Lactic acid buildup, maybe?
Doing better post effort cooldown and stretching might help a bit, as will deep tissue massage.
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. “He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”- Fredrick Nietzsche
"We can judge the heart of a man by his treatment of animals." - Immanuel Kant