Hitting the Sweet Spot: Applying the Berlin Protocol in Tendon Rehab

Tendon rehabilitation remains one of the most debated and misunderstood areas of musculoskeletal care. Despite widespread agreement that tendons need to be loaded to adapt, uncertainty persists around how much, how often, and what type of load actually drives positive change. Traditional approaches often focus on exercise type, volume, or pain response, yet growing mechanobiology research suggests these variables alone are not enough. Instead, tendon adaptation appears to be governed by a far more specific factor: the magnitude and duration of strain applied to the tendon tissue itself. The Berlin Protocol emerged from this body of work, offering a targeted, evidence-based framework for applying tendon load with greater precision, personalisation, and intent.

The Berlin Protocol is a tendon-loading framework developed from decades of mechanobiology research at Humboldt-Universität zu Berlin, aimed at optimising tendon adaptation. 

Unlike muscle, which can adapt across a wide variety of loading conditions, tendons appear to adapt most effectively to repeated high-strain loading (~4.5–6.5% strain) sustained for several seconds. There is also evidence that excessive strain may be detrimental: prospective studies show athletes with tendon strains of ≥9% have more than double the risk of developing tendinopathy (Mersmann et al., 2023). Although the exact overload threshold in humans is not fully established, these findings underscore the importance of targeting the tendon’s mechanobiological “sweet spot” — applying enough strain to stimulate anabolic adaptation, while avoiding underloading or excessive fatigue damage (Arampatzis et al., 2007; Arampatzis et al., 2010).

Standard prescription:

• 3-second isometric contraction (eg. calf raise isometric for the Achilles tendon, or leg extension isometric for the patellar tendon)

• 3-second rest

• 4 repetitions per set

• 5 sets per session

• 2–3 min rest between sets

• 3–4 sessions per week, for at least 12 weeks

Evidence Supporting the Berlin Protocol

Mechanobiological Foundations

• High strain is essential: Only high-intensity loading (~90% MVC, ~4.6% strain) increases stiffness and modulus; moderate (~55% MVC, ~2.9%) has no effect (Arampatzis et al., 2010; Bohm et al., 2015).

• Strain duration matters: ~3 seconds under high strain is optimal; too brief (plyometrics) or too prolonged (>12s) is less effective (Bohm et al., 2015).

• Tendon vs. muscle mismatch: Tendons adapt more slowly than muscles. Rapid strength gains without tendon adaptation elevate tendon strain and injury risk (Mersmann et al., 2017; Lambrianides et al., 2024).

Clinical and Preventive Trials

• Achilles tendinopathy: A 12-week high-loading program improved tendon stiffness (+20%), cross-sectional area (+9%), and reduced maximum strain (−12%), outperforming eccentric or passive therapy (Radovanović et al., 2022).

• Patellar tendinopathy prevention: In adolescent handball players, adding high-load tendon training twice per week during the season reduced pain prevalence to near zero, compared to ~30% in controls (Mersmann et al., 2021).

• Longitudinal risk evidence: Athletes with ≥9% tendon strain had a 2.3-fold higher risk of developing patellar tendon pain (Mersmann et al., 2023).

• Personalised loading: Tailoring exercise intensity to keep strain within 4.5–6.5% reduced muscle–tendon imbalances in both adolescent and adult athletes (Domroes et al., 2024; Weidlich et al., 2024).

How to Implement the Berlin Protocol

Step 1 – Exercise Selection:

• Isometric plantarflexion (Achilles) or knee extension (patellar tendon) using a leg press, dynamometer, or immovable resistance.

• Functional options: heavy slow squats with pauses at ~90° knee flexion.

Step 2 – Loading Parameters:

• Intensity: ~90% MVC (approx. 8–9/10 effort if no dynamometer available).

• Prescription: 3s load, 3s rest, 4 reps, 5 sets, 2–3 min rest.

• Frequency: 3–4 sessions/week for ≥12 weeks.

Step 3 – Monitoring & Progression:

• Keep pain ≤3–4/10 on NRS during/after loading.

• Use VISA-A/P scores for tracking.

• When available, ultrasound + dynamometry provides precision strain monitoring, though %MVC approximations are practical clinically.

Advantages of the Berlin Protocol

• Mechanobiology-based: Built on decades of in vivo and in vitro research, the protocol directly targets the strain levels shown to stimulate tendon adaptation (Arampatzis et al., 2007; Bohm et al., 2015).

• Time-efficient: Each session requires only a few minutes, making it practical to integrate into rehabilitation or training routines.

• Evidence of structural adaptation: Multiple trials show improvements in tendon stiffness, cross-sectional area, and reduced tendon strain, particularly in Achilles and patellar tendons (Radovanović et al., 2022; Mersmann et al., 2021).

