Gear ratio

What it gear ratio, why is it important, what does the evidence say and how can you apply this understanding assessing running gait

What is gear ratio?

Gear ratio is defined as the ratio of external ground reaction force moment arm to internal muscle-tendon moment arm. The next question, is what are these two components. The external ground reaction force moment arm is the distance between the joint and the point of force application e.g. ankle joint centre to centre of pressure. The internal muscle-tendon moment arm is the distance between the joint and muscle-tendon insertion e.g. ankle joint centre to Achilles tendon insertion. The 'effective mechanical advantage' is the inverse of the gear ratio.

How can I calculate effective mechanical advantage?

Very few studies have actually fully measured gear ratio, as it requires either an MRI or ultrasound of the muscle-tendon insertion to accurately calculate. However, the internal muscle-tendon arm is unmodifiable. Therefore, measuring the external ground reaction force moment arm using a force plate and movement data can give a useful insight into the gear ratio, as it is the only modifiable component in the equation. Video analysis will only be able to provide an approximate measure of the external ground reaction force moment arm.

Why is gear ratio important for runners?

Gear ratio was first explained by Carrier and colleagues in 1994 using an car analogy. For peak power and efficiency, a car's engine speed operates over a narrow speed range, which is a result of the piston speed. For the engine to operate efficiently across different driving speeds, the ratio of engine and drive speed must be varied by a 'gear ratio'. For a runner, the contractile elements in a muscle work efficiently in a narrow range of velocities. Therefore, changing the gear ratio during ground contact could enhance force production by allowing the muscle-tendon units to operate at favourable velocities according to the force-velocity relationship.

What does the evidence say about external ground reaction force moment arms?

Older runners (60 - 69 year olds) have a higher gear ratio than younger runners (21 - 32 year olds). This appears to be a safety strategy that may reduce the mechanical load through the Achilles tendon, whilst also potentially resulting from the lower plantarflexion strengththat older runners exhibit.

As runners become more economical they produce a more favourable gear ratio during braking by aligning the leg axis with the ground reaction force. They also maintain a high gear ratio during propulsion, which is the most efficient strategy.

Trainers can also influence the gear ratio. Evidence shows that there is a critical level of stiffness that optimises running economy, lengthens the external ground reaction force moment arm and reduces propulsive forces, as long as it does not interfere with the natural toe flexion.

Applying this understanding during a gait assessment

Whilst you may not be able to accurately determine the external ground reaction force moment arm, an approximation could be calculated based on the point of contact during foot impact. This means a forefoot strike would have a longer external ground reaction force moment arm and therefore, lower gear ratio, than a rearfoot strike. This links back to the mechanical load through the Achilles tendon mentioned above, because we know that a forefoot strike increases the demand on the Achilles tendon compared to a rearfoot strike.

It is perhaps also important to consider the relationship between a runner's strength capacity and their gear ratio, which may be particularly important in older runners or runners returning from injury.