Tendon Adaptation: Mechanobiology to Practical Application

In the previous installment of The Debrief, we discussed how our historical ignorance of tendon biology has led to an incomplete understanding and treatment of tendinopathy. Falk Mersmann has spent a significant portion of his career investigating tendon mechanics, along with the biological underpinnings that support their various functional roles, in a concerted effort to drive the field forward.

It is widely known that tendons act to transfer muscle force to the skeleton and their elasticity allows them to efficiently store and recoil mechanical energy [1]. However, Mersmann and his colleagues have drilled down on the less appreciated aspect regarding the dynamic interaction between the muscle and tendon during varying task constraints.

Muscle–tendon units (MTUs) do not behave like simple springs. Their behavior reflects both the structural design of the system and the way that structure is tuned to meet specific functional demands [2].

Design: refers to its architectural features, including fiber length, pennation, tendon length, and aponeurosis geometry [2].

Tuning: describes how this architecture shapes the interaction between muscle and tendon, including fascicle behavior, strain distribution, and the flow of mechanical energy during movement [2,3].

Mersmann’s work makes it clear that the interaction between muscle and tendon does more than transfer force. It shapes the contractile environment the muscle operates within. In his analysis of jumping and sprinting, he showed that muscle performance is enhanced through three mechanisms: a high force–length potential, a high power–velocity potential, and high muscle activation. These mechanisms arise because the muscle and tendon do not experience the same mechanical conditions at the same time. Their contributions shift with the demands of the task [1].

He summarizes this by noting that human movement performance depends on the interplay between muscles and their tendons. This interplay reflects the contractile capacities of the muscle, the mechanical properties of the tendon, and the neural drive provided by the central nervous system. Together, these factors determine the operating conditions of the muscle and how the MTU responds to different loading scenarios [1].

The key implication is that MTU behavior is fundamentally task dependent. Because muscle and tendon experience different mechanical conditions and contribute differently across movement demands, the system reorganizes itself to meet the specific requirements of the task. This is why Mersmann frames MTU function through three distinct contexts:

1. Energy conservation

2. Work production

3. Maximum power

Each represents a different solution to the same underlying problem, which is optimizing the muscle’s operating conditions under varying constraints.

1) Energy Conservation

This refers to activities where we’re managing tasks requiring force generation over long durations.

(i.e. steady-state running or walking)

Energy conservation is achieved when a muscle generates the required force with the lowest possible metabolic cost. The collective unit achieves these conditions through a phenomenon called gearing. In essence, gearing describes how the MTU can undergo large length changes while the fascicles themselves change length very little. Most of the MTU excursion is absorbed by tendon stretch and by fascicle rotation within the muscle belly, buffering the fascicles from the joint‑level motion. This allows the fascicles to remain near their optimal operating length and to shorten at very low velocities even when the joint is moving quickly [4,5].

In that region of the force–length and force–velocity curves, each fiber produces more force per unit activation, meaning fewer fibers need to be recruited to generate the same joint moment. The result is a meaningful reduction in metabolic demand, enabling sustained, economical force production during repetitive tasks like steady‑state running [4,5].

During steady‑state running: the Achilles tendon stretches and recoils through a large range while the soleus fascicles shorten only slightly and remain close to their optimal length. This gearing effect keeps fascicle velocities low and force potential high, meaning the soleus can generate the required plantarflexion force with modest activation while the tendon manages most of the mechanical work [4].

2) Work Production

Work production refers to tasks where the system must generate mechanical energy on each cycle. These are repetitive efforts where force output is prioritized more than pure economy.

(i.e. running propulsion at moderate to higher speeds, accelerating the body, or the concentric portion of squatting)

In these contexts, the muscle–tendon unit still relies on gearing, but the fascicles now shorten more substantially and contribute direct contractile work. Tendon stretch and fascicle rotation continue to buffer the fascicles from the full MTU excursion, yet the fascicles operate at lengths and velocities that favor efficient work production rather than pure force conservation. They remain near optimal length while shortening at moderate velocities close to the efficiency optimum (~20% of their maximum shortening velocity). In this region of the force–length, force–velocity, and efficiency curves, the muscle can generate both force and mechanical energy with reasonable activation [4,5].

A clear example comes from the soleus during running, where the fascicles actively shorten throughout the entire stance phase and consistently produce mechanical work. As the MTU lengthens in early stance, part of this contractile work is stored as elastic strain energy in the Achilles tendon. As the MTU shortens in late stance, the soleus continues to produce work while the tendon recoils, and the total work produced by the contractile element remains similar across speeds [6].

3) Maximum Power

Maximum power refers to tasks where the system must generate the highest possible rate of mechanical energy. These are short‑duration efforts where both force and velocity are pushed toward their upper limits.

