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How do tendons adapt? Going beyond tissue responses to understand positive adaptation and pathology development: A narrative review

Docking, S.I. and Cook, J., 2019. How do tendons adapt? Going beyond tissue responses to understand positive adaptation and pathology development: A narrative review. Journal of musculoskeletal & neuronal interactions, 19(3), p.300.

In today’s letter

• Overview of how injured tendons adapt to load

• Audio version of the newsletter

• Rapid Results = Its not fully proven, however it seems like the affected tendons due not heal. The improvements come from healthy collagen fibres becoming stronger

• 3 Reads to check out to further you knowledge about tendons

• Meme of the week: Mmm, can you point to where you discomfort is again? 😄 

Aim of study

1. This narrative review compiles evidence of how normal and pathological (injured) tendons adapts to load, and how this relates to adaptation of load capacity & function

2. This paper aims to define the mechano-responses related to both normal and pathological tendon adaptation

Did you know?

• Understanding mechano-biology, which is how the body senses and responds to mechanical stimuli, is essential for anyone involved in sports medicine and science.

• Excessive load can lead to tendon pathology, resulting in pain and impaired function, commonly known as tendinopathy.

• While tendons were once thought to be metabolically inactive in response to movement & exercise, research has shown numerous times that tendons do respond to load

• This paper aims to define the mechano-responses related to both normal and pathological tendon adaptation, explore their impact on load capacity and function

What is adaptation?

• Adaptation refers to how an organism, organ system, or tissue changes its structure or function to better suit its environment.

• Tendon adaptation is mainly driven by the presence OR absence of mechanical stimuli, such as tensile strain, compression, or shear stress (Activity

• On an individual level, adaptation results in increased load capacity.

• In tendons, this increased load capacity leads to improved athletic performance.

• Enough load capacity theoretically reduces the risk of developing tendon pathology or clinical symptoms.

• Load capacity is considered to be dynamic, which can be enhanced with proper application of load or diminished in the absence of load.

• The 'mechanostat point' is a tissue-level threshold that determines if the applied load causes an adaptive or maladaptive response.

• In tendon cell cultures, applying 6% tensile strain produced a potential adaptive response (increase in collagen I mRNA and inhibition of degradation enzymes), whereas the absence of load induced markers of degradation (increase in matrix metalloproteinases and collagenase enzymes).

• Changes within the tendon contribute to improved athletic performance but do not entirely account for it.

• Other factors such as musculature, the nervous system, and other connective tissues also contribute to individual-level adaptation, though this paper will primarily focus on tendon tissue properties and their relation to adaptation

How do tendons adapt?

Tendon size

• Changes in tendon dimensions in response to load have been thoroughly studied to understand tendon adaptation.

• Studies have shown increases in collagen synthesis markers 24 hours post-exercise, both in the tendon, in the space between the tendon and peritendinous sheath.

• This increase in collagen synthesis may lead to larger tendon dimensions,

• It has also been detected that within 24-72 hours post-exercise the collagen degradation markers and digestive enzymes also rise

• Increases in collagen mRNA or protein do not necessarily result in the brand new collagen being integrated into the tendon matrix, as collagen degradation can occur both in and out of the cells

• Studies suggest that tendon turnover is limited after adolescence.

• Tissue-based adaptation through increases in tendon dimensions may only be possible during puberty.

• Limited turnover in adults may explain the mixed results when investigating changes in tendon dimensions in response to load.

• Bohm et al. reported a small overall effect for an increase in cross-sectional area in response to various exercises (e.g., isometric, eccentric, concentric, and eccentric combined).

• Increases in tendon dimensions in response to exercise have been consistently observed in younger participants (mean age <25 years old).

• No significant change in tendon cross-sectional area has been observed in participants over 60 years old.

• There are limited studies investigating the elderly population, and further research is needed to determine if increases in tendon dimensions through exercise are age-dependent

• It is logical to assume that increases in tendon cross-sectional area are adaptive and increase load capacity, as they decrease the stress placed on the tendon for the same force (stress = force/cross-sectional area).

• However, a link between increased tendon dimensions and a reduced risk of injury has not been established.

• Puberty may provide a critical window of opportunity where the tendon can adapt by building new collagen tissue and be conditioned to tolerate high loads later in life.

• Inactivity during puberty may be a risk factor for the development of tendinopathy.

• That being said, absence of change in tendon cross-sectional area does not mean absence of tissue adaptation.

• Post-skeletal maturation, the tendon may adapt via other mechanisms such as alteration of mechanical properties or changes in the extracellular matrix composition.

Mechanical properties

• The term ‘tendon mechanical properties’ is used to refer to both the aponeurosis and the free tendon.

• Studies comparing mechanical properties between these two regions indicate that changes in the aponeurosis do not directly translate to the free tendon.

• Understanding the temporal sequence of changes in mechanical properties in response to load is crucial.

• A reduction in stiffness at both the aponeurosis and free tendon is observed immediately after exercise.

• Some authors suggest these transient changes (lasting hours to days) increase the susceptibility to tendon pain and pathology, though evidence is limited.

• These changes might result from tendon creep due to viscoelastic properties, rather than mechanical fatigue.

