Learn the gaston in bouldering: grab a sidepull with the thumb down, elbow out and push yourself away. With shoulder safety notes and step-by-step milestones.
The gaston is a sidepull you grab with the thumb pointing down and push yourself away from, instead of pulling it toward you. The elbow points outward. The honest answer to "Why is the gaston so uncomfortable?": because your shoulder muscles work in a pattern here that hardly ever occurs with a normal grip. Whoever climbs it cleanly reaches holds that are unreachable face-on.
Sports science describes the gaston as a sidepull for the other hand: if the left hold can only be loaded from the right side, you can no longer pull on it but push yourself away from it [1]. The muscle chains for this are completely different from a natural grip and place high demands on the shoulder girdle muscles. That is why the technique is also called the "shoulder move" [1]. The international federation IFSC defines the sidepull as a hold where you turn your weight away from the direction it faces [2]. In the gaston, the direction of force is reversed compared with the normal sidepull.
In practice that means: in a normal sidepull you pull the hand toward you, in a gaston you work in the opposite direction. Your body usually stands in a turned-in position, and the hip sets the direction. Once you have felt it, you recognize gastons just by looking at the route.
Expect it to take several sessions until the gaston no longer feels foreign (an experience value, not a measurement).
Entry criteria (practice standard):
Look at the sidepull: if the surface faces the wrong side for the hand that should take it, it is a gaston. You don't pull, you push away.
Milestone: You name the right hand and pressure direction at five holds.
Place your feet so they work against your pressure, and turn the hip toward the wall. Without counter-pressure your body swings open [1].
Milestone: You hold the counter-pressure for five seconds without a shoe moving.
Grab the sidepull with the thumb down, the elbow points outward, the shoulder stays low. The arm works in a different rotation than with a hold from above [3].
Milestone: You hold the gaston stable for three seconds.
Push yourself away from the hold and reach on with the other hand. The pressure is not arm strength alone but comes from the back and the feet. Let the hip work toward the wall and keep your eyes on the next hold, not on the hand.
Milestone: Five clean gaston moves per side.
Climb a medium boulder with a gaston. Decide on each move whether you really need it.
Milestone: One calm run without slipping off.
The gaston is a move with high shoulder demand [1]. In a lab study with men with no climbing experience, arm position changed which parts of the trapezius worked, and one-handed support demanded much more of the muscles than two-handed support [3]. That was a therapeutic hold, not bouldering, but it shows that rotation and one-handed support matter. In a prospective study of 507 boulderers, 63 percent of all injuries affected the upper extremity [5]. A systematic review found that a strength training intervention prevented shoulder and elbow injuries [4]. Warm up shoulders and back, start at jump-down height and stop if your shoulder pinches. Persistent shoulder pain belongs in medical hands. This guide explains and motivates but does not replace supervised training.
Imagine you are pushing a heavy door away from you: the elbow goes out, the chest opens, and the power comes from the back. This is practical experience and not proven, but the image stops you from pulling the gaston toward you like a normal hold.
A gaston is a sidepull for the other hand: you grab it with the thumb down and push yourself away from it instead of pulling it [1]. The elbow points outward.
Because the shoulder muscles work completely differently than with a natural grip and place high demands on the shoulder girdle [1]. That is why the technique is also called the "shoulder move".
Set your feet against the pressure, turn the hip toward the wall, grab the hold with the thumb down, keep the shoulder low and push away. Reach on quickly.
In a sidepull you pull the hold toward you and push back with your feet. In a gaston you grab with the thumb down and push yourself away from the hold. The muscle chains are completely different, which is why the gaston doesn't work with normal sidepull technique [1].
It demands a lot of the shoulder muscles [1]. Warm up, hold the gaston briefly and stop if it pinches. For persistent pain, see a doctor.
Lock-Off: Fix the Pulling Arm and Reach On