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.
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」.