Rhomboids, like triangles, may take various different shapes, but they always look like a lopsided diamond or rectangle. As both a noun and an adjective, rhomboid can be applied to anything with those shapes, such as certain muscles of the upper back when viewed from behind. Whenever you hear about rhomboid exercises, rhomboid strain, or rhomboid pain, it involves those muscles, which attach your shoulder blades to your spine and can be strained by carrying a heavy backpack, serving a tennis ball, or just slumping in your chair in front of a computer all day.