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Could the defining factor between good and great sprinters be leg length?

Can exercise science tell us who is the next Usain Bolt?

What makes a fast runner, fast?

Elite sprinters can reach run up to 44km per hour. ECU Professor of Biomechanics Tony Blazevich has a look at what allows these incredible athletes to reach such speeds.

There are 2 major players biomechnically - Muscle Force and Muscle mass. The fastest sprinters on average take longer strides than slower sprinters, but at a similar stride rate. This results from larger forces being delivered to the ground. Of course, having longer legs can benefit stride length, which appears to be a significant reason for Usain Bolt's superior top speed.

But it's not all good news for taller runners. Longer legs have a greater moment of inertia (that is, their legs are heavier and their mass is generally located further from their hips), so they attain less speed for a given hip torque production (i.e. muscle force). In this case, there are different costs-benefits for shorter versus taller sprinters; shorter sprinters must attain faster limb movement speeds, but taller sprinters need to generate sufficient torque to rapidly accelerate their longer limbs.

So in basic terms - Long legs are bigger and take more effort to turn over as quickly as shorter legs. The best sprinters therefore have very low limb masses, which enables them to cyclically move their arms and legs at high speeds. By this way of thinking the ‘smaller' (but not necessarily shorter) the sprinter the faster.

So while you might not be able to pick the fastest sprinters through muscle fibre type, fibre bundle length, or tendon stiffness tests, you can be sure their limb masses will be small and their techniques will be the most efficient. And after the races are over, perhaps we'll be able to answer the ultimate question: long legs or short?

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