Muscle--Tendon Transformation During Resistance Loading: Force References, Adaptation Timing, and Clinical Interpretation Critical Narrative Review With Analytical Illustration

Abstract

Muscular force capacity and tendon resistance to elongation respond differently to resistance loading. In initially untrained men, 12 weeks of explosive and sustained contractions increased patellar tendon stiffness by 20% and 16%, respectively; tendon--aponeurosis stiffness increased only with sustained contractions. In nine men, Achilles tendon stiffness increased by 50.3% after three months of training and returned to the pre-training value within one month of detraining. Across two years of adolescent development, 18 volleyball athletes increased patellar tendon cross-sectional area by 27% and stiffness by 25%, while tendon stress decreased by 9%. These changes cannot be interpreted solely from muscular strength or from a stiffness slope estimated over a changing force interval. A nonlinear calculation gives lower elongation at 1200 N but higher strain when maximum force increases to 2100 N. Prospective adolescent observations associate strain of at least 9\% with subsequent symptoms (risk ratio 2.3; 95% confidence interval 1.1--4.9), without establishing a universal injury threshold. Common-force elongation, current-maximum strain, anatomical dimensions, seasonal variation, and symptoms therefore require separate assessment. Error propagation must retain the dependence between force, elongation, and derived stiffness.