Too much to read:

• Running speed and load are tightly linked — but not in a one-size-fits-all way.
• Impact magnitude and repetition combine to shape total mechanical stress.
• Every runner’s curve is unique — knowing it helps prevent re-injury.

OnTracx’s load-based approach turns these insights into action, empowering clinicians to create data-driven, individualized return-to-run programs that rebuild resilience safely and effectively.

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Introduction

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Changing running speed can have an enormous effect on the biomechanical load you experience during running. It feels intuitive: lower speed, lower load. That’s true — but the story is more complicated than that.

Running speed doesn’t just change how fast you cover ground, it also changes how your body absorbs, transfers, and responds to impact.

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Every small change in pace shifts your biomechanical load — the forces your muscles, bones, and tendons experience with each step.

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For anyone guiding athletes through injury rehab or towards a certain goal, understanding the relationship between load and speed is key to prevent setbacks.

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What Actually Happens When We Run

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Each time your foot strikes the ground, a shock travels up your leg. This is captured as vertical tibial acceleration, or impact load — a direct measure of how hard your body hits the ground at heel strike.

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Every step adds a small load to your muscles, tendons, and bones. A single step is harmless, but repeated thousands of times it becomes accumulated load — or the total load you experienced during for example a running session or an entire week.  

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Edwards et al. (2018) described running injuries through a mechanical fatigue framework: injury doesn’t result from one stride, but from the combined effect of impact magnitude and repetition exceeding what a certain tissues can recover from. Importantly, impact magnitude matters more than repetition because load doesn’t accumulate linearly. Doubling the impact per step doesn’t simply double the load — it multiplies it.

That’s why:

• Many gentle steps (low magnitude, high repetition) typically allows tissues to adapt safely.
• Fewer but harder impacts (high magnitude, low repetition) accelerates fatigue and microdamage.

The body can handle a lot of small hits, but only a few large ones — which is why (even small) speed increases can create large jumps in total load, and thus increase injury risk.

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How Speed Changes Impact Load

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As speed increases, stride lengthens, contact time shortens, and impact per step rises. Multiple studies confirm that impact load per step increases with running speed (Sheerin et al., 2020).

As Edwards described, for most runners, the increase in impact magnitude outweighs the reduction in step count — meaning that faster running usually produces a higher total load, even over the same distance.

That’s why running “just a bit faster” might feel fine on the surface, but internally, your tissues could be facing a significantly greater mechanical demand.

Understanding how your body responds to those changes is essential for training — and especially for returning to running — safely and effectively.

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Why Every Runner Handles Load Differently

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Once you understand how speed and load interact, the next question becomes: how can two runners experience such different outcomes at the same pace?

The answer lies in individual biomechanics. Each runner’s technique, leg stiffness, and neuromuscular control determine how effectively they absorb and redistribute impact (Van Waerbeke et al., 2023; Fadillioglu et al., 2022).

This means that runners not only can experience different impact at the same speed, but their load–speed relationships can also differ:

• Some show a linear increase, where load rises evenly with each increase in speed
• Others show a biphasic pattern, where load stays stable until a breakpoint, then rises sharply.

Neither is inherently better or worse, they simply describe different biomechanical responses.
The key is knowing your individual profile — so you can progress safely and build load tolerance gradually.

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Case Example:

Let’s look at two runners tested in the OnTracx Lab load screening.

For Lukas, speed and impact load rise together in a clear, linear pattern.

Every 1 km/h increase in pace raises his tibial impact by around 0.9 G — roughly 10% per step.

This reflects consistent technique but also a steep load gradient: small increases in speed cause meaningful jumps in cumulative load.

Managing how quickly he increases pace — and how often he trains at higher intensities — is key to preventing overload.

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Tom’s curve on the other hand tells a different story. Up to about 13 km/h, his load increases only slightly — 0.4 G per km/h. Beyond that speed, load spikes dramatically — rising 2.1 G per km/h.

That 13 km/h mark is his speed-load breakpoint — the pace where impact load starts rising disproportionately. Running faster might feel easy, but internally, load escalates quickly beyond this

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Why This Matters for Return-to-Run

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In rehabilitation, understanding a runner’s load–speed relationship transforms how you plan their return to running. OnTracx Lab quantifies how impact changes with speed, while the OnTracx Pro platform enables follow-up over time — step by step, session by session.

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Even more, it allows clinicians and coaches to prescribe personalized, gradual load progressions programs instead of relying on generic speed or distance programs.

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The two cases presented above make one thing clear: load progression is never one-size-fits-all. Some runners experience large increases when speed increases, while others don’t. OnTracx’s load-based training schemes embed this philosophy, empowering clinicians to design safer, evidence-driven return-to-run programs.

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Take-Home Message

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Running speed and biomechanical load are tightly linked — but not in a one-size-fits-all way.

• Impact load per step rises with speed, while total load also depends on step count and distance.
• Each runner has a unique curve — linear or biphasic — that defines their individual load response to speed.
• Objective screening with OnTracx Lab gives clinicians the insights to guide recovery with precision — turning biomechanics into practical, data-driven return-to-run programs.

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