Mechanical, material and morphological adaptations of healthy lower limb tendons to mechanical loading: a systematic review and meta analysis

Key points

1. 01Mechanical loading produced moderate increases in tendon stiffness (SMD 0.74), large increases in modulus (SMD 0.82), and small increases in cross-sectional area (SMD 0.22)
2. 02Increased tendon modulus - not increased size - was the main driver of greater stiffness, based on meta-regression of 20 studies
3. 03Resistance training was the only intervention type to clearly increase all three tendon properties; jump-based training improved stiffness only
4. 04High-strain protocols (~5% strain) produced larger stiffness and modulus gains than low-strain protocols
5. 05Adults showed greater adaptations than elderly participants; elderly participants still responded but to a smaller degree

How it was conducted

Design

Systematic review and meta-analysis (PRISMA-compliant, PROSPERO registration CRD42019141299)

Databases

PubMed, Scopus, CINAHL, SPORTDiscus, EMBASE searched from inception to 4 September 2021

Included studies

61 articles meeting pre-specified criteria for lower limb tendon adaptation to mechanical loading

Participants

763 unique participants (615 male, 135 female, 13 unspecified); healthy adults, elderly, and one paediatric study

Interventions

Resistance training (46 studies), jump-based training (9 studies), concurrent training (2 studies), aerobic training (1 study); durations 3-52 weeks

Primary outcomes

Tendon stiffness, Young's modulus, and cross-sectional area (CSA); SMDs calculated using Hedges' g with random-effects model

What they found

• Overall mechanical loading: stiffness SMD 0.74 (95% CI 0.62-0.86; 81 intervention groups), modulus SMD 0.82 (95% CI 0.58-1.07), CSA SMD 0.22 (95% CI 0.12-0.33)
• Meta-regression (20 studies): modulus was the predominant moderator of increased stiffness (p < 0.001); CSA was not a significant moderator
• Resistance training: stiffness SMD 0.80 (95% CI 0.67-0.94; 68 groups), modulus SMD 0.90 (95% CI 0.65-1.15; 35 groups), CSA SMD 0.23 (95% CI 0.12-0.34; 59 groups)
• Jump-based training: stiffness SMD 0.49 (95% CI 0.22-0.77; 11 groups); no clear modulus or CSA change
• Concurrent training: stiffness SMD 0.03 (95% CI -1.03-1.10; 2 groups) - no significant effect
• High-strain (~5%) protocols: stiffness SMD 1.04 (95% CI 0.65-1.43), modulus SMD 0.82 (95% CI 0.44-1.20); high-strain produced significantly greater stiffness (p = 0.009) and modulus (p = 0.009) than low-strain
• High-intensity (>=70% 1RM/MVC) resistance training: stiffness SMD 0.86, modulus SMD 0.91 (95% CI 0.63-1.18), CSA SMD 0.22 (95% CI 0.09-0.35)
• Adult participants: modulus SMD 1.05 (95% CI 0.75-1.34), stiffness SMD 0.91 (95% CI 0.74-1.09), CSA SMD 0.24 (95% CI 0.10-0.37)
• Elderly participants (>60 years): modulus SMD 0.21 (95% CI -0.21-0.63), stiffness SMD 0.40 (95% CI 0.02-0.78); significantly lower than adults (p = 0.002)
• All contraction modes (concentric, eccentric, isometric, combined) produced moderate-to-large increases in stiffness (SMD range 0.67-1.17) and modulus (SMD range 0.98-1.22) without significant differences between modes

Limitations

• Significant heterogeneity existed between protocols; isometric contraction subgroups were not matched for time under tension
• Only 3 included studies directly measured tendon strain, limiting dose-response analysis for strain
• Tendon adaptations in free tendons and aponeuroses were aggregated despite structural and functional differences
• Most studies involved short-to-medium durations (under 52 weeks), limiting conclusions about long-term morphological adaptation and tendon hypertrophy

Why it matters

For patients

People who exercise can expect their tendons to become stiffer and stronger through training, particularly with resistance exercise performed at higher loads, which may reduce injury risk and improve athletic performance.

For clinicians

Resistance training at high tendon strains (~5%) should be prioritised for tendon conditioning programs; contraction mode and training volume matter less than ensuring adequate strain magnitude, and elderly patients show attenuated but real responses.

For readers

This large meta-analysis establishes that tendon material quality (modulus), not size growth, is the main mechanism behind training-induced stiffness gains, and identifies localised tendon strain as the key programming variable.