Learn Spanish (Latin America): Siento el arnés apretar mi cuerpo.

With perception verbs like sentir (to feel), using the infinitive (apretar) focuses on the action as a whole fact or a completed event. If you used the gerund (apretando), the focus would shift to the ongoing, continuous action happening in that exact moment (meaning "I feel the harness tightening"). Both are correct, but they offer a slightly different perspective on the action.

How would I rewrite this sentence to say "I feel it tighten my body"?

You would place the direct object pronoun lo before the main conjugated verb: Lo siento apretar mi cuerpo. When dealing with perception verbs, the pronoun for the object you are perceiving (the harness) goes before the main conjugated verb, and cannot be attached to the end of the infinitive in this specific structure.

Does apretar literally mean "to tighten"? I originally learned it as "to squeeze".

You learned right! Apretar usually means "to squeeze" or "to press". However, when talking about clothing, straps, shoes, or harnesses, it is the standard verb used to express that something fits tightly, pinches, or tightens around you.

Why is the verb just siento and not the reflexive me siento?