Measuring Dynamic Stability
The R&D team just came back from the lab. This time, they were investigating whether it was possible to use MOTI to evaluate dynamic stability. Dynamic stability is probably something that many of you have heard about elsewhere. However, the term is often used interchangeably to describe many different aspects of human function…..unfortunately, these are not always accurate and often describe phenomena that are difficult or even impossible to measure.
What do we mean by dynamic stability?
Dynamic stability may be a confusing as it describes two contradicting factors i.e. something that is moving (dynamic) and something that is stable. To simplify what we mean by this, we can take an example from sport where dynamic stability plays an important role. Many sporting disciplines require the athlete to quickly change directions. This is seen in e.g. soccer where the player needs to initiate a run after landing from heading a ball. Here, the time it takes to find the balance after the landing is a deciding factor in terms of when the player can start running.
The central nervous system plays a vital role in this process and it does so by coordinating sensory feedback with motor output. More specifically, signals from receptors (also called proprioceptors) lying in muscles, ligaments and tendons are used to coordinate the muscle activity needed to control the movement (regain balance). The sensory feedback from these receptors ensures that visual feedback is not necessarily needed to know which position our joints are in. To see how this works, close your eyes and straighten your elbow. Then (with your eyes still closed) bend your elbow to 90 degrees. Then open your eyes and see if your elbow joint is approximately where it is supposed to be. Was it close? Yes? This is because the brain estimated how far the elbow joint needed to move to reach desired position by using information from the proprioceptors. Pretty clever (pssst measuring joint angles is actually a feature in MOTI has as well ).
Why is measuring dynamic stability important?
Many injuries in sport involve a damage to ligaments and other joint structures, which also means that part of the sensory feedback is lost. The best known examples are probably injuries to the knee’s ligamentous structures e.g. the anterior cruciate ligament (ACL) or the ligaments on the outside of the ankle. These ligamentous structures play a pivotal role in maintaining dynamic stability in the lower extremities. Surgically repairing the damage is possible in e.g. the knee but the big challenge comes afterwards. The new ACL does not have the same sensory receptors as the original ligament. For this reason, athletes need a long rehabilitation period after the injury (6-12 months) as the athlete needs to train his sensory and motor system to control the movements differently than before. Returning to sport is also an important factor to consider i.e. when has the athlete regained full control over the injured extremity? This can be difficult to determine without measuring it accurately.
In essence, the rehabilitation period after a ligament injury focuses on re-training motor skills by integrating new sensory input with appropriate (and meaningful) motor output. But, what if the motor skills were never there? Many children have difficulties performing activities that require gross-motor coordination. This can be reflected in many ways but often these kids are described as simply being “clumsy”. When referred to a physiotherapist, the complete motor spectrum (fine and gross motor tasks and coordination) is usually evaluated. For these children, evaluating their performance and improvements following rehabilitation is equally as important as it is for the athletes described above. If anything, it is even more important. Having an accurate assessment method to evaluate their dynamic stability in the clinic can therefore be very valuable.
So how can MOTI help?
We compared the measurements from a force platform (gold standard) for dynamic stability with the data from MOTI. Following a verbal cue, the subject jumped and landed on one leg on the force platform. The subjects were instructed to try and regain quiet stance as quickly as possible after landing. The marker to determine dynamic stability was the time it took until the person could stand quietly on the platform. In this pilot study, the person performed three identical jumps. The results are presented in the figures below.
Figure 1 Data from the force platform from jump number 1 showing the force (y-axis) over time (x-axis). The foot contact is indicated with the green dot and the point where stability was reached is indicated with the red dot. The time between these two was 0.9 seconds
The time for from landing until the stability was reached on the force platform was 0.8 – 1.0 seconds while the time measured with MOTI was 0.7 - 1.1 seconds. The average difference between the two methods was 130ms.
What does this mean?
The findings indicate that MOTI can give a very accurate measurement of dynamic stability as it consistently measures almost the same as the force platform (average difference of 130ms). For comparison, the time a normal person uses to blink their eye is somewhere between a 100 – 150ms. Put more simply, identifying the difference between the two assessment methods would not be possible to detect with the naked eye. Moreover, identifying progression e.g. following targeted exercise to improve dynamic stability would be possible with the help of MOTI. The findings are encouraging, and the R&D team has now begun to collect data from a larger group of people to validate the findings from this pilot project. We look forwards to get back to you with our findings.
MOTI Research and development team