A muscle shirt is basically a men’s T-shirt without sleeves. You can try out the look by converting a basic T-shirt into a muscle shirt in just a few minutes. For an authentic muscle shirt look, pick a tighter-fitting T-shirt to convert into a muscle shirt.
Use a seam ripper or scissors to remove one of the sleeves of the T-shirt at the seam. Try to just cut the threads holding the sleeve to the T-shirt without damaging the t-shirt itself.
Repeat on the other sleeve. Discard the sleeves or keep them for another project; you don’t need them to make a muscle shirt.
Fold the raw edges of one sleeve under by about 1/2 inch, and pin in place.
Repeat on the other sleeve. These folds will create the hems on your newly modified armholes.
Iron both pinned hems flat.
Sew the hems around each of the armholes using a sewing machine or needle and thread. During this step, be careful not to accidentally sew the armholes shut. This is especially possible if you’re using a sewing machine. Remove the pins.
If sewing is outside of your comfort zone, leave the armholes unfinished after removing the sleeves. This will give the shirt a rugged look. The drawbacks to this option are that the shirt may begin to unravel, and it will be much easier to accidentally rip at the armholes.