Estimated read time: 4.5 minutes (about as long as it takes to explain to your PT why you're done with calf stretches🤔)

Hey Performance Nerds! Jonah here. 🤓

You've been stretching your Achilles when you should be stiffening it.

A stiff Achilles acts like a biological carbon plate. It reduces how hard your calf works and can improve running economy by about 4% (super shoes).

Six months ago we broke down why stiffer tendons mean free speed.

Today: the part we left out. Why your stretching routine might be the problem, and what actually builds a durable Achilles:

(Augie thinks heavy calf raises sound like a lot of work for what could be a perfectly good nap. He's not wrong.)

You're confused. You're worried. A carb paper is making the rounds, and now you're second-guessing everything you thought you knew about fueling.

I get it.

Gels, timing, how much, conflicting advice from every corner of the internet... fueling is overwhelming. And when science gets weaponized into fear-mongering headlines, it makes everything worse.

Here's what I want you to know: you deserve context, not fear.

I'm working on more content to break this down properly. But in the meantime, if you have ANY questions about fueling, carbs, metabolism, or topics you want me to cover, reply to this email.

I will personally answer you.

My goal is to support you with science that helps, not headlines that scare. I've got your back. 🤝

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That "tight" feeling in your Achilles isn't what you think.

It's your nervous system putting on the brakes. Protective tension, because the tendon can't handle the load.

So you stretch. The alarm quiets for an hour. Then it comes right back.

Stretching a weak Achilles is like cold plunging after your runs. Might feel nice, solves nothing.

Chronic stretching may actually reduce tendon stiffness over time. And stiffness isn't injury risk. It's protection.

A stiffer tendon deforms less under load. That means less strain on every single step.

What you need isn't flexibility. It's strength.

(Look, I get it. Sitting down to stretch looks a lot more "recovery" than loading up a barbell for calf raises. And it definitely is a lot easier to talk to your gym crush. But your Achilles doesn't care about that.)

We covered the spring mechanics back in August. Quick refresher, plus what we've learned since:

Research on trained athletes found heavy calf training increased Achilles stiffness by ~31%. Running economy improved by roughly 4%.

Why? A stiffer tendon lets your calf hold its length instead of shortening with every step. Muscles burn the least energy when they hold still.

Early research suggests soleus fibers operate at 15% higher efficiency this way.

The payoff:

That ~4% economy gain is comparable to super shoe benefits. Except it's permanent.

This is what that August issue didn't cover. And it might matter more than the speed gains.

A stiffer Achilles isn't just faster. It's more durable.

Under repeated loading, tendons "creep," gradually lengthening during a long run.

Research found Achilles stiffness dropped about 9% after a 90-minute run, increasing energy cost by roughly 12 joules per stride (that’s a fancy way to say you became a less efficient runner).

The chain reaction: tendon creep forces your calf to shorten more. Energy cost climbs. Form breaks down. The late-race shuffle to pizza begins.

Masters runners may benefit even more. Research found older runners showed greater stiffness loss after half-marathons compared to younger athletes.

This connects stiffness to performance, injury risk reduction, and late race durability. (Your super shoes can't do that.)

Running alone won't build this. Each step lasts a fraction of a second. Tendons need sustained strain to adapt.

Research shows tendons respond to strain in the 4.5-6.5% range. Below that, less happens. Above that, damage risk climbs.

Why slow holds? It takes about 300 milliseconds for strain to reach the cellular level. A 1-second contraction gives your tendon cells almost no signal.

A 3-4 second hold gives them real adaptation time.

Tendons adapt slower than muscles. But the adaptations stick.

Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.

Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.

High-mileage running looks bone-friendly on the surface. Lots of steps, lots of impact, lots of time on your feet. But this question exposed whether people understand what actually tells bone to get stronger.

The correct answer?

A. Running loads bone repeatedly but rarely with enough strain magnitude to trigger new bone formation 🦴⚡ ✅

Bone adapts to magnitude and novelty of load, not just volume. Running delivers thousands of highly repetitive forces in the same directions. That’s excellent for aerobic fitness, but often below the threshold needed to stimulate osteoblast activity.

Strength training and high-force movements work differently. Muscular contractions pull hard on bone, creating larger, less predictable strain signals. That is the stimulus that actually drives increases in bone density.

This is why runners can accumulate massive mileage and still show flat or declining bone density, while lifters and jump-based athletes often see improvements with far fewer total loading cycles.

Here’s how the votes shook out:
🟩 A. Running loads bone repeatedly but rarely with enough strain magnitude to trigger new bone formation 🦴⚡ – 139 ✅
⬜️ B. Bone adapts mainly to fatigue, not mechanical loading 😮‍💨🧠 – 4
🟨 C. Running prioritizes aerobic adaptations over skeletal adaptations 🫁🏃‍♂️ – 40
⬜️ D. Bone density only responds to impact, not muscular force 💥🚫 – 12

Bottom line?

If you want resilient bones to match your aerobic engine, mileage alone isn’t enough. You need occasional high-force signals—think heavy lifts, jumps, or sprint-level loading—to give bone a real reason to adapt.