Explosiveness: Your Achilles Tendon Is the Real Engine

Explosiveness: Your Achilles Tendon Is the Real Engine

by Laurent Glatz – for Athletic Carnivore

For a long time, explosiveness was reduced to three variables: muscle mass, the proportion of fast-twitch fibers, and the quality of neural activation. This triad still dominates training culture. Yet, a decisive part of performance lies in a structure you don’t see: the tendon.

In 2007, Kiyoshi Kubo published a series of studies in the European Journal of Applied Physiology that changed the mechanical understanding of explosive performance. The Achilles tendon is not just a passive cable. It is an active mechanical organ whose stiffness determines the speed of force transmission.

What Kubo Actually Measures

The study uses a combined approach: ultrasound to measure Achilles tendon elongation under load, dynamometry to quantify maximal voluntary force, and calculation of tendon stiffness from the force/elongation ratio.

Stiffness is defined as the slope of the force–deformation relationship. Mathematically, the more force required to slightly elongate the tendon, the higher its stiffness.

After specific training, researchers observe:

- A significant increase in tendon stiffness,
- An improvement in rate of force development (RFD),
- And enhanced performance in explosive movements.

Crucially, the increase in maximal muscle force alone does not explain these gains. The key variable is the mechanical transmission speed.

Understanding Tendon Biology

The tendon is mainly composed of type I collagen organized into hierarchical bundles: fibrils, fibers, fascicles. This structure gives it viscoelastic properties.

Two components determine its behavior:

- The elastic component, linked to collagen structure.
- The viscous component, related to interfibrillar sliding and the extracellular matrix.

When the muscle contracts, it applies force to the tendon. If the tendon is compliant (flexible), part of the initial contraction goes into "stretching the spring" before the force is transmitted to the bone. This creates a mechanical delay.

In an explosive movement where the expression window is under 150 milliseconds, this delay is critical.

A stiffer tendon reduces this latency phase. It transmits force faster, improving RFD, which is central in sprinting, jumping, and changing direction.

Simple Explanation of the Phenomenon

Imagine two systems.

In the first, the muscle pushes on a soft elastic band before moving the bone segment.

In the second, it pushes on a firm spring.

In both cases, the muscle produces the same maximal force. But in the second system, energy reaches the ground faster.

Explosiveness isn’t just about how much force you produce, but how quickly that force is transmitted.

Muscle–Tendon Interaction

The muscle-tendon complex functions as an integrated unit.

A stiffer tendon allows the muscle to operate in a more favorable zone of its force–velocity curve. By limiting excessive elongation, it optimizes fascicle length at the moment of force production.

During the stretch–shortening cycle, the tendon stores elastic energy during the eccentric phase and returns it during the concentric phase. The more efficient this return, the higher the performance.

Elite sprinters often exhibit:

- High tendon stiffness,
- Extremely short ground contact times,
- Remarkable ability to return elastic energy.

Why Stiffness Adapts

Tendon adaptation mainly involves collagen modifications:

- Increased fiber density,
- Better organization of bundles,
- Increased cross-linking,
- Changes in the extracellular matrix.

These adaptations are slower than muscle hypertrophy. Collagen remodeling takes several weeks or even months.

How to Increase Achilles Tendon Stiffness

Scientific data converge on two main stimuli.

The first is high-rate force development loading.

Drop jumps with ground contact times under 200 milliseconds impose a very rapid force peak. This rapid mechanical load seems particularly effective at stimulating stiffness increases.

Pogo jumps—short bounces on the forefoot with minimal amplitude—specifically target the Achilles tendon. The goal is reactivity, not fatigue.

Short sprints with full recovery complement this work. The tendon must be exposed to high but brief loads.

The second stimulus is prolonged maximal tension.

High-intensity isometric contractions lasting three to five seconds, especially heavy calf raises or specific positions near the functional sprint angle, also induce stiffness increases.

Sustained tension promotes collagen adaptation and strengthens internal structure.

How to Reduce Stiffness

In some cases, especially for prevention or management of tendinopathies, excessive stiffness can be problematic.

An overly stiff tendon transmits force very quickly but absorbs less. This can increase stress on the bone interface and muscle.

Slower loads, through full range of motion with controlled eccentric phases, promote a more compliant adaptation. They improve absorption capacity and mechanical tolerance without excessively increasing stiffness.

Balancing stiffness and absorption capacity is fundamental.

What This Changes in Training Design

Developing explosiveness doesn’t just mean producing more force.

It means reducing the mechanical delay between muscle force production and external transmission.

A high-performing muscle-tendon system:

- Produces force rapidly,
- Transmits that force without excessive latency,
- Efficiently returns elastic energy.

Explosive performance is not only muscular. It is mechanical, structural, and temporal.

Perhaps the fundamental question is no longer “how many kilos can you lift?” but “how fast does your system convert that force into useful movement?”

To connect your explosiveness, recovery, and nutrition to how you function, you can [take the free questionnaire](/en/questionnaire-neuroprofil.html).

FAQ

Does explosiveness mainly come from the muscle?

Are pogo jumps useful?

Can a tendon be too stiff?

How long does it take to adapt a tendon?

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