Sources of error when measuring Achilles tendon mechanics during the stance phase of running

In recent years, the use of methods to investigate muscle-tendon unit function that combine motion capture with ultrasound (MoCapUS) has increased. Although several limitations and individual errors of these methods have been reported, the total error from all the potential sources together has not been estimated. The aim of this study was to establish the total error in the Achilles tendon (AT) measurements, specifically its length (AT_L), strain (AT_S), and moment arm (AT_MA) acquired with MoCapUS during running. The total error from digitizing, marker movement, ultrasound calibration, and probe rotation errors caused mean AT_L error of 4.2 ± 0.6 mm, mean AT_MA error of 0.1 ± 0.1 mm, and could potentially alter measured AT_S by a mean 2.9 ± 0.2%. Correcting both the calcaneus insertion position (CIP) and properly synchronizing ultrasound and motion capture data caused changes of up to 5.4 ± 1.7 mm in AT_L and 11.6 ± 1.3 mm in AT_MA. CIP correction and synchronization caused a similar amount of change in AT_L, as well as AT_S. However, the AT_MA change was almost exclusively due to the CIP correction. Finally, if all sources of error were combined, the total AT_L error could reach 13.1 mm, the total AT_MA error could reach 14.4 mm, and AT_S differences could reach up to  ± 6.7%. The magnitude of such errors emphasizes the fact that MoCapUS-based AT measurements must be interpreted within the scope of their corresponding errors.