THE SECRETS OF AN ECONOMICAL STRIDE [SPECIAL RUNNING]
PART 1 - EXPLANATIONSRunning is practiced by over 6 million people in France. No doubt because it’s the easiest sport to do. A pair of shoes and off you go! But that doesn’t mean it’s the easiest sport to progress in. And when it comes to understanding the underlying mechanisms of running, it gets even more complicated. I’m here to shed some light on the subject of running economy and give you some ideas on how to improve your running stride!
To put into context just how specific a sport running is in terms of economy, we need to understand the basic mechanism of running with the spring model. See the diagram below:
Diagram of the running spring model :
- L0 represents the length of the leg
- ΔL represents the maximum compression during the support phase
- Δy represents the maximum change in displacement of the rider’s center of gravity
- θ represents half the angle swept by the leg (the spring) during the support phase.
Always use images to better understand!
The legs function like a spring, converting kinetic energy (created by movement) into elastic energy (so as not to waste this kinetic energy by simply cushioning it) on impact with the ground. Their role is therefore to store and release this kinetic energy into elastic energy in the best possible way. This energy is also known as “free energy”, as our musculo-tendinous structures do some of the work, without us having to exert the slightest effort.
Above all, this model shows that it is possible to be very economical. The efficient use of elastic energy will greatly improve running economy. Without this efficient mechanism, it would be very difficult to exceed 10 km/h. For example, let’s take the case of a good marathon runner who has optimized the efficiency of his “spring model”: the proportion of elastic energy (through the spring) can be greater than 50% of total work. This means that 50% of the power supplied comes from this ingenious spring mechanism! It’s thanks to this level of economy (among others) that he’ll be able to keep up these breathtaking speeds for so long (note: world record (homologated) in 2h01’39”, i.e. an average of 20.81 km/h).
It’s worth understanding how it works and trying to optimize it through training. 😉
In running, the legs use this efficient mechanism to move us forward economically, using elastic energy thanks to the stretch-shorten cycle(in red , the neuromuscular qualities in play that will be the focus of our training):
- The role of the legs is to absorb the force of impact (otherwise we’d regularly have fractures) and store kinetic energy in the form of elastic energy ⇒ Eccentric musculotendinous capacity ( eccentric force)
- The legs function like a spring that will release the elastic energy created by the impact on the ground while generating force and power to create movement (and fight gravity) ⇒ The ability to use elastic energy to produce concentric muscular force ( reactive force)
- We need to be able to measure the efficiency of this stretch-shortening cycle to ensure that these energies are properly transferred during the run ⇒ Leg Spring Stiffness (LSS)
Understanding these 3 qualities is essential for anyone wishing to improve their running economy. Studies show that eccentric strength, reactive strength (reactive strength index), and leg stiffness are significantly correlated with running economy. So let’s take a look at these 3 parameters, which seem to be essential for running. Let’s start by understanding the first stage in this stretch-shortening cycle , which is eccentric force!
Eccentric force
During the stride cycle, muscles are eccentrically loaded during the first phase (landing) of ground contact and produce concentric force during the second phase (take-off) of ground contact.
Landing on the ground will stretch the muscles to absorb the shock, and the eccentric force will make it easier to absorb the shock.
Eccentric force therefore plays an important role in the stretching-shortening cycle, helping to convert kinetic energy into elastic energy and thus reduce the metabolic cost of running (as already seen above).
Studies have shown that the 2 main functions of eccentric force in running are :
- The ability of ankle, knee and hip extensor muscles to absorb mechanical energy on contact with the ground (they are stretched by the impact and absorb it as well as possible).
- Potentiate this kinetic energy into elastic energy (to waste as little energy as possible, even if some is also dissipated in the form of heat).
So a runner with greater eccentric capacity will be able to store more energy by absorbing this force. And therefore potentially transmit more force during the concentric phase. Indeed, the reuse of stored elastic energy is considered a major determinant of energy savings.
Reactive force
Reactive force represents a runner’s ability to use the stretch-shorten cycle efficiently. The reactive strength index (RSI) is calculated by dividing the time suspended in the air by the ground contact time during a sprint (several methods are possible). The result demonstrates an athlete’s ability to move quickly and efficiently from eccentric to concentric contraction.
This means that runners with greater reactive strength and the ability to use elastic energy through the stretch-shorten cycle will be better able to conserve energy when running. Equally interesting, as running speed increases, more elastic mechanisms will be brought into play, and concentric contraction will also be more efficient.
The greater the elastic energy released, the stronger the concentric contraction (thanks to the coupling of the two forces).
Reactive strength is therefore an interesting quality to take into account in training, even for endurance runners, as it enables more efficient concentric contraction.
Leg stiffness
Leg spring stiffness (LSS) is an important parameter that defines the amount of energy stored and released for a given compression (imagine a stiff spring: it will be less deformable than a soft spring, and therefore potentially able to store and release more energy). It is defined as the ratio between the peak force at ground contact and the maximum crushing of the leg during the stance. Basically, it’s the capacity to resist deformation (see diagram above for a better understanding).
Leg stiffness is correlated with running economy (see figure below):
Correlation between leg stiffness and running economy at 16 km/h. Kleg = leg stiffness; RE = running economy.
Greater leg stiffness theoretically allows more elastic energy to be stored during the eccentric phase, reflecting an efficient stretch-shortening cycle. As leg stiffness is closely linked to ground contact time, increasing it will reduce ground contact time.
These improvements will optimize several economic mechanisms:
- Less loss of speed on ground impact due to braking force
- Increased time in the air, i.e. more relaxation time between muscular contractions, less accumulation of fatigue
- Improved muscle oxygenation and blood perfusion (O2 supply)
Basically, you’re more economical and can maintain a sub-maximal pace for longer!
CONCRETE EXAMPLES :
For example, I analyze this value during VMA tests to see the neuromuscular threshold for leg stiffness (LSS). Below is the example of a “beginner” athlete, who did a first test with a decreasing LSS despite increasing speed. The second test showed progress, with LSS increasing with speed up to a certain threshold. In fact, his support time at 11.5 km/h also increased by 12%, a sign of better economy at this pace (his VMA increased by 4%, so doesn’t explain the progress on its own).
Here’s another example from an experienced athlete. His “economic” threshold comes into play at 15 km/h, i.e. from this point onwards, the force of impact is too great and the deformation of his “spring” is more consequent. This means he’ll have to provide a greater concentric contraction as speed increases. It’s easy to imagine that he’ll soon be exhausted.
In itself, this may not be such a bad sign, as he may be better able to call on his eccentric capacities and potentially make better use of his reactive force towards a tenfold concentric contraction (coupling elastic energy/concentric energy) to sustain the imposed pace. That’s just a guess.
Except that studies show that a significant inverse correlation exists between leg stiffness and running economy (see figure below). However, the studies analyze LSS at a given speed and we don’t have the evolution of this LSS as a function of speed. As far as I know, no study has worked on this specifically. Who’s keen to join my own study? 😉
So the question remains as to the interpretation of this data (from the Stryd sensor), although I have my own ideas.
The next article will cover training methods to improve eccentric strength, reactive strength and leg stiffness to optimize your stretch-shorten cycle and become more economical and faster!
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