What is Leg Stiffness?

Let's start with what Leg Stiffness is not: it is not about how tight your muscles feel.  It's not about whether you can touch your toes. 

The Spring Mass Model (the technical term for Leg Stiffness) is a simple model often used to assess running and hopping. In this model, the body is considered as a point mass (or simply a mass) that is connected to the ground through a series of springs (muscles + tendons), which represents the leg.

When the foot comes into contact with the ground during running, the 'spring' is compressed under the weight of the 'mass,' storing potential energy. Think of the knee bending as you transition from initial contact to midstance. Then, as the 'spring' extends (midstance to toe-off), it releases this built-up potential energy in the form of kinetic energy, propelling the 'mass' upward and forward (hopefully more forward than upward). 

Leg stiffness is a key factor often overlooked in running biomechanics. It refers to the body's resistance to deformation when a load is applied, in this case, when the foot hits the ground while running. Our ability to efficiently retrieve and redistribute impact forces is directly linked to our running performance and prevention of damage or injury. 

As mentioned above, leg stiffness, in simple terms, is the spring-like quality of your legs when you run. Picture the following scenario: when you drop a steel ball on a hard surface, it bounces repeatedly, losing a small amount of energy with each bounce until it stops. That’s essentially what your legs attempt to do while running — retain as much energy as possible from each ground contact for the following stride. This balance of absorption and propulsion is facilitated by leg stiffness.

When we talk about stiffness, it's important to note that it's not inherently negative. In the context of running, optimal leg stiffness can enhance performance and efficiency. Think of it like a balanced spring: too soft, and it absorbs too much energy; too stiff, and it doesn’t absorb enough, potentially leading to injury. The right amount of leg stiffness helps maintain an effective and energy-efficient running stride.

What Leg Stiffness Looks Like in Running:

Why Most Runners have Low Muscular Elasticity.

Low muscular elasticity in runners is often due to a lack of strength and conditioning for running-specific motions. Running involves repetitive, high-impact movements, and without the necessary strength and elasticity, muscles can become less effective at absorbing and releasing energy.

But more than that, running is a very eccentrically dominant sport (in comparison say to cycling or swimming). Eccentric muscle contractions refer to when a muscle is under load and lengthening, which poses a greater neuromuscular challenge compared to a concentric muscle contraction (when the muscle shortens). The eccentric phase in running is primarily the first half of the stance phase: the ground absorption phase from initial contact to midstance (it also occurs during the terminal swing phase with the hamstrings eccentrically decelerating the shin bone). 

Muscular elastic recoil refers to the inherent ability of muscles to store and release energy during movement. As mentioned above, when a muscle is stretched or lengthened during an eccentric contraction, it stores potential energy within its elastic structures, and this stored energy is then released during the subsequent concentric contraction, allowing for a more efficient and powerful muscle action. The elastic recoil mechanism enables muscles to generate greater force and produce quick, explosive movements. It minimises energy expenditure and enhances performance by harnessing the stretch-shortening cycle, where the muscle acts as a spring-like mechanism. The muscular elastic recoil should be noted as being speed sensitive: low cadence, most notably, will inhibit the muscular elastic response.

So why do runners typically have low leg stiffness/elasticity: because they don't train it into their system - running technique-wise, and strength-wise.  They don't do enough plyometrics. They don't do enough specific eccentrically based exercises.

Enhancing Leg Stiffness and Muscular Elasticity 101.

There are two easy ways in which you can start to increase your leg stiffness:

In Running: get your cadence into the correct Cadence Window:

HEIGHT:                                          CADENCE (PER FOOT):

At Home: SKIP!  It's a very, very good tool for building elasticity and stiffness into the lower limb.  It build a lot of resilience within the Achilles and tendinous tissue.  2-3x/week.

The Alternate Step Jump appears to be the most effective skipping technique (Effect of Rope Skipping Techniques on Kinematics and Dynamics of Motion; Ratna et al. 2014).  Start with the traditional Bounced Jump, then with time add in the single leg version. 

Understanding and improving leg stiffness and muscular elasticity will ultimately help improve your running efficiency and reduce injury risk, making your running journey more enjoyable and fruitful. Happy running!