Abstract
BackgroundThe present article systematically reviews recent literature on the in vivo adaptation of asymptomatic human tendons following increased chronic mechanical loading, and meta-analyzes the loading conditions, intervention outcomes, as well as methodological aspects.MethodsThe search was performed in the databases PubMed, Web of Knowledge, and Scopus as well as in the reference lists of the eligible articles. A study was included if it conducted (a) a longitudinal exercise intervention (≥8 weeks) on (b) healthy humans (18 to 50 years), (c) investigating the effects on mechanical (i.e., stiffness), material (i.e., Young’s modulus) and/or morphological properties (i.e., cross-sectional area (CSA)) of tendons in vivo, and was reported (d) in English language. Weighted average effect sizes (SMD, random-effects) and heterogeneity (Q and I2 statistics) of the intervention-induced changes of tendon stiffness, Young’s modulus, and CSA were calculated. A subgroup analysis was conducted regarding the applied loading intensity, muscle contraction type, and intervention duration. Further, the methodological study quality and the risk of bias were assessed.ResultsThe review process yielded 27 studies with 37 separate interventions on either the Achilles or patellar tendon (264 participants). SMD was 0.70 (confidence interval: 0.51, 0.88) for tendon stiffness (N=37), 0.69 (0.36, 1.03) for Young’s modulus (N=17), and 0.24 (0.07, 0.42) for CSA (N=33), with significant overall intervention effects (p<0.05). The heterogeneity analysis (stiffness: I2=30%; Young’s modulus: I2=57%; CSA: I2=21%) indicated that differences in the loading conditions may affect the adaptive responses. The subgroup analysis confirmed that stiffness adaptation significantly (p<0.05) depends on loading intensity (I2=0%), but not on muscle contraction type. Although not significantly different, SMD was higher for interventions with longer duration (≥12 weeks). The average score of 71±9% in methodological quality assessment indicated an appropriate quality of most studies.ConclusionsThe present meta-analysis provides elaborate statistical evidence that tendons are highly responsive to diverse loading regimens. However, the data strongly suggests that loading magnitude in particular plays a key role for tendon adaptation in contrast to muscle contraction type. Furthermore, intervention-induced changes in tendon stiffness seem to be more attributed to adaptations of the material rather than morphological properties.
Highlights
The present article systematically reviews recent literature on the in vivo adaptation of asymptomatic human tendons following increased chronic mechanical loading, and meta-analyzes the loading conditions, intervention outcomes, as well as methodological aspects
Study selection and inclusion criteria Two independent reviewers (S.B. and F.M.) evaluated the titles of the studies that resulted from the search and included studies when the title indicated that the following inclusion criteria were fulfilled: (a) a longitudinal exercise intervention (≥8 weeks) was conducted, (b) healthy humans (18 to 50 years) served as participants, and (c) the effects on mechanical, material (Young’s modulus), and/or morphological (CSA) properties of asymptomatic tendons in vivo were reported (d) in the English language
Subgroup analysis The subgroup analysis on the loading intensity showed that pooling interventions using muscle contraction intensities higher than 70% of MVC or RM (N = 27) and those using lower intensities (N = 5) resulted in a weighted averaged effect size of tendon stiffness of 0.90 (CI 0.71, 1.08) and 0.04 (CI −0.46, 0.53), respectively
Summary
The present article systematically reviews recent literature on the in vivo adaptation of asymptomatic human tendons following increased chronic mechanical loading, and meta-analyzes the loading conditions, intervention outcomes, as well as methodological aspects. Two mechanisms could account for an increase of tendon stiffness: a) changes of the tendon material (i.e., increase of Young’s modulus) and b) changes of the tendon morphological properties (i.e., increase of cross-sectional area) [24,28,29,30,31]. Both tendon material and morphological changes result from an increase of collagen synthesis and from changes of collagen fibril morphology and levels of collagen molecular cross-linking [19,32,33]. Excessive mechanical loading (i.e., overloading) was considered as an important factor in the etiology of tendinopathy [20,34,35], which is associated with pain, focal tendon tenderness, and decreased strength and movement [32]
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