I've been reading this interesting study on posture and body weight. The authors found a strong correlation between increased body weight and a loss of balance. What's interesting to me is that they also connect being overweight to receiving less tactile information from the feet.
The study states that being overweight reduces the senesitivty of the receptors in the feet. Specifically, the authors state that, "the greater pressure values and larger contact areas for the obese persons may reduce the quality and/or quantity of the sensory information arising from the plantar mechanoreceptors."
They also mention a previous study which shows the reverse - that is, losing weight improves posture. Overall, this relationship doesn't bode well for the United States. With 2/3 of the population being overweight or obese, you now have 2/3 of the population susceptible to poor posture.
