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 (ATL), strain (ATS), and moment arm (ATMA) acquired with MoCapUS during running. The total error from digitizing, marker movement, ultrasound calibration, and probe rotation errors caused mean ATL error of 4.2 ± 0.6 mm, mean ATMA error of 0.1 ± 0.1 mm, and could potentially alter measured ATS 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 ATL and 11.6 ± 1.3 mm in ATMA. CIP correction and synchronization caused a similar amount of change in ATL, as well as ATS. However, the ATMA change was almost exclusively due to the CIP correction. Finally, if all sources of error were combined, the total ATL error could reach 13.1 mm, the total ATMA error could reach 14.4 mm, and ATS 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.

Measurement of instantaneous Achilles tendon moment arm and force during the stance phase of running.

Accurate estimates of the Achilles tendon (AT) moment arm (ATMA) are necessary for investigating triceps surae muscle-tendon unit loading and function. There are limited reported values of ATMA during running. By combining ultrasound and motion capture, ATMA was estimated during the stance phase of running. Group mean ATMA was estimated at 49.2 ± 3.8 mm and 37.5 ± 5.3 mm, relative to the centre of rotation (malleoli markers midpoint) and the ankle finite helical axis respectively. Differences in the corresponding estimated AT forces reached up to 3100 N approximately. Such discrepancies can lead to misinterpretation of the whole muscle-tendon unit function.