Sources of Error When Measuring Achilles Tendon Mechanics During the Stance Phase of Running

In recent years, the use of methods 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 digitising, 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 the calcaneus insertion position (CIP) and properly synchronising ultrasound and motion capture data combined caused ATL and ATMA changes up to 5.4 ± 1.7 mm and 11.6 ± 1.3 mm, respectively. Changes in ATL and ATS due to the CIP correction and synchronisation individually were similar. 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 such errors emphasises the fact that MoCapUS based AT measurements must be interpreted within the scope of their corresponding errors.