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I was surprised the authors did not take into consideration heel-to-toe height variances in SRSs, which can be significant (10+ mm). While this may have less effect on one's ability to land "softly" when running uphill, the opposite holds true on downward slopes; depending on the degree of slope, the heel is more likely to contact ground before or concurrent to the forefoot. At least, this has been my experience.
Secondly, while it may be true that a directive to "run softly" will effect the gait and form of a runner upon hearing the instructions, and may even hold sway for a few minutes, the truth is that we all tend to revert to habits after a time, and doubly so when fatigue sets in.