As you read this article, you're probably sitting in a chair. Your lumbar vertebrae are supporting your weight. The longer you sit, the more your bones will actively remodel – modifying your vertebrae to better support your weight. However, the older you are, the less your bones will remodel in response to a load. This raises the question – to what extent are the benefits of bone remodeling retained with age? Is it possible to future-proof bone?
Stuart Warden and his colleagues at Indiana University, USA, and several other international institutions thought that bone remodeling caused by physical activity during youth might provide some benefits even years later and the team found a great system to test their hypotheses: professional baseball players. These athletes all undergo the same training routines and have similar levels of activity. They start early in childhood, most stop physical activity altogether upon retirement and they come with their own internal control – their non-dominant hand. The researchers took CT scans of baseball players and non-players of all ages, early career to long retired, and compared the humeri of their dominant and non-dominant arms. Then, they used finite element analysis and musculoskeletal models to simulate the strain on each humerus during a fastball. Finally, the researchers compared age cohorts of players to see which, if any, changes were maintained later in life.
In accordance with earlier studies, the humeri in the throwing arms of the baseball players were much larger than those in the non-throwing arms. The throwing arm humerus in the active baseball players had a greater cross-sectional area, greater cortical bone area, greater cortical bone mass and smaller spongy bone area than the non-throwing arm. These changes considerably reduced the torsional and shear strain on the humerus during throwing – protecting the bone from damage.
When the researchers compared the bones of former baseball players, accounting for the number of years since they had last trained, a surprising find emerged: though the cortical bone benefits of training disappeared over time in the absence of training, some benefits remained. The increase in total bone size that resulted from years of throwing was maintained decades later. This extra size meant that even former baseball players had more than one-third of their original benefits in bone torsional strength – even when their throwing muscles had long since diminished. The few players who continued throwing post-retirement lost less cortical bone than their colleagues, resulting in greater retention of their earlier humeral strength.
The authors stress that their work shows the benefit of physical activity, especially during youth. Even 90 year old former baseball players retained some of the benefits of their training, even though they stopped training more than half a century ago. The benefits of physical activity accrue most rapidly when young, and do not completely diminish with time. Perhaps we should get up off the couch when young, so that we can get up off the couch when we're older.