(i.e. sprinting, maximal jumping, or explosive lifting) 

In these situations, the muscle–tendon unit organizes itself to maximize power output rather than efficiency or sustained force. The fascicles shorten rapidly and operate at higher velocities. Tendon stretch and fascicle rotation still buffer the fascicles from the full MTU excursion, but the fascicles now work at lengths and velocities that favor maximal power (~30% maximum shortening velocity) rather than economical force or efficient work [4,5]. A key determinant of this state is that muscle can load the tendon at relatively slow, favorable shortening velocities while the tendon later recoils much more rapidly. This decoupling allows elastic tissues to release stored energy faster than the muscle fibers can shorten, producing joint power outputs that exceed the instantaneous power of the muscle alone. As a result, tendon recoil contributes substantially to the high power demands of explosive tasks such as sprinting or maximal jumping [7].

A clear example comes from the vastus lateralis during maximal jumping, where fascicles shorten quickly through the force‑producing range while the tendon stores and releases elastic energy to amplify total power output. The MTU undergoes a large stretch–shortening cycle, and the fascicles operate at velocities that allow the system to reach peak power. This pattern reflects the defining feature of the maximum‑power role: rapid fascicle shortening, high activation, and coordinated tendon recoil to produce the greatest possible rate of mechanical energy.

In The Recordings

Time is a finite resource, and it is a near‑impossible task to devote the necessary attention to all aspects pushing the field forward. Our goal with the Tendon Summit Insider is to help curate and streamline this process for working professionals. We will continue to highlight important emerging research and trends, but we also encourage readers to take advantage of the depth offered by our presenters. Their expertise provides an opportunity to engage with these topics at a level that is difficult to achieve through passive reading alone and to build the kind of conceptual fluency that strengthens clinical reasoning and supports better decision making in day‑to‑day practice.

This newsletter covers only one aspect of Dr. Mersmann’s presentation. His session was dense with information spanning not only the functional roles of tendons discussed here but also how this knowledge has been leveraged to design loading protocols that reliably elicit adaptation. He walks through the why and the how of tendinopathy management, outlining how specific loading parameters influence the therapeutic response. Understanding these mechanistic foundations gives clinicians a clearer rationale for selecting and progressing interventions. It shifts loading from a generic prescription to a targeted manipulation of strain magnitude, strain duration, and strain distribution, variables that directly shape tendon adaptation. 

We encourage anyone interested to read his written publications on the topic and watch his full lecture below (and probably rewatch it again to catch everything).

Next up, we review the lecture from Stephanie Cone — the functional relevance of the muscle-specific Achilles subtendons.

See you in two weeks!

Debrief written by: Jason Eure, PT, DPT, OCS, CSCS

Reference List

1. Mersmann F, Bohm S, Domroes T, Weidlich K, Arampatzis A. Personalized Injury Risk Assessment and Exercise Prescription Based on Tendon Strain. Exerc Sport Sci Rev. 2025 Apr 1;53(2):77-86. doi: 10.1249/JES.0000000000000359. Epub 2025 Feb 7. PMID: 39919310.

2. Holt NC, Mayfield DL. Muscle-tendon unit design and tuning for power enhancement, power attenuation, and reduction of metabolic cost. J Biomech. 2023 May;153:111585. doi: 10.1016/j.jbiomech.2023.111585. Epub 2023 Apr 13. Erratum in: J Biomech. 2024 Mar;166:112002. doi: 10.1016/j.jbiomech.2024.112002. PMID: 37126884; PMCID: PMC10949972.

3. Mersmann F, Bohm S, Arampatzis A, Karamanidis K and Seynnes O. Editorial: Muscle and Tendon Plasticity and Interaction in Physiological and Pathological Conditions. Front. Physiol. 2021. 12:678801. doi: 10.3389/fphys.2021.678801

4. Sebastian Bohm, Falk Mersmann, Alessandro Santuz, Adamantios Arampatzis; The force–length–velocity potential of the human soleus muscle is related to the energetic cost of running. Proc Biol Sci 1 December 2019; 286 (1917): 20192560. https://doi.org/10.1098/rspb.2019.2560

5. Sebastian Bohm, Falk Mersmann, Alessandro Santuz, Arno Schrol, lAdamantios Arampatzis. Muscle-specific economy of force generation and efficiency of work production during human running eLife. 2021. 10:e67182. https://doi.org/10.7554/eLife.67182

6. Sebastian Bohm, Falk Mersmann, Arno Schroll, Adamantios Arampatzis; Speed-specific optimal contractile conditions of the human soleus muscle from slow to maximum running speed. J Exp Biol 15 November 2023; 226 (22): jeb246437. doi: https://doi.org/10.1242/jeb.246437

7.  Roberts TJ. Contribution of elastic tissues to the mechanics and energetics of muscle function during movement. J Exp Biol. 2016 Jan;219(Pt 2):266-75. doi: 10.1242/jeb.124446. PMID: 26792339; PMCID: PMC6514471.