Do Strength and Tendon Stiffness Adapt in Parallel? A Systematic Review and Multilevel Meta-Analysis of Chronic Mechanical Loading in Healthy Humans

Background and Objectives: This review evaluated whether strength and tendon stiffness show similar proportional responses to chronic mechanical loading. Methods: Six databases and citation chasing identified 57 reports from 50 cohorts. The primary analysis paired within-group longitudinal log response ratios (lnRRs) across 70 matched units, 62 participant samples, and 37 analytic cohorts, using multilevel models and cohort-clustered inference. This estimand was not a control-adjusted treatment effect. Results: Strength increased by 16.12% (lnRR 0.1495, 95% CI 0.1210 to 0.1779) and stiffness by 24.06% (lnRR 0.2156, 95% CI 0.1638 to 0.2674). The strength–stiffness contrast was −0.0661 (95% CI −0.1190 to −0.0132). Restriction to controlled designs attenuated the contrast, and its interval included zero. A reviewer-requested control-adjusted sensitivity analysis was directionally supportive but exploratory, with only 11 cohorts and low effective degrees of freedom. Strict maximal tendon strain showed no clear change (−4.77%, 95% CI −11.45% to 2.42%). Conclusions: The paired within-group analysis identified a small average proportional contrast, with a larger average stiffness response. The functional significance of this difference remains uncertain, and causal interpretation is limited.