Eccentric Quasi-Isometric Loading: A Smarter Way to Load the Achilles and Treat Achilles Tendinopathy? | Find Your Stride | Edinburgh Podiatrist

Introduction

Managing Achilles tendon load is one of the biggest challenges in podiatry, sports medicine, and running performance. Traditional heavy–slow resistance (HSR) and eccentric protocols work, but it has been suggested that they are time-consuming, painful, and often poorly tolerated during high training loads.

A recent study by Križaj et al. (2025) investigates whether eccentric quasi-isometric (EQI) exercise might offer a more efficient and less painful way to load the ankle plantar flexors, with clear implications for Achilles tendinopathy rehab and performance-oriented strength training.

Križaj et al. (2025) suggests that traditional heavy–slow resistance (HSR) and eccentric protocols work well as rehab following achilles tendinopathy. But, the authors also describe these exercises as time-consuming, painful, and often poorly tolerated during high training loads. They have looked into an alternative approach.

What Did the Study Test?

The authors compared EQI contractions with isokinetic heavy–slow resistance (IHSR) in healthy, physically active adults.

• EQI protocol: Sustained isometric plantar-flexion until failure, followed by controlled eccentric yielding

  
  

• IHSR protocol: 90 slow isokinetic repetitions (30°/s), similar to traditional heavy–slow calf training

Outcomes measured:

• Torque impulse (total mechanical loading)

• Maximal voluntary isometric contraction (MVIC)

• Pain (general and activity-related)

• Ankle dorsiflexion range of motion

• Perceived effort and fatigue

This matters because tendon adaptation is driven by mechanical impulse, not simply reps or sets.

Key Findings (Why Clinicians Should Care)

1. More Load, Less Time

Despite attempting to “match” workloads, EQI produced significantly greater torque impulse in less total time under tension.

Clinical takeaway:

EQI may deliver the mechanical stimulus needed for tendon adaptation more efficiently. A major advantage in rehab settings with limited tolerance or time.

2. Less Pain and Soreness

Participants performing EQI reported:

• Lower immediate pain

• Less activity-related pain

• Reduced DOMS at 24 and 48 hours

Meanwhile, IHSR caused:

• Higher pain scores peaking at 24 hours

• Persistent soreness at 48 hours

Why this matters:

Pain is one of the biggest barriers to adherence in Achilles rehab. A protocol that loads the tendon without provoking excessive soreness is clinically valuable.

3. Strength Was Preserved

• EQI: No significant drop in MVIC strength

• IHSR: Strength remained suppressed even at 48 hours

This suggests EQI induces less neuromuscular fatigue and muscle damage, despite higher mechanical impulse. For runners; this could allow strength training closer to key run sessions without compromising performance.

4. Range of Motion Was Not Harmed

Neither protocol caused clinically meaningful reductions in ankle dorsiflexion. This reassures clinicians that high-load EQI does not acutely stiffen the ankle, an important consideration for running gait and calf–Achilles function.

Strengths of the Study

✔ Objective biomechanical measurement (torque–time impulse)

✔ Clinically relevant outcomes (pain, function, fatigue)

✔ Clear comparison to a gold-standard Achilles loading method

✔ Practical implications for both rehab and performance contexts

Important Limitations (What This Study Does Not Prove)

As promising as the findings are, clinicians should interpret them carefully:

• Healthy participants only - no Achilles tendinopathy patients were studied.)

• Short-term outcomes - this study measured responses over 48 hours—not long-term tendon adaptation.)

• Quasi-randomized design - group allocation was not fully blinded, introducing potential bias.

• Specialized equipment - EQI was performed on an isokinetic dynamometer—not easily replicated in clinics (yet).

👉 This study does not replace established eccentric or HSR protocols—but it strongly challenges the assumption that “more reps = better tendon loading.”

Practical Implications for Podiatrists & Athletes

For clinicians:

• EQI may be useful for high-load, low-pain Achilles loading

• Potential option for patients who flare with traditional eccentrics

• Promising for early-stage or load-sensitive tendinopathy (pending clinical trials)

For runners and strength coaches:

• Efficient calf loading with less soreness

• May reduce interference with running sessions

• Useful during in-season strength maintenance

Conclusion

This study suggests that eccentric quasi-isometric exercise delivers a higher mechanical stimulus with less pain and fatigue than heavy–slow resistance calf training.

For podiatry and sports-injury professionals, EQI represents a high-potential loading strateg, but one that still requires validation in Achilles tendinopathy and running-specific populations.

Expect EQI to feature more prominently in future discussions on tendon-friendly strength training.

Citation

Križaj L, Kozinc Ž, Šarabon N. Eccentric Quasi-Isometric Exercise Produces Greater Impulse with Less Pain than Isokinetic Heavy–Slow Resistance Exercise in Ankle Plantar Flexors. Applied Sciences. 2025;15:11177.

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