We’ve all woken up sore the day after a hard work-out or long run. I often hear my patients say they feel stiff in their muscles and tendons. The relationship between stiffness and tendons is actually a very intricate one, and the stiffness of our tendons can be a good predictor of our performance as well as our injury risk.
What does stiffness mean?
In the normal world, when most people say they are stiff, they are usually describing soreness or tightness. Some people may label their lack of flexibility as stiffness. But let us consider for a minute what stiffness means in the scientific world, as it is an engineering concept.
Stiffness is the mechanical property describing the relationship between the force applied to the tendon and the change in the length of the unit. In other words, how much a tendon will stretch with a certain force applied to it. If we think of a tendon as an elastic band, stiffness is the amount of stretch you will get by pulling it with a defined force. The more stiffer the tendon, the less it will stretch.
It can be summarised by the following equation:
Stiffness (N/m) = force/change in muscle-tendon complex (MTC) length
Why do we need stiff tendons?
Consider this scenario. You are a 100m sprinter. Would you want to have tendons that are stiff and transmit forces from your muscles quickly to create movement? Or more a ‘compliant’ tendon that is stretchy and uses up more time and energy. The former right? The truth is, stiffness is not a bad thing in some tendons. Those tendons that are shorter and are used for more explosive movements e.g. knee and ankle tendons, will benefit from being a bit stiffer in comparison to tendons which surround joints that benefit from larger ranges of motion e.g. the shoulder.
If stiffness is so good for me, why does my Achilles hurt?
Imagine you have been for a long run, much longer than what you are used to. It is likely the next day your knee or ankle tendons will feel stiff. In fact, this is exactly what is happening. The first stage of a tendon reaction is called the reactive phase. The tendon swells with water as a short term adaptation. This is the body’s way of trying to stiffen the tendon to reduce the stress it is being put through. If we were to continue running long distances without the appropriate training our tendon will then change in cellular structure and breakdown. By training properly and increasing your running volume and intensity gradually, your tendons will naturally adapt and stiffen accordingly. Remember, if you are suffering from any tendon pain, you need a full assessment by an appropriately qualified clinician.
How can I train tendon stiffness into my tendons?
Different types of training will influence the stiffness of tendons differently. Endurance training will generally increase the stiffness across some joints. This could be beneficial for movements such as running because of the more powerful recoil effect, which results in more economical movement. On the other hand flexibility and pylometric training such as stretching and jumping exercises have been shown to decrease tendon stiffness.
In reality, by combining resistance training and stretching exercises, tendon load and resistance can be increased to deal with the biomechanical demands of running, while muscle compliance can be improve our range of movement and strength. However, striking the right balance between tendon compliance and stiffness perhaps needs more consideration than we have previously given it.