If you’ve ever been frustrated by the refusal of your tired muscles to continue a particularly exertive activity, feel free to blame your old bones, in particular a bone hormone called osteocalcin. Researchers led by Columbia University’s Gerard Karsenty report that osteocalcin is necessary for muscle cells to adapt to increased energy requirements during exercise (Cell Metab. 2016, DOI: 10.1016/j.cmet.2016.05.004). The bone hormone—a small protein—activates muscle cells to increase uptake and catabolism of glucose and fatty acids so that exertive activity can be maintained. Working with mice, the team found that circulating levels of osteocalcin double during endurance exercise in young adults, decrease sharply before or around midlife, and do not increase during exercise in older animals to the same extent as in younger ones. When the team gave osteocalcin to 15-month-old mice, the animals’ exercise capacity returned to that of three-month-old mice. If the results hold true in humans, one wonders whether osteocalcin will become as common in gym bags as a towel and water bottle, or whether the hormone will end up banned and added to international doping lists.