Cat-crawls on a railing will rip everything from your feet to your core to your fingertips. They are hard as hell for a reason.
Um...Unless you found someway to weight your ankles, wrists, and back, and do these for a prescribed number of steps, it'd be hard to get a whole lot of strength out of them. It's more of a proprioceptive exercise, with a little bit of endurance (yes, after a while they tire you out. Usually quads/forearms/triceps). Just because it's hard, though, doesn't mean you'll be necessarily getting strength out of it.
Keep in mind doing strength that invlolves holding like say doing a pull up and stopping at half way will tone your muscles, while say doing just pull ups will make your muscles bigger.
Tone=Endurance
Repition=Short time but lots of power
....
Tone is just a mixture of how big (hypertrophied) the muscle is, and more importantly, how low your bodyfat is.
The other factor, which was originally called tone (and since now the term is perverted), is sometimes referred to as 'tonus'. This is when the muscle stays in a somewhat automatically contracted state, do to training.
Holding yourself halfway in a pullup, if done for a while, could work isometric endurance in the arm, but since the Brachialis is more responsible for such an action, more than the bicep (which is better for dynamic flexion of the elbow), you may not be getting the endurance for what you want it.
Doing a set of pull-ups for some people is endurance, and for others strength.
Why?
Well if you can do 11ish+ pullups, you're pushing into endurance work, it's not so much strength.
But if your max of pullups is 9-10 or lower, you're probably still staying in the strength area. (especially if your max is closer to 1 than it is to 10)
