Strongman athlete Dan Jones suffered a gruesome injury during a recent competition ... when BOTH of his knees snapped during a race, and it was caught on video!

The freak accident happened during a farmer's carry-type race at a competition in Southport (UK) earlier this month when Jones, a senior IT analyst by day, was approaching the finish line.

Jones was carrying 80kg (176 lbs.) when he suddenly collapsed ... and the way he fell will make your stomach turn.

Both of Jones's kneecaps buckled and he fell backward... and you can tell he was in excruciating pain.

Jones revealed he ruptured both patellar tendons and his kneecaps shot up into his quads.

Ouch!

Luckily, the 30-year-old had successful surgery and is currently rehabbing. He also said he's starting to move normally again.

"There's progress every day," Jones said on Wednesday, "so we're getting confident of getting home soon."

Gruesome injuries like this are a part of being a strongman ... from amateur competitions to the absolute elite.

In fact, Hafthor Bjornsson -- arguably the strongest man that's ever lived -- tore his chest muscle attempting a 555+ lb. bench press a few years back.