How to Train a Tendon: How Much Strain is Enough?
By Titled Sport and Exercise Physiotherapist, Michael Phipps
Tendons connect muscle to bone, and they get stronger with the right training. The issue is they are pickier than muscle, as muscle responds to almost any hard work. Tendons respond mainly to one thing: how far they are strained under load.
Strain and stiffness, simplified
Strain is how much a tendon stretches compared with its resting length. For simplicity, I'll refer to strain as stretch in this blog. Stiffness is how hard it is to stretch, like a thick resistance band versus a thin one.
A stiffer tendon passes force from muscle to bone more quickly, and it stretches less under the same load, which may help protect it from injury (Tsai et al., 2024). It doesn't add extra spring, though: at the same force, a stiffer tendon stores slightly less energy.
The sweet spot
Tendons seem to need a minimum amount of stretch before they adapt; below it, little changes. Around 4.5–6.5% strain appears to be the range that best triggers stiffening adaptation. In animal tendons, repeated strain of about 9% caused damage, although that upper limit hasn't been confirmed in people (McMahon, 2022).
Why tendons get sore: the muscle–tendon mismatch
Muscle adapts faster than tendon and responds to more kinds of training, including running, jumping and lighter, fatiguing work (Mersmann et al., 2021; Domroes et al., 2025). When strength outpaces tendon stiffness, the tendon is stretched further with every maximal effort. In adolescent athletes, strain of 9% or more during maximal contractions has been linked to a 2.3-fold higher risk of tendon pain (Domroes et al., 2025). This is one of many reasons a tendon can become sore (know as tendinopathy).
The Berlin method
Methods of tendon training have changed over the years: moving from high-rep, lowering-only (eccentric) heel drops, to heavy slow resistance (HSR) exercises, to more contemporary approaches, which target how far the tendon itself is stretched (strained).
The Berlin methods was developed at Humboldt University Berlin, the classic version is 5 sets of 4 isometric holds at about 90% of maximum effort (or dosed at a personalized effort range), each held for 3 seconds with 3 seconds' rest, about four times a week (Tsai et al., 2024). Typically push (or overcoming) isometric exercises are used. This is the type of isometric where a person pushes against an immovable object (like pushing against a strap in calf raise or knee extension fashion).
What the research shows:
A little goes a long way. Just 15 sets per week was found to be effective, with main adaptations typically occurring in the first 8 weeks (Tsai et al., 2024).
The same effort stretches tendons differently. During maximal contractions, strain ranged from 4.6% to 8.9% between people (Tsai et al., 2024). Setting each person's load by their own tendon strain prevented the rise in high strain seen in control players over a volleyball season (Weidlich et al., 2024), and cut the share of teenage female handball players with a muscle–tendon imbalance from 32% to 6% (Domroes et al., 2025).
Heavy, slow gym lifting counts. Triathletes added heavy leg work three times a week for 12 weeks. Exercises included: single-leg calf raises, knee extensions, squats, leg press and leg curls. They did 3–10 reps at about 80–90% of their one-rep max, each rep taking about 3 seconds (1 second up, 2 seconds down). Achilles tendon stiffness rose by about 39% and patellar tendon stiffness by about 16% (Mersmann et al., 2021; Jacobs et al., 2025).
Limitations
Most of the evidence comes from young, healthy men, and mostly from the Achilles and patellar tendons (McMahon, 2022). Strain is hard to measure outside a lab, although heavy, slow loading still reduced tendon pain without it (Mersmann et al., 2021), and personalising often means lighter loads for tendons that stretch easily (Weidlich et al., 2024). Anyone with tendon pain should have a personalised assessment and follow a progressive programme kept at a symptom-tolerable level.
Plain-language summary
Tendons get stronger (stiffer) when they are stretched (strained) enough under heavy load.
A small amount of the right training, a few times a week, is enough; most changes show within about two months.
The same exercise stretches everyone's tendons differently, so a one-size-fits-all load can be too light for some people and too heavy for others.
Get a personalised assessment. If you have tendon pain, play a jumping or running sport, or are returning from injury, ask a physiotherapist to assess your tendon, set a load that suits it, and progress it gradually at a level your symptoms tolerate.
References
Domroes T, et al. A personalized muscle–tendon assessment and exercise prescription concept reduces muscle–tendon imbalances in female adolescent athletes. Sports Med Open. 2025;11:14.
Jacobs MW, et al. Maximal strength training improves muscle-tendon properties and increases tendon matrix remodulation in well-trained triathletes. Sci Rep. 2025;15:27333.
McMahon G. No strain, no gain? The role of strain and load magnitude in human tendon responses and adaptation to loading. J Strength Cond Res. 2022;36(10):2950–2956.
Mersmann F, et al. A functional high-load exercise intervention for the patellar tendon reduces tendon pain prevalence during a competitive season in adolescent handball players. Front Physiol. 2021;12:626225.
Tsai M-S, et al. Effect of the temporal coordination and volume of cyclic mechanical loading on human Achilles tendon adaptation in men. Sci Rep. 2024;14:6875.
Weidlich K, et al. Addressing muscle–tendon imbalances in adult male athletes with personalized exercise prescription based on tendon strain. Eur J Appl Physiol. 2024;124:3201–3214.