Margot Robbie's personal trainer says something the whole fitness industry survives on not saying out loud: there is no programme that works forever. David Higgins, who prepared the actress for filming, explains that he adapts every session to how she feels that day. Sometimes it's strength, sometimes reformer pilates, sometimes a lighter load. The plan exists, but it isn't sacred.

This collides head-on with the industry's best-selling idea, which is that there is one true programme you should buy and repeat. Repetition is necessary, especially at the start. The body needs time to learn the movement, to build strength and to start showing change. But the same chart with the same weight and the same reps, month after month, at some point stops being a demand on the body.

When the stimulus stops being a stimulus

Physiotherapist Ines Jimenez, founder of a barre training studio, explains it without padding: "To achieve changes in the body, you have to give it time and repeat certain stimuli. We're not going to get a ballerina's legs because we went to barre once. The body needs a process of adaptation and consistency." So far this is what every guide says.

The problem comes later. "If for a whole month every Tuesday we do exactly the same shoulder exercises, with the same weight and the same reps, there will come a moment when that stimulus is less demanding for the body," she says. The fix isn't dramatic: a little more weight, a few more reps, shorter rest, a more intense tempo, or simply a different movement. None of that requires a new subscription.

The body, not the calendar

Higgins says he always asked how the actress felt before deciding what they would work on that day. To Jimenez that makes complete sense: "Not everyone has the same needs or reacts the same way to a session. You have to take into account what our goal is, our level and our physical capacity, but also how we are that day." Rest, stress, nutrition and workload change performance entirely.

Here comes the part repeated in every gym here too: the belief that muscle soreness proves a good workout. "Having soreness after introducing a new stimulus can be normal, but suffering very intense soreness constantly should not become the goal. If the body isn't recovering, it needs more time or the intensity needs to come down," the physiotherapist says. Constant fatigue, slow recovery, dropping performance and pain that doesn't pass are signs the load isn't right.

Her final piece of advice is the least attractive and the most useful: technique before intensity. "Many times we want to take too much weight because we think that way we'll get results faster, but if we lose our technique, we needlessly increase the risk of injury." Progress is built with slightly more reps, slightly longer time under load, or slightly more kilos - but only once the movement is mastered.

During the shooting of one film Margot Robbie trained five days a week when it was possible, and when the schedule allowed only half an hour, the session was adapted. That is the whole point. There is no perfect week, there is a week that repeats. And if someone is selling you a programme promising results without a single adjustable moment in it, they are selling you paper, not training.