Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of longitudinal intervention studies
PubMed 27747846 · doi:10.1186/s40798-015-0009-9
What was done
The authors systematically searched PubMed, Web of Knowledge, and Scopus for longitudinal exercise interventions (8 weeks or longer) in healthy adults aged 18 to 50 years that measured in vivo mechanical (stiffness), material (Young's modulus), or morphological (cross-sectional area) properties of Achilles or patellar tendons. Standardized mean differences (SMD) were calculated using random-effects models, with subgroup analyses for loading intensity, muscle contraction type, and intervention duration.
What was found
The meta-analysis included 27 studies with 37 interventions and 264 participants. Significant overall intervention effects occurred for tendon stiffness (SMD 0.70; 95% CI: 0.51, 0.88; N=37; I²=30%; p<0.05), Young's modulus (SMD 0.69; 95% CI: 0.36, 1.03; N=17; I²=57%; p<0.05), and cross-sectional area (SMD 0.24; 95% CI: 0.07, 0.42; N=33; I²=21%; p<0.05). Stiffness gains significantly depended on loading intensity (p<0.05, I²=0%) but not muscle contraction type. Longer interventions (12 weeks or more) had higher SMD without statistical significance.
Why it matters
Tendon adaptation to mechanical loading is primarily driven by loading magnitude rather than contraction mode. Increases in tendon stiffness stem largely from altered material properties rather than structural hypertrophy.
Limits
The review included only healthy asymptomatic adults aged 18 to 50 and evaluated only the Achilles and patellar tendons. Total sample size was small relative to the number of interventions, averaging about seven participants per intervention arm. Moderate heterogeneity was present for Young's modulus.
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