Learn the lock-off in bouldering: fix the pulling arm in a bent position, reach on with the other hand and protect elbow and shoulder. With milestones.
In a lock-off you hold one arm bent and fixed while the other hand reaches statically to the next hold. The honest answer to "Is locking off just strength?": no, but it doesn't work without strength, and without good leg work it costs you double. Whoever locks off cleanly reaches far without swinging.
The international federation IFSC describes the lock-off like this: you hold one arm with tension in about a 90-degree position to reach up or clip a rope [1]. Joints and muscles pay for it. In a lab study with 17 recreational climbers, elbow flexion moments rose with increasing flexion, and at 90 and 135 degrees of flexion some moments at the shoulder (flexion, adduction, rotation) were also higher than with straight arms, and the activity of biceps, trapezius and latissimus changed with arm position. The finger flexors stayed unchanged [2]. The result fits the picture from sports science: the ideal position is the straight arm, because bent arms engage additional elbow and shoulder muscles that aren't needed to transfer force to the hold [3]. So locking off is not a goal but a tool for places where a straight arm isn't an option.
Typically the lock-off shows up where the next hold is far away and no good foothold is available for a dyno or rockover. Then one arm pulls you to the wall and holds you there while the other hand reaches statically. Static means: no swing, no jump, with control. That is exactly what makes the lock-off the counterpart of the dyno.
Expect it to take several weeks until locking off feels stable (an experience value, not a measurement).
Entry criteria (practice standard):
Stand with both feet on footholds, bend one arm to about 90 degrees and fix it. The feet work along; the arm doesn't carry alone.
Milestone: You hold three seconds per side.
From the position you guide the other hand to the next hold and grab it. Keep the movement calm, without swinging.
Milestone: Five repetitions per side without the pulling arm shaking.
Practice the 90-degree position first. Don't bend the arm deeper than necessary: with increasing flexion, the moments at elbow and shoulder rise [2].
Milestone: Three clean lock-offs per side at 90 degrees.
Get height through the legs. A high, tight hip relieves the pulling arm and shortens the time you have to lock off [3].
Milestone: You solve a move with less locking off than before.
Climb a boulder in which you lock off once deliberately. Decide on each move whether a straight arm wouldn't be better.
Milestone: One calm run without the pulling arm shaking.
Locking off loads elbow and shoulder in a bent-arm position [2]. The authors of the lab study recommend hangs with straight arms for training forearm strength, because they load elbow and shoulder the least, and the study showed no higher finger flexor activity with bent arms [2]. A systematic review on overuse injuries found that a strength training intervention prevented shoulder and elbow injuries, and that reduced grip and finger strength was associated with more overuse [4]. Warm up, lock off only on big, secure holds and build up slowly. A lock-off on a small edge is an injury trap. This guide explains and motivates but does not replace supervised training.
Lock off the feet first, then the arm: when your feet press into the wall, the pulling arm is already half relieved. This is practical experience and not proven, but it is the fastest way to make the lock-off shorter and cheaper.
A lock-off means holding one arm bent and fixed while the other hand reaches for the next hold. The IFSC describes an arm position of about 90 degrees for this [1].
Practice the position on big holds with foot help, about 90 degrees first, three seconds per side. Build up slowly, stay on big holds and use the legs to shorten the time spent in the lock-off.
Bent arms increase the moments at elbow and shoulder [2]. Don't lock off cold, not on small edges and only as deep as necessary.
As briefly as possible. The lock-off is only a waypoint in which the other hand reaches the hold. The longer you stay in the bent position, the more it loads elbow and shoulder [2]. Plan the move so that you only need the position for the hold.
Strength helps, but technique counts more: good legs, a high hip and a straight arm where it fits save more than extra pull-ups. Bring both together.
Compression: Pressing Two Surfaces Toward Each Other