The process used could help prevent human diseases, according to scientists who led the study, which was published by the Journal of Molecular Cell Biology
"There is certainly the potential for this model to help fight human diseases. The process we have been developing could help prevent muscular dystrophy and Parkinson's disease," Professor Xiang Gao told CNN.
Gao, who led the project with Liangxue Lai, is a specialist in genetic engineering at Nanjing University in China.
The two modified beagles, named Hercules and Tiangou, had the myostatin gene deleted at the embryo stage. Myostatin inhibits muscle growth in animals, so the dogs were able to grow unnaturally bigger, more muscular, and stronger.
The process involved the introduction of gene-editing chemicals into around 60 dog embryos. The myostatin gene was successfully knocked out of only two of the dogs.
"The mutant dogs look much stronger than the others. They are overgrown in the thighs," Gao said. "We have not observed any negative side effects. The dogs are not in pain."
Their larger size and strength is expected to allow them to be better and stronger runners.
"Their extra strength means that they may have uses in hunting, even the military," said Professor Gao.
Elaine Ostrander, a scientist at the National Institutes of Health, told the MIT Technology Review
that we should not rush to conclusions from the study. "The number of dogs is still small ... It will be interesting to see what types of variation come up as more dogs undergo the process," she said.
Dogs were used because they have metabolic and neurological features that are similar to humans -- and the same mutation has occurred in humans before.
In 2003, a baby boy in Berlin was determined to be born without myostatin. As a result, he was incredibly strong.
According to the New England Journal of Medicine, "He appeared extraordinarily muscular, with protruding muscles in his thights and upper arms."
The Journal report states that by the time the child was less than five years old, he had increased strength and bulk, and was able to, "hold two 3-kg dumbbells in horizontal suspension with his arms extended."
Hercules and Tiangou -- named after the "heaven dog" in Chinese myth -- will remain at the Guangzhong Pharmaceutical Research Institute, where they will be bred.
"The next step in our study is to see whether or not the dogs can pass on the mutation to future generations," Gao said. "This will be another huge breakthrough."
Prof. Gao told CNN it was possible that humans could be genetically modified, like the beagles, to make stronger athletes or better soldiers.
"However, genetically modifying humans raises other issues," he added.
Advancement in genetic engineering is creating a wealth of new opportunities in medical science, but it also raises difficult ethical dilemmas.
Penny Hawkins, head of Research Animals Department at the Royal Society for the Prevention of Cruelty to Animals
, told CNN: "The creation of genetically engineered animals can involve painful, invasive procedures on animals; including the removal of eggs and hormone treatment.
"Genetic alteration is never predictable and can result in oversized embryos, resulting in painful births. It can leave the animals severely affected in a way which is impractical for life. The process also very wasteful."
In reference to the dog study, Hawkins said, "The genetic alteration of animals simply to make them stronger, or to have greater running ability, is completely unacceptable.
"If the purpose of the study was to help cure human diseases, then there is more justification. Yet, even so, we ought to look for alternatives to genetic engineering, because the effect on these animals can be so great."
A different Chinese institute, BGI, caused controversy earlier this year when they genetically engineered "micro-pigs"
to sell as pets. However, Prof. Gao made clear that there are no plans for the extra-muscular dogs to end up on sale as pets.