Keith Baar is extremely well known in the world of tendon research. For those following at home, you may remember his name from one of the short education videos presented by MAD’s Jack Williams. Keith has been researching the properties of tendon, how they respond to different loading environments and how they can change their structure with such loading. 

For those of you familiar with the Pacey Performance Podcast Keith, was a recent guest on this podcast. Link to the video and podcast can be seen at

A concept that he describes is one of stiffness. And when you speak with athletes and coaches there appears to be a misconception in what stiffness is and what role in plays in sport performance and injury. To set it straight the term stiffness is taken from the engineering realm and is related to the ability of the tissue to resist deformation. I.e if a force is applied will it change length, shape etc.  And this can be broken down into a further subset which is the difference between active and passive stiffness. Passive stiffness is that which most people think of when they hear an athlete describe that they are stiff. This is increased muscle tone, limited joint range of motion or a lack of overall flexibility. This tissue has limited compliance even when the tissue is not being used. 

Active stiffness is the ability for the joint, muscle, tendon to resist compliance when under tension such as when running, jumping or striking an implement. This is the property that allows an athlete to jump and land effortless, without injury and crumbling to the ground under their own weight and the acceleration of gravity. 

Now that we have that out of the way, an interesting concept that Keith Baar discusses is the fact that those that can create very high levels of active stiffness are often those that are fast, can jump high and are powerful. However from an injury perspective they appear to be the athletes that sustain the most muscle strain type injuries. This concept appears to be associated with the inability for these individuals muscles to tolerate the resistance to deformation that their tendon structures can create. They are able to create these movements and tissue stiffness properties at such a rate and speed that their muscle tissue will often fail under such high tension loads.  Conversely those that are unable to create such levels of stiffness tend to suffer more joint injuries. And this makes sense because often greater tendon stiffness leads to greater abilities to maintain joint stability. So when these stiffness properties go down (less active stiffness, particularly in tendons) our joints are placed under greater stress and the ligament structures are asked to create the joint stability. 

So what is the balance?

The balance appears to be that those who have greater active stiffness capability require heavy muscle strengthening tasks particularly at longer lengths, under eccentric or isometric work. Conversely those with increased compliance and general flexibility require tasks that increase active tendon and joint stiffness properties, such as fast resistance and power work and plyometric tasks. 

This concept is in its infancy, but it definitely provides food for thought for those working in sports performance and injury.