I disagree with most everything here. Front kick is not hard on the knee. Straight punch is not hard on the elbow.
It really depends on how you do what you do. Our lineage of wing chun punches to full extension, letting the elbow lock out (if we are "air punching" or, perhaps if we miss our target).
If you use the classical WC vertical fist and keep the elbow pointed down, and are very relaxed, it doesn't seem to cause any problems. But if people are stiff and use force,jamming against the joint, they may cause problems for themselves.
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