People with ME/CFS usually find pacing the best strategy to try to avoid crashes and give the best chance of improvement over time. Some use activity diaries, others use step counters (pedometer/actometer), others use heart rate monitoring with the aim of using it to alert you to rises that take you over the aerobic threshold. This tells you when to stop and take a break rather than pushing through and trying to complete an activity. A common way of calculating a personal threshold is (220 - age in years) multiplied by 0.6. A description I found helpful is here: http://www.cfidsselfhelp.org/librar...our-heart-rate-to-stay-inside-energy-envelope I have started this thread to invite people to share their experiences of using heart rate monitoring.