BRINGING SCIENCE TO TREATMENT

How tendons adapt: learning more and knowing less

Angham Khaza’al, 25, an Iraqi sprinter ties her shoelaces during an exercise unit in Basra, 2012

Knowledge of tendon pathology, the bugaboo of athletic injury, has evolved over the last few decades. Once thought of as a purely inflammatory condition, a diseased tendon demonstrates inflammation and degeneration within the tendon. Science continues to expand our knowledge of tendon health. A sports scientist from La Trobe University recently (2019) undertook the daunting task of summarizing the currently accepted views regarding tendon pathology, loading, and adaptation(1).

As the junction between bones and muscles, tendons respond to the mechanical stimuli of the tension placed upon it by the muscles. Some term this response to load- tendon adaptation. Adaptation is

“how an organism, organ system, or tissue alters its structure or function to best suit its environment.”(1)

There are two indicators of adaptation. One is the ability to execute a desired function called person-level changes. This marker takes into account the entire system required for a task – meaning the whole kinetic chain. The other is what happens on a physiological level within the tendon itself called tissue-level changes. This level of change is specific to the individual tendon.

Most believe that if a tendon can respond to greater loads positively, it is more resilient and less susceptible to injury (tissue-level). This positive response likely translates to improved athletic performance as well (person-level).  The healthy tendon then develops a load capacity, meaning it performs at the desired functional level without incurring injury. Athletic training, or lack thereof, alters the load capacity in either a positive or negative direction.

Herein lies the basis of acute:chronic workload theory. Athletic training must include enough loading to stimulate positive changes acutely while avoiding a progression of chronic load that exceeds the tendon’s loading capacity (see figure 1).

Figure 1: Mechanostat point for tendons(1)

The ‘mechanostat point’ is a term borrowed from bone physiology. The results of strain on tendon cell cultures show that there is a point at which the tendon can maintain homeostasis. Tension below or above that load produces a maladaptive response within the tendon(1)


Do tendons adapt?

While a tendon’s load capacity can increase or decrease, does it adapt? Muscle tissue responds to loading by increasing the number of myofibrils in the muscle fiber. Thus the size of the overall muscle increases. However, a tendon cannot increase its cross-sectional area after puberty(1). Therefore the supposition that tendons increase their load capacity and resistance to injury by increasing their cross-sectional area as muscles do is likely false.

This finding supports the movement to encourage fitness in young people. One thought is that tendon adaptability, and load tolerance may be determined during development. Inactivity during adolescence may place individuals at a higher risk of developing a sports-related tendon injury later in life.

Other factors beyond tendon size may contribute to adaptation and load capacity. One is the mechanical property known as stiffness. Stiffness is the relationship between the change in the length of the tendon in response to the stress applied to it(2). Tendon stiffness decreases immediately after bouts of acute exercise. This more compliant state may leave the tendon vulnerable to injury. Therefore, overtraining without adequate recovery may cause a tendon injury if performed while the tendon is less able to handle the load.

The role the mechanical state of the tendon plays in performance remains equivocal. However, it appears that isolated exercises have a more significant impact on tendon stiffness than functional movements. Thus, increase tendon load tolerance in rehab before progressing to functional tasks (see this previous newsletter highlighting strength deficits despite full functional movement).

Internal structure

Another aspect of tendon structure is its internal composition. Sampling tendons in live species is difficult. Therefore, knowledge of the physiological response to activity within a tendon is usually based on MRI and ultrasound studies. Researchers don’t know if the observed changes are positive or negative responses(1). However, they have seen more aligned fibers within the tendon on ultrasound after gradual and prolonged loading, such as in a pre-season training program(1). Whether these observations are true adaptations within the tendon structure is uncertain.

Improving loading capacity

These observations regarding tendon structure and responsiveness were made in healthy tendons. Sports injury professionals, however, are more concerned with how to heal and increase the load capacity of an injured tendon. The limiting factor in the rehabilitation of tendon injuries is usually pain. Athletes avoid movement and loading painful structures, and the subsequently decreased loading further lowers the tendon’s structural integrity. Interestingly, resolving the pain doesn’t automatically guarantee normal tendon function(1).

When it comes to tendons, load capacity, pathology, and pain do not directly correlate with one another(1). As found in other pain studies of other diagnoses, many pathological tendons are asymptomatic. Additionally, many of these diseased tendons tolerate full loads and demonstrate normal function. Thus, the physiological and structural state of the tendon plays less of a role in symptomatic tendons and load capacity than previously thought. Perhaps, the author of this review suggests, the tendons themselves have little to do with adaptation to load. Rather, the surrounding muscles and nervous system may modulate the tolerance to loading.

Figure 2: Response at the tissue level versus adaptation at the tissue and person level(1)


What we know

Tendons demonstrate compromised structure after an injury. The observed tissue changes seldom return to their previous state. Degeneration within the tendon may leave ‘holes’ or islands of dead tissue which compromise the structure. Some loading of the injured tendon, however, shows promise as a stimulus for the normalization of tendon structure. While this seems good news for tendon health, it shows little correlation with improved functional abilities and load tolerance(1). At the same time, heavy, slow resistance training appears to increase the loading capacity of the tendon, yet doesn’t significantly change the mechanical properties(1).

There is still much to learn about tendon health. As the author suggests, true adaptations must be demonstrated at the person-level as well as the tissue-level (see figure 2). Otherwise, the changes in the tendon are merely responses to stimuli. These responses may be more system-wide than at the tendon itself.

References

  1. J Musculoskeletal Neuronal Interact. 2019;19(3):300-10
  2. . 2018 Jun; 17(2): 223–228

 

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