What Is Heavy Slow Resistance Training for Achilles Tendinopathy?

If you've been researching Achilles tendinopathy, you've probably come across "heavy slow resistance" — often abbreviated HSR. You may have seen it compared to the Alfredson eccentric protocol, or mentioned alongside Silbernagel's combined approach. But what actually is it, and why does it work?

Let me break it down clearly, because understanding the rationale behind HSR changes the way you approach the exercise — and a lot of how well it works comes down to doing it correctly.

The basics: what heavy slow resistance actually means

Heavy slow resistance is exactly what the name suggests: loading a muscle-tendon unit with significant resistance, moved slowly and with full control through the range of motion.

For the Achilles tendon, HSR typically means calf raises performed with enough external load to make the exercise genuinely challenging in the 6–15 repetition range, at a pace of roughly 3 seconds up and 3 seconds down. Sometimes described as a 3-0-3 tempo — three seconds through the concentric phase, no pause at the top, three seconds through the eccentric phase.

The "heavy" part is important. A weight that allows you to complete 20–25 reps easily is not heavy enough to drive the adaptation you're looking for. The load needs to be challenging in the 8–12 rep range for HSR to work as intended.

The "slow" part is equally important, and it's where HSR differs most from plyometric or sport-specific loading.

Why slow tempo matters for tendons specifically

Tendons respond differently to different types of loading. Fast, elastic loading — like jumping or running — relies heavily on the tendon's ability to store and release energy rapidly. This type of loading is important for return to sport, but it's not where you start with a reactive or painful tendon.

Slow, sustained loading produces a different mechanical signal at the tendon level. It creates prolonged tensile stress through the tendon tissue, which is the primary driver of collagen synthesis and structural adaptation. The tendon responds to slow load by remodelling — gradually reorganising its collagen structure to better handle that load.

"The tendon doesn't respond much to how many times you load it. It responds to the magnitude of that load and how long it's sustained. Slow tempo maximises that signal."

This is also why HSR tends to be better tolerated than fast loading in the middle phases of rehab. Fast loading amplifies the tendon's elastic response, which can drive symptoms in a reactive tendon. Slow loading bypasses that mechanism and delivers the adaptation stimulus more directly.

The research behind it

The original comparison study between HSR and the Alfredson eccentric protocol was published by Beyer and colleagues in 2015. It was a randomised trial in which participants with chronic mid-portion Achilles tendinopathy performed either the traditional Alfredson eccentric heel drop program or an HSR protocol using a seated calf raise machine.

Both groups improved significantly over 12 weeks. But the HSR group reported higher satisfaction, better adherence, and similar or slightly better outcomes — without the pain that Alfredson's protocol deliberately imposes. This is clinically meaningful, because the Alfredson protocol specifically instructs people to exercise through pain — a requirement that limits who can adhere to it.

Subsequent research by Silbernagel and others has supported a combined approach: progressive loading that includes both concentric and eccentric components at challenging loads, guided by pain monitoring. This is the framework most current tendinopathy clinicians use, and it's what the Achilles rehabilitation protocols I follow are built around.

How HSR fits into a full Achilles rehab program

Heavy slow resistance doesn't stand alone — it's Phase 4 in a logical progression that starts with isometrics and builds progressively.

Phase 1 — Isometrics (Weeks 1–2): Sustained calf holds against a fixed resistance. Reduces pain quickly by modifying the nervous system's pain response, and begins stimulating the tendon with minimal reactive risk.

Phase 2 — Bilateral heel raises (Weeks 2–4): Full-range calf raises with both feet. Introduces movement and progressive loading with the load shared between both limbs.

Phase 3 — Single-leg loading (Weeks 4–8): Single-leg calf raises, progressing in volume and range. This is the central phase of Achilles rehabilitation — where the most significant tendon adaptation begins to occur.

Phase 4 — Heavy slow resistance (Weeks 6–12): Adding external load to make the exercise genuinely challenging. A weighted backpack, a gym machine, or a barbell if available. The goal is to reach a load where 8–10 reps is legitimately hard.

HSR begins overlapping with Phase 3 rather than replacing it. As load increases, the volume of bodyweight single-leg work can reduce. By weeks 10–12, HSR is the primary loading stimulus.

What HSR looks like in practice

No gym required for the basics. A weighted backpack and a step or stair is enough to perform HSR effectively at home.

A typical HSR session for Phase 4 Achilles rehab might look like:

• 3–4 sets of 8–12 single-leg calf raises
• 3-second concentric (up), 3-second eccentric (down)
• Load chosen so that the last 2–3 reps of each set are genuinely challenging
• 90–120 seconds rest between sets
• Sessions every 48 hours (tendons need recovery time between heavy loading sessions)

The pain monitoring framework applies throughout: aim to keep pain at or below 3–4/10 during the exercise, and check in the next morning. If morning stiffness is significantly elevated from your baseline, the load or volume was too high — adjust before the next session.

Common mistakes with heavy slow resistance

Not going heavy enough. This is the most common one. People perform single-leg calf raises with bodyweight and call it HSR. For most people beyond the early phases of rehab, bodyweight is not sufficient load to drive continued tendon adaptation. You need to add external load.

Moving too fast. If you're bouncing through the reps or using momentum to get through the top of the range, you're reducing the mechanical signal you're trying to create. Slow and controlled, with a genuine pause or reversal at the end of each rep, is what drives adaptation.

Starting HSR too early. Adding heavy load before the tendon has been prepared through isometrics and progressive bodyweight work often drives flare-ups. Phase 4 is called Phase 4 because Phases 1–3 need to come first.

Not recovering between sessions. Tendons are slow to remodel and need 48 hours between heavy loading sessions. Daily HSR is counterproductive — you get more adaptation from three quality sessions per week than from seven mediocre ones.

Getting started

If you're at the point where single-leg calf raises feel manageable and your baseline symptoms are low, Phase 4 loading is likely appropriate to introduce. The 12-week Achilles program walks you through the full progression — including exactly how to add load, how to monitor your response, and how to progress through Phase 4 toward return to running and sport. The Phase 4 exercise card includes a load tracking log so you can see your progression week over week.

If you're not sure where you are in the process, a session with me can help clarify that — and give you a starting point that matches your current capacity.