What is Speed Reserve?

Speed Reserve is the idea that by improving top speed, we get the benefit of our sub-maximal speeds feeling easier because we are operating further away from what our maximum velocity is. To put it simply, a car with a top speed of 200mph will have an easier time going 100mph than a car whose top speed is only 120mph.

In reality, there’s a lot that goes into how much a higher top speed translates into easier sub-maximal speeds. We need to consider:

• Energy Systems
• Fiber type
• Mechanical changes at different speeds
• Joint stiffness
• And probably a whole lot more

Fiber Types

Physiologists don’t consider skeletal muscle as just Type I and Type II. Out of convenience it’s a good model, but one that more commonly considers Type I (Slow Oxidative – 50%), Type IIa (Fast Oxidative/Glycolytic – 25%), Type IIx (Fast Glycolytic – 23%), and Type IIc (1-3%) (average composition of muscle; Kenny, Wilmore, Costill, Physiology of Sport and Exercise); some research even suggest more branches that better explain muscle. The differences in these fibers isn’t nearly as distinct as we’d like to think.

Type I is our slowest and most oxidative with peak tension taking around 110ms. Type II fibers as a whole are generally around 50ms which comes from a much a more developed sarcosplasmic reticulum which supplies Calcium into muscle cells, as well as having a much more powerful alpha neuron that innervates a lot more fibers than we see in type I.

Below is the order of capacity for each of the main fiber types (all taken from Physiology of Sport and Exercise)

• Oxidative Capacity: Type I > Type IIa > Type IIx
• Glycolytic Capacity: Type IIx > Type IIa > Type I
• Contractile Speed: Type IIx = Type IIa > Type I
• Fatigue Resistance: Type I > Type IIa > Type IIx
• Motor Unit Strength: Type IIx = Type IIa > Type I

Motor Neuron characteristics follow almost exactly what you’d expect. Type II are both similar and generally superior to type I.

Our bodies have a preference to fire from smaller to larger and from slower to faster. This is due to conservation of energy (biologically speaking). Why would we fire the biggest most fatigue-able fibers when a smaller more efficient one will work just fine?

This means our recruitment always involves Type I before we start recruiting Type II. This is true even for sprinters. They’ve done their events long enough to have the neural coding to know that when they’re about to sprint, they had better recruit as many fibers as quickly as possible. Someone brand new to sprinting likely has some biological handicaps built in where it doesn’t know how to call on every motor neuron it needs to run at its maximum right from the gun.

For the folks that might be wondering, well you said earlier Type I makes up 50% of our fibers, doesn’t that mean that it’s a large muscle and should be recruited later? The difference here is that we don’t recruit muscles, we recruit motor neurons. And Type I has the smallest number of muscle fibers attached (innervated) by each alpha-motor neuron so for each of those units being used we are activating less total muscle fibers. As the force needed increases we will ask more motor units to also kick in.

Now with the bulk of the general physiology out of the way we can shift the discussion toward how it affects running and speed reserve.

How Speed Reserve Fits in the Picture

All of those fibers exist on a continuum. There is no, just Type I or mostly Type II, we use all of them in different ratios at all times. A distance runner largely relies on Type I, but there’s no way Eluid Kipchoge is running 4:34’s per mile for 26.2 miles and not having a healthy amount of Type IIa helping to keep up that pace. Remember, our bodies have no idea what a mile is, they care about fatigue, force, and contraction velocity. I can’t think of any reason why a muscle fiber would know the difference between a HS boy running 4:34 and an elite marathoner running 4:34 from the standpoint of force or velocity (neurons and the integration of movement aside).

So how does speed reserve play into this? I don’t have the full picture, I won’t pretend that I do. But someone who has a high speed reserve would expect to also have more freedom of muscle choices. They are good at tapping into everything they’ve got. Think about a sports car that has it’s limiter taken off. I would also imagine that makes using those Type IIa (fast oxidative/glycolytic) fibers much easier to call upon. Remember that these are both fast and have good fatigue resistance.

A lot of circles call these intermediate fibers and imply that they merely have the capacity to change back and forth between slow and fast twitch, but really they always retain properties of both. I’m betting these tell a lot of the story for why people can use these to both run fast, but also learn to hold some of that speed for longer events. If they’ve already learned to contract similarly to the more pure Type IIx then they will likely have an even easier time when we ask them to move kind of fast, but not at their maximum – they still have the contractile qualities of fast twitch, but have more headroom to let the oxidative part keep up.

The other part I want to mention is that fast athletes have some wild card qualities that go more into the structures that their legs have and less about specific energetics. Some of the ones that stick out are tendon stiffness, ground contact times, neural coding, etc. I’m sure these have their place in the discussion as to why some can translate speed into their endurance events.