Under Armour uses a stretchy knit for better mobility.
Under Armour UA Tech 2.0 SS Tee Red Large Loose Fit Short Sleeve Men’s T-Shirt
A nice touch: the solid shade here is saturated without being overwhelming.
Mens Under Armour Tech 2.0 SS Tee Size M Gray-New With Tags
A nice touch is the flat seam construction on this shirt.