The elbow pain ruleEmerging evidence
If there was a pop, bruising or a bulge, don't start this plan: get it assessed first.
Some discomfort during forearm exercises is fine. Keep it mild: no more than about 3 to 4 out of 10. It should settle within an hour or so, and your elbow should be no worse the next morning. If it is worse, go lighter next time (a lighter weight, a bendier bar, fewer reps or a shorter hold), not nothing. When a set no longer causes any discomfort, it's time to progress.
Tingling is the exception: stop any exercise or position that brings on pins and needles or numbness in your hand, especially in the ring and little fingers, and avoid holding the elbow fully bent while it settles.
This is lower than the 5 out of 10 used on this site's back and shoulder pages, because most elbow trials used pain-free or nearly pain-free exercise. A UK consensus paper that reviewed the tennis elbow trials reports that all but one asked for pain-free exercise; the exception, as that paper describes it, allowed mild pain below 4 out of 10. The JOSPT guideline says the load should not make symptoms worse, and elbow specialists suggest doing the flexible-bar exercises with discomfort of about 3 to 4 out of 10.
Pushing harder backfires at the elbow. Still holds done above people's own pain threshold left more pain 30 minutes later, while holds below it did not, and in one trial most people gave up heavy loading, mainly because it aggravated their pain. In long-lasting muscle and joint pain generally, allowing some pain did no harm. The exact 3 to 4 out of 10 figure and the settling times are editorial. If pain climbs from week to week, or you notice a warning sign, stop and get it checked.
Sources 15, 14, 16, 17, 18, 19