Leg Stiffness: Implications for Injury & Performance

I was excited as only a complete run geek can be, when I saw last night that my friend Ian Griffiths had posted another video on his YouTube account!

Ian (@Sports_Pod) will no doubt agree that some will find the subject matter a bit dry! However in this presentation he manages to do a brilliant job of translating the mechanical concepts surrounding leg stiffness in runners into real-world practical examples, discussing both implications for injury rehabilitation and running performance.

The target audience for this video presentation is physios and therapists in the clinical world. That said, it’s truly a fascinating watch for anybody interested in the mechanics of how we run…

If you haven’t also seen Ian’s video on running shoe selection, I’d suggest it as a must watch for all runners and triathletes

Last updated on March 2nd, 2021.


  1. I’ve been thinking lately that maybe we need another model for the leg besides the spring because although it is certainly nice to run on ‘springy’ legs, in fact, unlike a spring which springs as a result of being compressed (shorten and then lengthen) our legs become springy as a result of tissues being stretched…eg the calf and Achilles’ tendon on landing: lengthen and then shorten

    1. Hi Malcolm,

      Thanks for taking the time to leave a comment. I know what you’re getting at, for sure. I’m completely open to hearing an alternative description for the complex mechanism that is the loading of the leg, foot and ankle in running gait…

      For me the spring analogy still works. Let me explain my thoughts on the similarities:

      We often think of springs in terms of their reaction to being compressed, as you describe above. Once the load causing compression of the spring is released, the spring ‘springs’ back to original shape, this is true for sure and describes the elastic nature of springs.

      However, consider what happens once mechanical strain is applied to a spring and it is stretched / lengthened with a load pulling one end of the spring away from the other (think springs on a pilates reformer, for example), just like the eccentric loading of a muscle. Under normal conditions, once the load (and thus strain) is released, the elastic nature of the spring will cause it to return to it’s resting state. If the strain acting upon is too great and takes it past the spring material’s yield point, we will see the effect of the non-elastic (plastic) changes to the spring as it returns to a longer condition than it’s original state once the stress is removed. Load too far, too fast or too frequently past this yield point into this non-elastic region and you risk mechanical failure of the spring. Pretty much all of the above statement can be applied to soft tissues of the body, including muscle, tendon, ligaments and fascia.

      Anyway – thinking about the content of Ian’s presentation and the consideration of the leg as a spring-like mechanism; rather than simply considering the effects of loading on an isolated soft tissue, it is important to consider how ground reaction force causes a loading of the leg as a whole. In simplistic terms, considering the ankle, knee and hip in the sagittal plane alone: from foot flat to mid-stance we obviously see ankle dorsiflexion, knee flexion and hip flexion as three combined movements to ‘take the strain’ of many times body weight as vGRF reaches a peak when the runner’s CoM reaches it’s lowest point. This loading action of the leg as a whole is akin to the compression of your spring mentioned in the comment above. However as this multiple joint spring-like compression is achieved, soft tissues such as the soleus/achilles are also being lengthened as these joints are loaded through range. As long as these soft tissues are allowed to function within their zone of optimal strain/stress they will return towards their original length as load is released from mid-stance through to terminal stance, helping to drive the ‘whole leg spring’ out of it’s compressed position.

      I possibly could have summed that up more succinctly in saying that while many of the soft tissues of the leg lengthen as we land and load, the leg as a multi-segmental spring mechanism is also compressing under load, like your description of the spring in your original comment, ready to unload and push you through terminal stance and onto the next step to start the process again! Both lengthening under tension, and compression under load are happening at the same time!

    2. This is in fact the essence of a linear spring. The leg as a whole (pelvis, thigh, shank and foot) act as a spring that is compressed and returns to shape. However, this occurs not only due to the mechanical action of muscle, but also because the passive ELASTIC elements of our muscles, tendons and fascia (connective tissues) return energy when they are stretched. This passive return of energy is the spring action of those tissues. While the stretch-shortening cycle you speak of is not passive, the resistance to stretch at the tissue level is well modeled as a linear spring.

      If you read the scientific literature, you will also note that there are linear springs (leg stiffness, vertical stiffness) and torsional springs (ankle, knee and hip joint stiffness). These are different types of springs and most of your reading thus far will be on the linear spring stiffness.

  2. Hi Malcolm,

    Thanks for your comment. Just to add to James’ very good and extensive reply, I think it should always be remembered that the human leg is NOT a spring; we merely model it as one. We use models for the human body when explaining its various functions and when trying to understand it better – often in an attempt to simplify very complex processes. I think it is fair to say that when modelling the human leg (during running) there is nothing currently superior to the spring-mass model.