Cycling Muscle Structure and Function

How do you see your legs? Are they big lumps of meat moved by your willpower? Or are they more like Guns of Destruction, ready to attack the peloton? In either sense, do you know what’s inside your gleaming Big Guns? You see, they are more than simple meaty bags of random muscle fibres. Understand your cycling muscle structure and you’ll appreciate your training and nutrition needs.

Let’s pull the Big Guns apart and poke around inside.

A Muscle Biopsy

A real muscle biopsy is nasty. Take a big needle. Jab it deep into your thigh and suck out a chunk of muscle tissue.

What would you find in that chunk?

It would have 2 major types of muscle fiber: Type I and Type II. Type IIs are further divided into 2 sub-types: Type IIa and IIb. I know, these are very creative sounding names. Big Guns are more impressive … but far less scientific.

Let’s put the Big Gun metaphor aside for a moment. Instead let’s think of your muscle fibres like 3 types of guitar strings. The music of your motion comes from strumming different chords.

Genetics determines your unique combination of these fibres. Training won’t change their quantities, but it can change how you express them.

Type I Muscle Fibers

These strings are the Slow-Twitch fibers. Use them like a base string to give solid resonance to your effort. They are the slow and steady contractors. They’re also slow to fatigue. If you meet their fueling needs, these fibers will power you over long distances.

Blood capillaries feed them oxygen and nutrients and flush out carbon dioxide. Long steady training rides will grow even more capillaries. That’ll super-saturate the fibres with blood flow and improve their efficiency.

Adenosine triphosphate (ATP) is the basic molecule of muscle cell energy. Your muscle mitochondria make ATP from fat, glucose and sometimes protein.

Type I ATP comes from fat and glucose burned in the presence of oxygen. This is the aerobic energy cycle — energy produced in the presence of oxygen. It’s your blood flow that delivers this oxygen.

Fat releases the biggest energy punch. One molecule of fat produces 100 molecules more ATP than one molecule of glucose.

During long steady all-day efforts (no more than 60% of your max), Type I fibers strum along using fat as their primary fuel.

But fat metabolism is slow. It isn’t very useful for moderate to intense bursts of muscle power. Glucose conversion to ATP is faster.

So as you work harder than 60%, all your muscle fibres need energy faster. Glucose becomes the primary fuel source. And you begin strumming the Fast-Twitch Type IIa and IIb muscle fibers to power your riding too.

It’s these Fast-Twitch fibres that make the difference in competitive cycling.

Type IIa Muscle Fibers

Want to throw down the hurt on a climb? Jump to make a breakaway? Then you’ll be strumming your Type IIa muscle fibres to get your legs booming along.

IIa fibers are 5-times stronger than Type I fibers, but not as fatigue resistant. Long endurance rides will recruit them when Type I fibres start fatiguing.

Type IIa fibers burn both fat and glucose for their energy needs. But because they need energy fast, most fuel comes from glucose.

Under extreme power requirements, they can use anaerobic energy metabolism. That’s getting ATP without oxygen. It’s obvious when you hit this wall. Your breathing gets heavy and muscles start burning.

The longer you can stay out of the anaerobic energy zone, the better your high power endurance.

The main reason EPO and blood doping are popular cheats is because they increase the oxygen levels in your blood. More oxygen means staying out of the anaerobic metabolism zone longer. Often much longer.

Lactic acid is a by-product of energy metabolism. Your body is always producing some lactic acid. Mitochondria will recycle it for further energy needs.

But when your body can’t recycle lactic acid fast enough it leads to an increase in blood pH. Your body can only stand a rise in pH for so long. Then self-preservation kicks in. You begin to feel fatigued. Your Big Guns stop making coordinated musical chords. You settle down to an effort level where your pH can drop to normal levels.

Type IIb Fibers Are the Sprinter’s Machine Guns

Type IIb fibers enter the picture above 80% of your max effort. These guys are the sprinter’s fibers. But they fatigue fast, usually within 15 seconds of effort. They can be used again, but need a period of complete energy recovery.

Type IIb fibers don’t use fat. They use the little ATP that’s immediately available. Or they get their ATP anaerobically from glucose. They have twice the contractile strength of Type IIa fibers – 10 times the force of Type I fibers.

Fiber Recruitment Chord

Your fiber recruitment “chord” is very important to know. Every cycling effort strikes a chord with your three muscle fibre types.

Type I fibers are the base strings in your chord. They dominate your long, low intensity efforts. You ALWAYS play these strings. Think of a well-structured melody. It has a steady base rhythm. Type I fibres provide the foundation for every cycling effort.

As power output increases, muscles continue striking the Type I fibers. Then you call Type IIa fibers into the melody. Remember, Type IIa fibres are 5-times stronger than your Type I fibres.

As you demand still more speed from your legs, you blast out cycling’s equivalent of rock’s power chord. The amplifier goes past 10 to 11. Type IIb fibers do their best to complete your power chord. You don’t just pluck the Type IIb fibres by themselves. It’s a full explosion of power coming from all three muscle fibre types.

Your 2 Take-Home Messages

There are two take-home messages from understanding this fibre recruitment chord analogy. One about your diet and one about your training.

First, muscles always use your aerobic metabolism

So ignore any “fat burning zones” that fitness equipment tells you. You are always burning fat while you exercise. Always! And at a general steady-state rate too.

As you increase your effort, muscles start drawing energy from your anaerobic metabolism. As effort goes up, the proportion of energy created anaerobically starts to outweigh aerobic energy.

Muscles can burn fat only in the aerobic metabolic process. Glucose can be used in both the aerobic and anaerobic processes. You must eat carbohydrates to provide your body its glucose. So, carbohydrate in your diet is fundamentally important. If you want to perform high quality power training and racing efforts, you need to be eating carbs.

Second, Type IIa fibers are your race-winning fibers

You need workouts that specifically call on them to train them.

All training of Type IIa fibers will also train the Type I fibers. Remember the musical chord analogy? Type I fibres do not shut down as your power output increases. They keep humming along and Type IIa fibers jump in to bring on the extra force.

It usually takes less than 10 minutes to establish a race-winning break. Sprints take around 15 seconds. So focus your interval training on 15 second to 10 minute efforts and you’ll have the best Type IIa fibers to win races.

If you have good genes, you’ll be blessed with a good degree of Type IIb fibers. You’ll know this if sprinting comes easy to you. Then you should train using 15 second to 1 minute sprint intervals. And find strategies to use your sprinting to win at the end of races.

But for most cyclists, working Type IIa fibres gives your training the best return on time invested.

Pressed for time? Then warm up for about 10-15 minutes. Ride a set of 5 intervals. Go hard for 4 minutes. Recover easy for 2 minutes between each. Cool down for about 10 minutes to keep blood flowing to kick-start the body’s rebuilding process. In less than an hour you’ll have worked on your endurance with Type I fibres and power with Type IIa fibres.