Can holding exercises alone make you stronger?

Short answer

Yes. Holding alone increased both strength and muscle, and holding strength rose more than with moving exercises. Holding shallow made only the range near that angle stronger, while holding in a deep position, with the muscle stretched, raised strength at other angles and in moving too.

Can holding exercises alone make you stronger?
'You have to move to get stronger': is it true?
Some exercises, like a wall squat, have you push hard and hold without moving the joint. A common claim is that they build less strength because you never lift and lower a weight. We checked whether holding alone builds strength, where the strength shows up, and whether muscle grows too.
Holding alone builds strength
Holding strength rose more than with moving exercises
An analysis that pooled 32 studies with 621 people found that holding exercises raised strength more than no exercise. Compared with lifting and lowering weights, holding strength also rose more with holding exercises. Strength measured while moving at a steady speed showed no difference between the two.
But it rose at the angle held
Only the range near the held angle got stronger
6 people held a maximal calf push for 5 seconds at a time, 3 times a week for 6 weeks. Strength rose only at the ankle angle trained and the two angles right next to it. In a study where 9 people held the front thigh with the knee only slightly bent for 12 weeks, strength also rose only near the trained angle.
Holding deep was different
A stretched-muscle position raised strength at other angles too
In the same 9-person study, holding with the knee deeply bent, so the front thigh was stretched, raised strength at every angle measured. In another 8-week holding study, the group that held at a 90-degree knee bend gained 23.5% in strength measured while moving. The group that held at a shallower 50 degrees did not gain meaningfully (10%).
Muscle grew too
Holding deep showed no difference from lifting and lowering
A review of 26 studies found that with the same amount of holding, the stretched-muscle position grew muscle by 0.86-1.69% a week and the shortened position by 0.08-0.83% a week. 23 people trained for 6 weeks, holding with one leg deeply bent and lifting and lowering through the full range with the other. Front thigh muscle thickness showed no difference.
The other way: what about lifting strength?
Lifting and lowering raised lifting strength 2 times more
An analysis that pooled 43 studies with 1,660 people found that lifting and lowering weights raised lifting strength 2 times more than holding strength. The reverse holds too: holding alone raises holding strength the most. Strength follows the movement and angle that were practiced.
Summary: holding alone builds strength
Hold deep, and practice the target lift separately
Holding alone increased strength and muscle. For the strength to spread to other angles, holding in a deep position with the muscle stretched works. A review of the research named 80-100% of maximal effort, 1-5 seconds per hold, 30-90 seconds of holding in total per session, as the approach for strength. If a barbell record is the goal, practicing with the barbell too works.