As an addendum: know your body well enough that you can distinguish muscle fatigue from injuries. Sometimes it's obvious (something pops and you get an immediate pain feedback), sometimes it's not. DOMS doesn't set in until 48 hours or so - if you're sore immediately after a workout, you probably injured yourself. Don't try to be a hero about it - you're expected to work through muscle fatigue, but there is simply no reason to try to work through an injury. You'll only end up making it worse, possibly even something chronic. I'm not a physical therapist or a doctor, so that's the extent of the advice I can give.