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Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults

Bohm, S., Mersmann, F. and Arampatzis, A., 2015. Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults. Sports medicine-open, 1, pp.1-18..

In today’s letter

• Overview of how tendons adapt to load

• Rapid Results = Tendons respond well to increased mechanical loading by adjusting their mechanical, material, and morphological properties, leading to increased stiffness primarily due to changes in the material rather than just the structure

• 3 Reads to check out to further you knowledge about tendon adaptation

• Meme of the week: Me, a doctor? oh please good sir 🙈

Aim of study

The study aimed to review recent research on how tendons adapt to increased mechanical loading (through training) in asymptomatic (no pain/discomfort) people. It will analyse loading levels, outcomes, and methods, which hasn't been done before.

Did you Know?

• Tendons are flexible and can store and release energy during movements, helping muscles to produce more force

• They help in every-day activities like walking/running and stability, as well as contributing heavily in athletic performance, such as sprinting.

• After increased mechanical stress, tendon stiffness may rise to maintain normal levels of strain during the movements mentioned above

• The increase in stiffness could come from changes in tendon material (e.g., higher Young’s modulus) and morphological properties (e.g., increased cross-sectional area).

Youngs modules = It tells us how much a material (tendon) stretches or compresses when a force is applied. Simplified, it's a measurement of how stiff or rigid a material is.

Increased cross sectional area = Refers to tendon being thicker or wider in a certain direction

• These changes result from increased collagen creation, alterations in collagen fibril structure, and levels of cross-linking.

• Advances in measurement techniques, particularly ultrasound for tendon elongation during muscle contractions and MRI for cross-sectional area determination, have enabled detailed studies of tendon properties and adaptive responses to increased loading.

• Numerous experimental studies have shown tendon adaptation following exercise interventions with various protocols:

1. Intensity

2. Duration

3. Repetitions

4. Sets

5. Intervention duration

6. Training frequency per week.

• Differences in applied loading conditions can influence the extent of tendon adaptation observed in studies.

• Conducting a meta-analysis of relevant experimental studies could increase the understanding of how different loading conditions affect tendon adaptation.

Results

27 studies met all criteria and were included in the meta-analysis

• All included studies assessed the effect of mechanical loading on either the patellar tendon (N=12) or the Achilles tendon (N = 15).

• 9 studies applied a different loading protocol on the two legs of the participants of the exercise group, and one study investigated 3 different intervention groups.

• The meta-analysis included in total 37 interventions (participants in total N = 264)

• In all 37 interventions, the parameter tendon stiffness was used in order to quantify the training effect on the adaptive tendon responses.

• 33 of these also examined the tendon cross sectional area, and 17 studies further included the parameter Young’s modulus.

• 17 interventions applied the mechanical stimulus on the tendon by means of isometric muscle contractions, 11 interventions used a combination of concentric and eccentric contractions or solely concentric (N = 1) or eccentric contractions (N = 3), 5 interventions performed plyometric training, 1 intervention added stretching to the resistance training, and 1 study investigated the effect of running on the tendon properties

  

• The loading conditions were set to different levels between studies, using high and low intensities, short and long durations of the single loading, and different numbers of repetitions and sets

• 35 of the 37 interventions were performed for 8 to 14 weeks, and the participants exercised on 2 to 4 days per week

• Except for four interventions, which included both female and male participants and one intervention including solely women

• All other interventions were performed with men.

Discussion

Tendon change

• This meta-analysis examined the impact of chronic mechanical loading on tendon properties like stiffness, Young’s modulus, and cross-sectional area

• Despite variations in loading protocols, there was a significant overall effect observed for stiffness, Young’s modulus, and cross-sectional area.

• However, there was notable difference in stiffness and Young’s modulus across interventions, suggesting that different loading levels could influence adaptive responses differently.

• Subgroup analysis show that high loading intensities were more effective than low intensities at bringing about adaptive responses, while the type of muscle contraction didn't seem to matter.

• This analysis suggests that the tendon's adaptive response to chronic mechanical loading may be more pronounced in terms of material properties compared to morphological properties.

Type of exercise

• Tendons are able to adapt to increased mechanical loading despite diverse loading principles

• However, significant variability in tendon stiffness changes across studies dependent on the conditions below:

1. Intensity

2. Duration

3. Repetitions

4. Sets

5. Intervention duration

6. Training frequency per week

• Various conditions, including muscle contraction type (isometric, concentric, or eccentric), repetitiveness, and joint angles affecting tendon lever arm length and stress, can significantly influence tendon adaptive responses.

