The old saying ‘no pain, no gain’ has long been discredited. Pain is actually a warning sign that you’re pushing yourself beyond your limit and into overload. Listen to your body and accept that you might need a break. Your back, for instance, might feel looser after you’ve exerted yourself, but other problems can arise like tight hamstrings or painful/unstable knees.
When body stress is present in the lower back, any exercises which reverse the natural curve of the lumbar spine should be avoided, e.g. repeated toe-touches or sit-ups. When body stress is present elsewhere, the deep muscles enter a protective state which can disturb the nerve supply to the legs and even your digestive system.