Ah, but here's the thing - how are we defining 'commoner'? Do we mean Joe the Pigfarmer from somewhere in rural England? What about Andreas the Guildsman from Amsterdam, a schutter of the crossbow guild? He's certainly not noble, but does he meet your definition of 'commoner'? (Indeed, as some of the answers below explore, 'noble vs commoner' may not even signify - there were rich peasants and poor nobles, after all.)

As with everything, more can always be said on the topic, so if anyone would like to try out their military history knowledge, please feel free to make a post of your own! Just be advised that if it's good, I'll save the link for the Medieval Armies Compilation. Which is also what we're drawing on today; observe these following answers:

• u/Rittermeister has three posts that examine participation in combat and why they'd fight;
• u/Goiyon examines the military human resources of the County of Holland;
• and u/Hergrim looks at a healthy male English peasant called up for service around 1100.

Note, of course, that the Middle Ages stretched a long time and encompassed a lot of places; what held for England in 1100 might not hold for England in 1355. If you have a specific time or place in mind, there might be something more in the Compilation that's closer.