Finger strength, hold spacing, and height Aug 8, 2019 19:17:40 GMT -7
Post by psathyrella on Aug 8, 2019 19:17:40 GMT -7
Feel free to remove if this is too tenuously related to training, but I thought some people here might find it mildly entertaining.
So despite being profoundly tired of irl discussions of the relative benefits of being tall vs short, since despite everyone's best efforts they inevitably devolve into I-didn't-send-what's-an-excuse (myself of course included), I have one persistent question. Setting aside the relative prevalence of reachy vs scrunchy moves, which depends on where you climb and what the last climb was that you didn't send, it's intuitively obvious to me why reachy moves are hard for my shorter partners. It's much less obvious to me, though, why my taller partners get shut down by scrunchy moves, so I thought I'd run through a quick calculation to try to work it out. I'm defining "scrunchy" as moves where feet and hands are forced to be close together vertically, either because hands are forced low (e.g. underclinging a roof or setting up for a deadpoint to a high hold) or feet are forced high (e.g. when you've just gone over a roof).
I'm attaching the actual work, but the result I get is that force at the fingers is roughly
F = mgl / 2z
for m (weight), g (gravitational constant), l (total length finger tips to toes), and z (vertical distance from foot holds to hand holds). This suggests that scrunchy difficulty is just a result of your mass acting on a longer lever arm (i.e. your butt is further out), requiring larger finger force to counteract the resulting torque. Note also the divergence as z goes to zero: keeping your feet low when laybacking is not a small effect.
To compare numbers, we have to decide on a mass to plug in. Now of course force required and "inherent" finger strength both tend to increase with weight, and we could plug in some proportionality between the two. For instance Fig 6 in this suggests that if you double a human's weight (e.g. make them taller), that their maximum trainable strength to weight ratio decreases by about a third. But whatever, let's just stick to the geometric effect of hold height, since the natural level of finger strength varies way tf more between people than any of this other stuff anyway. So to compare a couple different heights and z values I just use a common weight of 145lb. To convert length to height I used a proportionality of 1.25, which for sure varies from person to person (higher ape index means more finger strength required in this scenario), but this is fine for a rough estimate.
Anyway, the numbers I get are that for hand holds 4 feet above foot holds, going from 5'5" to 6'0" increases finger force by maybe 10lb. If the holds are 2 feet apart, this difference grows to about twenty pounds. This conforms to my expectation of at most perhaps a letter grade or two of difference from watching different sizes of people on various moves.
Here's a plot from google, if you plug in this search text it makes it easy to read off the numbers (units are inches and pounds for x and y), and the three lines are 5'0", 5'6", 6'0":
And the calculation:
And lastly, I wouldn't be remotely surprised if I messed up the algebra, do lmk if you see something funky.