• Prevention potential: Prospective studies suggest it can reduce muscle–tendon imbalances and lower the risk of tendinopathy in youth and adult athletes (Mersmann et al., 2023; Domroes et al., 2024; Weidlich et al., 2024).

• Personalisation possible: Strain-based loading can be adjusted to the individual, ensuring the tendon is trained within its “sweet spot” and not over- or under-stimulated (Domroes et al., 2024; Weidlich et al., 2024).

Limitations and Considerations

• Structure does not equate to pain: Structural changes (improvement or deterioration) in the tendon do not necessarily equate to pain or functional limitation. Patients can show imaging abnormalities without symptoms, and conversely, pain may persist despite structural improvements (Radovanović et al., 2023)

• Population & locations studied: Most intervention trials involve males; evidence in females, older adults, and other tendon sites (outside of Achilles & patellar tendon) remains limited.

• Overloading risk: Performing the protocol too frequently without sufficient recovery can be detrimental. Epro et al. (2023) showed that when high-strain loading was performed three times per day, tendon stiffness dropped, tendon strain increased, and participants developed pain within 8 days. By contrast, once-daily sessions preserved tendon integrity. This underlines the importance of at least 24 h recovery between sessions.

• Upper strain limits: While the “sweet spot” for tendon adaptation is ~4.5–6.5% strain, prospective evidence suggests that operating at ≥9% strain is linked to a more than two-fold increased risk of tendinopathy (Mersmann et al., 2023). The exact upper threshold in humans remains uncertain, but excessive strain likely increases mechanical demand and overuse risk.

• Equipment needs: Gold-standard strain monitoring requires dynamometry/ultrasound, though %MVC approximations are usable clinically.

• Not stand-alone, tendinopathy is multifactorial: The protocol should be integrated with other contributing factors, including metabolic and general health, biomechanics, and sport-specific rehabilitation

Clinical Takeaways

• The Berlin Protocol offers a mechanobiology-based loading strategy for tendons.

• It enhances tendon stiffness, reduces strain, and may prevent tendinopathy in high-risk athletes.

• Personalised application: identifying and targeting tendon strain “sweet spots” for each individual is emerging.

• Implementation requires balancing practicality (using %MVC) with mechanistic insights (strain thresholds).

To hear more about this approach, listen to the Rehab Runway podcast episode featuring Professor Peter Malliaras here

References

• Arampatzis A, Karamanidis K, Albracht K. Adaptational responses of the human Achilles tendon by modulation of the applied cyclic strain magnitude. J Exp Biol. 2007.

• Arampatzis A, Peper A, Bierbaum S, Albracht K. Plasticity of human Achilles tendon mechanical and morphological properties in response to cyclic strain. J Biomech. 2010.

• Bohm S, Mersmann F, Arampatzis A. Human tendon adaptation in vivo in response to mechanical loading: a systematic review. Front Physiol. 2015.Domroes T, Weidlich K, Bohm S, Mersmann F, Arampatzis A. Personalized tendon loading reduces muscle–tendon imbalances in male adolescent elite athletes. Scand J Med Sci Sports. 2024.

• Lambrianides Y, Epro G, Arampatzis A, Karamanidis K. Evidence of different sensitivity of muscle and tendon to mechano-metabolic stimuli. Scand J Med Sci Sports. 2024.

• Mersmann F, Bohm S, Arampatzis A. Imbalances in muscle and tendon development as risk factors for tendinopathies in youth athletes. Front Physiol. 2017.

• Mersmann F, Bohm S, Schroll A, Arampatzis A. Muscle and tendon adaptation in adolescent athletes: a longitudinal study. Scand J Med Sci Sports. 2016.

• Mersmann F, Pentidis N, Tsai M-S, Bohm S, Arampatzis A. Patellar tendon strain associates to tendon structural abnormalities in adolescent athletes. Front Physiol. 2019.

• Mersmann F, Domroes T, Tsai M-S, Pentidis N, Bohm S, Arampatzis A. Longitudinal evidence for high-level patellar tendon strain as a risk factor for tendinopathy in adolescent athletes. Sports Med Open. 2023.

• Radovanović G, Bohm S, Arampatzis A, Legerlotz K. Evidence-based high-loading exercise for Achilles tendinopathy improves tendon structure and function. Sports Med Open. 2022.

• Weidlich K, Domroes T, Bohm S, Mersmann F, Arampatzis A. Addressing muscle–tendon imbalances in adult male athletes with personalized exercise prescription based on tendon strain. Eur J Appl Physiol. 2024.