not a doctor...
Yes, it does tighten muscles. My explanation is triggerpoints, which is a perfect example of oxygen deprivation in tissue. Using the muscle will give pain which disables you to keep up physical activity as you would like to. Of course you are more likely to hurt yourself when muscles are tense and not functioning (temporarily) like they should, pain is meant to protect you from that. If I loose control of my knee, I am more prone to twist it for example. Tearing a muscle with a triggerpoint in it is pretty difficult, because the pain will probably stop you beforehand. Keeping up with physical activity in my mind should not be read as pushing through despite terrible pain and lack of control. Not doing any physical activity at all isn't the way to go either. You need to find a level of activity that is doable and that helps you to break the idea that you are suffering from something structural. Seeing the discomfort as a symptom of the mind instead of getting scared or angry about the symptom is the way to go.