Breeding for increased breast muscle means that the broilers’ centre of gravity has moved forward and their breasts are broader compared with their ancestors, which affects the way they walk and puts additional stresses on their hips and legs. There is a high frequency of skeletal problems in broilers, mainly in the locomotory system, including varus and valgus deformities, osteodystrophy, dyschondroplasia and femoral head necrosis. These leg abnormalities impair the locomotor abilities of the birds, and lame birds spend more time lying and sleeping. The behavioural activities of broilers decrease rapidly from 14 days of age onwards. Reduced locomotion also decreases ossification of the bones and results in skeletal abnormalities; these are reduced when broilers have been exercised under experimental conditions.