Running requires a balance of loading and unloading. Give and take. Yielding and unyielding. Stiffness and compliance.
A simple model that I find useful for return to run decision making is defining mechanical issues and/or mechanisms of injury as either an excessive stiffness or over compliance issue. While this way of looking at the problem is overly simplified, like it any model, it can be a useful guide despite its limitations. Stiffness lives on a spectrum. Knowing which end of this spectrum the recovering running deviates towards and the end of the spectrum that the injury occurred can serve as an entry point for early return to run interventions to both prime for mechanical improvements and protect vulnerable tissues.
The concept of stiffness in rooted in physics. It is a the amount of force required to move a deformable object, like a spring, a certain distance. For athletic performance and running, stiffness can be a super power. It is our ability to store and release energy. Increased leg stiffness is related to increased running speeds, decreased stride lengths, and decreased energy requirements. With stiff leg contacts we see decreased joint ranges of motion and an increased contribution from the ankle rather than the hip or knee. With a decreased joint excursion, we will rely more heavily on the lengthening and shortening of non-contractile tissue (tendon/fascia) to store and release passive energy rather than the active contribution from contractile tissue (muscle).
However, just like any good thing too much can be a problem. Excessive stiffness can lead to increased peak forces and loading rates due to reduced joint excursions. Think of a boxer taking a punch to the face. If the boxer rides with the punch, the forces are dispersed over a longer time. If the boxer takes it square to the jaw, the rate of impact is very high. In these situations, the force will be distributed to a smaller number of tissues, thus more focal shock. Williams et. al (2001 & 2003) demonstrated that runners with higher arches (greater leg stiffness and vertical loading rates) had a higher incidence of bony stress injuries compared to low arch runners.
On the flip side of the coin, too little stiffness increases joint excursion putting more of the force demand on contractile tissue. Williams et. all (2001 & 2003) found that runners with lower arches (decreased leg stiffness and lower rates of loading) had higher rates of soft tissue related injuries. With increased joint excursion, more lengthening is required from contractile tissue. Repeated eccentric stress can lead to micro-trauma and may predispose the runner to either an accumulated stress issue or a magnitude related issue (muscle strain).
So how does this guide our decision making for return to run following an injury? As mentioned in a previous post, a common mistake seen is treating jogging at the main entry point back into running. For most people, jogging is playing to the middle ground of stiffness and compliance. Forces are relatively high and joint excursions are as well. We need preparation to handle these forces. We can start on the opposite end of the stiffness or compliance stress from where our issue is classified or from where our vulnerable tissue is most compromised. If we had a stiffness related issues (i.e stress fracture) we can start with our compliance bucket of drills. Starting with longer ground contacts and larger joint excursion will help to distribute the forces to a larger area. We are allowing the runner to ride with the punch thus reducing loading rates and focal shock to the recovering tissue. These include activities such as longer contact time plyometrics, box step downs to a soft landing, and slow tempo strength training.
For a compliance related issues, we will prioritize activities more biased towards decreased joint excursion to limit the lengthening stress of a vulnerable tissue. The menu here features short contact plyometrics, box step down with a stiffer landing, dibble drills, and isometrics.
Using these categorizations, we can make more calculated decisions to create the best path forward for a recovering runner or field sport athlete. Manipulating the stiffness and compliance of an activity should allow the runner to maintain the highest possible training volume to maintain desirable qualities (endurance, mechanical competency, neurological preparedness) while decreasing the risk of adverse effects during injury recovery.
1) Butler RJ, Crowell HP, Davis IM. Lower extremity stiffness: implications for performance and injury. Clin Biomech (Bristol, Avon). 2003;18(6):511-517.
2) Williams, D.S., McClay Davis, I., Scholz, J.P., Hamill J., Buchanan, T.S., 2003. Lower extremity stiffness in runners with different foot types. Gait and Posture, in press.
3) Williams, D.S., McClay, I.S., Hamill, J., 2001. Arch structure and injury patterns in runners. Clin. Biomech. 16, 341–347.