As onbuhimos take the babywearing world by storm, I'm increasingly seeing pictures of babe on the back and a note about loving the look and compactness of the onbu, but the wearer just doesn't think it's for them because they can't get comfy in it. While explaining to someone what they needed to do differently and why (the science behind it), they suggested I write a post about it for all the folks who will soon be sporting a G&S onbu. So here you have it. The Science of Babywearing and how it will make your onbu more comfortable.
The short answer is, T = d * F
T = torque, or in this case, how heavy your baby is feeling and how much he/she is pulling on your shoulders and back (or in Science terms, the force perpendicular to the arm).
d = the distance between the mass (ie, baby) you are carrying and your personal center of gravity axis (the aforementioned "arm").
F = Force, or in this case, the weight of the baby
(Note: I actually did consult a Physics teacher before writing this. I didn't remember that formula, either, just the concept behind it.)
The natural bio-mechanics of the human body is to lean slightly forward. It's not a massive lean, and hardly even noticeable to look at someone, but it plays an important factor in this discussion.
Our center of gravity is roughly around our navel.
(Please excuse the slightly alien aspect of my mama here - my talent is in textiles, not drawing!)
When you put your child on your back and position too low, there's not enough room for your center of gravity axis to absorb that weight, and so it draws the fulcrum out. You've just created the "d" in the formula. That means you've just made your child feel heavier than he or she actually is. Torque is what you're feeling, and you've just multiplied their weight (F) x wherever they've landed low (d). The lower it is, the more it will force open that "hinge" of the axis, and the more torque will be felt. Pity that lower back!!
Likewise for high wearing but leaving enough room for the child to be able to move around. I hear it often - "if I tighten it, she feels like she's pinned there and she likes to have some room!". Except, if we apply our formula, we're going to see what's happening again. It looks like this:
The degree to which you give the child extra space is the degree to which "d" becomes your multiplier. What?!?!?
How much your shoulders hurt = F (25 lb child) x 1.2 units from you center of gravity = feel of a 30 lb child.
Got a real leaner? 25 lbs x 1.5 = 37.5 lbs! No wonder your shoulders hurt and you hate your onbu!! It's like that weather app that has the actual temperature and the "feels like" temperature, but in terms your chiropractor and massage therapist (aka, husband) deal with.
The trick to an onbu (or a mei tai, for that matter), is high and tight. Decrease the amount the child's body can open that fulcrum angle while minimizing how much space the baby can create at the end of the lever (otherwise known as your torso!):
Want to know the real secret to that amazing babywearing sweet spot you hear tell about? Simple math:
Child is high, tight, and incredibly close to your own center of gravity axis (which, unless they're on your head will never be '0'), you can actually get into multipliers less than one. So the 25 lb child who is, say, .75 units off your center now weighs....wait for it....19 lbs! (please note, since it has come up repeatedly in one forum, this is a perceived, relative weight and not an actual measurement of the child's weight. You cannot cause actual weight loss in your child by tying them higher or lower, tighter or looser on your body. They will weigh the same when you take them off as they did when you put them on.)
Soft Structured Carriers give you a huge margin of error around the center of gravity aspect because of the padded waist belt. What you don't absorb into your own axis of carrying is absorbed by the closed cell foam at your waist. So you cannot, cannot, CANNOT try to wear your onbu (or your unpadded mei tai) like your full buckle. It just won't be as comfortable. You really need to take the care to find the right position and adjustments.
Fortunately, since the onbu is designed to allow for arms out, you often won't need to worry so much about whether the child "feels pinned" or not. They still have freely hanging legs (because you checked the knee to knee didn't extend past the knee) and arms out to hold a juice cup, dolly, or your ponytail. The rest of the transition from a deep well of room to move in an SSC to the higher vantage point of the onbu often works itself out.
So enjoy your onbu, and carry on!