Chris Hemsworth's ripped arms were iconic in his role as Thor in the Marvel Cinematic Universe.
Hemsworth says he achieved these results by using blood-flow restriction in his arms.
This technique can help boost muscle size, endurance, and recovery.
Chris Hemsworth's ripped arms are some of his most recognizable features, especially given their prominence in his role as Thor in the Marvel movies.
Hemsworth posted a video of the blood-flow-restriction bands being strapped to his arms by the Australian trainer Ross Edgley before a weight-lifting session. Hemsworth then did dumbbell curls, and the device caused the veins in his arms to noticeably protrude.
"By restricting blood flow and oxygen the muscles are forced to work harder in a shorter period of time and a bunch of other 'sports sciency' stuff happens," Hemsworth wrote in the caption. "Basically it's one of the most uncomfortable training methods I've experienced but part of the puzzle in growing Thor's arms to look like the legs of a racehorse."
Blood-flow restriction is designed to help build muscle without lifting very heavy weights, and it can also be used to boost endurance and aid recovery. The technique was also heavily used by athletes at the Tokyo Olympics this year.
Blood-flow restriction causes your arms to build up more lactic acid as they lift
To get blood-flow restriction in your arms, you tightly wrap your arms in an elastic band. This traps blood in the arms and causes lactic acid to build up.
"It's very useful for training around an injury that doesn't allow for heavier loads to be used," the personal trainer Harry Smith previously told Insider's Rachel Hosie. "Blood-flow restriction training forces you to use much lighter loads than you usually would as the restricted venous return traps blood in the muscle, limiting its range of motion to an extent and causes a huge build-up of metabolites and lactic acid."
This technique can allow bodybuilders, or anyone looking to add muscle gains and definition to their biceps and triceps, to do so without the risks that come with using heavier weights.
"The trapping of the blood is going to trap lactic acid," Zachary Long, the director of physical therapy at Carolina Sports Clinic, told Hosie. "As you're exercising, you're building up more and more lactic acid," he continued, adding that the resulting growth-hormone release was "actually higher than what we see with heavy resistance training."
Blood-flow restriction can also prevent limbs from becoming asymmetrical in terms of muscle bulk after an injury. Often when someone breaks an arm or a leg and has to wear a cast, that limb will become smaller and less muscular than the opposite limb once the cast comes off.
But blood-flow restriction can prevent this from happening, Jeremy P. Loenneke, a professor of health and exercise science at the University of Mississippi, told Tonic.
"There's some evidence that applying blood-flow restriction in the absence of exercise may be useful for helping to maintain muscle size and strength," Loenneke said.
There can be a downside and dangers to using blood-flow restriction
Using blood-flow restriction to build that muscle and improve arm aesthetics may come with some painful side effects as well.
"This 'pumps' up the muscle, really stretching it - this sensation is incredibly painful and may stimulate muscular hypertrophy because of this swelling effect," Smith told Hosie.
The technique also shouldn't be used by people with high blood pressure, varicose veins, or deep-vein thrombosis.
Still, a randomized controlled trial by J Sport Rehabilitation found that blood-flow restriction might nevertheless cause less pain than either high- or low-intensity resistance exercise performed muscular failure.
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