• Subgroup analysis highlighted the importance of high tendon loading intensities for adaptation, with exercises exceeding 70% of 1 rep max showing best adaptive results

• This paper showed that no matter the contraction type (isometric, concentric-eccentric, or purely eccentric). the tendons will have similar adaptations

Loading intensity

• The deformation of tendon cells due to loading is an important stimulus, that impacts cellular and molecular adaptation

• Increased strain leads to observable changes like collagen crimp loss and enhanced fiber recruitment, likely resulting in increased cell deformation and adaptive processes, influenced by the intensity of loading.

• Apart from loading intensity, various other factors such as loading frequency, rate, joint angle, duration, and type (repetitive vs. static) of loading were found to influence tendon adaptation

Plyometrics

• The impact of plyometrics on tendon properties remains uncertain, as findings from five interventions included in this meta-analysis showed conflicting results.

• The reported changes in tendon stiffness varied widely, ranging from a significant increase of +28% to a decrease of -9%.

• Only one study reported a statistically significant 27% increase in tendon stiffness.

• Discrepancies in findings could be attributed to different types of jumping exercises, varying tendon load magnitudes (some uncontrolled or relatively low), and differences in intervention durations (ranging from 8 to 14 weeks).

• When comparing dynamic (concentric-eccentric) and isometric training with plyometric training, 2 studies found a significant increase in Achilles tendon stiffness only after dynamic and isometric training, not plyometric training.

• 1 study suggested that the short duration of loading during jumping exercises may limit the transmission of mechanical stimuli to the cellular level due to tendon tissue viscosity, potentially affective optimal adaptive responses of the tendon.

Duration of the exercise intervention

• Several studies with an 8-week duration demonstrated significant increases in tendon stiffness, which indicted that 2 months is a good timeframe to gain adaptation

• Intervention durations of 8 to 12 weeks were shown to induce tendon adaptive responses in the analysis.

• However, longer durations of 12 weeks or more appeared to be more effective

Tendon stiffness

• As discussed above the Increases in tendon stiffness may result from alterations in tendon material properties (e.g., Young’s modulus) and/or morphological properties (e.g., cross-sectional area, tendon rest length).

• Some studies have observed increases in tendon cross-sectional area following training interventions.

• However, exercise-induced changes in tendon rest length are lacking, suggesting it may not be a relevant adaptive mechanism to increased loading.

• The similarity and higher average effect sizes of stiffness and Young’s modulus compared to cross-sectional area suggest that stiffness increase may primarily come from material property changes (Youngs modulus) rather than morphological changes.

• Material property changes are considered an early mechanism for stiffness increase, while tendon size increase may represent a longer-term effect of mechanical loading.

• Additionally, studies have noted an increase in tendon Young’s modulus without changes in cross-sectional area, indicating that material properties show greater plasticity and respond quicker to increased chronic loading.

Duration

• With intervention durations averaging 12.9 ± 4.5 weeks, including two longer studies (14 weeks or more), the relatively short durations may explain the small effect size of cross-sectional area compared to Young’s modulus.

• Tendon hypertrophy might be more significant with frequent loading over longer periods ( e.g training a sport for a very long time), as opposed to the durations typically seen in exercise ( e.g going to the gym once in a while for a year)

Excessive mechanical loading (overloading)

• Excessive loading has been identified as a significant contributor to tendinopathy development.

• Tendinopathy is marked by activity-related pain, focal tendon tenderness, and decreased strength and flexibility.

• Repetitive strains, even below the tendon's failure threshold, can lead to micro-injuries and inflammation, potentially contributing to tendon degeneration.

• Additionally, interventions included in studies reported a notable dropout of participants due to clinical symptoms of overloading

📚 Top 3 resources 📚

That will further your knowledge about tendon adaptation to exercise

1. How tendons adapt - short & sweet

   https://www.sciencedaily.com/releases/2021/05/210524161800.htm

2. Mechanical properties of tendons & the implication in training

   https://simplifaster.com/articles/tendons-mechanical-properties-training-athletes/

3. Nicely explained science behind tendon training for late rehab (VIDEO)

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

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