Should my knee bend like that?
Knee injuries can be nasty things, particularly if you are a professional athlete who earns a living with your legs. To give you an idea, injuries to the major ligaments can keep you out of action for anywhere between 6 weeks to 18 months. In some cases the injuries can be so significant that athletes are forced to retire early. During his playing days David Beckham is reported to have insured his legs for over $195 million!
These injuries sometimes happen as a result of the legs being in positions that they shouldn’t be.
The knee joint is designed to bend and straighten with a very small amount of twisting. In bio-mechanics we call the bending and straightening “flexion and extension” and it looks something like this…
The knee joint however was not designed to move from side to side and this is when injuries happen. In bio-mechanics we call this movement either “abduction and adduction” or “valgus and varus” and it looks something like this…
In an attempt to prevent or predict knee injuries, healthcare professionals working in sports and exercise medicine have created tests which try to reproduce the factors related to injury. They also measure what the joints are doing using 3D motion analysis.
This is why you sometimes see athletes jumping off boxes with lots of reflective markers on their legs.
In the last blog, we wrote about some of our research, in which we used 3D motion analysis to investigate if a movement test was able to predict injury.
For 3D motion analysis we place reflective markers on the skin. This is done to identify the movement of the bones in body parts such as the thigh or lower leg. It is assumed that the markers do not move in relation to the bones and that joint centres, can be found accurately using the markers. We call the sequence of mathematical equations which calculate the joint centre positions and angles a biomechanical model.
Using similar bio-mechanical models, some studies have reportedly found that larger knee valgus angles (the side to side movement) are associated with knee injuries such as ACL ruptures. We found some the reported values were suspiciously high for real movements. If the markers move over the bone, or aren’t put in the correct place, the bio-mechanical model can think that the flexion-extension movement is a valgus-varus movement. Bio-mechanical models are based on marker sets originally developed to measure walking. As I am sure you’ve figured out, there is a big difference between walking and jumping. Sporting activities therefore challenge some important assumptions about the models. We felt it was important for clinicians to understand the possible sources of error when making decisions based on these results.
So what did we do?
Using 3D motion analysis, we did some more research looking at different ways of estimating the knee joint position. This is important because the position of the knee joint is used for calculating the knee angles. We looked at the activities of walking, squatting and stepping over a hurdle and compared the results of each method.
What did we find?
Our results showed that small differences in position of the knee joint made very little difference (less than 4⁰) to the flexion-extension movements, which are large anyway. However, the position of the knee joint had a bigger effect on the valgus-varus values with differences of up to 10⁰ between some methods and activities.
We found that this experimental error was larger than some of the differences reported in other studies which reported knee valgus as a cause of either ACL injury or knee pain. We concluded that the amount of “dynamic knee valgus” observed in our group of non-injured football players probably came from the flexion-extension movement being mistaken for valgus-varus movement and not actual side to side movement of the lower leg bone (tibia) in relation to the upper leg bone (femur).
Another finding in our study was that skin and muscle and bone didn’t always move together and this had a surprisingly large effect on the results. One of the main reasons for this was the amount of skin and muscle movement associated with completing bigger and faster movements. This is also likely to be true for other studies which report similar numbers. We were right to be suspicious!
And what does it all mean?
For the sports boffins it means we have provided them with some different ways to correct the knee position and hopefully get better results. We also wanted this to be an educational paper for clinicians and help them understand how the angles are calculated and where the potential errors come from.
A really important finding in our study was that the experimental differences between methods were larger than previous differences which have been considered clinically important. Standard methods for 3D motion analysis are unlikely to achieve an error low enough for true differences to be measured if they exist. We therefore k-need to be careful when making decisions for performance or injury based on these tests and measures. A better understanding of the size and sources of error will result in more accurate testing procedures, saving time (more efficient rehabilitation), money (reduced injury burden) and knees!
