A finger jam holds because you twist your fingers inside the crack. Learn it on top rope, dose the load and see why leading cracks is a course of its own.
A finger jam is a hold that exists only in a crack: you put your fingers into a narrow gap and twist them until they jam. The honest answer to "Doesn't that hurt like hell?": at first yes, because the skin rubs at the crack opening. With time you get used to it. What you should not train away is respect for your fingers.
Fingers are the most common weak point in climbing. A systematic review concluded that overuse injuries, above all of fingers and shoulders, are more frequent than acute injuries in adult climbers [1]. A motion analysis names rupture of a finger pulley as one of the most common climbing injuries [2].
Part of the truth is also: whether finger jams in a crack load the pulleys like a crimp, I did not find in any examined study. That is why you dose carefully and learn the technique where you are protected.
In a crack it is not the strongest finger that climbs, but the one that stays healthy longest.
Entry criteria (practice standard):
A finger jam needs a crack into which only the fingertips up to the first or second joint fit. For starters look for a ground-level crack or a crack machine and test whether the fingers hold with a slight twist. The rock should be clean and not sharp-edged.
Milestone: You hang 10 seconds with one hand in a finger jam, feet on the ground, without slipping. (Rule of thumb from course practice.)
In a finger jam you put the fingers into the crack, and the thumb usually points down. You rotate the hand around the axis of the arm so that the fingers tilt in the crack and wedge. The force comes from the twist, not the pull.
Milestone: You place the jam cleanly three times with both hands and release it just as cleanly.
In the jam you hang on the skeleton, not on the biceps. Keep the arm long and let the feet work, as in crack technique in general. Pulling creates pull on the fingers.
Milestone: You stand two moves in the crack in such a way that you can shake out each arm once.
Finger jams are intermediate holds, not rest places. Plan changes so each finger hold carries only a few seconds, and allow yourself breaks. Finger strength protects: reduced grip and finger strength is associated with more overuse injuries [3].
Milestone: You climb an easy crack length on top rope and change hand and hold in good time.
Anyone who wants to lead in a crack needs removable protection. The DAV writes about this: its safe use requires experience and is best learned in further courses [4]. Until then you stay on top rope.
Milestone: You can explain why you do not lead the crack yet.
Two risks run in parallel in a crack: the acute one and the creeping one.
The acute one is the fall on rock. In an analysis of Austrian climbing rescues the risks of severe or fatal injury rose when climbing without a helmet or without a rope [5]. The DAV recommends a helmet on rock generally because rockfall and impact cannot be ruled out [4].
The creeping one is overuse. In an overview of 34 studies, higher climbing intensity, bouldering, low grip strength, crimp grip and previous injuries were associated with more overuse injuries. Strength training prevented shoulder and elbow injuries [3]. Tape, stretching or warm-up showed no such association [3].
Try the same finger jam in two directions, thumb down and thumb up. Depending on crack width and direction, one of the two positions holds better, and you learn to feel before going in which one. That is practice from crack climbing, not a study finding.
It is uncomfortable, mainly on the skin. With habituation it gets better. But pain in the finger itself, especially at the finger joint, is a warning signal; then you pause.
It protects the skin from abrasion. As protection against overuse it is not proven: in a systematic review taping was not associated with fewer overuse injuries [3].
I found no reliable study on that. Practice: plan breaks and progress slowly, especially if you had finger problems before.
I found no direct comparison. On rock the risk depends on terrain, helmet and protection [5]. In a crack removable protection comes in, which you learn in a course [4].
Only on a real crack machine or a ground crack. Without a crack there is no jam.
Foot Jam: Standing in a Crack Without Paying With Your Foot