Between ages 38 and 50, activity levels for many people drop by an average of about a half hour a day over the following decade and are replaced by more sedentary behavior, according to a study from the University of Texas Health Science Center at Houston.

The researchers used data from the Coronary Artery Risk Development in Young Adults Study (CARDIA), which began more than 30 years ago and involves 5,115 participants whose activity levels are monitored with a device similar to a pedometer. Participants were between 18 and 30 when the study started.

Over a 10-year period in middle age, the researchers observed decreases in physical activity levels in both men and women. The decline was most significant in African-American men, who reduced their physical activity levels by an average of nearly an hour a day. African-American women were the least active at the start of the study and were also the most sedentary 10 years later.