Tendon creep = The gradual elongation of a tendon under a constant load over time

• Exercise shows contrasting effects over different time frames; a short-term decrease in stiffness is seen, but long-term actoivity show significant increases in tendon stiffness and Young’s modulus.

Young’s modulus = a measure of the stiffness of a tendon, indicating how much it will stretch in response to a given amount of stress

• A strong correlation exists between the rate of torque development and aponeurosis stiffness.

• Increased aponeurosis stiffness may enhance the transmission of contractile forces from muscle, improving rate of force development requiring activities (i.e jumping, sprinting).

• In addition, improved jump performance has been linked to a more compliant aponeurosis, which enhances energy storage capability.

• Mechanical adaptations that improve performance may be specific to the sport, with optimal tendon mechanical properties likely falling within a bell curve, where extremes in stiffness increase the risk of injury

• Increased stiffness may benefit power athletes by optimising force transference through the tendon.

• The link between tendon mechanical properties and maladaptation remains unestablished.

• Reduced flexibility in the musculotendinous unit, related to tendon stiffness, is a risk factor for Achilles and patellar tendinopathy.

• There is substantial evidence suggesting that person-level adaptation is influenced by aponeurosis and/or tendon mechanical properties.

• It is unclear which specific tissue-level changes induce specific person-level adaptations, such as whether stiffer or less stiff tendons are preferable for endurance athletes.

• Studies have reported changes in mechanical properties following multi-joint exercises, though more research is needed to clarify these adaptations.

Tendon blood flow

• The response of blood flow to exercise has been suggested as a tissue-based change that may influence maladaptation.

• Using real-time contrast-enhanced ultrasound, Pingel et al. observed an increase in blood volume immediately after a 1-hour run, which returned to baseline 24 hours post-exercise.

• Interestingly, these changes in blood flow were not affected by the presence of pathology or pain.

• Since pathological or painful tendons show increased blood flow, any increase in blood flow in response to exercise has been suggested as a maladaptive response.

• To date, changes in blood flow in response to exercise are best described as a tissue-based temporal response, as they have not been linked to the development of pathology or pain and return to baseline levels within days.

Tendon load capacity in tendinopathy

• Improving load capacity in normal tendons is key for injury prevention and athletic performance.

• Tendon pain is linked to abnormal tendon structure, reduced load capacity, and decreased performance, potentially due to compromised structure.

• Pain in tendinopathy limits the tendon’s capacity to tolerate load, leading to further reductions in structural and mechanical properties

• Simply removing pain through medical intervention does not immediately increase load capacity or shift the tendon’s 'mechanostat point,' as it does not address local tendon maladaptation.

• While reducing pain is important in tendinopathy, improving load capacity through adaptation is necessary to prevent re-injury.

• Load capacity is not directly related to the presence or extent of tendon pathology.

• The high prevalence of non-symptomatic tendon pathology and similar training volumes and performance between normal and pathological tendons indicate that structural disorganisation is not critical in determining load capacity.

• The body adapts and compensates for tendon pathology.

• This compensation may occur in more metabolically active tissues, such as muscles and/or the central nervous system.

  It remains unclear what specific changes, if any, occur at the local tendon level that increase positive adaptation.

How does the pathological tendon adapt?

• The mechanisms by which a degenerative tendon increases load capacity are not well understood, and it rarely recovers normal structure.

• Load-based interventions have shown improvements in tendon structural properties (Doppler signal, anteroposterior thickness, UTC echo pattern).

A Doppler signal in medical imaging refers to the use of Doppler ultrasound to measure the flow of blood or other fluids within the body, often used to assess vascular health and detect abnormalities

Anteroposterior thickness is the measurement of the depth or width of a structure from front (anterior) to back (posterior)

UTC echo pattern refers to the specific patterns seen in ultrasound tissue characterisation (UTC) imaging, used to analyse the structure and integrity of tendons and muscles

• However, these structural improvements do not come with clinical improvements.

• The pathological tendon may have an adaptive mechanism to maintain homeostasis and compensate for disorganised areas.

• Docking and Cook demonstrated that pathological Achilles and patellar tendons have greater levels of aligned fibrillar structure compared to normal tendons.

• This study showed that pathological tendons compensate for disorganisation by increasing dimensions to maintain sufficient aligned fibrillar structure to tolerate load.

• Although this aligned fibrillar structure is likely abnormal with subtle compositional changes, it may allow the tendon to tolerate load by maintaining parallel collagen fibrils.

• Adaptation and increases in load capacity might not occur in the degenerative area due to its inability to sense mechanical stimuli.

• Thornton and Hart proposed that non-resolving pathology can have significant matrix turnover without forming mature tissue, with adaptation occurring in the surrounding aligned fibrillar structure instead.

• While changes in mechanical properties are a plausible explanation for adaptation in pathological tendons, current evidence does not definitively support this.

Top 3 reads of the week

1. Hamstring strain rehab

   https://www.sportsmith.co/articles/a-short-guide-to-rehabilitating-a-hamstring-tendon-repair/

2. Diet and tendinopathy

   https://www.running-physio.com/diet-and-tendinopathy2023/

3. Tendon adaptation to load (VIDEO)

   https://www.youtube.com/watch?v=oq1ACPZPJ6M

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