Experts fear he is misleading his fan base by promoting £350 toning belts that give the illusion they create bulging biceps and Hollywood abs.
Ronaldo is the ambassador for SIXPAD, a Japanese tech company which specialises in 'electric muscle stimulation' (EMS) products.
Cristiano Ronaldo has been accused of endorsing a multi-million pound fitness sham for teaming up with Japanese brand SIXPAD
They come in the form of training belts that zap currents into muscles after being wrapped around users' abs, arms and legs.
The Portuguese footballer's rock-hard six pack is plastered on the firm's website and flagship store in Westfield Shopping Centre, West London.
But experts have told MailOnline that, while the equipment is effective for recovery, it does nothing for muscle growth.
The firm's futuristic-looking Abs Belt will set you back £350, but experts say it won't help you build muscle
Ronaldo wraps the device around his midriff in one of the company's glossy, high production TV adverts
Dr Niall MacFarlane, a professor of physiology and sports science at the University of Glasgow, said: 'Sports scientists use EMS techniques all the time for rehabbing injuries and recovery.
'Elite athletes do use EMS devices for recovery, but not while they're in the gym. You won't see Ronaldo wearing one of those while lifting weights, that's for sure.'
In one of the adverts, Ronaldo appears to be wearing the belts while exercising in his home gym - but experts told MailOnline it was unlikely the star would wear one while workout out
Dr MacFarlane said the SIXPAD products make users feel good because they give users a 'pump' by increasing blood flow into the muscle.
'This makes you feel like you've got more muscle there, because the muscles are harder and stronger,' he added.
'But in reality the muscle is just a bit swollen and 30 to 40 minutes later that 'pump' will disappear again.
'There's no convincing evidence addition of electrical stimulation has additional benefit to growing muscle.'
But on the website, the company says it has studied 'the mechanism of muscle development' and cracked the most effective way to 'train the muscle'.
But Dr MacFarlane said the company is being misleading, adding: 'They're very careful about what they say.
The SIXPAD Abs Fit 2 (left) costs a hefty £230, while the Body Fit 2 (right) - which can be wrapped around you waist, arms or legs - is priced at £175.00
'If I didn't know any better I'd be left with impression that adding this to training is helping me get bigger muscles.
'But it doesn't benefit muscle development. That's not what it does. If you have 300-400 pounds to spend, get quality personal training sessions instead.'
Impulses are sent from the device to a series of electrodes, which are placed on the skin over the muscles being targeted.
But while an EMS device may be able to temporarily strengthen or firm a muscle, no EMS devices has been proven for muscle growth or weight loss.
EMS has been used in physical therapy settings for years, particularly in patients recovering from injuries.
Dr Theo Bampouras, a lecturer in biomechanics and sports science at Lancaster University, described SIXPAD's advertising as clever.
He told MailOnline: 'The way the technology is marketed it almost seems as though you don't have to put in the work.
'These devices are great for recovery, and helping people who cannot contract their muscles fully due to injury.
'But they will not improve hypertrophy. If you're a recreational athlete with no injuries, and don't need rehab. It's probably not worth your money.'
In a nutshell, EMS applies an electric current to muscles to trigger involuntary twitches similar to those created by the central nervous system in a traditional workout.
EMS has been used in physical therapy settings for years and has now made its way into the fitness world.
During a quick 20 to 30-minute workout those electrodes apply a current that creates a 'fluttering' feeling, caused by small spasms occurring in muscles throughout the body.
Proponents of the technique say that it could revolutionise the fitness community by packing significant results into a much shorter workout.
A 2011 study published in the European Journal of Applied Physiology found there was a small increase in strength associated with EMS training.
They concluded: '[EMS] has been acknowledged as an efficient modality leading to significant improvements in isometric maximal voluntary strength.
'However, the resulting changes in dynamic strength, motor performance skills and explosive movements (i.e., jump performance, sprint ability) are still ambiguous.'
The researchers added the results could 'only be obtained when [EMS] is combined with voluntary dynamic exercise such as plyometrics'.
SIXPAD's director Masafumi Arakawa said: 'Although it is true that the EMS technology is widely used for recovery, there’s a range of other ways to benefit from EMS training and the difference lies on the frequency.
'While physiotherapy and rehabilitation devices are set in several different frequencies, the SIXPAD products are developed with a trademark pulse combining a 20Hz frequency with a built-in training session and backed by a Japanese authority on sports medicine who proved the 20Hz frequency to be effective for muscle development.'
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