The gluteus maximus is the largest muscle in the human body and is the main muscle used to help walk upstairs.
Notes on verification
Well corroborated by multiple independent anatomical sources (Wikipedia, Physio-pedia, ScienceDirect, Healthline) confirming both the size claim and its role in stair climbing.
Sources
- 14 Fun Facts About the Muscular System - Healthline (healthline.com)
- https://www.physio-pedia.com/Gluteus_Maximus (physio-pedia.com)
- https://www.sciencedirect.com/topics/neuroscience/gluteus-maximus-muscle (sciencedirect.com)