Poor cardiovascular health, bone density issues, and poor muscle health all have been attributed to lack of exercise. The decreased amount of muscle mass makes even simple movements difficult, while one study found that crated sows had two thirds the bone density of non-crated sows, with researchers concluding that weakened bones led to a higher incidence of broken bones or injuries and another study finding that a common factor in mortality cases was leg weakness. The lameness contributed to these weakened bones is also in part due to the hardness of the concrete flooring, which inevitably leads to a higher incidence of injuries to the feet and or legs, not to mention shoulder sores and abrasions to the skin from rocks, sharp edges, and bolts fastening the crates in place. This same flooring is thought to contribute to toe lesions, with some reports claiming that up to 80% of crated sows suffer from this illness. Due to lack of exercise, sows may develop a number of additional physical conditions that compromise their overall well-being. Urinary tract infections may develop as a result of the pigs immobility, which causes them to drink less frequently and consequently develop bacteria within the urinary tract. Though this same illness is contributed to the fact that sows are forced to lie, or sit, in their own feces - another reason some think there is a higher incidence of respiratory disease among sows confined in gestation crates.