Recreational runners with Achilles tendinopathy have symmetrical single leg hopping mechanics, and contact duration is associated with vertical stiffness

Background: Runners with unilateral Achilles tendinopathy (AT) may exhibit inter-limb differences in lower limb mechanics during single leg hopping, and clinic friendly temporospatial measures may relate to stiffness characteristics. Objective: To test whether runners with unilateral AT have symmetrical lower limb mechanics across during hopping, and to examine the association between contact duration and stiffness. Design: Within-subject study. Method: Seventeen recreational runners with unilateral AT performed single leg hopping tasks: self-selected cadence (20 s), maximal height hopping (≥ 3 consecutive hops), cued hopping at 132 hops·min^{− 1} (20 s), and a progressive test at 120, 108, and 96 hops·min^{− 1} (15 hops each). Inter-limb comparisons evaluated temporospatial variables, kinematics, kinetics, and stiffness (vertical, leg, joint). When no inter-limb differences were detected for a variable within a task, a participant level, pooled value (mean of both sides) was calculated. Pearson correlations examined the associations between contact duration and stiffness at the participant level. Results: No inter-limb differences were observed for vertical or leg stiffness, ankle mechanics, or temporospatial variables across tasks. Small differences were observed at the hip and knee (e.g., greater hip flexion angle by 1–3° at 120 and 108 hops·min^{− 1}, p = 0.02; greater knee flexion by 3° during maximal height hopping, p = 0.02), and greater peak hip power on the affected side at 120 hops·min^{− 1} (0.82 (0.38) vs. 0.58 (0.33) W·kg^{− 1}, p = 0.03). Contact duration was strongly and negatively associated with vertical and leg stiffness during self-selected hopping (r = − 0.86 and − 0.85), and with ankle stiffness (r = − 0.74). Conclusion: Recreational runners with unilateral AT, had symmetrical lower limb mechanics during single leg hopping tasks, with subtle proximal differences of uncertain clinical relevance. The strong association between contact duration and stiffness supports the clinical utility of temporospatial metrics during self-selected hopping as practical indicators of lower limb mechanical behaviour.