When running downhill, keep your stride short and avoid excessive heel strike. This protects your anterior tibialis which is keeping your toes from slapping down as well as protecting your quadriceps, psoas, and gluteus medius that are stabilizing and controlling your speed.
When you are fatigued to the point that you cannot train with good form any longer, it is time to quit because you're treating compensation instead of good form. You are also increasing stress that will cause you to have to recover longer before you can train again.
At the end of a long training run your hamstrings are fatigued and not decelerating your leg when it swings forward. This increases your stride length, causing heel strike. At the same time you're glutes are fatiguing and not controlling hip sway which is then controlled eccentrically by the TFL - IT. Your glutes are external rotators and when they are fatigued the deep external rotators such as the piriformis have to become overactive to compensate.
Look at the attached study if you are interested in further information. I have edited and reorganized the paper to make it more readable. The paper in its entirety is available for free on pub med. See appendix A at the end of the paper for their walk run program.
I like this paper because they attempt to find cues that were effective in changing behavior rather than simply stating an analysis which as we know frequently does not change what is performed. The fact that this group of subjects had this particular problem is a good indication that they probably have running form issues that are either causative or at least contributory. We do see this problem and problems with similar mechanisms a lot in our novice runners.
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