Test-Retest Reliability of In Vivo Structure and Mechanical Properties of the Achilles Tendon in Youth With Obesity.

PURPOSE: To assess the test-retest reliability of in vivo Achilles tendon (AT) structure and mechanical properties in youth with obesity. METHODS: Participants (11 males; 12 females; age: 12.00 [2.07] y; body mass index z score: 2.93 [0.59]) were assessed for AT (1) length, (2) cross-sectional area, (3) moment arm, (4) force, (5) deformation, (6) stress, (7) strain, (8) stiffness, (9) Young's modulus, (10) plantar flexor maximal isometric voluntary contraction, and (11) tibialis anterior co-activation. Reliability was assessed using the intraclass correlation coefficient, coefficient of variation, Bland-Altman, standard error of the measurement, and minimum detectable change. The intraclass correlation coefficient values were classified as excellent (≥.90), good (≥.75 to <.90), moderate (>.49 to <.75), and poor (≤.49). Moreover, the threshold for CV was set at < 10% to be considered excellent. RESULTS: The intraclass correlation coefficient ranged from .88 (P < .001) (AT stiffness) to .99 (P < .001) (AT stress), while the CV ranged from 0.91% (AT length) to 9.98% (AT Young's modulus). Bland-Altman (%) of mean bias ranged from 0.24% (AT length) to -6.65% (AT stiffness). The standard error of the measurement (%) ranged from 0.13% (AT length) to 2.96% (AT stiffness), and the minimum detectable change (%) of the mean ranged from 0.35% (AT length) to 8.20% (AT stiffness). CONCLUSION: The in vivo measurement of AT structure and mechanical properties showed a good to excellent degree of test-retest reliability in youth with obesity.