When it comes to training hard, few bodybuilders have quite the pedigree of six-time Mr Olympia Dorian Yates. This is a man whose approach to building muscle was literally dubbed ‘Blood and Guts’. His sessions involved heavy weights, brutally hard sets and tagging on a few more reps beyond the point where most of us would have said we’re done.

So when Yates says there’s an exercise even he won’t do, it’s probably worth hearing him out.

‘No, no, no, no. Never, ever ******* do this exercise. Never.’ Yates protested in a recent Instagram post.

The movement getting this fairly unequivocal review? The barbell preacher curl.

‘Massive tension on the tendon here. Nothing at the top,’ he explains, demonstrating the difference between the bottom and top of the rep. ‘So very unproductive and dangerous at the same time.’

Strong words. That being said, there is some science here, as well as a decent lesson in how resistance actually works, and why sometimes the best tool for building muscle is a machine.

Whatever weight you put on a barbell, it weighs the same at the beginning of your rep as it does at the end. What changes is how much leverage that weight has against your joints. This changing demand is what we’re talking about when we refer to an exercise’s ‘resistance curve’.

During a curl, gravity pulls the bar straight down. The force on the working muscles (the elbow flexors in this case) is greatest when your forearm is horizontal, falling away as it approaches vertical. This is why it’s considerably easier to rest at the top of a bicep curl than it is in the middle.

An angled preacher bench puts more of that demand towards the lower portion of the rep, with considerably less as you curl towards the top.

‘Because this is gravity at the bottom, there’s loads of tension and then they come up, it falls off,’ says Yates.

His concerns about injuries aren’t unfounded, either. Biceps tendon tears have been documented during preacher curls, while filmed ruptures that you’re likely to see on Instagram (beware if you have a weak stomach) frequently show an almost straight elbow and a palm-up grip when the injury occurs. Basically, the exact portion of the lift Yates is referring to.

‘Nautilus machine is the same tension from the beginning all the way through the movement,’ says Yates. ‘So that’s the beauty of a Nautilus machine.’

In practice, that depends on the machine. Cables don’t automatically mean constant muscular tension, either. But both offer a way of changing where the movement is hardest. Get the angle of a cable right, for instance, and it can keep your biceps working where the barbell starts letting you off.

Does that make the barbell preacher curl ‘unproductive’? Not quite. A ten-week study comparing barbell and cable preacher curls found similar biceps growth.

But it does mean machines and cables can be better suited to the job you want them to do. And when the job is building muscle, there are no bonus points for choosing the most hardcore-looking piece of equipment.

Train hard, by all means. Yates would almost certainly insist on that part. But choose your tools wisely.