The so-called post-workout 'pump' typically leaves muscles looking bigger, stronger and fuller.

But an expert has warned that, particularly for men, it may have the opposite effect below the belt.

Known as 'gym penis', the term has been coined to describe genitals looking smaller-than-usual after intense exercise.

And according to Chris Bustamante, an aesthetic nurse practitioner and the founder of Lushful Aesthetics, this is a little-known, yet normal, symptom of working out.

He told the New York Post: 'Many men experience some degree of temporary retraction while working out, but the amount varies dramatically.'

Mr Bustamante said some people will 'barely notice it' while others 'may feel like they are turtling' – a term used for when the penis retracts closer to the body.

It may be triggered by exercises that activate the body's 'fight or flight' reflexes.

These include intense cardio, such as running, which can increase adrenaline levels, as well as heavy weightlifting. 

During these activities the body prioritises blood flow to the muscles doing the work, directing it away from inactive areas like the penis.

Mr Bustamante stressed that it is usually nothing to worry about. 

'Once your body cools down, your nervous system settles and blood flow redistributes, the penis typically returns to its usual flaccid appearance,' he explained.

'For most men, that can happen within minutes to an hour after working out.'

The process may take longer for people who are dehydrated or who have consumed large amounts of caffeine or pre-workout supplements. 

Cold temperatures or a particularly intense training session can also delay the penis returning to its natural size. 

Despite being temporary, Mr Bustamante warned that the phenomenon can have a significant psychological impact on some men.

He said it can make them anxious about their size and reluctant to use shared changing rooms. 

According to 2024 statistics, men in Britain have the 68th-biggest penises in the world, on average.

The Daily Mail's interactive map from the time shows that the average size of an erect penis in the UK was 5.17 inches. 

In first place with 6.93in, meanwhile, was Ecuador, followed by Cameroon and Bolivia.

The US was also ahead of Britain in the rankings. 

While 'gym penis' itself is generally harmless, Mr Bustamante advised men to pay attention to persistent or unexplained changes. 

He said changes in penis size that do not resolve after exercise, or symptoms such as erectile dysfunction, pain or numbness, warrant being seen by a healthcare professional.