I've been using a pedometer(~$10) which measures the number of steps you take. I could not accurately figure out how much physical exertion(walking) makes me crash or PEM/DPEM. It drove me crazy and miserable with the up and down unpredictable crashing. Using the pedometer has been the most accurate way for me to test my walking limits and to prevent crashing. I want to share with you my numbers to help you figure out your numbers. 100 steps=1min 1000 steps=10min( based on normal walking speed) Most days i'm housebound and laying on my back getting up only to goto the bathroom or kitchen. I average about 700 steps(7min) over my waking 16hours before i goto sleep. I was shocked when i first saw 700 steps because it sounded like alot, but it's only 7min on my feet over 16hours. When i am "crashed"/PEM, i average about 400steps(4min) over the 16hours i'm awake. On a very good day, i can walk 4 residential blocks back and forth(8blocks) which is about 2000 steps(20min). I have a good day 1X or 2X a week. So now, i know that on my "good" days, i can walk up to 2000 steps or 20minutes continuously. A year ago, i couldn't even walk 10minutes w/o crashing. I improve my walking distance very slowly about every 4months. However, I am still unable to walk 2500 steps/25min w/o crashing so i am stuck at 2000/20 right now. I also tried 2000/20 on a "normal" day, but i cannot walk the 2000/20 without DPEM(delayed) which crashes me for about 2 days. I will only go out and run short errands on my good days knowing what i know with the pedometer. Just going to the supermarket or the doctor's office will easily run up the pedometer 1500steps or more. I'm fine running 1 or 2 short errands on my good days as long as i know that i can't pass 2000 steps on my pedometer. For comparisons, healthy sedentary people who do not exercise averages 10,000-12,000 steps per day. I encourage everyone to get a pedometer. It has really helped to prevent crashing myself and to accurately track the status of my physical improvement. Those are my numbers. What are your numbers?