Measuring Stiffness – The Principles

If you missed my introduction to stiffness post then it’s worthwhile catching up with it by clicking on the link. This post will take this on a step further and will look at the principles of trying to measure and quantify stiffness.

What do we need?

To determine any sort of stiffness we need two measurements:

  • Force
  • Length change

Step 1 – Measure your force output

How you measure force will depend on the type of stiffness (i.e. physiologic level) you are trying to calculate and the activity you’re trying relate it to. The two common ‘functional’ outputs that are used are:

  • Ground reaction force (GRF) during jumping
  • GRF during gait

These give you a global force output for an entire system (i.e. the whole body) and therefore work well for determining leg spring stiffness. With some clever calculations we can also estimate the contribution of different joints to the overall force.

For a more isolated measure of stiffness the following is typically used:

  • Tendon SetupForce output during isometric maximal voluntary contraction (iMVC)

This can be done with a range of different instruments (i.e. force plate, isokinetic dynamometer), as long as they provide you with force measurement. This gives you a global measure of joint stiffness from which you could then determine components forces such as tendon force or muscle fascicle force.

Assessing relative force contribution

During a global movement (such as a jump) you may wish to determine the relative force contribution from different muscles. We can estimate this by using EMG:

First off, you’d start by assessing the EMG activity of a muscle in isolation during an iMVC. Let’s take the medial gastrocnemius as an example. The idea is that this tells you how much plantar flexion force is produced for a given level of EMG activity in the gastroc.

Then you go on to perform your global movement (i.e. the jump) with EMG attached to the medial gastroc. You work out how much EMG activity was present and use the data from your iMVC to estimate how much force the gastroc was exerting.

This process is not without its problems, but it can provide a good indication for your relative contributions.

Step 2 – Measure your length change

Force output done, now we need to determine a length change. If we’re looking at the big whole body movements then we’d typically measure one of the following:

  • Displacement of the centre of mass
  • Displacement of a specific joint

These can both be determining easily using video analysis of the movement. The latter could also be achieved using electric goniometers.

  • Displacement of tendons/muscle fibres

Ultrasound ImageWe can measure the displacement of individual components of the muscle-tendon unit (MTU) using ultrasonography. An ultrasound probe is position over the relevant point of the muscle/tendon and the point is then recorded during the performance of the movement. Given an appropriate probe this can be used during whole body movements as well as with the isolated single joint movements.

 

So there you have the principles of calculating stiffness. In the next part I’ll talk through what we can do with the data and how we can make sense of it.

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