Why do knuckles crack?
Show answer & explanation
Answer: Gas bubbles popping in fluid
Ligaments snapping back — Wrong. Ligaments can snap creating sounds, but classic knuckle crack is cavitation—gas bubble formation/collapse in joint fluid.
Gas bubbles popping in fluid ✓ — Correct! Joints contain synovial fluid lubricating movement. Pulling/bending joint rapidly decreases pressure in fluid—dissolved gases (CO₂, nitrogen, oxygen) form bubbles—cavitation. Bubble formation/collapse creates popping sound. Can't crack same joint immediately—gases need ~20 minutes to redissolve. Studies show cracking doesn't cause arthritis! Some people habitual crackers. Harmless reflex.
Cartilage breaking slightly — Wrong. Cartilage isn't breaking. Sound comes from gas bubble cavitation in synovial fluid when joint pressure drops suddenly.