The full crimp in bouldering, honestly explained: how it works, why it loads the finger pulleys most and how to use it sparingly. With milestones.
The full crimp is the grip with the highest holding power and the highest finger pulley load: the fingers are strongly bent, the end joint is hyperextended, the thumb lies over the index finger. The honest answer to "Should I learn the full crimp?": you learn to recognize it and to use it sparingly, but not to make it your default. It is a tool for a few moments on small edges, not an everyday grip.
The international federation IFSC distinguishes the closed crimp, where the thumb wraps over the fingers, from the open one, where the thumb is removed [1]. A motion analysis with eleven elite climbers showed how differently the finger joints work: the middle finger joint (PIP) was bent on average 87 degrees in the crimp, 70 degrees in the half crimp and 39 degrees in the open hand [2]. A 3D model calculation showed that the A2 pulley was loaded 36 times less in the slope grip than in the crimp, and the A4 pulley four times less [3]. The reason: in the crimp the tendon presses hard against the pulleys like a bowstring. In measurements on living fingers, the load over the A2 pulley reached up to 116 newtons, as if a good 11 liters of water hung on a single pulley. Warming up also increased the distance by which the tendon lifts off the bone by about 0.6 millimeters [4].
Expect it to take months until the pulleys are used to the full crimp (an experience value, not a measurement). Get them used to it slowly.
Entry criteria (practice standard):
The half crimp is your main grip. In a study of 32 advanced and elite male boulderers, half-crimp strength explained 48 to 58 percent of bouldering performance and was thus the best single indicator [7]. Set it cleanly and without pain.
Milestone: You hold ten seconds on a big edge in the half crimp without pain.
Deliberately set open, half and full crimp, still without full load. Feel how the bend angle in the middle finger joint changes [2].
Milestone: You switch the three forms cleanly with little load.
Test the full crimp only briefly and with plenty of foot help on a big edge. The thumb lies over the index finger, the feet carry most of the load.
Milestone: You hold three seconds on a big edge, feet carrying most of the load.
Use the full crimp only when the edge won't hold any other way, and only for one move. Afterwards you open again.
Milestone: One move in full crimp, then immediately back to half or open hand.
After the move go back to a more relaxed form and rest. Plan your boulder so that you need at most one full-crimp move.
Milestone: You climb a boulder with at most one full-crimp move.
This is a safety-critical guide. Pulley injuries are the most common overuse injury in climbers, and the crimp grip is the mechanism; the A2, A3 and A4 pulleys are most at risk, especially under eccentric load [5]. A systematic review found the crimp grip to be a risk factor for overuse injuries, as were bouldering, higher climbing intensity, lower grip strength and earlier injuries. Warming up and stretching were not associated with less overuse in that review, so the evidence is mixed [6]. Diagnosis runs through ultrasound; treatment for grades I to III is conservative with immobilization, tape or splint, for grade IV surgical [5].
So: build up instead of taking shortcuts. Don't make the full crimp a training stimulus, don't use it on small edges when fatigued and not when your fingers hurt. For pain at the finger base, rest and get a medical check. This guide explains and motivates but does not replace supervised training or medical advice.
Before every move ask yourself: "Does the edge also hold in the half crimp?" If yes, stay there. The full crimp is the exception when the edge is truly too small. This is practical experience and not proven, but the question saves you hundreds of full-crimp moves a year.
The full crimp is the grip where the fingers are strongly bent, the end joint is hyperextended and the thumb lies over the index finger. It has the highest holding power and the highest pulley load [1][3].
It loads the pulleys much more than the open hand [3]. Pulley injuries are the most common overuse injury in climbing, and the crimp grip is the mechanism [5]. Use it sparingly and with a good warm-up routine.
Only when an edge won't hold in the half crimp or the open hand, and only for a single move. Afterwards you open again. It is not suited as a default.
Not as a foundation. In one study, half-crimp strength explained bouldering performance better than any other grip tested [7]. Build the half crimp and technique first before you use the full crimp.
Pinch Grip: Hold With Pressure, Not Just Pull