Relation between "muscles" and "tendons"

In Uberon, tendon is textually defined as “dense regular connective tissue that connects muscle to bone”, and is a subclass of connects some 'muscle organ'.

Several individual tendon classes do fit that definition, such as tendon of triceps brachii, which is attached to 'triceps brachii, or calcaneal tendon, which connects some 'muscle of leg'.

However in some cases, the tendon term is said to be part of the associated muscle, e.g.

• tendon of quadriceps femoris, which is part of some 'quadriceps femoris';
• central tendon of diaphragm, which is part of some diaphragm;
• infraspinatus tendon, which is part of some 'infraspinatus muscle' (and also connects some 'infraspinatus muscle').

So should tendons be generally considered to be part of the associated muscles or rather be considered distinct from the muscles and merely attached to them?

(Context: In FBbt we are currently considering introducing proper terms to represent muscles – we only have terms to represent muscle cells at the moment –, and we are not sure yet what to do with the tendons with respect to the muscles. I’d like to be somewhat consistent with what Uberon is doing.)