolf shares his hypothesis.

“The truth of the matter is, we don’t fully know why this is the case,” he says. “But we do observe that, as a rough rule of thumb, if you are able to gain 10kg of muscle in your first year of lifting, it will probably take around twice that time to gain the same amount of muscle again.”

Similar diminishing returns are often observed when training for strength, he adds. “Let’s say you gained 40kg on your squat in a year. It might take you around two years to gain another 40kg, and then from there, another four years.”

The reason he proposes for beginners gaining muscle faster than seasoned gymgoers is that “their muscle growth pathways are a lot more desensitised”.

Remember the first time you ate your favourite dessert? It probably packed more of a punch than the 100th time you sat down for the exact same sweet treat. The idea here is somewhat similar, albeit with more complex underlying physiology at play.

“When you’ve been lifting for several years, you probably get a greater degree of anabolic resistance,” Wolf explains. “Effectively, your body has initiated that muscle growth pattern so many times that pressing that switch becomes incrementally harder with time.

“Even though you’re training hard and you’re still giving your body the same stimulus, your body is not as sensitive to that stimulus anymore. Whereas, when you’re new to lifting, that stimulus is so novel that the response your body gives, in terms of muscle growth, is a lot larger.”

Wolf says the size of your frame could also be a limiting factor for muscle growth.

“There’s some evidence that certain people grow faster than others simply because of the size of their skeleton,” he says. “Some people have used ankle circumference or wrist circumference as predictors of how much muscle you are likely to gain in a given time frame, and what your genetic ceiling might be.”

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