So, if exercise isn’t necessarily the key to physical prowess, then what is? One clue comes from a broader view of the meaning of physical fitness. Biologically speaking, all it means is that the body has undergone changes that make it stronger and more efficient. In humans, these changes are induced by exercise. In animals such as bears and migratory birds, they appear to be triggered by seasonal changes that alert their bodies to a coming challenge. For bears, that cue might be falling temperatures or a lack of available food. Whatever the exact tip-off, it seems to prompt the release of muscle-protecting compounds in their blood. In experiments in which rat muscle was bathed in the blood of hibernating bears, muscle loss was reduced by 40 per cent compared with muscle put in blood from bears that were not hibernating.

Our ancestors’ lives were unpredictable. They had to do a lot of running to catch food and escape danger, but they also needed to keep muscle mass to a minimum because food was limited. Seen through this lens, losing condition is an adaptation in itself. Muscle is biologically expensive. Each kilogram contributes about 10 to 15 kilocalories a day to our resting metabolism – which doesn’t sound like much until you realise that muscles account for about 40 per cent of the average person’s body mass. “Most of us are spending 20 per cent of our basic energy budget taking care of muscle mass,” says Daniel Lieberman, an evolutionary biologist and marathon runner from Harvard University.

posted 2555 days ago