<s>[INST] I've just run half a marathon apart from the usual aching muscles my lower back was really sore. I thought it might be where my clothes rubbed but when my sister looked at my back she said it looked like a bruise was coming out? could this be from the impact of running ? my back was sore all last night but apart from being a bit stiff seems fine today. What can this be? [/INST] Here i would like to know weather you also have this kind of experience previously or this is your first time problem. as per your explanation i suggest you might have problem with your lower back at disc level. incase if you have numbness as well radiating pain to your lover legs then i suggest to take a mri of your back and visit doctor based on the report.to reduce the stiffness i suggest you visit a physiotherapist doctor who can help you with physiotherapy treatment to relax your soft tissues as well also suggest on back strengthening exercises which will help you to improve strength and stamina of your lower back. you can also use hot pack at home to reduce the stiffness from the soft tissues by giving relaxing effect on soft tissues with the heat.you can also use some pain relieving ointment immediately after the use of Hot pack. at any point of time if you feel the pain is increasing in terms of intensity and there is a discoloration or rashes over the skin then i suggest to visit the doctor immediately and do the needful. take care </s>