A genetic tweak has converted mice into endurance runners by enriching a little-known form of muscle fibre. The discovery could help boost sporting abilities, or reveal ways to slow muscle wasting. Human muscles are made of four main types of fibre, including two ‘slow-twitch’ varieties and one ‘fast-twitch’ muscle type that are suited to endurance and sprint activities respectively. Little has been known about the fourth type, called IIX fibre, because it is scattered throughout different muscles.
Now a Boston team has hit upon a genetic switch that converts almost all mouse muscle fibres into type IIX. The result is startling. “Damn, they’re good athletes,” says Bruce Spiegelman of Harvard Medical School, who led the team. The mice were able to run on a mouse treadmill for 25% longer than normal before reaching exhaustion.
The discovery hints that the elusive type IIX muscle fibres are an underappreciated contributor to athletic ability. It is possible, for example, that world-class athletes are naturally endowed with more of these fibres than the average person — or that hard training generates more of them.