What is the difference between the following characters: 腱 vs 筋 （すじ）
Both translate to tendon (as in the connective tissue between muscles and bones)
腱 would generally be read けん and is the word (generally and medically speaking) that means tendon.
The kanji 筋 itself has numerous meanings and uses, so I'll limit this answer to biology.
筋 is much more general, applying to any fibrous tissue, muscle being the most common, not tendon. Also, it is not limited to animals as it includes plant fiber as well 「ふきの筋をとる」(this is a video of someone removing the fibrous outer layer of the fuki plant). Other common usages include the string of snap pea pods and the digestive tract in the back of shrimp.
So 腱（けん） is a specific subset of 筋（すじ）。 筋（すじ）pretty much only means tendon in layman's terms. Just like when we say "I pulled a muscle" even though we may not really know at the time if it was a muscle, tendon, or ligament that we injured, in Japanese people say 「筋を違えた」.