This question was the reason why we developed the NIMBEL baby carrier! With any regular carrier or baby wrap, the weight is largely carried on your shoulders. Many carriers have a waistband, but only a small portion of the weight is directed to it. If you were to loosen the shoulder straps, your baby would fall to the ground. It's the shoulder straps that carry most of the weight, which is understandable given that the carrier is made of a piece of fabric without reinforcement.
This is actually quite odd. With hiking backpacks, we've known for years that it's much more comfortable, and much more sustainable, to carry the weight primarily on the hips. That's why backpacks have frames that help direct the weight to the hips.
We've applied this simple principle to the NIMBEL baby carrier. Two rods on the outside direct the weight of your baby directly to your hips. If you loosen the shoulder straps of the NIMBEL carrier, your baby will still remain seated (for added safety, we recommend always using both shoulder straps!).
To prevent shoulder, neck, and upper back pain, carry your baby in the NIMBEL carrier and adjust the shoulder straps to your liking. By tightening and loosening them, you can play with how much weight is on your hips versus your shoulders.