How Do Tendons Adapt? Going Beyond Tissue Responses to Understand Positive Adaption and Pathology Development: A Narrative Review.

Review written by Dr Carlo Wood info

Key Points

  1. We have not fully explained how specific tissue level changes affect person-level alterations as other systems contribute to performance.
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BACKGROUND & OBJECTIVE

It is crucial to understand the mechanobiology of tendon adaptation, the effect of load, and how to improve tendon quality. Negative consequences can create pathology, pain and impair function, while positive responses increase load capacity, can decrease pain and improve function. Tendons respond to mechanical stimuli in multiple ways. Our understanding of how these complex biochemical/mechanical responses contribute to positive adaptation of the tendon or person is incomplete. Person level alterations measure the capacity of several tissues and systems in the kinetic chain, but we also need to consider specific tissue level alterations. It is unclear how these tissue level changes affect the person. It is also important to note that adaptation in one tissue or system may not explain adaptation of the individual. Changes within the tendon may contribute to, but not fully explain, improved performance due to the fact that muscle, nerve and other connective tissues also contribute to person-level adaptation.

The main purpose of this narrative review was to define the mechanoresponses potentially related to adaptation of the normal and pathological tendon and how they may relate to improved load capacity or function.

Tendons respond to mechanical stimuli in multiple ways.
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Improving pain in tendinopathy does not directly translate into increased load capacity.

How do Tendons Adapt? Tendon size

  • Integration of new collagen in the tendon appears to be limited after skeletal development
  • Loading is associated with greater tendon dimensions
  • There is a small increase in cross-sectional area (CSA) with exercise
  • There is no link between increased CSA and injury reduction
  • Lack of exercise during puberty is a possible risk factor for developing a tendinopathy as an adult
  • After puberty, tendon may alter its mechanical properties or extracellular matrix

Mechanical Properties

  • Reduced stiffness immediately after exercise: returns to baseline within 1 day
  • Long-term loading exhibits increased stiffness: unclear whether these changes contribute to person-level adaptation
  • Unclear whether stiffness or compliance is advantageous for performance
  • Positive correlation between stiffness and both squat and countermovement jump height
  • Large correlation between stiffness and rate of torque development
  • Stiffness may improve transmission of contractile force from the muscle to the bone, improving power
  • Compliance seems to lead to greater energy storage, improved jumping, more elongation and storage of potential energy
  • No link between tendon mechanical properties and maladaptation
  • Person-level adaptation is influenced by tendon mechanical properties: unclear what specific tissue-level changes are required for this

Internal Tendon Structure

  • Specific proteins involved (these are unknown: beyond the resolution of imaging modalities)
  • Loading may alter proteoglycan content and therefore water content which creates separated fibers

Tendon Blood Flow

  • Changes in blood flow in response to exercise have not been linked to pathology/pain, and this returns to baseline levels within days

How do Pathological Tendons Adapt? The degenerative tendon rarely returns to normal structure and it is uncertain how it increases its load capacity. Its attempt at normalization have been demonstrated with loading, however, these changes do not equate with clinical improvement. Improvement in structure is not the only way a pathological tendon adapts.

Pathological tendon contains greater levels of aligned fibrillar structure compared to structurally normal tendons. This compensates for areas of disorganization by increasing in dimensions to ensure a sufficient level of aligned fibrillar structure to tolerate load.

Adaptation and increased load capacity may not occur within the degenerative area due to inability to sense mechanical stimuli which is responsible to stimulate a cellular response and required for adaptation. Adaptation may occur in the aligned fibrillar structure, rather than changes within the degenerative area. How mechanical stimuli affects the tendinopathic tendon is poorly understood.

Eccentric training and heavy slow resistance training demonstrate little effect on mechanical properties. While changes in mechanical properties are a likely candidate to explain adaptation in the pathological tendon, the current evidence is a long way from stating this as fact.

CLINICAL IMPLICATIONS

There is a void in our understanding of mechanistic evidence regarding load capacity. We do know that even a 6% tensile strain can increase collagen I mRNA and inhibit degradation enzymes. Without load, markers of degradation increase in matrix metalloproteinases

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