This is an interesting question and not as straightforward as you might think. Obesity is described as a BMI (body mass index) more than 30. A BMI of between 25 and 30 is considered ‘overweight’. In fact, being underweight (a BMI less than 18.5) is known to increase the risk of some fractures. However, it is a complex pattern, as highlighted by a study this year which looked at the relationship of weight, height, and BMI with fracture risk at different sites in post-menopausal women. There was some evidence that hip, spine and wrist fractures were more likely with lower BMI. For upper arm, shoulder and clavicle fractures, only height was significantly associated.
The authors concluded that the relationships between fracture and weight, BMI and height are site-specific. The different associations may be at least partly explained by effects on bone density, bone structure and geometry, and patterns of falling.
While I am no expert in mechanics, from basic principles it seems likely that obesity would make it more difficult to balance, particularly at low speeds.
My advice to any overweight rider considering a return to cycling would be to go for it. Cycling is an excellent low-impact activity which offers great benefits to health and these should far outweigh any risks. And it’s a good way to lose weight. So get back on the bike but take it steadily to start with, until you build up confidence and improve fitness.
This was first published in the October / November 2014 edition of CTC's Cycle magazine.